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GBH Enterprises, Ltd.

Process Engineering Guide:
Hygiene: Estimating and
Understanding Personal Exposure to
Inhalation of Vapors on Chemical
Plants
Process Disclaimer
Information contained in this publication or as otherwise supplied to Users is
believed to be accurate and correct at time of going to press, and is given in
good faith, but it is for the User to satisfy itself of the suitability of the Product for
its own particular purpose. GBHE gives no warranty as to the fitness of the
Product for any particular purpose and any implied warranty or condition
(statutory or otherwise) is excluded except to the extent that exclusion is
prevented by law. GBHE accepts no liability for loss, damage or personnel injury
caused or resulting from reliance on this information. Freedom under Patent,
Copyright and Designs cannot be assumed.
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Process Engineering Guide:
Hygiene: Estimating and Understanding Personal Exposure to
Inhalation of Vapors on Chemical Plants
CONTENTS

0

INTRODUCTION

1

SCOPE

2

FIELD OF APPLICATION

3

DEFINITIONS

4

KEY CONCEPTS
4.1
4.2
4.3
4.4

Occupational Exposure Limits
Exposures to Vapor - Background, Task and Incident
Variability and Target Exposure Levels
Units

5

OVERVIEW OF METHOD

6

APPLICATION OF METHOD
6.1
6.2
6.3
6.4
6.5

Chemical Substances – List and Rank
Personnel - List and Rank
Plant – Define Zones
Zone Information
Integrate Exposures Over All Zones

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7

UNDERSTANDING

8

INCIDENTS

APPENDICES

A

VENTILATION

B

ESTIMATING CONCENTRATION AND EXPOSURE

C

VARIABILITY OF EXPOSURE LEVELS

D

CALCULATION OF INCIDENT QUANTITIES

E

SUBSTANCE INDEX

F

GLOSSARY

G

REFERENCES

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TABLES

1

STEPS IN A STUDY

2

TYPICAL EMISSION RATES FROM EQUIPMENT

3

TYPICAL EMISSION RATES FROM EQUIPMENT

4

EFFECTIVE WIND SPEED

5

NATURAL VENTILATION RATES OF BUILDINGS

6

SUBSTANCE INDICES FOR ACUTE INCIDENTS

FIGURES
1

FLOWCHART

2

RESULTS OF MONITORING

3

(LOG) NORMAL DISTRIBUTIONS

DOCUMENTS REFERRED TO IN THIS PROCESS ENGINEERING GUIDE

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INTRODUCTION
It is a vital aspect of chemical plant design and operation to minimize any contact
of the operating and maintenance personnel with the chemicals on the plant.
Often the most significant route of exposure to chemicals is by inhalation, and in
this case it is relatively easy to measure exposure. This has led to the
development of standards which indicate levels of exposure which should
minimize any risk to health. These will be called Occupational Exposure
Limits (OELs) in this Guide as a generic name.
Given an OEL as a design criterion it might seem essential to be able to estimate
personal exposures. In practice the complexity of the problem has prevented any
systematic approach being developed. Consequently plant design has tended to
be evolutionary, based on experience of earlier plant, and levels of exposure
found thereon.
There are some major problems with this evolutionary approach. Firstly there
may be very little previous experience of a particular plant design, or standards
may have tightened dramatically. Secondly there is very little appreciation of
which control measures are really effective, and which are more trouble than they
are worth.
This Guide is intended to provide a means to estimate personal exposures on
chemical plants to aid in such design problems. It also provides a means to rank
emission sources in terms of their importance in health terms. Control measures
can then be applied to problems of real significance, with an increased likelihood
of achieving genuinely cost-effective solutions. The problem is found to be well
suited to analysis on a computer using a spreadsheet program, and this
approach is recommended in the text.
The whole topic of monitoring exposure levels and designing appropriate controls
and procedures is central to Occupational Hygiene. It is anticipated that this
approach will normally be carried out by a combination of Occupational
Hygienists and Process Engineers, with other disciplines being consulted as
required.

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The recommended method for estimating exposures considers the materials
handled, the personnel at risk, and the potential sources of exposure. This Guide
considers primarily chronic exposure to toxics by vapor inhalation. The method
should however highlight acute problems and is potentially useful in assessing
risks due to other exposure routes e.g. dusts. This method has been used as part
of hygiene studies on plants in Europe.
The total exposure of an individual is considered to be a combination of
background and task exposures. Methods of estimating such exposures, by
dividing the plant into zones, determining release sources and local dispersion,
are presented. To make progress on the problem of estimating exposures which
may be random both in time and space, an 'averaging' approach has been
adopted. Variations around the 'average' and short term exposures due to
incidents are also considered. Potential acute problems due to unplanned
incidents are also examined.
This Guide does not attempt to define or recommend good practice in plant
design or operation. Its purpose is to help assess alternative designs or operating
procedures. The inherent uncertainty in the information used to estimate
exposures can be considerable; errors in estimates can be large. However
sensible application of this Guide should improve understanding of potential
exposures on a plant, rank exposure hazards and indicate areas for improvement
or of over-design.

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1

SCOPE

This Engineering Guide covers the estimation of the likely exposures of
personnel to toxic vapors on chemical plants. A methodology is given which
should be used as a complement to the standard design and Hazard Study
procedures.

2

FIELD OF APPLICATION

This Guide applies to the process engineering community worldwide.

3

DEFINITIONS
For the purposes of this Guide no specific definitions apply.

4

KEY CONCEPTS

4.1

Occupational Exposure Limits

In this Guide, the term Occupational Exposure Limit indicates the criterion set for
plant design and operation. Different countries have different terms for such
standards. Well known examples include Threshold Limit Values (TLVs) and
OSHA Permissible Exposure Limits (PELs), both US, and MAKs and TRKs from
Germany. The UK system of Occupational Exposure Limits has changed from
Control Limits and Recommended Limits to Maximum Exposure Limits (MELs)
and Occupational Exposure Standards (OESs). These will be more strictly
enforceable under the new Control of Substances Hazardous to Health
Regulations. The legal status and rules for deciding compliance with such
standards vary from country to country. This Clause can only lay out some
general guidance, based mainly on our current understanding of the UK system.
Specific guidance may need to be obtained from the relevant Occupational
Health Department to ensure that the correct criterion, either national or inhouse, has been selected, and is being applied correctly.

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OELs are based either on animal experiments (toxicology) or human experience.
It should be understood that the data underlying such standards are often not
very extensive, or reliable. The safety margin provided by the standards for
different substances can vary also. For this reason it is often stated that OELs
should not be regarded as fine lines dividing safe and dangerous conditions.
They can be thought of as limits which should be met with as much leeway as is
reasonably practicable.
In the UK system (see Guidance Note EH40) it is stated that OELs should 'not
normally be exceeded'. This phrase has never been defined. In GBHE it has
been taken to mean that 90% or 95% of all measured exposures for a workgroup should be below the OEL. It is important to understand that this means that
the average exposure of a work-group meeting the criterion will be significantly
less than the OEL - typically between 25% and 50% of the OEL in fact.
Consequently the target criterion for average personal exposure is not the OEL
itself but a fraction of it.
Most OELs are set for measurements over 8-hour shifts, known as 8-hour timeweighted averages (TWAs). For some substances it is more important to
measure short-term peaks, as the effects occur rapidly - an obvious example
would be carbon monoxide. Such OELs are called Short-Term Exposure Limits
(STELs), and in the UK are 10-minute TWAs. The calculations in this Guide are
based on 8-hour TWAs, but the procedure can handle other time bases
as appropriate.
More complex questions e.g. on the effects of exposure to mixtures of
substances, or the effect of unusual work-patterns, should be referred to the
relevant Occupational Health Department. A clear understanding of the criterion
to be applied is essential.

4.2

Exposures to Vapor - Background, Task and Incident

It is helpful to classify some different mechanisms by which people might be
exposed to vapors on a plant. Just walking around a plant causes some
exposure, which we label as 'BACKGROUND' exposure. The source of this can
be from vapor or dust clouds as they dissipate or from pools of liquid as they
evaporate. The vapor or dust clouds can be the fugitive emissions from plant
equipment (i.e. minor leaks from seals, glands, flanges etc.), or the consequence
of earlier emissions associated with a task (see below).

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The second class is the exposure directly associated with a person carrying out
some work, i.e. a 'TASK' exposure. The average exposure of people in a workgroup is thus the sum of their average 'TASK' and 'BACKGROUND' exposures.
For both 'BACKGROUND' and 'TASK' exposures a medium-term view should be
taken. In general any source of emission likely to occur more frequently than say
once a month should be included in the daily averages. Rarer events should be
included only if they are sufficiently sizeable to affect the daily average.
A further class of exposure is that due to 'INCIDENTS'. Incidents by definition are
infrequent but may involve significant releases of material. Although such events
are unlikely to make a noticeable contribution to the time weighted average
exposure, incidents may have an acute impact on the health of the exposed
person. The identification and quantification of potential incidents should occur
during Hazard Studies. This Guide should be used to complement the
Hazard Studies by contributing to quantifying the consequences of significant
releases.
'INCIDENTS' involve a different definition of toxic hazard from longer-term
exposures. For instance lead compounds are well known toxins, but are most
unlikely to cause an acute injury by inhalation. Thus the Occupational Exposure
Limit - 8 hour TWA - is not relevant for 'INCIDENTS'. It is suggested that the
IDLH (Immediately Dangerous to Life or Health) criterion be used for this
purpose.
Further consideration of incidents is given in Clause 8.

4.3

Variability and Target Exposure Levels

There are many reasons why exposure levels measured in the field do vary
considerably (see Guidance Note EH42). In the process outlined above, only the
mean exposure is estimated at each stage.
This is clearly the simplest procedure rather than considering ranges. The
variability of the exposure levels is compensated for by setting a target at a
fraction of the Occupational Exposure Limit (OEL). In general we recommend the
value of one-quarter of the OEL for plant design purposes (see 4.1).
Specific cases can be considered in more depth by comparisons with similar
types of plant which have been closely monitored.
This topic is developed in more detail in Appendix C.
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4.4

Units

The units used in this Guide are normally milligrams, seconds and meters. Thus
emission rates of contaminants are expressed in mg/s, air speeds in m/s, flow
rates of air through the plant in m3/s and concentrations in mg/m3.
Concentrations are often expressed in ppm (vol/vol). The conversion factor
involves the Molecular Weight of the substance MW. The formula is:

(24.45 is the volume in m3 occupied by 1 kmol of gas at 25 °C and 1
atmosphere).
Occasionally, different time bases are used e.g. the units of exposure are taken
as mg.min/m3.

5

OVERVIEW OF METHOD

On a plant a variety of people will be exposed to a variety of chemicals escaping
from a variety of process equipment. This method for estimating exposure offers
a way of handling these different variables. It considers the chemicals handled
the personnel on the plant, and the layout and types of equipment. Particular
emphasis is given to the distribution of equipment on the plant, and the plant is
considered as a series of 'zones'. The method considers background and task
exposure.
The key steps in the method are to:
(a)

Define the extent of the study.

(b)

Divide the plant into zones.

(c)

Gather the information.

(d)

Compute the exposures.

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Figure 1 and Table 1 describe the steps. Each step in the process is examined in
more detail in Clause 6. The use of spreadsheet techniques to collate data and
calculate the various exposures is highly recommended. Spreadsheets simplify
the repeat calculations for different chemicals or personnel, or for examining
design changes.
The cumulative exposure by vapor inhalation is estimated by considering the
various exposures that personnel may experience on a plant. Each particular
exposure is calculated from the exposure time and the concentration of the target
material in the local atmosphere i.e.:

The local concentration is estimated from the local vapour emission rate and the
effective local ventilation rate, where:

To provide a basis for operating the (existing) process outlined in Figure 1, in the
most efficient manner.

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FIGURE 1

FLOWCHART

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6

APPLICATION OF METHOD

6.1

Chemical Substances - List and Rank

The first stage is to consider the flowsheet of the process. From this, it is possible
to develop a list of substances which might present a significant inhalation
hazard. These substances can then be ranked in order of importance by taking
account of the following:
(a)

The quantity.

(b)

The conditions of containment (quality, temperature, pressure).

(c)

The Occupational Exposure Limit (OEL).

(d)

The concentration Immediately Dangerous to Life or Health (IDLH).

(e)

The saturated vapor pressure (svp) at ambient temperature.

The IDLH and svp are both intrinsic properties of the substance and can be
combined into a single Substance Index for ranking purposes (see Appendix E).
The Substance Index can be compared to a number of other substances to give
an understanding of the significance of the inhalation hazard. This should then be
combined with data on the quantities being handled, and the quality of
containment to decide for which substances, if any, a detailed analysis is
required.
For relevant substances the following physical properties should be obtained:
(1)

The molecular weight.

(2)

The solubility in water, and in main components of relevant process
streams.

(3)

The saturated vapor pressure across the range of process temperatures
and process streams.

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6.2

Personnel - List And Rank

Personnel who might be exposed then need to be identified. The most obvious
categories generally are process operators, and regular maintenance tradesmen,
especially fitters. Other tradesmen and supervisors should be considered. The
aim is not to produce a complete listing of those who might be exposed to any
extent, but a practical short list of those who might be the most significantly
exposed.
Given this selection of the more relevant personnel, a list of tasks, which might
cause significant exposure, should be produced for each chosen work-group.

6.3

Plant - Define Zones

Normally a plant is not homogeneous in terms of exposure. All aspects of
ventilation, emission sources, and occupacity by personnel will cause the
exposure to vary significantly. The recommended method is to divide the plant
into zones (say 10 - 20) which are at least reasonably homogeneous. The
principal division should be in terms of the quality of ventilation provided. For
example outside areas should be separated from internal areas. Normally,
different floors should also be separated.
Further sub-division of zones can be carried out if there are sub-areas where a
significantly different exposure pattern is likely. This will mainly be due to the
items of equipment, or operations in these sub-areas.
This method also uses the concept of Zone 0, where personnel spend time, but
there is no exposure. This can include time off-plant, and time in amenities areas
and control rooms. Careful thought should be given as to whether these areas
are indeed zero-exposure. Control rooms for example can easily be
contaminated, especially by low-volatility materials brought in on people’s
clothing. The relatively long time spent in the control room can then cause
significant exposure.

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6.4

Zone Information
6.4.1 Process Equipment
Having defined the extent of the plant zones, information is now required
on the equipment which could be a source of exposure. This is achieved
by listing in each zone:
(a)

all equipment which could contain significant quantities of the
material under consideration,

(b)

and for that equipment, the process stream data (pressure,
temperature, state, composition and in particular concentration of
target material).

The list of equipment should include plant items and any other potentially
significant sources such as drains, sumps or pipelines passing through the
zone. Note should also be made of any equipment in other zones which
releases material into the zone. For example, if an item has a remote vent
or drain pipe in another zone, then the location of such outlets should also
be noted.
Process stream data should be available from the process flowsheet.

6.4.2 Fugitive Sources
The equipment identified above should now be examined to determine the
possible escape of material to atmosphere. There are several routes for
these fugitive emissions; these include:
(a)

Seals on moving shafts. Such shafts include pumps, compressors,
agitators, control valves etc. Release rates will depend upon the
complexity of the design and the wear and maintenance
performance. Tables 2 and 3 give an indication of typical rates.

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(b)

Vents. These include process vents, where allowance has been
made for a recognized gas flow to atmosphere, breathing vents,
where flow is occasional and due to displacement of gas (often
contaminated air) during filling or temperature changes, and
emergency relief vents. Estimates can be made of the quantities of
vapor vented either by calculating vapor flows in vents or by
calculating the volumes and compositions of displaced
contaminated air.

(c)

Open vessels. This category covers atmospheric-pressure
uncovered tanks, pits, drain systems etc. often containing
potentially contaminated water.

(d)

'Leaks'. Material can leak through joints, drain valves, corrosion
holes etc. In a well designed and maintained system, leaks from
joints may be negligible. However when corrosion is a problem, that
is when materials of construction have a short life, serious leaks
can occur.

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For possible sources not covered above (e.g. fume cupboards and laminar
flow booths) specialist advice may be required.
For each fugitive source, an estimate is required of the quantity,
properties, and frequency of chemicals which can be released into each
zone. It is recognized that the accuracy of these estimates will be limited.
Particular problems occur with sources where conditions vary with time.
Examples include mechanical equipment which wears (pump seals),
where corrosion is significant, and where material is released
occasionally, either due to an infrequent process operation or say tank
breathing. It may be necessary in these cases to estimate both an
average and worst release.

These figures, which are based on CMA guidance (ref 8) should be used
only as preliminary estimates. Actual leak rates will depend upon the
suitability of the equipment for the duty, the severity of the duty, the
standard of maintenance etc. Advice should be sought from Machinery or
Piping Sections where necessary.

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6.4.3 Time Spent In Zones
In order to estimate people’s exposure to contaminants it is necessary to
know where they spend their time and what they do. For an existing plant,
the plant manager should be able to provide the information. For a new
plant the information may have to be inferred from the operating
instructions. The information needed is the average number of minutes
per shift spent in each zone and on each task. It is often necessary to
consider what is done over a week or a month to arrive at an average
figure.

6.4.4 Tasks
Material can also be released into the atmosphere as a result of carrying
out tasks. Such tasks are usually those where material is transferred into
open vessels or where process joints have to be broken. Examples
include:
(a)

Filling/emptying containers. Containers include tankers, cylinders, drums,
bottles, sacks etc. Typical tasks are packing product, charging, sampling,
draining, purging etc. Material escapes either due to displacement of
contaminated air, spillage, or evaporation from wetted equipment (e.g. dip
pipes).

(b)

Maintenance of equipment and/or contents. Work on process equipment
can lead to release of material either during decontamination
(purging/washing) or maintenance (replacement or repair of damaged
parts, catalyst, packing etc.). Particular problems occur when material
cannot be removed effectively due to blockages.
A list of all tasks performed in each zone with potentially significant
exposure should be prepared. These tasks should include both process
and maintenance operations. The frequency and extent should be noted.
Estimates of the quantities of material associated with each task are
required. For example in a sampling operation, how much material has to
be purged before the sample is taken? How big is the sample? What form
is it in? How much vapor is displaced?

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6.4.5 Estimate Effective Local Ventilation Rates
Ventilation is as yet one of the least certain areas in the estimation of
personal exposure. To ease calculation it is assumed that contaminated
air leaves a zone at a steady rate to be replaced by air from another zone
or the outside environment. An estimate of the volumetric rate is required
for the estimation of zone concentrations given later.
For an indoor area three approaches are possible-calculation,
measurement and simple estimation. There are methods available
(Goodfellow, ref 3, provides an introduction and further references) for the
prediction of the ventilation in a building when full details are known of its
construction. However these are involved and unlikely to be worth using
for the type of study envisaged here.
There are also methods for measuring the air changes per hour for
existing buildings. When designing a new structure, it may be possible to
take measurements on an analogous existing system. Where the
ventilation is forced, the air flow rate should be known.
Simple estimation involves predicting a 'normal' number of air changes per hour
and from this a volumetric flow rate. A simple table is given for guidance in
Appendix A.
Local ventilation can be applied to major sources, such as a sample booth or a
hooded plating bath. A way of dealing with these is to ascribe an efficiency factor.
For example, a well designed fume cupboard allows only 0.1% of the material
released in it to escape from the front. So effective source strength can be used
in the study, which is one thousandth of the actual source strength.
For outdoor areas the problem should be simpler. At its simplest the air can be
assumed to travel through the plant at a steady speed, equal to the wind speed,
under plug flow. However there are a number of complicating factors. Choosing
the appropriate wind speed is only one (see Appendix A). A chemical plant
represents a permeable obstruction to the wind. Air flow accelerates through the
gaps between plant items and eddies behind them. Overall the wind speed is
attenuated. However some estimate can be made of the volumetric flow rate
through a zone (see Appendix A).

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6.4.6 Estimate Fugitive Vapor Emission Rates
The CMA (ref 8) give guidance on how to measure vapor emission rates
from items of equipment, and the Appendices refer to the use of existing
methods, particularly as developed by the Environmental Protection
Agency (EPA).
For material to be a hazard by the vapor inhalation route, the released
material needs to be present as a vapor. For direct vapor/gas releases,
the vapor rate is the same as the release rate. For liquid releases, the fate
of the liquid and mass transfer into the vapor phase should be considered.
The simplest assumption is to assume all released material evaporates
into the zone; this may be true for a small leak of hot liquid onto a poorly
drained surface. For other liquid releases, particularly where material is
drained away or treated, only some fraction of the released material
evaporates locally. Removal of liquid may cause problems in other zones
e.g. where liquid in drains meets hot water.

Mass transfer may occur by flashing if a superheated liquid is released, or
more generally, by evaporation from a wetted surface. Although it is
relatively straightforward to calculate the rate of vapor release from a
flashing stream if the process stream conditions are known, calculations of
evaporation rates are less precise. The evaporation rate is a function of
the wetted area, pool temperature and composition (defining vapor
pressure) and local wind speed.
A useful correlation, due to Clancey, (ref 2) for evaporation from a well mixed
pool is:

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For evaporation of a component in a dilute solution, mass transfer of the
component within the solution may be limiting. In this case the vapor rates
will be overestimated using the above equation.

6.4.7 Estimate Task Vapor Emission Rates
From the information obtained in 6.4.4, estimates of the material released
may be made. The approach adopted will be similar to that for estimating
vapor rates for fugitive sources. The quantity and nature of material
escaping from the equipment, and the fraction of that material evaporating
locally should be calculated. As task releases usually only last for a limited
time, an average vapor emission rate should be calculated.

6.4.8 Estimate Preliminary Background Concentrations
When making a preliminary estimate of background concentrations it is
assumed that the air entering a zone is free of the contaminant. Allowance
for the air being already contaminated is made later. The preliminary
background concentration is the sum of the fugitive emission rate and the
task emission rate in the zone divided by the volumetric air flow rate and
multiplied by an inefficiency factor, see Appendix B.

6.4.9 Estimate Task Exposures
When carrying out a task, personnel may be exposed to concentrations in
excess of the background, as the task may be the cause of material
release, and the personnel may be working close to the source. The task
exposure can be calculated as:

The effective task ventilation rate is estimated by assuming the person
works in a 'task zone', a nominal 3m cube around the equipment or
source. Further details are given in Appendix B.

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Knowing the task concentration and the duration of the task or exposure
time, the task exposure can be calculated as:

6.5

Integrate Exposures Over All Zones
6.5.1 Revise Background Concentrations
In the preliminary estimation (6.4.8) of background concentrations it was
assumed that the air entering a zone was uncontaminated. Frequently this
is not the case and an allowance has to be made for the contamination of
the incoming air. The easiest way to do this is to add the quantity of
contaminant brought into the zone to the emission rate in the zone before
calculating the concentration. Where a number of zones interact iteration
may be required but that should not be difficult using a spreadsheet.
6.5.2 Estimate Exposure Due To Background
The average background exposure in mg.min/m3 for a shift is estimated
by multiplying the average number of minutes per shift spent in each zone
by the background concentration in the zone and summing over all zones.
If the pattern of work is very variable it may be necessary to investigate
the various patterns to check that none gives rise to an unacceptably high
exposure over a shift.

6.5.3 Estimate Average Personal Exposure
Since incidents are by definition relatively rare events, they are ignored in
estimating average personal exposure. The average personal exposure in
mg.min/m3 per shift is estimated by adding the average background
exposure to the average exposure due to tasks. This per shift figure may
need to be converted to the correct time basis (usually 8 hr Time
Weighted Average, TWA) for comparison with the relevant exposure
criterion.

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7

UNDERSTANDING

The procedure described so far enables an estimate to be made of personal
exposure which may be compared with the OEL. If this is fully satisfactory no
more may need to be done. However, much information is contained in the way
the estimate is built up which may be used to understand the main sources of
exposure. This understanding may then be used to reduce personal exposure.
This reduction may be necessary to meet the OEL, although in general it is not
enough simply to comply with OELs; exposures should be reduced as far as
reasonably practicable.
The following understanding is likely to be obtained
(a)

Which chemicals are particularly troublesome?

(b)

Who is most at risk and whether they are likely to be overexposed;

(c)

The distribution of background concentrations and the location of any 'hotspots';

(d)

A ranking of emission sources;

(e)

A ranking of tasks.

The understanding may demonstrate the need for changes such as those listed
below:
(1)

The process may need to be altered to avoid the use of a chemical if no
satisfactory way can be found of containing it.

(2)

Items of equipment may need to be changed or modified to improve
containment if they are shown to be major emission sources.

(3)

The method of operation can often be changed to separate people from
high concentrations once the location of the latter has been identified.

(4)

The ventilation may need to be improved, either generally or by the
provision of local extraction.

(5)

Additional personal protection may be needed, but this should be seen as
a last resort.

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If the calculations have been computerized it is relatively simple to evaluate the
expected effect of each change. This should enable the provision of a
satisfactory working environment in a cost effective manner.
The estimates for background concentrations and task exposures should be
checked to ensure that STELs are not exceeded and that acute problems do not
occur.

8

INCIDENTS

As discussed in 4.2, INCIDENTS are single exposures of such a magnitude as to
possibly cause direct damage to health. Not all toxic substances fall into this
category; for instance lead compounds are most unlikely to cause an acute injury
by inhalation. So the first step in defining possible incidents is to determine what
level of exposure, if any, could cause immediate health effects.
The criterion proposed here is IDLH (Immediately Dangerous to Life or Health).
This was developed by NIOSH in the USA to assist respirator selection. The
IDLH concentration represents the maximum level from which one could escape
in 30 minutes without any escape impairing symptoms, or any irreversible health
effects. Values for a wide variety of substances are listed in Reference
NIOSH/OSHA. A new system of Emergency Response Planning Guides
(ERPG) is currently being developed by the American Industrial Hygiene
Association (AIHA). These standards will probably be more thoroughly
researched than the IDLH ones but are, as yet, only available for a few
substances.
Using the approach of this Guide, it is possible to estimate the scale of events
which will lead to concentrations above the IDLH value in occupied areas of the
plant. This scale will depend on whether the emission is inside or outside. Such
estimates give quantitative criteria for Hazards and Operability Studies.
The acute hazard associated with any substance can be estimated using the
Substance Index approach - see Appendix E. This can usefully be refined in
early stages of Hazard studies using rough estimates of typical indoor and
outdoor ventilation. An example is given in Appendix D.

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There is no agreement as yet as to what frequency events causing exposures to
IDLH concentrations might be allowed. It is strongly recommended that the
relevant Occupational Health Department is involved in such decisions, both to
confirm that the IDLH value is reasonable, and also to agree any choice of
frequency.

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APPENDIX A
A.1

VENTILATION

OUTDOOR VENTILATION

As stated in the main text, the ventilation rate for outdoor zones is based on wind
speed. The problem then remains of choosing an appropriate wind speed. Wind
speed is approximately log-normally distributed and the geometric mean wind
speed frequently lies between 3 and 4!m/s. Places such as Baton Rouge,
Lousiana is at the bottom of the range; Newark, New Jersey is in the middle with
Cardiff Wales at the top end. Lower values are found for Gladstone, Australia
(1.8 m/s) and Tokyo, Japan (2.8!m/s) and higher ones for Wattisham, Suffolk,
England (4.6 m/s) and Anglesey, Wales (5.6!m/s). Users should satisfy
themselves that they are using a value appropriate to the area concerned.
Wind speed is normally measured at a height of 10m. The wind speed varies with
height, e.g. at 2m in average conditions and surroundings it is 70% of its value at
10m. The effect of plant will be to slow the wind further and reduce the local
effective ventilation. Estimation of ventilation is particularly difficult for outside
zones which contain walls. A tentative set of effective wind speeds is suggested
in Table 4. This suggestion assumes that the wind speed is reduced uniformly,
from an 'exposed' value to an 'indoor' value, as the plant zones become more
confined.

The air flow rate for an outdoor zone is found by multiplying the effective wind
speed by a suitable cross-sectional area. If the wind direction is assumed to be
uniformly distributed, the mean cross-sectional area of a rectangular zone is the
height of the zone multiplied by the sum of the length and width of the zone
multiplied by 0.64.

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A.1.1 Downwash from Vents
If vents are set close to the roof, vented material is likely to be entrained in the
building wake. This is discussed clearly by Bryant, (ref 1) 'When the actual stack
height is equal to or less than 1.5 building heights i.e. the effective stack height is
appreciably less than one building height, effluent entrained in the wake could be
carried to ground level in the immediate vicinity of the building on which the stack
stands.' This was considered by Scorer and Barrett (ref 6) with respect to the
short-term concentration likely to be found near a building with a low chimney.
They assumed the pollution to be spread over a cone of semi-angle 12° bent
round the building, in a moderate wind of about 5 m/s, this representing the worst
weather conditions. In winds of higher speeds the pollution would be less, and in
lower speeds the wake would not be well developed.
From the formula given by Scorer and Barrett:

where Ps is the typical maximum short term concentration per unit release rate
(unit/m3 per unit/sec) and is the length of the trajectory to the place where the
pollution is measured (m)
In general, on the assumption that the place of interest is at ground level outside
the building, the length of trajectory is given by the height of the stack above
ground level plus the distance from the stack to the outside edge of the building.
To convert Ps into the long-term concentration PL it is suggested by Bryant that,
corresponding to the assumed semi-vertical angle of 12°, a swing of the wind
factor of 360/(2 x 12) = 15 may be applied.

This is tantamount to assuming a uniform wind rose and spreading released
material uniformly in all directions. In light winds the pollutant may not be brought
down to ground level. The above assumes a vent on the roof of a closed building.
If one side of the building is open or has an open structure built against it,
eddying may cause contaminant to enter the building or open structure.

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A.2

NATURAL VENTILATION OF INDOOR AREAS

The following table (Table 5) has been taken from the 1970 Institute of Heating
and Ventilation Engineers (IHVE) Guide. It does not apply if there are large
doorways or forced ventilation.

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APPENDIX B
B.1

ESTIMATING CONCENTRATION AND EXPOSURE

ESTIMATING ZONE CONCENTRATIONS

The choice of K factor is something of an art. The factor allows for three items:
(a)

How uniformly the air is distributed through the zone.

(b)

How uniformly the sources are distributed through the zone.

(c)

How uniformly the worker 'samples' the air throughout the zone.

For most zones K=1 should be used. If there is doubt about the uniformity of any
of the three items, a value of K greater than one should be used. The plug flow
value should be used only when all three items are favorable. Air distribution is
favorable in uncluttered outdoor zones or where the ventilation has been
specially designed to achieve plug flow. Source distribution is favorable where it
is uniform or sources are near the air exits (and flow reversal does not happen).
Worker distribution is favorable when it is uniform or the worker stays deliberately
upwind of sources.
Some users may wish to make the inefficiency factor a product of three individual
terms, each representing one of the contributing items.

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B.2

ESTIMATING TASK EXPOSURES

There are two major routes for exposure when carrying out a task:
(a)

Evaporation from a wetted surface
This is the most frequent source of exposure with liquids. Tasks usually
relate to transferring materials. Examples include tanker discharge where
material can be spilled during hose handling, draining equipment, and
sampling where excess liquid wets bottles and gloves etc.

(b)

Release of gas or vapor
There are several tasks when contaminant vapor is released directly into
the local environment where people are working. Examples include
maintenance work on equipment which has not been completely purged,
filling containers where contaminated air is displaced, dipping tanks etc.
The gas release may be of limited duration (i.e. less than the time
someone is present) or may occur throughout the task.
To estimate the task exposure, estimates are needed of the average
vapor emission rate, the local ventilation, and the time spent on the task.

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B.2.1 Worked Examples
(a)

Liquid spillages
Consider an operation where a small quantity of material is spilled. This
could be a filling or maintenance operation. Assume 500 g is spilled and
this covers a floor area of say1m2. Assume operation is outside and task
lasts 15 min. Assume material is spilled at ambient say 10 °C; vapor
pressure is 0.01 bara; mol wt 100.
Assume wind speed 2 m/s, then evaporation rate from equation (4) is

Assume task takes place in 3 x 3 x 3 m cube.

Assume K = 1 , no obvious maldistribution of worker or ventilation, then

As pool persists for longer than the 15 min of the task, the worker will be
exposed to an average 7 mg/m3 concentration throughout the task.

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(b)

Release of gas
Consider operation where non-toxic solid is added to an
atmospheric vessel containing a volatile toxic liquid. Exposure
occurs as operator is exposed to the displaced contaminated air.
Assume 0.2 m3 of solid added. Assume vapor pressure of liquid is
0.1 bara; mol. Wt. = 120; temperature of vessel 50 °C. Assume
operation takes place inside a building with 2 air changes/hour.

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APPENDIX C

VARIABILITY OF EXPOSURE LEVELS

When sufficient personal monitoring is carried out there are two obvious
limitations on the type of distribution. Firstly no result can be less than zero, and
secondly occasional high results will occur, with the frequency tailing off at higher
values. The simplest distribution with these characteristics is the log-normal
distribution (lnd), and this often fits experimental data very well.
In practice the fit of a set of results to a lnd is very easily demonstrated by
plotting the cumulative percentage of results below a certain value, against that
value on Log-probability paper. Figure 2 shows plots of two distributions, with a
median of 10. NIOSH (ref 11) discusses deviations from lnd in more depth.
The distribution of a log-normal distribution is defined by the geometric mean and
the geometric standard deviation (gsd). The gsd has been found to vary in
practice from about 1.3 to 10, although more commonly between 1.5 and 2.5.
Figure 3 demonstrates the difference between a normal distribution, and lnd’s
with gsd’s of 1.5 and 3. It will be seen that a gsd of 1.5 is not dissimilar to the
normal distribution. The gsd of 3 however gives a much more skewed
distribution, with the arithmetic mean largely due to occasional high level
excursions. This is more characteristic of plants where background levels are
very low, relative to the occasional excursions.
Compliance with a standard is best considered as the percentage of results
below the standard. This should be at least 90%, and preferably 95%, to show
good compliance. The results of the method developed in this Guide generate
arithmetic means, so the relationship between this mean and 90 and 95
percentiles is vital. Fortunately, the ratio of the arithmetic mean to the 90 and 95
percentiles does not vary much with gsd. This means that if the arithmetic mean
of a set of results, which are log-normally distributed is less than one-quarter the
OEL, then 95% compliance is guaranteed, whatever the gsd. With favorable
gsd’s of 1.6 or less the arithmetic mean can be one-half the OEL for 95%
compliance.
As stated earlier, we generally recommend the value of one-quarter of the OEL
for plant design purposes. This errs slightly to caution, but care has been taken in
earlier stages not to accumulate safety factors.

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APPENDIX D

CALCULATION OF INCIDENT QUANTITIES

The quantity to cause an incident, as discussed in Clause 8, depends on:
(a)

The IDLH concentration

(b)

The effective ventilation of the zone. In the earlier stages of plant design
rough estimates of outdoor and indoor ventilation rates can be made
following Appendix A.

As an example consider a substance with an IDLH concentration of 100 mg/m3,
over 30 minutes. The zone will be given dimensions of 10m x 5m x 5m (250 m3).
In Appendix A.1 it was suggested that a value of 1 m/s be used as an average
wind speed, together with a formula for Mean flow area. Assuming a K of 1 this
gives an effective ventilation rate (outdoors),

Refinery Process Stream Purification Refinery Process Catalysts Troubleshooting Refinery Process Catalyst Start-Up / Shutdown
Activation Reduction In-situ Ex-situ Sulfiding Specializing in Refinery Process Catalyst Performance Evaluation Heat & Mass
Balance Analysis Catalyst Remaining Life Determination Catalyst Deactivation Assessment Catalyst Performance
Characterization Refining & Gas Processing & Petrochemical Industries Catalysts / Process Technology - Hydrogen Catalysts /
Process Technology – Ammonia Catalyst Process Technology - Methanol Catalysts / process Technology – Petrochemicals
Specializing in the Development & Commercialization of New Technology in the Refining & Petrochemical Industries
Web Site: www.GBHEnterprises.com
APPENDIX E
E.1

SUBSTANCE INDEX

INTRODUCTION
There is a need for a simple way of ranking substances for their difficulty
of control for hygiene purposes. The Substance Index as described below,
is relevant to short-term exposures and incidents, rather than chronic
exposures.
Two principal characteristics of a substance that indicate its relative
difficulty of control for hygiene purposes are its 'volatility' and its 'toxicity'.
As will be seen both can be expressed as atmospheric vapor
concentrations, so the ratio of the two is a dimensionless index of toxic
hazard.
Here 'volatility' is an indication of the ease with which a substance may
pass into the vapor phase. A simple measure of this is the partial pressure
exerted by the specified substance. For pure substances, rather than
mixtures, the saturated vapor pressure (svp) can be used. For the
examples below, svp at ambient conditions is used - care should be taken
to use relevant data for any real-life examples.
This comparison applies strictly to vapors arising from liquids and solids,
e.g. evaporation from a pool. Emissions of gases can be treated in the
same way, as they produce a gas cloud of the pure substance, i.e. at 1
bar, rather than the svp of a liquid. This underestimates the immediate
impact of a gaseous emission, but has the merit of simplicity. Strictly,
Substance Indices for gases should be compared only to other gases.
Liquefied gases behave like a mixture of a gas and a liquid. Gas flashes
off until the pool cools to the boiling point of the substance, then it
behaves as a liquid with svp of 1 bar. The simplified formula for gases is
appropriate in this case.

Refinery Process Stream Purification Refinery Process Catalysts Troubleshooting Refinery Process Catalyst Start-Up / Shutdown
Activation Reduction In-situ Ex-situ Sulfiding Specializing in Refinery Process Catalyst Performance Evaluation Heat & Mass
Balance Analysis Catalyst Remaining Life Determination Catalyst Deactivation Assessment Catalyst Performance
Characterization Refining & Gas Processing & Petrochemical Industries Catalysts / Process Technology - Hydrogen Catalysts /
Process Technology – Ammonia Catalyst Process Technology - Methanol Catalysts / process Technology – Petrochemicals
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Web Site: www.GBHEnterprises.com
E.2

INCIDENTS

When considering 'INCIDENTS' a suitable measure of the 'toxicity' is the IDLH
concentration. The simple index is thus:

where P is the relevant pressure in bars, converted in this formula to ppm by
assuming ideality. As discussed this pressure is 1 bar for a gas, the saturated
vapor pressure for a pure liquid, and the partial pressure for liquid mixtures. For
liquids the pressures should be at the liquid temperature. Table 5 assumes 25°C.

E.3

SHORT TERM HAZARDS

The same logic can be used when considering short term emissions, against the
Short Term Exposure Limit (STEL), averaged over 10 minutes. In this case the
STEL value replaces IDLH. This is recommended only when the STEL presents
the primary standard for compliance, i.e. short-term exposures are more relevant
than longer term ones. Of the compounds in Table 5 this applies only to chlorine,
and possibly trichlorofluoromethane. Advice should be sought from the relevant
Occupational Health Department for specific substances.

E.4

LONGER TERM

When considering long term (CHRONIC) effects the effect of volatility is more
complex. A highly volatile material will reach higher concentrations more quickly,
but over longer time periods a lower volatility substance can exert the same
average concentration. High volatility can even aid control in that it may be
possible to leave a known emission to disperse. On the other hand low volatility
substances can drain away, or be treated with absorbent.
The impact of volatility can be discounted, provided it is realized that the
emission rates used in calculations are the effective emissions, i.e. the quantity
left to evaporate after alternative outlets have been considered, such as losses to
drains, or conversion to different compounds over time.

Refinery Process Stream Purification Refinery Process Catalysts Troubleshooting Refinery Process Catalyst Start-Up / Shutdown
Activation Reduction In-situ Ex-situ Sulfiding Specializing in Refinery Process Catalyst Performance Evaluation Heat & Mass
Balance Analysis Catalyst Remaining Life Determination Catalyst Deactivation Assessment Catalyst Performance
Characterization Refining & Gas Processing & Petrochemical Industries Catalysts / Process Technology - Hydrogen Catalysts /
Process Technology – Ammonia Catalyst Process Technology - Methanol Catalysts / process Technology – Petrochemicals
Specializing in the Development & Commercialization of New Technology in the Refining & Petrochemical Industries
Web Site: www.GBHEnterprises.com
Then the best index to the hazard presented by a substance is simply the value
of the OEL. Comparisons may be better in mg/m3 rather than ppm because
emissions are normally considered in weight rather than molar terms.

The usefulness of Table 6 can be judged by considering two well-known toxic
hazards, chlorine and mercury. Chlorine ranks easily the worst INCIDENT
substance. Mercury on the other hand presents essentially no INCIDENT hazard,
but is very difficult to contain on a long term basis.

Refinery Process Stream Purification Refinery Process Catalysts Troubleshooting Refinery Process Catalyst Start-Up / Shutdown
Activation Reduction In-situ Ex-situ Sulfiding Specializing in Refinery Process Catalyst Performance Evaluation Heat & Mass
Balance Analysis Catalyst Remaining Life Determination Catalyst Deactivation Assessment Catalyst Performance
Characterization Refining & Gas Processing & Petrochemical Industries Catalysts / Process Technology - Hydrogen Catalysts /
Process Technology – Ammonia Catalyst Process Technology - Methanol Catalysts / process Technology – Petrochemicals
Specializing in the Development & Commercialization of New Technology in the Refining & Petrochemical Industries
Web Site: www.GBHEnterprises.com
Refinery Process Stream Purification Refinery Process Catalysts Troubleshooting Refinery Process Catalyst Start-Up / Shutdown
Activation Reduction In-situ Ex-situ Sulfiding Specializing in Refinery Process Catalyst Performance Evaluation Heat & Mass
Balance Analysis Catalyst Remaining Life Determination Catalyst Deactivation Assessment Catalyst Performance
Characterization Refining & Gas Processing & Petrochemical Industries Catalysts / Process Technology - Hydrogen Catalysts /
Process Technology – Ammonia Catalyst Process Technology - Methanol Catalysts / process Technology – Petrochemicals
Specializing in the Development & Commercialization of New Technology in the Refining & Petrochemical Industries
Web Site: www.GBHEnterprises.com
Refinery Process Stream Purification Refinery Process Catalysts Troubleshooting Refinery Process Catalyst Start-Up / Shutdown
Activation Reduction In-situ Ex-situ Sulfiding Specializing in Refinery Process Catalyst Performance Evaluation Heat & Mass
Balance Analysis Catalyst Remaining Life Determination Catalyst Deactivation Assessment Catalyst Performance
Characterization Refining & Gas Processing & Petrochemical Industries Catalysts / Process Technology - Hydrogen Catalysts /
Process Technology – Ammonia Catalyst Process Technology - Methanol Catalysts / process Technology – Petrochemicals
Specializing in the Development & Commercialization of New Technology in the Refining & Petrochemical Industries
Web Site: www.GBHEnterprises.com
Refinery Process Stream Purification Refinery Process Catalysts Troubleshooting Refinery Process Catalyst Start-Up / Shutdown
Activation Reduction In-situ Ex-situ Sulfiding Specializing in Refinery Process Catalyst Performance Evaluation Heat & Mass
Balance Analysis Catalyst Remaining Life Determination Catalyst Deactivation Assessment Catalyst Performance
Characterization Refining & Gas Processing & Petrochemical Industries Catalysts / Process Technology - Hydrogen Catalysts /
Process Technology – Ammonia Catalyst Process Technology - Methanol Catalysts / process Technology – Petrochemicals
Specializing in the Development & Commercialization of New Technology in the Refining & Petrochemical Industries
Web Site: www.GBHEnterprises.com
Refinery Process Stream Purification Refinery Process Catalysts Troubleshooting Refinery Process Catalyst Start-Up / Shutdown
Activation Reduction In-situ Ex-situ Sulfiding Specializing in Refinery Process Catalyst Performance Evaluation Heat & Mass
Balance Analysis Catalyst Remaining Life Determination Catalyst Deactivation Assessment Catalyst Performance
Characterization Refining & Gas Processing & Petrochemical Industries Catalysts / Process Technology - Hydrogen Catalysts /
Process Technology – Ammonia Catalyst Process Technology - Methanol Catalysts / process Technology – Petrochemicals
Specializing in the Development & Commercialization of New Technology in the Refining & Petrochemical Industries
Web Site: www.GBHEnterprises.com

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Hygiene: Estimating and Understanding Personal Exposure to Inhalation of Vapors on Chemical Plants

  • 1. GBH Enterprises, Ltd. Process Engineering Guide: Hygiene: Estimating and Understanding Personal Exposure to Inhalation of Vapors on Chemical Plants Process Disclaimer Information contained in this publication or as otherwise supplied to Users is believed to be accurate and correct at time of going to press, and is given in good faith, but it is for the User to satisfy itself of the suitability of the Product for its own particular purpose. GBHE gives no warranty as to the fitness of the Product for any particular purpose and any implied warranty or condition (statutory or otherwise) is excluded except to the extent that exclusion is prevented by law. GBHE accepts no liability for loss, damage or personnel injury caused or resulting from reliance on this information. Freedom under Patent, Copyright and Designs cannot be assumed. Refinery Process Stream Purification Refinery Process Catalysts Troubleshooting Refinery Process Catalyst Start-Up / Shutdown Activation Reduction In-situ Ex-situ Sulfiding Specializing in Refinery Process Catalyst Performance Evaluation Heat & Mass Balance Analysis Catalyst Remaining Life Determination Catalyst Deactivation Assessment Catalyst Performance Characterization Refining & Gas Processing & Petrochemical Industries Catalysts / Process Technology - Hydrogen Catalysts / Process Technology – Ammonia Catalyst Process Technology - Methanol Catalysts / process Technology – Petrochemicals Specializing in the Development & Commercialization of New Technology in the Refining & Petrochemical Industries Web Site: www.GBHEnterprises.com
  • 2. Process Engineering Guide: Hygiene: Estimating and Understanding Personal Exposure to Inhalation of Vapors on Chemical Plants CONTENTS 0 INTRODUCTION 1 SCOPE 2 FIELD OF APPLICATION 3 DEFINITIONS 4 KEY CONCEPTS 4.1 4.2 4.3 4.4 Occupational Exposure Limits Exposures to Vapor - Background, Task and Incident Variability and Target Exposure Levels Units 5 OVERVIEW OF METHOD 6 APPLICATION OF METHOD 6.1 6.2 6.3 6.4 6.5 Chemical Substances – List and Rank Personnel - List and Rank Plant – Define Zones Zone Information Integrate Exposures Over All Zones Refinery Process Stream Purification Refinery Process Catalysts Troubleshooting Refinery Process Catalyst Start-Up / Shutdown Activation Reduction In-situ Ex-situ Sulfiding Specializing in Refinery Process Catalyst Performance Evaluation Heat & Mass Balance Analysis Catalyst Remaining Life Determination Catalyst Deactivation Assessment Catalyst Performance Characterization Refining & Gas Processing & Petrochemical Industries Catalysts / Process Technology - Hydrogen Catalysts / Process Technology – Ammonia Catalyst Process Technology - Methanol Catalysts / process Technology – Petrochemicals Specializing in the Development & Commercialization of New Technology in the Refining & Petrochemical Industries Web Site: www.GBHEnterprises.com
  • 3. 7 UNDERSTANDING 8 INCIDENTS APPENDICES A VENTILATION B ESTIMATING CONCENTRATION AND EXPOSURE C VARIABILITY OF EXPOSURE LEVELS D CALCULATION OF INCIDENT QUANTITIES E SUBSTANCE INDEX F GLOSSARY G REFERENCES Refinery Process Stream Purification Refinery Process Catalysts Troubleshooting Refinery Process Catalyst Start-Up / Shutdown Activation Reduction In-situ Ex-situ Sulfiding Specializing in Refinery Process Catalyst Performance Evaluation Heat & Mass Balance Analysis Catalyst Remaining Life Determination Catalyst Deactivation Assessment Catalyst Performance Characterization Refining & Gas Processing & Petrochemical Industries Catalysts / Process Technology - Hydrogen Catalysts / Process Technology – Ammonia Catalyst Process Technology - Methanol Catalysts / process Technology – Petrochemicals Specializing in the Development & Commercialization of New Technology in the Refining & Petrochemical Industries Web Site: www.GBHEnterprises.com
  • 4. TABLES 1 STEPS IN A STUDY 2 TYPICAL EMISSION RATES FROM EQUIPMENT 3 TYPICAL EMISSION RATES FROM EQUIPMENT 4 EFFECTIVE WIND SPEED 5 NATURAL VENTILATION RATES OF BUILDINGS 6 SUBSTANCE INDICES FOR ACUTE INCIDENTS FIGURES 1 FLOWCHART 2 RESULTS OF MONITORING 3 (LOG) NORMAL DISTRIBUTIONS DOCUMENTS REFERRED TO IN THIS PROCESS ENGINEERING GUIDE Refinery Process Stream Purification Refinery Process Catalysts Troubleshooting Refinery Process Catalyst Start-Up / Shutdown Activation Reduction In-situ Ex-situ Sulfiding Specializing in Refinery Process Catalyst Performance Evaluation Heat & Mass Balance Analysis Catalyst Remaining Life Determination Catalyst Deactivation Assessment Catalyst Performance Characterization Refining & Gas Processing & Petrochemical Industries Catalysts / Process Technology - Hydrogen Catalysts / Process Technology – Ammonia Catalyst Process Technology - Methanol Catalysts / process Technology – Petrochemicals Specializing in the Development & Commercialization of New Technology in the Refining & Petrochemical Industries Web Site: www.GBHEnterprises.com
  • 5. INTRODUCTION It is a vital aspect of chemical plant design and operation to minimize any contact of the operating and maintenance personnel with the chemicals on the plant. Often the most significant route of exposure to chemicals is by inhalation, and in this case it is relatively easy to measure exposure. This has led to the development of standards which indicate levels of exposure which should minimize any risk to health. These will be called Occupational Exposure Limits (OELs) in this Guide as a generic name. Given an OEL as a design criterion it might seem essential to be able to estimate personal exposures. In practice the complexity of the problem has prevented any systematic approach being developed. Consequently plant design has tended to be evolutionary, based on experience of earlier plant, and levels of exposure found thereon. There are some major problems with this evolutionary approach. Firstly there may be very little previous experience of a particular plant design, or standards may have tightened dramatically. Secondly there is very little appreciation of which control measures are really effective, and which are more trouble than they are worth. This Guide is intended to provide a means to estimate personal exposures on chemical plants to aid in such design problems. It also provides a means to rank emission sources in terms of their importance in health terms. Control measures can then be applied to problems of real significance, with an increased likelihood of achieving genuinely cost-effective solutions. The problem is found to be well suited to analysis on a computer using a spreadsheet program, and this approach is recommended in the text. The whole topic of monitoring exposure levels and designing appropriate controls and procedures is central to Occupational Hygiene. It is anticipated that this approach will normally be carried out by a combination of Occupational Hygienists and Process Engineers, with other disciplines being consulted as required. Refinery Process Stream Purification Refinery Process Catalysts Troubleshooting Refinery Process Catalyst Start-Up / Shutdown Activation Reduction In-situ Ex-situ Sulfiding Specializing in Refinery Process Catalyst Performance Evaluation Heat & Mass Balance Analysis Catalyst Remaining Life Determination Catalyst Deactivation Assessment Catalyst Performance Characterization Refining & Gas Processing & Petrochemical Industries Catalysts / Process Technology - Hydrogen Catalysts / Process Technology – Ammonia Catalyst Process Technology - Methanol Catalysts / process Technology – Petrochemicals Specializing in the Development & Commercialization of New Technology in the Refining & Petrochemical Industries Web Site: www.GBHEnterprises.com
  • 6. The recommended method for estimating exposures considers the materials handled, the personnel at risk, and the potential sources of exposure. This Guide considers primarily chronic exposure to toxics by vapor inhalation. The method should however highlight acute problems and is potentially useful in assessing risks due to other exposure routes e.g. dusts. This method has been used as part of hygiene studies on plants in Europe. The total exposure of an individual is considered to be a combination of background and task exposures. Methods of estimating such exposures, by dividing the plant into zones, determining release sources and local dispersion, are presented. To make progress on the problem of estimating exposures which may be random both in time and space, an 'averaging' approach has been adopted. Variations around the 'average' and short term exposures due to incidents are also considered. Potential acute problems due to unplanned incidents are also examined. This Guide does not attempt to define or recommend good practice in plant design or operation. Its purpose is to help assess alternative designs or operating procedures. The inherent uncertainty in the information used to estimate exposures can be considerable; errors in estimates can be large. However sensible application of this Guide should improve understanding of potential exposures on a plant, rank exposure hazards and indicate areas for improvement or of over-design. Refinery Process Stream Purification Refinery Process Catalysts Troubleshooting Refinery Process Catalyst Start-Up / Shutdown Activation Reduction In-situ Ex-situ Sulfiding Specializing in Refinery Process Catalyst Performance Evaluation Heat & Mass Balance Analysis Catalyst Remaining Life Determination Catalyst Deactivation Assessment Catalyst Performance Characterization Refining & Gas Processing & Petrochemical Industries Catalysts / Process Technology - Hydrogen Catalysts / Process Technology – Ammonia Catalyst Process Technology - Methanol Catalysts / process Technology – Petrochemicals Specializing in the Development & Commercialization of New Technology in the Refining & Petrochemical Industries Web Site: www.GBHEnterprises.com
  • 7. 1 SCOPE This Engineering Guide covers the estimation of the likely exposures of personnel to toxic vapors on chemical plants. A methodology is given which should be used as a complement to the standard design and Hazard Study procedures. 2 FIELD OF APPLICATION This Guide applies to the process engineering community worldwide. 3 DEFINITIONS For the purposes of this Guide no specific definitions apply. 4 KEY CONCEPTS 4.1 Occupational Exposure Limits In this Guide, the term Occupational Exposure Limit indicates the criterion set for plant design and operation. Different countries have different terms for such standards. Well known examples include Threshold Limit Values (TLVs) and OSHA Permissible Exposure Limits (PELs), both US, and MAKs and TRKs from Germany. The UK system of Occupational Exposure Limits has changed from Control Limits and Recommended Limits to Maximum Exposure Limits (MELs) and Occupational Exposure Standards (OESs). These will be more strictly enforceable under the new Control of Substances Hazardous to Health Regulations. The legal status and rules for deciding compliance with such standards vary from country to country. This Clause can only lay out some general guidance, based mainly on our current understanding of the UK system. Specific guidance may need to be obtained from the relevant Occupational Health Department to ensure that the correct criterion, either national or inhouse, has been selected, and is being applied correctly. Refinery Process Stream Purification Refinery Process Catalysts Troubleshooting Refinery Process Catalyst Start-Up / Shutdown Activation Reduction In-situ Ex-situ Sulfiding Specializing in Refinery Process Catalyst Performance Evaluation Heat & Mass Balance Analysis Catalyst Remaining Life Determination Catalyst Deactivation Assessment Catalyst Performance Characterization Refining & Gas Processing & Petrochemical Industries Catalysts / Process Technology - Hydrogen Catalysts / Process Technology – Ammonia Catalyst Process Technology - Methanol Catalysts / process Technology – Petrochemicals Specializing in the Development & Commercialization of New Technology in the Refining & Petrochemical Industries Web Site: www.GBHEnterprises.com
  • 8. OELs are based either on animal experiments (toxicology) or human experience. It should be understood that the data underlying such standards are often not very extensive, or reliable. The safety margin provided by the standards for different substances can vary also. For this reason it is often stated that OELs should not be regarded as fine lines dividing safe and dangerous conditions. They can be thought of as limits which should be met with as much leeway as is reasonably practicable. In the UK system (see Guidance Note EH40) it is stated that OELs should 'not normally be exceeded'. This phrase has never been defined. In GBHE it has been taken to mean that 90% or 95% of all measured exposures for a workgroup should be below the OEL. It is important to understand that this means that the average exposure of a work-group meeting the criterion will be significantly less than the OEL - typically between 25% and 50% of the OEL in fact. Consequently the target criterion for average personal exposure is not the OEL itself but a fraction of it. Most OELs are set for measurements over 8-hour shifts, known as 8-hour timeweighted averages (TWAs). For some substances it is more important to measure short-term peaks, as the effects occur rapidly - an obvious example would be carbon monoxide. Such OELs are called Short-Term Exposure Limits (STELs), and in the UK are 10-minute TWAs. The calculations in this Guide are based on 8-hour TWAs, but the procedure can handle other time bases as appropriate. More complex questions e.g. on the effects of exposure to mixtures of substances, or the effect of unusual work-patterns, should be referred to the relevant Occupational Health Department. A clear understanding of the criterion to be applied is essential. 4.2 Exposures to Vapor - Background, Task and Incident It is helpful to classify some different mechanisms by which people might be exposed to vapors on a plant. Just walking around a plant causes some exposure, which we label as 'BACKGROUND' exposure. The source of this can be from vapor or dust clouds as they dissipate or from pools of liquid as they evaporate. The vapor or dust clouds can be the fugitive emissions from plant equipment (i.e. minor leaks from seals, glands, flanges etc.), or the consequence of earlier emissions associated with a task (see below). Refinery Process Stream Purification Refinery Process Catalysts Troubleshooting Refinery Process Catalyst Start-Up / Shutdown Activation Reduction In-situ Ex-situ Sulfiding Specializing in Refinery Process Catalyst Performance Evaluation Heat & Mass Balance Analysis Catalyst Remaining Life Determination Catalyst Deactivation Assessment Catalyst Performance Characterization Refining & Gas Processing & Petrochemical Industries Catalysts / Process Technology - Hydrogen Catalysts / Process Technology – Ammonia Catalyst Process Technology - Methanol Catalysts / process Technology – Petrochemicals Specializing in the Development & Commercialization of New Technology in the Refining & Petrochemical Industries Web Site: www.GBHEnterprises.com
  • 9. The second class is the exposure directly associated with a person carrying out some work, i.e. a 'TASK' exposure. The average exposure of people in a workgroup is thus the sum of their average 'TASK' and 'BACKGROUND' exposures. For both 'BACKGROUND' and 'TASK' exposures a medium-term view should be taken. In general any source of emission likely to occur more frequently than say once a month should be included in the daily averages. Rarer events should be included only if they are sufficiently sizeable to affect the daily average. A further class of exposure is that due to 'INCIDENTS'. Incidents by definition are infrequent but may involve significant releases of material. Although such events are unlikely to make a noticeable contribution to the time weighted average exposure, incidents may have an acute impact on the health of the exposed person. The identification and quantification of potential incidents should occur during Hazard Studies. This Guide should be used to complement the Hazard Studies by contributing to quantifying the consequences of significant releases. 'INCIDENTS' involve a different definition of toxic hazard from longer-term exposures. For instance lead compounds are well known toxins, but are most unlikely to cause an acute injury by inhalation. Thus the Occupational Exposure Limit - 8 hour TWA - is not relevant for 'INCIDENTS'. It is suggested that the IDLH (Immediately Dangerous to Life or Health) criterion be used for this purpose. Further consideration of incidents is given in Clause 8. 4.3 Variability and Target Exposure Levels There are many reasons why exposure levels measured in the field do vary considerably (see Guidance Note EH42). In the process outlined above, only the mean exposure is estimated at each stage. This is clearly the simplest procedure rather than considering ranges. The variability of the exposure levels is compensated for by setting a target at a fraction of the Occupational Exposure Limit (OEL). In general we recommend the value of one-quarter of the OEL for plant design purposes (see 4.1). Specific cases can be considered in more depth by comparisons with similar types of plant which have been closely monitored. This topic is developed in more detail in Appendix C. Refinery Process Stream Purification Refinery Process Catalysts Troubleshooting Refinery Process Catalyst Start-Up / Shutdown Activation Reduction In-situ Ex-situ Sulfiding Specializing in Refinery Process Catalyst Performance Evaluation Heat & Mass Balance Analysis Catalyst Remaining Life Determination Catalyst Deactivation Assessment Catalyst Performance Characterization Refining & Gas Processing & Petrochemical Industries Catalysts / Process Technology - Hydrogen Catalysts / Process Technology – Ammonia Catalyst Process Technology - Methanol Catalysts / process Technology – Petrochemicals Specializing in the Development & Commercialization of New Technology in the Refining & Petrochemical Industries Web Site: www.GBHEnterprises.com
  • 10. 4.4 Units The units used in this Guide are normally milligrams, seconds and meters. Thus emission rates of contaminants are expressed in mg/s, air speeds in m/s, flow rates of air through the plant in m3/s and concentrations in mg/m3. Concentrations are often expressed in ppm (vol/vol). The conversion factor involves the Molecular Weight of the substance MW. The formula is: (24.45 is the volume in m3 occupied by 1 kmol of gas at 25 °C and 1 atmosphere). Occasionally, different time bases are used e.g. the units of exposure are taken as mg.min/m3. 5 OVERVIEW OF METHOD On a plant a variety of people will be exposed to a variety of chemicals escaping from a variety of process equipment. This method for estimating exposure offers a way of handling these different variables. It considers the chemicals handled the personnel on the plant, and the layout and types of equipment. Particular emphasis is given to the distribution of equipment on the plant, and the plant is considered as a series of 'zones'. The method considers background and task exposure. The key steps in the method are to: (a) Define the extent of the study. (b) Divide the plant into zones. (c) Gather the information. (d) Compute the exposures. Refinery Process Stream Purification Refinery Process Catalysts Troubleshooting Refinery Process Catalyst Start-Up / Shutdown Activation Reduction In-situ Ex-situ Sulfiding Specializing in Refinery Process Catalyst Performance Evaluation Heat & Mass Balance Analysis Catalyst Remaining Life Determination Catalyst Deactivation Assessment Catalyst Performance Characterization Refining & Gas Processing & Petrochemical Industries Catalysts / Process Technology - Hydrogen Catalysts / Process Technology – Ammonia Catalyst Process Technology - Methanol Catalysts / process Technology – Petrochemicals Specializing in the Development & Commercialization of New Technology in the Refining & Petrochemical Industries Web Site: www.GBHEnterprises.com
  • 11. Figure 1 and Table 1 describe the steps. Each step in the process is examined in more detail in Clause 6. The use of spreadsheet techniques to collate data and calculate the various exposures is highly recommended. Spreadsheets simplify the repeat calculations for different chemicals or personnel, or for examining design changes. The cumulative exposure by vapor inhalation is estimated by considering the various exposures that personnel may experience on a plant. Each particular exposure is calculated from the exposure time and the concentration of the target material in the local atmosphere i.e.: The local concentration is estimated from the local vapour emission rate and the effective local ventilation rate, where: To provide a basis for operating the (existing) process outlined in Figure 1, in the most efficient manner. Refinery Process Stream Purification Refinery Process Catalysts Troubleshooting Refinery Process Catalyst Start-Up / Shutdown Activation Reduction In-situ Ex-situ Sulfiding Specializing in Refinery Process Catalyst Performance Evaluation Heat & Mass Balance Analysis Catalyst Remaining Life Determination Catalyst Deactivation Assessment Catalyst Performance Characterization Refining & Gas Processing & Petrochemical Industries Catalysts / Process Technology - Hydrogen Catalysts / Process Technology – Ammonia Catalyst Process Technology - Methanol Catalysts / process Technology – Petrochemicals Specializing in the Development & Commercialization of New Technology in the Refining & Petrochemical Industries Web Site: www.GBHEnterprises.com
  • 12. FIGURE 1 FLOWCHART Refinery Process Stream Purification Refinery Process Catalysts Troubleshooting Refinery Process Catalyst Start-Up / Shutdown Activation Reduction In-situ Ex-situ Sulfiding Specializing in Refinery Process Catalyst Performance Evaluation Heat & Mass Balance Analysis Catalyst Remaining Life Determination Catalyst Deactivation Assessment Catalyst Performance Characterization Refining & Gas Processing & Petrochemical Industries Catalysts / Process Technology - Hydrogen Catalysts / Process Technology – Ammonia Catalyst Process Technology - Methanol Catalysts / process Technology – Petrochemicals Specializing in the Development & Commercialization of New Technology in the Refining & Petrochemical Industries Web Site: www.GBHEnterprises.com
  • 13. Refinery Process Stream Purification Refinery Process Catalysts Troubleshooting Refinery Process Catalyst Start-Up / Shutdown Activation Reduction In-situ Ex-situ Sulfiding Specializing in Refinery Process Catalyst Performance Evaluation Heat & Mass Balance Analysis Catalyst Remaining Life Determination Catalyst Deactivation Assessment Catalyst Performance Characterization Refining & Gas Processing & Petrochemical Industries Catalysts / Process Technology - Hydrogen Catalysts / Process Technology – Ammonia Catalyst Process Technology - Methanol Catalysts / process Technology – Petrochemicals Specializing in the Development & Commercialization of New Technology in the Refining & Petrochemical Industries Web Site: www.GBHEnterprises.com
  • 14. 6 APPLICATION OF METHOD 6.1 Chemical Substances - List and Rank The first stage is to consider the flowsheet of the process. From this, it is possible to develop a list of substances which might present a significant inhalation hazard. These substances can then be ranked in order of importance by taking account of the following: (a) The quantity. (b) The conditions of containment (quality, temperature, pressure). (c) The Occupational Exposure Limit (OEL). (d) The concentration Immediately Dangerous to Life or Health (IDLH). (e) The saturated vapor pressure (svp) at ambient temperature. The IDLH and svp are both intrinsic properties of the substance and can be combined into a single Substance Index for ranking purposes (see Appendix E). The Substance Index can be compared to a number of other substances to give an understanding of the significance of the inhalation hazard. This should then be combined with data on the quantities being handled, and the quality of containment to decide for which substances, if any, a detailed analysis is required. For relevant substances the following physical properties should be obtained: (1) The molecular weight. (2) The solubility in water, and in main components of relevant process streams. (3) The saturated vapor pressure across the range of process temperatures and process streams. Refinery Process Stream Purification Refinery Process Catalysts Troubleshooting Refinery Process Catalyst Start-Up / Shutdown Activation Reduction In-situ Ex-situ Sulfiding Specializing in Refinery Process Catalyst Performance Evaluation Heat & Mass Balance Analysis Catalyst Remaining Life Determination Catalyst Deactivation Assessment Catalyst Performance Characterization Refining & Gas Processing & Petrochemical Industries Catalysts / Process Technology - Hydrogen Catalysts / Process Technology – Ammonia Catalyst Process Technology - Methanol Catalysts / process Technology – Petrochemicals Specializing in the Development & Commercialization of New Technology in the Refining & Petrochemical Industries Web Site: www.GBHEnterprises.com
  • 15. 6.2 Personnel - List And Rank Personnel who might be exposed then need to be identified. The most obvious categories generally are process operators, and regular maintenance tradesmen, especially fitters. Other tradesmen and supervisors should be considered. The aim is not to produce a complete listing of those who might be exposed to any extent, but a practical short list of those who might be the most significantly exposed. Given this selection of the more relevant personnel, a list of tasks, which might cause significant exposure, should be produced for each chosen work-group. 6.3 Plant - Define Zones Normally a plant is not homogeneous in terms of exposure. All aspects of ventilation, emission sources, and occupacity by personnel will cause the exposure to vary significantly. The recommended method is to divide the plant into zones (say 10 - 20) which are at least reasonably homogeneous. The principal division should be in terms of the quality of ventilation provided. For example outside areas should be separated from internal areas. Normally, different floors should also be separated. Further sub-division of zones can be carried out if there are sub-areas where a significantly different exposure pattern is likely. This will mainly be due to the items of equipment, or operations in these sub-areas. This method also uses the concept of Zone 0, where personnel spend time, but there is no exposure. This can include time off-plant, and time in amenities areas and control rooms. Careful thought should be given as to whether these areas are indeed zero-exposure. Control rooms for example can easily be contaminated, especially by low-volatility materials brought in on people’s clothing. The relatively long time spent in the control room can then cause significant exposure. Refinery Process Stream Purification Refinery Process Catalysts Troubleshooting Refinery Process Catalyst Start-Up / Shutdown Activation Reduction In-situ Ex-situ Sulfiding Specializing in Refinery Process Catalyst Performance Evaluation Heat & Mass Balance Analysis Catalyst Remaining Life Determination Catalyst Deactivation Assessment Catalyst Performance Characterization Refining & Gas Processing & Petrochemical Industries Catalysts / Process Technology - Hydrogen Catalysts / Process Technology – Ammonia Catalyst Process Technology - Methanol Catalysts / process Technology – Petrochemicals Specializing in the Development & Commercialization of New Technology in the Refining & Petrochemical Industries Web Site: www.GBHEnterprises.com
  • 16. 6.4 Zone Information 6.4.1 Process Equipment Having defined the extent of the plant zones, information is now required on the equipment which could be a source of exposure. This is achieved by listing in each zone: (a) all equipment which could contain significant quantities of the material under consideration, (b) and for that equipment, the process stream data (pressure, temperature, state, composition and in particular concentration of target material). The list of equipment should include plant items and any other potentially significant sources such as drains, sumps or pipelines passing through the zone. Note should also be made of any equipment in other zones which releases material into the zone. For example, if an item has a remote vent or drain pipe in another zone, then the location of such outlets should also be noted. Process stream data should be available from the process flowsheet. 6.4.2 Fugitive Sources The equipment identified above should now be examined to determine the possible escape of material to atmosphere. There are several routes for these fugitive emissions; these include: (a) Seals on moving shafts. Such shafts include pumps, compressors, agitators, control valves etc. Release rates will depend upon the complexity of the design and the wear and maintenance performance. Tables 2 and 3 give an indication of typical rates. Refinery Process Stream Purification Refinery Process Catalysts Troubleshooting Refinery Process Catalyst Start-Up / Shutdown Activation Reduction In-situ Ex-situ Sulfiding Specializing in Refinery Process Catalyst Performance Evaluation Heat & Mass Balance Analysis Catalyst Remaining Life Determination Catalyst Deactivation Assessment Catalyst Performance Characterization Refining & Gas Processing & Petrochemical Industries Catalysts / Process Technology - Hydrogen Catalysts / Process Technology – Ammonia Catalyst Process Technology - Methanol Catalysts / process Technology – Petrochemicals Specializing in the Development & Commercialization of New Technology in the Refining & Petrochemical Industries Web Site: www.GBHEnterprises.com
  • 17. (b) Vents. These include process vents, where allowance has been made for a recognized gas flow to atmosphere, breathing vents, where flow is occasional and due to displacement of gas (often contaminated air) during filling or temperature changes, and emergency relief vents. Estimates can be made of the quantities of vapor vented either by calculating vapor flows in vents or by calculating the volumes and compositions of displaced contaminated air. (c) Open vessels. This category covers atmospheric-pressure uncovered tanks, pits, drain systems etc. often containing potentially contaminated water. (d) 'Leaks'. Material can leak through joints, drain valves, corrosion holes etc. In a well designed and maintained system, leaks from joints may be negligible. However when corrosion is a problem, that is when materials of construction have a short life, serious leaks can occur. Refinery Process Stream Purification Refinery Process Catalysts Troubleshooting Refinery Process Catalyst Start-Up / Shutdown Activation Reduction In-situ Ex-situ Sulfiding Specializing in Refinery Process Catalyst Performance Evaluation Heat & Mass Balance Analysis Catalyst Remaining Life Determination Catalyst Deactivation Assessment Catalyst Performance Characterization Refining & Gas Processing & Petrochemical Industries Catalysts / Process Technology - Hydrogen Catalysts / Process Technology – Ammonia Catalyst Process Technology - Methanol Catalysts / process Technology – Petrochemicals Specializing in the Development & Commercialization of New Technology in the Refining & Petrochemical Industries Web Site: www.GBHEnterprises.com
  • 18. For possible sources not covered above (e.g. fume cupboards and laminar flow booths) specialist advice may be required. For each fugitive source, an estimate is required of the quantity, properties, and frequency of chemicals which can be released into each zone. It is recognized that the accuracy of these estimates will be limited. Particular problems occur with sources where conditions vary with time. Examples include mechanical equipment which wears (pump seals), where corrosion is significant, and where material is released occasionally, either due to an infrequent process operation or say tank breathing. It may be necessary in these cases to estimate both an average and worst release. These figures, which are based on CMA guidance (ref 8) should be used only as preliminary estimates. Actual leak rates will depend upon the suitability of the equipment for the duty, the severity of the duty, the standard of maintenance etc. Advice should be sought from Machinery or Piping Sections where necessary. Refinery Process Stream Purification Refinery Process Catalysts Troubleshooting Refinery Process Catalyst Start-Up / Shutdown Activation Reduction In-situ Ex-situ Sulfiding Specializing in Refinery Process Catalyst Performance Evaluation Heat & Mass Balance Analysis Catalyst Remaining Life Determination Catalyst Deactivation Assessment Catalyst Performance Characterization Refining & Gas Processing & Petrochemical Industries Catalysts / Process Technology - Hydrogen Catalysts / Process Technology – Ammonia Catalyst Process Technology - Methanol Catalysts / process Technology – Petrochemicals Specializing in the Development & Commercialization of New Technology in the Refining & Petrochemical Industries Web Site: www.GBHEnterprises.com
  • 19. 6.4.3 Time Spent In Zones In order to estimate people’s exposure to contaminants it is necessary to know where they spend their time and what they do. For an existing plant, the plant manager should be able to provide the information. For a new plant the information may have to be inferred from the operating instructions. The information needed is the average number of minutes per shift spent in each zone and on each task. It is often necessary to consider what is done over a week or a month to arrive at an average figure. 6.4.4 Tasks Material can also be released into the atmosphere as a result of carrying out tasks. Such tasks are usually those where material is transferred into open vessels or where process joints have to be broken. Examples include: (a) Filling/emptying containers. Containers include tankers, cylinders, drums, bottles, sacks etc. Typical tasks are packing product, charging, sampling, draining, purging etc. Material escapes either due to displacement of contaminated air, spillage, or evaporation from wetted equipment (e.g. dip pipes). (b) Maintenance of equipment and/or contents. Work on process equipment can lead to release of material either during decontamination (purging/washing) or maintenance (replacement or repair of damaged parts, catalyst, packing etc.). Particular problems occur when material cannot be removed effectively due to blockages. A list of all tasks performed in each zone with potentially significant exposure should be prepared. These tasks should include both process and maintenance operations. The frequency and extent should be noted. Estimates of the quantities of material associated with each task are required. For example in a sampling operation, how much material has to be purged before the sample is taken? How big is the sample? What form is it in? How much vapor is displaced? Refinery Process Stream Purification Refinery Process Catalysts Troubleshooting Refinery Process Catalyst Start-Up / Shutdown Activation Reduction In-situ Ex-situ Sulfiding Specializing in Refinery Process Catalyst Performance Evaluation Heat & Mass Balance Analysis Catalyst Remaining Life Determination Catalyst Deactivation Assessment Catalyst Performance Characterization Refining & Gas Processing & Petrochemical Industries Catalysts / Process Technology - Hydrogen Catalysts / Process Technology – Ammonia Catalyst Process Technology - Methanol Catalysts / process Technology – Petrochemicals Specializing in the Development & Commercialization of New Technology in the Refining & Petrochemical Industries Web Site: www.GBHEnterprises.com
  • 20. 6.4.5 Estimate Effective Local Ventilation Rates Ventilation is as yet one of the least certain areas in the estimation of personal exposure. To ease calculation it is assumed that contaminated air leaves a zone at a steady rate to be replaced by air from another zone or the outside environment. An estimate of the volumetric rate is required for the estimation of zone concentrations given later. For an indoor area three approaches are possible-calculation, measurement and simple estimation. There are methods available (Goodfellow, ref 3, provides an introduction and further references) for the prediction of the ventilation in a building when full details are known of its construction. However these are involved and unlikely to be worth using for the type of study envisaged here. There are also methods for measuring the air changes per hour for existing buildings. When designing a new structure, it may be possible to take measurements on an analogous existing system. Where the ventilation is forced, the air flow rate should be known. Simple estimation involves predicting a 'normal' number of air changes per hour and from this a volumetric flow rate. A simple table is given for guidance in Appendix A. Local ventilation can be applied to major sources, such as a sample booth or a hooded plating bath. A way of dealing with these is to ascribe an efficiency factor. For example, a well designed fume cupboard allows only 0.1% of the material released in it to escape from the front. So effective source strength can be used in the study, which is one thousandth of the actual source strength. For outdoor areas the problem should be simpler. At its simplest the air can be assumed to travel through the plant at a steady speed, equal to the wind speed, under plug flow. However there are a number of complicating factors. Choosing the appropriate wind speed is only one (see Appendix A). A chemical plant represents a permeable obstruction to the wind. Air flow accelerates through the gaps between plant items and eddies behind them. Overall the wind speed is attenuated. However some estimate can be made of the volumetric flow rate through a zone (see Appendix A). Refinery Process Stream Purification Refinery Process Catalysts Troubleshooting Refinery Process Catalyst Start-Up / Shutdown Activation Reduction In-situ Ex-situ Sulfiding Specializing in Refinery Process Catalyst Performance Evaluation Heat & Mass Balance Analysis Catalyst Remaining Life Determination Catalyst Deactivation Assessment Catalyst Performance Characterization Refining & Gas Processing & Petrochemical Industries Catalysts / Process Technology - Hydrogen Catalysts / Process Technology – Ammonia Catalyst Process Technology - Methanol Catalysts / process Technology – Petrochemicals Specializing in the Development & Commercialization of New Technology in the Refining & Petrochemical Industries Web Site: www.GBHEnterprises.com
  • 21. 6.4.6 Estimate Fugitive Vapor Emission Rates The CMA (ref 8) give guidance on how to measure vapor emission rates from items of equipment, and the Appendices refer to the use of existing methods, particularly as developed by the Environmental Protection Agency (EPA). For material to be a hazard by the vapor inhalation route, the released material needs to be present as a vapor. For direct vapor/gas releases, the vapor rate is the same as the release rate. For liquid releases, the fate of the liquid and mass transfer into the vapor phase should be considered. The simplest assumption is to assume all released material evaporates into the zone; this may be true for a small leak of hot liquid onto a poorly drained surface. For other liquid releases, particularly where material is drained away or treated, only some fraction of the released material evaporates locally. Removal of liquid may cause problems in other zones e.g. where liquid in drains meets hot water. Mass transfer may occur by flashing if a superheated liquid is released, or more generally, by evaporation from a wetted surface. Although it is relatively straightforward to calculate the rate of vapor release from a flashing stream if the process stream conditions are known, calculations of evaporation rates are less precise. The evaporation rate is a function of the wetted area, pool temperature and composition (defining vapor pressure) and local wind speed. A useful correlation, due to Clancey, (ref 2) for evaporation from a well mixed pool is: Refinery Process Stream Purification Refinery Process Catalysts Troubleshooting Refinery Process Catalyst Start-Up / Shutdown Activation Reduction In-situ Ex-situ Sulfiding Specializing in Refinery Process Catalyst Performance Evaluation Heat & Mass Balance Analysis Catalyst Remaining Life Determination Catalyst Deactivation Assessment Catalyst Performance Characterization Refining & Gas Processing & Petrochemical Industries Catalysts / Process Technology - Hydrogen Catalysts / Process Technology – Ammonia Catalyst Process Technology - Methanol Catalysts / process Technology – Petrochemicals Specializing in the Development & Commercialization of New Technology in the Refining & Petrochemical Industries Web Site: www.GBHEnterprises.com
  • 22. For evaporation of a component in a dilute solution, mass transfer of the component within the solution may be limiting. In this case the vapor rates will be overestimated using the above equation. 6.4.7 Estimate Task Vapor Emission Rates From the information obtained in 6.4.4, estimates of the material released may be made. The approach adopted will be similar to that for estimating vapor rates for fugitive sources. The quantity and nature of material escaping from the equipment, and the fraction of that material evaporating locally should be calculated. As task releases usually only last for a limited time, an average vapor emission rate should be calculated. 6.4.8 Estimate Preliminary Background Concentrations When making a preliminary estimate of background concentrations it is assumed that the air entering a zone is free of the contaminant. Allowance for the air being already contaminated is made later. The preliminary background concentration is the sum of the fugitive emission rate and the task emission rate in the zone divided by the volumetric air flow rate and multiplied by an inefficiency factor, see Appendix B. 6.4.9 Estimate Task Exposures When carrying out a task, personnel may be exposed to concentrations in excess of the background, as the task may be the cause of material release, and the personnel may be working close to the source. The task exposure can be calculated as: The effective task ventilation rate is estimated by assuming the person works in a 'task zone', a nominal 3m cube around the equipment or source. Further details are given in Appendix B. Refinery Process Stream Purification Refinery Process Catalysts Troubleshooting Refinery Process Catalyst Start-Up / Shutdown Activation Reduction In-situ Ex-situ Sulfiding Specializing in Refinery Process Catalyst Performance Evaluation Heat & Mass Balance Analysis Catalyst Remaining Life Determination Catalyst Deactivation Assessment Catalyst Performance Characterization Refining & Gas Processing & Petrochemical Industries Catalysts / Process Technology - Hydrogen Catalysts / Process Technology – Ammonia Catalyst Process Technology - Methanol Catalysts / process Technology – Petrochemicals Specializing in the Development & Commercialization of New Technology in the Refining & Petrochemical Industries Web Site: www.GBHEnterprises.com
  • 23. Knowing the task concentration and the duration of the task or exposure time, the task exposure can be calculated as: 6.5 Integrate Exposures Over All Zones 6.5.1 Revise Background Concentrations In the preliminary estimation (6.4.8) of background concentrations it was assumed that the air entering a zone was uncontaminated. Frequently this is not the case and an allowance has to be made for the contamination of the incoming air. The easiest way to do this is to add the quantity of contaminant brought into the zone to the emission rate in the zone before calculating the concentration. Where a number of zones interact iteration may be required but that should not be difficult using a spreadsheet. 6.5.2 Estimate Exposure Due To Background The average background exposure in mg.min/m3 for a shift is estimated by multiplying the average number of minutes per shift spent in each zone by the background concentration in the zone and summing over all zones. If the pattern of work is very variable it may be necessary to investigate the various patterns to check that none gives rise to an unacceptably high exposure over a shift. 6.5.3 Estimate Average Personal Exposure Since incidents are by definition relatively rare events, they are ignored in estimating average personal exposure. The average personal exposure in mg.min/m3 per shift is estimated by adding the average background exposure to the average exposure due to tasks. This per shift figure may need to be converted to the correct time basis (usually 8 hr Time Weighted Average, TWA) for comparison with the relevant exposure criterion. Refinery Process Stream Purification Refinery Process Catalysts Troubleshooting Refinery Process Catalyst Start-Up / Shutdown Activation Reduction In-situ Ex-situ Sulfiding Specializing in Refinery Process Catalyst Performance Evaluation Heat & Mass Balance Analysis Catalyst Remaining Life Determination Catalyst Deactivation Assessment Catalyst Performance Characterization Refining & Gas Processing & Petrochemical Industries Catalysts / Process Technology - Hydrogen Catalysts / Process Technology – Ammonia Catalyst Process Technology - Methanol Catalysts / process Technology – Petrochemicals Specializing in the Development & Commercialization of New Technology in the Refining & Petrochemical Industries Web Site: www.GBHEnterprises.com
  • 24. 7 UNDERSTANDING The procedure described so far enables an estimate to be made of personal exposure which may be compared with the OEL. If this is fully satisfactory no more may need to be done. However, much information is contained in the way the estimate is built up which may be used to understand the main sources of exposure. This understanding may then be used to reduce personal exposure. This reduction may be necessary to meet the OEL, although in general it is not enough simply to comply with OELs; exposures should be reduced as far as reasonably practicable. The following understanding is likely to be obtained (a) Which chemicals are particularly troublesome? (b) Who is most at risk and whether they are likely to be overexposed; (c) The distribution of background concentrations and the location of any 'hotspots'; (d) A ranking of emission sources; (e) A ranking of tasks. The understanding may demonstrate the need for changes such as those listed below: (1) The process may need to be altered to avoid the use of a chemical if no satisfactory way can be found of containing it. (2) Items of equipment may need to be changed or modified to improve containment if they are shown to be major emission sources. (3) The method of operation can often be changed to separate people from high concentrations once the location of the latter has been identified. (4) The ventilation may need to be improved, either generally or by the provision of local extraction. (5) Additional personal protection may be needed, but this should be seen as a last resort. Refinery Process Stream Purification Refinery Process Catalysts Troubleshooting Refinery Process Catalyst Start-Up / Shutdown Activation Reduction In-situ Ex-situ Sulfiding Specializing in Refinery Process Catalyst Performance Evaluation Heat & Mass Balance Analysis Catalyst Remaining Life Determination Catalyst Deactivation Assessment Catalyst Performance Characterization Refining & Gas Processing & Petrochemical Industries Catalysts / Process Technology - Hydrogen Catalysts / Process Technology – Ammonia Catalyst Process Technology - Methanol Catalysts / process Technology – Petrochemicals Specializing in the Development & Commercialization of New Technology in the Refining & Petrochemical Industries Web Site: www.GBHEnterprises.com
  • 25. If the calculations have been computerized it is relatively simple to evaluate the expected effect of each change. This should enable the provision of a satisfactory working environment in a cost effective manner. The estimates for background concentrations and task exposures should be checked to ensure that STELs are not exceeded and that acute problems do not occur. 8 INCIDENTS As discussed in 4.2, INCIDENTS are single exposures of such a magnitude as to possibly cause direct damage to health. Not all toxic substances fall into this category; for instance lead compounds are most unlikely to cause an acute injury by inhalation. So the first step in defining possible incidents is to determine what level of exposure, if any, could cause immediate health effects. The criterion proposed here is IDLH (Immediately Dangerous to Life or Health). This was developed by NIOSH in the USA to assist respirator selection. The IDLH concentration represents the maximum level from which one could escape in 30 minutes without any escape impairing symptoms, or any irreversible health effects. Values for a wide variety of substances are listed in Reference NIOSH/OSHA. A new system of Emergency Response Planning Guides (ERPG) is currently being developed by the American Industrial Hygiene Association (AIHA). These standards will probably be more thoroughly researched than the IDLH ones but are, as yet, only available for a few substances. Using the approach of this Guide, it is possible to estimate the scale of events which will lead to concentrations above the IDLH value in occupied areas of the plant. This scale will depend on whether the emission is inside or outside. Such estimates give quantitative criteria for Hazards and Operability Studies. The acute hazard associated with any substance can be estimated using the Substance Index approach - see Appendix E. This can usefully be refined in early stages of Hazard studies using rough estimates of typical indoor and outdoor ventilation. An example is given in Appendix D. Refinery Process Stream Purification Refinery Process Catalysts Troubleshooting Refinery Process Catalyst Start-Up / Shutdown Activation Reduction In-situ Ex-situ Sulfiding Specializing in Refinery Process Catalyst Performance Evaluation Heat & Mass Balance Analysis Catalyst Remaining Life Determination Catalyst Deactivation Assessment Catalyst Performance Characterization Refining & Gas Processing & Petrochemical Industries Catalysts / Process Technology - Hydrogen Catalysts / Process Technology – Ammonia Catalyst Process Technology - Methanol Catalysts / process Technology – Petrochemicals Specializing in the Development & Commercialization of New Technology in the Refining & Petrochemical Industries Web Site: www.GBHEnterprises.com
  • 26. There is no agreement as yet as to what frequency events causing exposures to IDLH concentrations might be allowed. It is strongly recommended that the relevant Occupational Health Department is involved in such decisions, both to confirm that the IDLH value is reasonable, and also to agree any choice of frequency. Refinery Process Stream Purification Refinery Process Catalysts Troubleshooting Refinery Process Catalyst Start-Up / Shutdown Activation Reduction In-situ Ex-situ Sulfiding Specializing in Refinery Process Catalyst Performance Evaluation Heat & Mass Balance Analysis Catalyst Remaining Life Determination Catalyst Deactivation Assessment Catalyst Performance Characterization Refining & Gas Processing & Petrochemical Industries Catalysts / Process Technology - Hydrogen Catalysts / Process Technology – Ammonia Catalyst Process Technology - Methanol Catalysts / process Technology – Petrochemicals Specializing in the Development & Commercialization of New Technology in the Refining & Petrochemical Industries Web Site: www.GBHEnterprises.com
  • 27. APPENDIX A A.1 VENTILATION OUTDOOR VENTILATION As stated in the main text, the ventilation rate for outdoor zones is based on wind speed. The problem then remains of choosing an appropriate wind speed. Wind speed is approximately log-normally distributed and the geometric mean wind speed frequently lies between 3 and 4!m/s. Places such as Baton Rouge, Lousiana is at the bottom of the range; Newark, New Jersey is in the middle with Cardiff Wales at the top end. Lower values are found for Gladstone, Australia (1.8 m/s) and Tokyo, Japan (2.8!m/s) and higher ones for Wattisham, Suffolk, England (4.6 m/s) and Anglesey, Wales (5.6!m/s). Users should satisfy themselves that they are using a value appropriate to the area concerned. Wind speed is normally measured at a height of 10m. The wind speed varies with height, e.g. at 2m in average conditions and surroundings it is 70% of its value at 10m. The effect of plant will be to slow the wind further and reduce the local effective ventilation. Estimation of ventilation is particularly difficult for outside zones which contain walls. A tentative set of effective wind speeds is suggested in Table 4. This suggestion assumes that the wind speed is reduced uniformly, from an 'exposed' value to an 'indoor' value, as the plant zones become more confined. The air flow rate for an outdoor zone is found by multiplying the effective wind speed by a suitable cross-sectional area. If the wind direction is assumed to be uniformly distributed, the mean cross-sectional area of a rectangular zone is the height of the zone multiplied by the sum of the length and width of the zone multiplied by 0.64. Refinery Process Stream Purification Refinery Process Catalysts Troubleshooting Refinery Process Catalyst Start-Up / Shutdown Activation Reduction In-situ Ex-situ Sulfiding Specializing in Refinery Process Catalyst Performance Evaluation Heat & Mass Balance Analysis Catalyst Remaining Life Determination Catalyst Deactivation Assessment Catalyst Performance Characterization Refining & Gas Processing & Petrochemical Industries Catalysts / Process Technology - Hydrogen Catalysts / Process Technology – Ammonia Catalyst Process Technology - Methanol Catalysts / process Technology – Petrochemicals Specializing in the Development & Commercialization of New Technology in the Refining & Petrochemical Industries Web Site: www.GBHEnterprises.com
  • 28. A.1.1 Downwash from Vents If vents are set close to the roof, vented material is likely to be entrained in the building wake. This is discussed clearly by Bryant, (ref 1) 'When the actual stack height is equal to or less than 1.5 building heights i.e. the effective stack height is appreciably less than one building height, effluent entrained in the wake could be carried to ground level in the immediate vicinity of the building on which the stack stands.' This was considered by Scorer and Barrett (ref 6) with respect to the short-term concentration likely to be found near a building with a low chimney. They assumed the pollution to be spread over a cone of semi-angle 12° bent round the building, in a moderate wind of about 5 m/s, this representing the worst weather conditions. In winds of higher speeds the pollution would be less, and in lower speeds the wake would not be well developed. From the formula given by Scorer and Barrett: where Ps is the typical maximum short term concentration per unit release rate (unit/m3 per unit/sec) and is the length of the trajectory to the place where the pollution is measured (m) In general, on the assumption that the place of interest is at ground level outside the building, the length of trajectory is given by the height of the stack above ground level plus the distance from the stack to the outside edge of the building. To convert Ps into the long-term concentration PL it is suggested by Bryant that, corresponding to the assumed semi-vertical angle of 12°, a swing of the wind factor of 360/(2 x 12) = 15 may be applied. This is tantamount to assuming a uniform wind rose and spreading released material uniformly in all directions. In light winds the pollutant may not be brought down to ground level. The above assumes a vent on the roof of a closed building. If one side of the building is open or has an open structure built against it, eddying may cause contaminant to enter the building or open structure. Refinery Process Stream Purification Refinery Process Catalysts Troubleshooting Refinery Process Catalyst Start-Up / Shutdown Activation Reduction In-situ Ex-situ Sulfiding Specializing in Refinery Process Catalyst Performance Evaluation Heat & Mass Balance Analysis Catalyst Remaining Life Determination Catalyst Deactivation Assessment Catalyst Performance Characterization Refining & Gas Processing & Petrochemical Industries Catalysts / Process Technology - Hydrogen Catalysts / Process Technology – Ammonia Catalyst Process Technology - Methanol Catalysts / process Technology – Petrochemicals Specializing in the Development & Commercialization of New Technology in the Refining & Petrochemical Industries Web Site: www.GBHEnterprises.com
  • 29. A.2 NATURAL VENTILATION OF INDOOR AREAS The following table (Table 5) has been taken from the 1970 Institute of Heating and Ventilation Engineers (IHVE) Guide. It does not apply if there are large doorways or forced ventilation. Refinery Process Stream Purification Refinery Process Catalysts Troubleshooting Refinery Process Catalyst Start-Up / Shutdown Activation Reduction In-situ Ex-situ Sulfiding Specializing in Refinery Process Catalyst Performance Evaluation Heat & Mass Balance Analysis Catalyst Remaining Life Determination Catalyst Deactivation Assessment Catalyst Performance Characterization Refining & Gas Processing & Petrochemical Industries Catalysts / Process Technology - Hydrogen Catalysts / Process Technology – Ammonia Catalyst Process Technology - Methanol Catalysts / process Technology – Petrochemicals Specializing in the Development & Commercialization of New Technology in the Refining & Petrochemical Industries Web Site: www.GBHEnterprises.com
  • 30. APPENDIX B B.1 ESTIMATING CONCENTRATION AND EXPOSURE ESTIMATING ZONE CONCENTRATIONS The choice of K factor is something of an art. The factor allows for three items: (a) How uniformly the air is distributed through the zone. (b) How uniformly the sources are distributed through the zone. (c) How uniformly the worker 'samples' the air throughout the zone. For most zones K=1 should be used. If there is doubt about the uniformity of any of the three items, a value of K greater than one should be used. The plug flow value should be used only when all three items are favorable. Air distribution is favorable in uncluttered outdoor zones or where the ventilation has been specially designed to achieve plug flow. Source distribution is favorable where it is uniform or sources are near the air exits (and flow reversal does not happen). Worker distribution is favorable when it is uniform or the worker stays deliberately upwind of sources. Some users may wish to make the inefficiency factor a product of three individual terms, each representing one of the contributing items. Refinery Process Stream Purification Refinery Process Catalysts Troubleshooting Refinery Process Catalyst Start-Up / Shutdown Activation Reduction In-situ Ex-situ Sulfiding Specializing in Refinery Process Catalyst Performance Evaluation Heat & Mass Balance Analysis Catalyst Remaining Life Determination Catalyst Deactivation Assessment Catalyst Performance Characterization Refining & Gas Processing & Petrochemical Industries Catalysts / Process Technology - Hydrogen Catalysts / Process Technology – Ammonia Catalyst Process Technology - Methanol Catalysts / process Technology – Petrochemicals Specializing in the Development & Commercialization of New Technology in the Refining & Petrochemical Industries Web Site: www.GBHEnterprises.com
  • 31. B.2 ESTIMATING TASK EXPOSURES There are two major routes for exposure when carrying out a task: (a) Evaporation from a wetted surface This is the most frequent source of exposure with liquids. Tasks usually relate to transferring materials. Examples include tanker discharge where material can be spilled during hose handling, draining equipment, and sampling where excess liquid wets bottles and gloves etc. (b) Release of gas or vapor There are several tasks when contaminant vapor is released directly into the local environment where people are working. Examples include maintenance work on equipment which has not been completely purged, filling containers where contaminated air is displaced, dipping tanks etc. The gas release may be of limited duration (i.e. less than the time someone is present) or may occur throughout the task. To estimate the task exposure, estimates are needed of the average vapor emission rate, the local ventilation, and the time spent on the task. Refinery Process Stream Purification Refinery Process Catalysts Troubleshooting Refinery Process Catalyst Start-Up / Shutdown Activation Reduction In-situ Ex-situ Sulfiding Specializing in Refinery Process Catalyst Performance Evaluation Heat & Mass Balance Analysis Catalyst Remaining Life Determination Catalyst Deactivation Assessment Catalyst Performance Characterization Refining & Gas Processing & Petrochemical Industries Catalysts / Process Technology - Hydrogen Catalysts / Process Technology – Ammonia Catalyst Process Technology - Methanol Catalysts / process Technology – Petrochemicals Specializing in the Development & Commercialization of New Technology in the Refining & Petrochemical Industries Web Site: www.GBHEnterprises.com
  • 32. B.2.1 Worked Examples (a) Liquid spillages Consider an operation where a small quantity of material is spilled. This could be a filling or maintenance operation. Assume 500 g is spilled and this covers a floor area of say1m2. Assume operation is outside and task lasts 15 min. Assume material is spilled at ambient say 10 °C; vapor pressure is 0.01 bara; mol wt 100. Assume wind speed 2 m/s, then evaporation rate from equation (4) is Assume task takes place in 3 x 3 x 3 m cube. Assume K = 1 , no obvious maldistribution of worker or ventilation, then As pool persists for longer than the 15 min of the task, the worker will be exposed to an average 7 mg/m3 concentration throughout the task. Refinery Process Stream Purification Refinery Process Catalysts Troubleshooting Refinery Process Catalyst Start-Up / Shutdown Activation Reduction In-situ Ex-situ Sulfiding Specializing in Refinery Process Catalyst Performance Evaluation Heat & Mass Balance Analysis Catalyst Remaining Life Determination Catalyst Deactivation Assessment Catalyst Performance Characterization Refining & Gas Processing & Petrochemical Industries Catalysts / Process Technology - Hydrogen Catalysts / Process Technology – Ammonia Catalyst Process Technology - Methanol Catalysts / process Technology – Petrochemicals Specializing in the Development & Commercialization of New Technology in the Refining & Petrochemical Industries Web Site: www.GBHEnterprises.com
  • 33. (b) Release of gas Consider operation where non-toxic solid is added to an atmospheric vessel containing a volatile toxic liquid. Exposure occurs as operator is exposed to the displaced contaminated air. Assume 0.2 m3 of solid added. Assume vapor pressure of liquid is 0.1 bara; mol. Wt. = 120; temperature of vessel 50 °C. Assume operation takes place inside a building with 2 air changes/hour. Refinery Process Stream Purification Refinery Process Catalysts Troubleshooting Refinery Process Catalyst Start-Up / Shutdown Activation Reduction In-situ Ex-situ Sulfiding Specializing in Refinery Process Catalyst Performance Evaluation Heat & Mass Balance Analysis Catalyst Remaining Life Determination Catalyst Deactivation Assessment Catalyst Performance Characterization Refining & Gas Processing & Petrochemical Industries Catalysts / Process Technology - Hydrogen Catalysts / Process Technology – Ammonia Catalyst Process Technology - Methanol Catalysts / process Technology – Petrochemicals Specializing in the Development & Commercialization of New Technology in the Refining & Petrochemical Industries Web Site: www.GBHEnterprises.com
  • 34. APPENDIX C VARIABILITY OF EXPOSURE LEVELS When sufficient personal monitoring is carried out there are two obvious limitations on the type of distribution. Firstly no result can be less than zero, and secondly occasional high results will occur, with the frequency tailing off at higher values. The simplest distribution with these characteristics is the log-normal distribution (lnd), and this often fits experimental data very well. In practice the fit of a set of results to a lnd is very easily demonstrated by plotting the cumulative percentage of results below a certain value, against that value on Log-probability paper. Figure 2 shows plots of two distributions, with a median of 10. NIOSH (ref 11) discusses deviations from lnd in more depth. The distribution of a log-normal distribution is defined by the geometric mean and the geometric standard deviation (gsd). The gsd has been found to vary in practice from about 1.3 to 10, although more commonly between 1.5 and 2.5. Figure 3 demonstrates the difference between a normal distribution, and lnd’s with gsd’s of 1.5 and 3. It will be seen that a gsd of 1.5 is not dissimilar to the normal distribution. The gsd of 3 however gives a much more skewed distribution, with the arithmetic mean largely due to occasional high level excursions. This is more characteristic of plants where background levels are very low, relative to the occasional excursions. Compliance with a standard is best considered as the percentage of results below the standard. This should be at least 90%, and preferably 95%, to show good compliance. The results of the method developed in this Guide generate arithmetic means, so the relationship between this mean and 90 and 95 percentiles is vital. Fortunately, the ratio of the arithmetic mean to the 90 and 95 percentiles does not vary much with gsd. This means that if the arithmetic mean of a set of results, which are log-normally distributed is less than one-quarter the OEL, then 95% compliance is guaranteed, whatever the gsd. With favorable gsd’s of 1.6 or less the arithmetic mean can be one-half the OEL for 95% compliance. As stated earlier, we generally recommend the value of one-quarter of the OEL for plant design purposes. This errs slightly to caution, but care has been taken in earlier stages not to accumulate safety factors. Refinery Process Stream Purification Refinery Process Catalysts Troubleshooting Refinery Process Catalyst Start-Up / Shutdown Activation Reduction In-situ Ex-situ Sulfiding Specializing in Refinery Process Catalyst Performance Evaluation Heat & Mass Balance Analysis Catalyst Remaining Life Determination Catalyst Deactivation Assessment Catalyst Performance Characterization Refining & Gas Processing & Petrochemical Industries Catalysts / Process Technology - Hydrogen Catalysts / Process Technology – Ammonia Catalyst Process Technology - Methanol Catalysts / process Technology – Petrochemicals Specializing in the Development & Commercialization of New Technology in the Refining & Petrochemical Industries Web Site: www.GBHEnterprises.com
  • 35. Refinery Process Stream Purification Refinery Process Catalysts Troubleshooting Refinery Process Catalyst Start-Up / Shutdown Activation Reduction In-situ Ex-situ Sulfiding Specializing in Refinery Process Catalyst Performance Evaluation Heat & Mass Balance Analysis Catalyst Remaining Life Determination Catalyst Deactivation Assessment Catalyst Performance Characterization Refining & Gas Processing & Petrochemical Industries Catalysts / Process Technology - Hydrogen Catalysts / Process Technology – Ammonia Catalyst Process Technology - Methanol Catalysts / process Technology – Petrochemicals Specializing in the Development & Commercialization of New Technology in the Refining & Petrochemical Industries Web Site: www.GBHEnterprises.com
  • 36. Refinery Process Stream Purification Refinery Process Catalysts Troubleshooting Refinery Process Catalyst Start-Up / Shutdown Activation Reduction In-situ Ex-situ Sulfiding Specializing in Refinery Process Catalyst Performance Evaluation Heat & Mass Balance Analysis Catalyst Remaining Life Determination Catalyst Deactivation Assessment Catalyst Performance Characterization Refining & Gas Processing & Petrochemical Industries Catalysts / Process Technology - Hydrogen Catalysts / Process Technology – Ammonia Catalyst Process Technology - Methanol Catalysts / process Technology – Petrochemicals Specializing in the Development & Commercialization of New Technology in the Refining & Petrochemical Industries Web Site: www.GBHEnterprises.com
  • 37. APPENDIX D CALCULATION OF INCIDENT QUANTITIES The quantity to cause an incident, as discussed in Clause 8, depends on: (a) The IDLH concentration (b) The effective ventilation of the zone. In the earlier stages of plant design rough estimates of outdoor and indoor ventilation rates can be made following Appendix A. As an example consider a substance with an IDLH concentration of 100 mg/m3, over 30 minutes. The zone will be given dimensions of 10m x 5m x 5m (250 m3). In Appendix A.1 it was suggested that a value of 1 m/s be used as an average wind speed, together with a formula for Mean flow area. Assuming a K of 1 this gives an effective ventilation rate (outdoors), Refinery Process Stream Purification Refinery Process Catalysts Troubleshooting Refinery Process Catalyst Start-Up / Shutdown Activation Reduction In-situ Ex-situ Sulfiding Specializing in Refinery Process Catalyst Performance Evaluation Heat & Mass Balance Analysis Catalyst Remaining Life Determination Catalyst Deactivation Assessment Catalyst Performance Characterization Refining & Gas Processing & Petrochemical Industries Catalysts / Process Technology - Hydrogen Catalysts / Process Technology – Ammonia Catalyst Process Technology - Methanol Catalysts / process Technology – Petrochemicals Specializing in the Development & Commercialization of New Technology in the Refining & Petrochemical Industries Web Site: www.GBHEnterprises.com
  • 38. APPENDIX E E.1 SUBSTANCE INDEX INTRODUCTION There is a need for a simple way of ranking substances for their difficulty of control for hygiene purposes. The Substance Index as described below, is relevant to short-term exposures and incidents, rather than chronic exposures. Two principal characteristics of a substance that indicate its relative difficulty of control for hygiene purposes are its 'volatility' and its 'toxicity'. As will be seen both can be expressed as atmospheric vapor concentrations, so the ratio of the two is a dimensionless index of toxic hazard. Here 'volatility' is an indication of the ease with which a substance may pass into the vapor phase. A simple measure of this is the partial pressure exerted by the specified substance. For pure substances, rather than mixtures, the saturated vapor pressure (svp) can be used. For the examples below, svp at ambient conditions is used - care should be taken to use relevant data for any real-life examples. This comparison applies strictly to vapors arising from liquids and solids, e.g. evaporation from a pool. Emissions of gases can be treated in the same way, as they produce a gas cloud of the pure substance, i.e. at 1 bar, rather than the svp of a liquid. This underestimates the immediate impact of a gaseous emission, but has the merit of simplicity. Strictly, Substance Indices for gases should be compared only to other gases. Liquefied gases behave like a mixture of a gas and a liquid. Gas flashes off until the pool cools to the boiling point of the substance, then it behaves as a liquid with svp of 1 bar. The simplified formula for gases is appropriate in this case. Refinery Process Stream Purification Refinery Process Catalysts Troubleshooting Refinery Process Catalyst Start-Up / Shutdown Activation Reduction In-situ Ex-situ Sulfiding Specializing in Refinery Process Catalyst Performance Evaluation Heat & Mass Balance Analysis Catalyst Remaining Life Determination Catalyst Deactivation Assessment Catalyst Performance Characterization Refining & Gas Processing & Petrochemical Industries Catalysts / Process Technology - Hydrogen Catalysts / Process Technology – Ammonia Catalyst Process Technology - Methanol Catalysts / process Technology – Petrochemicals Specializing in the Development & Commercialization of New Technology in the Refining & Petrochemical Industries Web Site: www.GBHEnterprises.com
  • 39. E.2 INCIDENTS When considering 'INCIDENTS' a suitable measure of the 'toxicity' is the IDLH concentration. The simple index is thus: where P is the relevant pressure in bars, converted in this formula to ppm by assuming ideality. As discussed this pressure is 1 bar for a gas, the saturated vapor pressure for a pure liquid, and the partial pressure for liquid mixtures. For liquids the pressures should be at the liquid temperature. Table 5 assumes 25°C. E.3 SHORT TERM HAZARDS The same logic can be used when considering short term emissions, against the Short Term Exposure Limit (STEL), averaged over 10 minutes. In this case the STEL value replaces IDLH. This is recommended only when the STEL presents the primary standard for compliance, i.e. short-term exposures are more relevant than longer term ones. Of the compounds in Table 5 this applies only to chlorine, and possibly trichlorofluoromethane. Advice should be sought from the relevant Occupational Health Department for specific substances. E.4 LONGER TERM When considering long term (CHRONIC) effects the effect of volatility is more complex. A highly volatile material will reach higher concentrations more quickly, but over longer time periods a lower volatility substance can exert the same average concentration. High volatility can even aid control in that it may be possible to leave a known emission to disperse. On the other hand low volatility substances can drain away, or be treated with absorbent. The impact of volatility can be discounted, provided it is realized that the emission rates used in calculations are the effective emissions, i.e. the quantity left to evaporate after alternative outlets have been considered, such as losses to drains, or conversion to different compounds over time. Refinery Process Stream Purification Refinery Process Catalysts Troubleshooting Refinery Process Catalyst Start-Up / Shutdown Activation Reduction In-situ Ex-situ Sulfiding Specializing in Refinery Process Catalyst Performance Evaluation Heat & Mass Balance Analysis Catalyst Remaining Life Determination Catalyst Deactivation Assessment Catalyst Performance Characterization Refining & Gas Processing & Petrochemical Industries Catalysts / Process Technology - Hydrogen Catalysts / Process Technology – Ammonia Catalyst Process Technology - Methanol Catalysts / process Technology – Petrochemicals Specializing in the Development & Commercialization of New Technology in the Refining & Petrochemical Industries Web Site: www.GBHEnterprises.com
  • 40. Then the best index to the hazard presented by a substance is simply the value of the OEL. Comparisons may be better in mg/m3 rather than ppm because emissions are normally considered in weight rather than molar terms. The usefulness of Table 6 can be judged by considering two well-known toxic hazards, chlorine and mercury. Chlorine ranks easily the worst INCIDENT substance. Mercury on the other hand presents essentially no INCIDENT hazard, but is very difficult to contain on a long term basis. Refinery Process Stream Purification Refinery Process Catalysts Troubleshooting Refinery Process Catalyst Start-Up / Shutdown Activation Reduction In-situ Ex-situ Sulfiding Specializing in Refinery Process Catalyst Performance Evaluation Heat & Mass Balance Analysis Catalyst Remaining Life Determination Catalyst Deactivation Assessment Catalyst Performance Characterization Refining & Gas Processing & Petrochemical Industries Catalysts / Process Technology - Hydrogen Catalysts / Process Technology – Ammonia Catalyst Process Technology - Methanol Catalysts / process Technology – Petrochemicals Specializing in the Development & Commercialization of New Technology in the Refining & Petrochemical Industries Web Site: www.GBHEnterprises.com
  • 41. Refinery Process Stream Purification Refinery Process Catalysts Troubleshooting Refinery Process Catalyst Start-Up / Shutdown Activation Reduction In-situ Ex-situ Sulfiding Specializing in Refinery Process Catalyst Performance Evaluation Heat & Mass Balance Analysis Catalyst Remaining Life Determination Catalyst Deactivation Assessment Catalyst Performance Characterization Refining & Gas Processing & Petrochemical Industries Catalysts / Process Technology - Hydrogen Catalysts / Process Technology – Ammonia Catalyst Process Technology - Methanol Catalysts / process Technology – Petrochemicals Specializing in the Development & Commercialization of New Technology in the Refining & Petrochemical Industries Web Site: www.GBHEnterprises.com
  • 42. Refinery Process Stream Purification Refinery Process Catalysts Troubleshooting Refinery Process Catalyst Start-Up / Shutdown Activation Reduction In-situ Ex-situ Sulfiding Specializing in Refinery Process Catalyst Performance Evaluation Heat & Mass Balance Analysis Catalyst Remaining Life Determination Catalyst Deactivation Assessment Catalyst Performance Characterization Refining & Gas Processing & Petrochemical Industries Catalysts / Process Technology - Hydrogen Catalysts / Process Technology – Ammonia Catalyst Process Technology - Methanol Catalysts / process Technology – Petrochemicals Specializing in the Development & Commercialization of New Technology in the Refining & Petrochemical Industries Web Site: www.GBHEnterprises.com
  • 43. Refinery Process Stream Purification Refinery Process Catalysts Troubleshooting Refinery Process Catalyst Start-Up / Shutdown Activation Reduction In-situ Ex-situ Sulfiding Specializing in Refinery Process Catalyst Performance Evaluation Heat & Mass Balance Analysis Catalyst Remaining Life Determination Catalyst Deactivation Assessment Catalyst Performance Characterization Refining & Gas Processing & Petrochemical Industries Catalysts / Process Technology - Hydrogen Catalysts / Process Technology – Ammonia Catalyst Process Technology - Methanol Catalysts / process Technology – Petrochemicals Specializing in the Development & Commercialization of New Technology in the Refining & Petrochemical Industries Web Site: www.GBHEnterprises.com
  • 44. Refinery Process Stream Purification Refinery Process Catalysts Troubleshooting Refinery Process Catalyst Start-Up / Shutdown Activation Reduction In-situ Ex-situ Sulfiding Specializing in Refinery Process Catalyst Performance Evaluation Heat & Mass Balance Analysis Catalyst Remaining Life Determination Catalyst Deactivation Assessment Catalyst Performance Characterization Refining & Gas Processing & Petrochemical Industries Catalysts / Process Technology - Hydrogen Catalysts / Process Technology – Ammonia Catalyst Process Technology - Methanol Catalysts / process Technology – Petrochemicals Specializing in the Development & Commercialization of New Technology in the Refining & Petrochemical Industries Web Site: www.GBHEnterprises.com