PRACTICAL GUIDE TO DEVELOPING PROCESS FLOW DIAGRAMS AND PRELIMINARY ENGINEERING LINE DIAGRAMS PROCESS

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Process Safety Guide:
GBHE-PSG-021
PRACTICAL GUIDE TO
DEVELOPING PROCESS
FLOW DIAGRAMS AND
PRELIMINARY ENGINEERING
LINE DIAGRAMS PROCESS
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Process Safety Guide: PFDs and ELDs
CONTENTS
1 INTRODUCTION
2 DESCRIPTION OF METHODOLOGY
2.1 Philosophy of the Design Process
2.2 Outline of Methodology
2.3 Additional Aspects of Methodology
3 WORKBOOK
4 EXAMPLE: BATCH NEUTRALIZATION AND STRIPPING PROCESS
4.1 Start of Stage 1
4.2 Stage 1
4.3 Stage 2
4.3 Stage 3
5 REFERENCES
TABLES
1 DESIGN OBJECTIVES
2 OUTPUT FROM EACH STAGE
3 STEP BY STEP APPROACH
4 FUNCTION
5 OPERATION
6 FAILURES
7 STAGE 3 PROMPTS (DATA SHEET)
8 PRELIMINARY MASS BALANCE
9 PRELIMINARY EQUIPMENT LIST
10 POTENTIAL PROBLEMS
11 STAGE 2: DEVELOPMENT OF OPERATING STATES AND
TRANSMISSIONS
12 POTENTIAL PROBLEMS

FIGURES
1 DESIGN PROCESS
2 DESIGN PHILOSOPHY.
3 SUMMARY OF METHODOLOGY
4 POSSIBLE OUTCOMES FROM A KEYWORD PROMPT
5 OPERATING STATES
6 OPERATING STATES/TRANSITIONS
7 EXAMPLE: OVERALL BLOCK DIAGRAM
8 EXAMPLE: “PRIMITIVE PFD”
9 EXAMPLE: STAGE 1 “WORKING DIAGRAM”.
10 EXAMPLE: MAIN OPERATING STATES AND TRANSITIONS
11 EXAMPLE: DETAILED PFD (END OF STAGE 1)
12 EXAMPLE: STAGE 2 “WORKING DIAGRAM”
13 EXAMPLE: STAGE 3 “REV 0” ELD

1 INTRODUCTION
This document presents a methodology to guide the development of
projects from preliminary process design into Process Flow Diagrams
(PFDs) and subsequently into Engineering Line Diagrams (ELDs). Key
features of this approach are an early understanding of design issues and
in particular a systematic way of designing out potential hazards. The
guide provides a structure and set of tools to improve the effectiveness of
the design process and especially to minimize design changes. The
methodology is applicable to both batch and continuous processes at any
scale of operation. It can be applied equally well to a plant complex or an
individual section of a plant. This "synthesis" tool may also be used for
analysis to develop a systematic understanding of an existing process or
plant. It should therefore also be useful to newly appointed plant
personnel, to those involved in purchasing licensed technology or to
teams involved in acquisitions.
The development of this methodology arose out of project OSPREY, a
collaborative research project between GBH ENTERPRISES and
RENALT ENERGY. The original remit of OSPREY was to produce a
formal methodology to aid in the development of ELDs from PFDs, and
thus to support and promote consideration of safety, health and
environmental concerns during this stage of design and in particular to
minimize changes identified during Hazard Studies. During the project it
became apparent that there was no formal methodology for the
development of PFDs. In order to bridge this gap, work on an extended
remit was carried out in collaboration with RENALTENERGY and
subsequently completed within GBH ENTERPRISES.
Figure 1 shows a schematic of the design process from the initial "idea"
through to a completed set of ELDs. There is a well-established
methodology for the development of an initial "idea" through to preliminary
process design ("Professor Douglas" Methodology, Ref.1). In contrast
there appears to be no published approach to guide the development of
the output from preliminary process design into PFDs and subsequently
into ELDs. During this latter development successive layers of detail are
added, the design team increases in size and it becomes more expensive
and it takes more time to do any rework associated with design changes.
In addition, the increase in detail makes it difficult to maintain a "global
view" of the whole process as a single PFD is replaced by a set of detailed
PFDs and ELDs. There is thus a need for a methodology to drive the
development of PFDs and ELDs. This process SHE Guide represents a
considered and documented approach for this step.

FIGURE 1 DESIGN PROCESS

2 DESCRIPTION OF METHODOLOGY
2.1 Philosophy of the Design Process
This methodology is based on the fundamental approach illustrated in
Figure 2. Here it is argued that any design process should start with an
agreed intent, continue by steps to determine possible solutions, screen
the possibilities against criteria such as cost, and work up the preferred
solution into the final design. The original intent should always be checked
to ensure that this is really what is wanted. This checking should occur
both at the start and throughout the design process. The list of possible
solutions should not be and cannot be exhaustive so some recycling back
to the solution stage will occur if the criteria are not met. If no acceptable
solution can be found, then there is a need to restate the intent.
Every design project will be different and it is important that the design
process takes a fresh view to meeting objectives and implementing
solutions. It should however be recognized that successful projects will
make use of existing knowledge and understanding and will integrate into
the local culture. There is no point in re-inventing wheels but the skill will
be in applying the right set of wheels.
This fundamental approach is scale independent; thus it could be applied
to the design of an entire chemical plant or to a minor modification. In the
context of this guide, the approach is applied in stages to provide a
structure and some ’milestones’ for the PFD and ELD development.
2.2 Outline of Methodology
The methodology comprises three stages as follows:
Stage 1: Design Strategy
This stage takes the preliminary process design information and uses a
structured approach to identify all the issues relevant to PFD
development. This allows development of an overall PFD, together with a
process description and operating strategy.

Stage 2: Design Intent
This stage uses the overall PFD as a basis for notional ELDs, and uses a
structured approach to add the next layer of detail with the emphasis on
what is required, rather than how requirements are to be implemented in
specific hardware.
Stage 3: Design Implementation
This stage turns intent into hardware proposals, which are displayed on
"Revision 0" ELDs. These ELDs can then be refined further in an ongoing
process of ELD development and review.
Any viable process design should operate efficiently at steady state or
throughout a batch cycle, should be capable of operating under different
conditions and should be capable of minimizing losses and shutting down
safely in the face of failures. Accordingly, the considerations at each stage
are divided into three sub-stages:
(a) Function: here the focus is on those design objectives that are key
to the normal operation of the plant.
(b) Operation: here the focus is on the various operating states and
transitions (e.g. start-up, shutdown) that the plant must be designed
to handle.
(c) Failures: here the focus is on identification of likely failures,
minimization of the risk of failure and limiting the effects of failures.
A risk assessment can then be made, and where necessary safeguard actions
proposed and implemented.

FIGURE 2 DESIGN PHILOSOPHY

Table 1 shows the various design objectives grouped into each substage.
TABLE 1 DESIGN OBJECTIVES
These design objectives are expanded and used as thought prompts or
guidewords in stages 1 and 2. The same prompts are used in both stages; this
has the advantage of maintaining some linkage between the stages and makes
for ease of cross-referencing. The distinction between function, operation and
failures is retained in Stage 3, although a separate set of guidewords, based on
defining equipment data, is used.
Figure 3 summarizes the outline methodology and indicates the method of
working. It will always be a matter of judgment as to the level of detail required at
each stage during the development. A key to efficient design is timing: providing
the right level of detail at the right time. Experience is vital to achieve that
efficiency. Developing confidence that problems can be solved later should lead
to a more effective approach. The possible outcomes from any guideword are
shown in Figure 4.
For each guideword the outcomes are recorded and any actions are identified
and noted. There is no need for elaborate records; notes on the notional PFD or
the notional ELD, together with a simple table of Guideword/Comments/Actions
should be sufficient for subsequent reference and to demonstrate that a
systematic method was used.
The methodology can be used by an individual; however it is probably most
powerful if used by a small group of individuals with complementary skills. An
ideal core group would comprise the lead process engineer, the project engineer
and a representative from operations. Such a team would require support from
functional specialists from other disciplines (e.g. R&T, Control, Vessels,
Machinery, etc).

2.3 Additional Aspects of Methodology
2.3.1 Operating States and Transitions
A key part of "Operation" in Stage 1 is the definition of the operating states that
the plant must be able to operate at and the transitions between these operating
states that should be possible. Figure 5 shows an example of possible operating
states for a plant together with transitions between them. Some transitions, such
as between regeneration and ready to start-up, will be carried out in a controlled
way; others, such as between normal operation and shutdown, may be initiated
by automatic detection of a fault by the plant shutdown system.
For each operating state it is possible to define the status of each process
stream, service or item of equipment and thus define the actions required during
any given transition (Figure 6). This provides a basis for the systematic
development of operating instructions.

FIGURE 3 SUMMARY OF METHODOLOGY

FIGURE 4 POSSIBLE OUTCOMES FROM A KEYWORD PROMPT

FIGURE 5 OPERATING STATES

FIGURE 6 OPERATING STATES/TRANSITIONS
2.3.2 Interactions
There are a number of global design issues, such as control, measurement and
analysis, that span the whole plant. Furthermore some of these issues, such as
isolation and relief, interact with each other. It is difficult to handle these issues
once the level of detail is such that the design is presented on more than one
PFD or ELD. It is also recognized that much of the detailed development,
particularly on large projects, may be carried out by different teams at different
times. It is therefore proposed to retain and use a single overall PFD in addition
to the more detailed sectional PFDs that might be developed. This overall PFD
should show the main plant items (MPIs) and lines, together with "global" details
such as relief, isolation, control and instrumentation relevant to control of the
whole plant (inventory control in particular). This PFD should suppress as much
"local" detail as possible, such as local instrumentation and control (except
control valves that might isolate vessels from relief devices), and local kick back
lines. This global PFD should be updated as the project progresses.

3 WORKBOOK
This clause describes how to develop a concept into a line diagram with
its associated supporting design information. The description is based
around a whole plant design and the development of a set of line
diagrams but is equally applicable to a single diagram, e.g. in a
modification to an existing plant. The methodology considers the
development in a series of defined stages and uses a set of guidewords to
define the purpose of the design and find acceptable solutions.
Figure 3 shows the stages in the proposed method. This guide starts with
a preliminary proposal and ends with a set of Rev "0" ELDs and
supporting data sheets. Further details of the output and the typical
support information, required at each stage, are given in Table 2.
The approach to be adopted is described in outline in Table 3. It will be
noted that at each stage, the proposed design is critically examined with
respect to function, operation and failure, using a set of guidewords.
These guidewords are described in Tables 4 to 6. A specific set, loosely
based on the information needed for datasheets, is also available for
stage 3 (Table 7).
It is difficult to be specific in this guide as to the level of critical
examination and detail necessary at each stage. Such detail will depend
upon the complexity of the project. The detail should be adequate to
identify and address the important issues and allow efficient progress. The
detail should not be so great as to divert attention or to complete design
work that may either change or be better carried out later. An experienced
team is necessary.
For a new plant, the overall PFD and section ELDs develop together. For
a modification to an existing plant, these drawings and supporting design
data should already be available. It is however critical to review the overall
PFD and ELDs to ensure that all new potential interactions are fully
considered.

4 EXAMPLE: BATCH NEUTRALISATION AND STRIPPING PROCESS
This example illustrates the application of the methodology to a fairly
simple batch process: the batch neutralization and steam stripping of an
effluent stream. The notes below are not intended to be exhaustive, but to
give some illustration of the use of the methodology and in particular to
illustrate the level of detail that is appropriate at each stage.
4.1 Start of Stage 1
At this point the information available is as follows:
An upstream plant produces an acidic effluent contaminated with acid
soluble organics which must be neutralized and stripped before
discharging to the site effluent system. Laboratory experiments show that
the organics are easily stripped from the neutralized effluent by steam
sparging, and that the resulting aqueous and organic layers in the
condensate are immiscible and easily separated.
Figure 7 shows a block diagram of the batch neutralization and stripping
process in the overall context of the main plant producing the effluent. The
effluent flow is continuous and averages 0.2m3/hr at 10% acid with about
1% organics, although there is considerable variability in both flow and
composition and on occasions there will be a need to process
decontamination washings from the main plant. The stripped organics can
be returned to the main plant for reuse provided they are substantially free
of free phase water. The organics have boiling points in the range 100-
120°C, have a low OEL and are flammable. The stripped alkaline effluent
can be fed to the Works Treatment Plant (TP) provided the level of
organics is below 10ppm. The Works TP can handle variations in pH,
although the temperature of the stream must be below 50°C.
Figure 8 shows a "primitive" PFD. The process concept is to neutralize the
effluent with 10% caustic, strip with live steam to remove the volatile
organics, condense the vapors produced and phase separate the
condensate. The organic phase is returned to the upstream plant,
the organic-saturated aqueous phase is recycled to D1 for stripping with
the next batch.
Table 8 is a preliminary mass balance, based on a 6 hr batch cycle. Table
9 is a preliminary equipment list:

TABLE 8 PRELIMINARY MASS BALANCE
amounts in kg, 6 hr batch
TABLE 9 PRELIMINARY EQUIPMENT LIST

FIGURE 7 EXAMPLE: OVERALL BLOCK DIAGRAM

FIGURE 8 EXAMPLE: “PRIMITIVE PFD”
4.2 Stage 1
The overall objective of this stage is to convert the primitive PFD to a more
detailed PFD using the guidewords under each of the three headings of
function, operation and failure. A copy of the primitive PFD can be used as
a working diagram (e.g. Figure 9) to record decision and comments.
(a) Function
The guidewords are used to check carefully the basis of design. A
number of issues surface such as:
(1) destination of recovered organics if main plant not able to
accept?
(2) destination for the vent: treat locally or tie into main plant
vent system?
(3) N2 padding on vent required to maintain an inert
atmosphere;
(4) etc.

(b) Operation
One of the key activities at this stage is to make explicit the main
operating states and the transitions between them. Figure 10 has
been developed for this example. This in turn allows the main
instrumentation and control requirements to be defined and added
to the working diagram. Measurement of pH and organics, together
with observation of the interface are flagged as areas that will need
additional consideration. The need for process water for
commissioning and for washout before maintenance is noted.
(c) Failures
At this stage, the potential problems and their possible solutions are
listed in Table 10:
TABLE 10 POTENTIAL PROBLEMS
A detailed PFD (Figure 11) can be produced from the working diagram. This diagram
becomes the starting point for the Stage 2 work.

FIGURE 9 EXAMPLE: STAGE 1 “WORKING DIAGRAM”
FIGURE 10 EXAMPLE: MAIN OPERATING STATES AND TRANSITIONS

FIGURE 11 EXAMPLE: DETAILED PFD (END OF STAGE 1)
4.3 Stage 2
The overall objective of this stage is to convert the detailed PFD to a
"concept" ELD that captures design intent, using the guidewords under
each of the three headings of function, operation and failure. A copy of the
detailed PFD can be used as a working diagram (e.g. Figure 11) to record
decision and comments.
(a) Function
The guidewords should be revisited to check that:
(1) problems and issues raised in Stage 1 have been resolved;
(2) any new information (e.g. from an experimental program) is
taken on board;
(3) any changes in project scope are addressed.
It is important to check that any "interface" issues raised by the guideword
"input/output" are resolved at this stage; issues for the example include:
(i) availability of caustic batches when required;
(ii) are feed pumps required for caustic and the acid effluent?
(iii) etc.

(b) Operation
The operating states and transitions that were defined in Stage 1 are
developed in more detail; this can be done most effectively in a tabular
format like Table 11. This table can be used to write outline operating
instructions: by way of example the outline instructions for the transitions
"add caustic" and "add acid effluent" are as follows:
(1) "add caustic";
(2) check exit valves shut, cooling water on, caustic available;
(3) analyze acid strength in buffer storage;
(4) calculate batch recipe;
(5) charge caustic;
(6) return aqueous "heel" from phase separator to D1;
(7) start agitator;
(8) check level and temperature;
(9) check caustic valve closed;
(10) "add acid effluent";
(11) add 90% of acid in recipe;
(12) agitate for 5 mins, check level and temperature;
(13) check pH (should be alkaline);
(14) recalculate "trim" addition (should be acid effluent);
(15) add "trim" addition;
(16) agitate, check pH.
Table 11, together with these outline operating instructions should enable
detailed consideration of the remaining guidewords and allow
development of the working diagram Figure 12.

TABLE 11 STAGE 2: DEVELOPMENT OF OPERATING STATES AND
TRANSITIONS

(c) Failures
A more detailed consideration is probably best carried out in a
tabular form such as Table 12 below:
TABLE 12 POTENTIAL PROBLEMS
Additions to the working diagram (Figure 12) are made as required. This
diagram becomes the starting point for the Stage 3 work.
4.4 Stage 3
The overall objective of this stage is to convert the working diagram
(Figure 12) which shows ELD intent into a "Revision 0" ELD which shows
hardware proposals. The guidewords in Tables 4 to 6 should be applied to
each line and/or vessel in turn.
Figure 13 is the completed "Revision 0" ELD.
5 REFERENCES
1 IC02290: Process Evaluation & Preliminary Design Procedure. D C
Woodcock et al. 1985

FIGURE 12 EXAMPLE: STAGE 2 WORKING DIAGRAM

FIGURE 13 EXAMPLE: STAGE 3 “REV 0” ELD

PRACTICAL GUIDE TO DEVELOPING PROCESS FLOW DIAGRAMS AND PRELIMINARY ENGINEERING LINE DIAGRAMS PROCESS

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