Instrumentation tech 1

Process Technician Training

Process Control
Instruments
Package number 1
1

Process Control Instrumentation
Process Control Instrumentation is a wide ranging
subject.
While only dealing with the subject in a necessarily
brief manner the packages are of some considerable
length
For this reason the subject has been broken into two
separate and distinct packages.
It is recommended that the packages are studied in
their correct sequence

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The first package deals with the principles of Flow,
Level, Temperature and Pressure and how they are
controlled
The second package deals with the Controllers and
the ancillaries that complement them
The package also deals with some of the more
complicated control systems such as PLC’s and DCS’s

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There are many video clips within the packages.

For the clips to activate and play the PowerPoint
presentation must be viewed in “View Show” from the
“Slide Show” drop-down menu

“Slide Show” can be selected from the Toolbar at the
top of the page. “View Show” can be selected from the
menu.

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Units in this Course
First Package
Unit 1 Introduction
Unit 2 Pressure Measurement
Unit 3 Level Measurement
Unit 4 Temperature Measurement
Unit 5 Flow Measurements

Second Package
Unit 6 Ancillary Control Equipment
Unit 7 Controllers
Unit 8 Control Loops
Unit 9 Overview of:
- Distributive Control Systems (DCS)
- Programmable Logic Controllers (PLC)

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First Package

Unit 1 Introduction
Unit 2 Pressure Measurement
Unit 3 Level Measurement
Unit 4 Temperature Measurement
Unit 5 Flow Measurements

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Objectives
The objectives of this course are that the participant:

 Knows the four variables being measured and
controlled
 Understands what a “Process Instrument” is.
 Understands how an instrument functions
 Gains an understanding of the many types of
instruments
 Learns of the many applications that control
systems can be used in

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Introduction to Process Control Instruments

Instrumentation plays an important part in the
efficient operation of any processing or production
plant.

Instruments enable the plant to operate smoothly
and safely with a minimum of operating staff.

This course will give a basic understanding of how
the instruments function.

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A process is the changing of a raw material into a
finished product.

As raw materials flow through the process
equipment they are subjected to various conditions.

These conditions may alter the composition of the
raw material and may be the chemical structure.

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It is important that the process conditions are
accurately controlled at all times.

The controlling is done by instruments.

An instrument cannot ‘think’. It can only respond to
parameters that are set by the Process Department

It is the Technicians that tell the instruments what to
do.

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The Technician who knows the instruments is
the master.
The instruments are his slaves or servants.
When the instruments are properly used, the
process equipment operates correctly.
When the process equipment operates correctly
the finished products are up to specification.
When the process equipment is running steadily
the company is making money.
An upset unit is costing money

13

Process Control
A simple example of a Manual process control.

Technician

The process is temperature control. The indicator is a
thermometer

Process Control

The correcting unit is the gas control valve.

The controller is the Technician who uses his own
judgment to keep the water temperature constant.

Manual control has its uses:
 It is cheap to install and maintain
 It is simple to operate.

15

Such a control system is seldom used in industry
because:

The Technician must remain in position at all times.
It cannot be used if the Technician is placed in a
dangerous area.
The process may change faster than the Technician
can react.
A mistake by the Technician can have dangerous
results.

16

These problems are avoided by using automatic control.

Modern household appliances now use automatic control
to make work easier. For example:-

 Refrigerators and air conditioners use automatic
temperature control.
 Air conditioners use automatic temperature control.
 An electric water heater uses automatic heating and
water control and a switch that will shut off when the
water boils.

17


This is a simple automatic controller.
The boiler now has the loop closed and no Technician
is required.

To install the automatic system the following items were
added:
The temperature transmitter (T.T) which senses the
temperature of the hot water and changes it to a
standard signal.
A signal line from the transmitter to the automatic
controller (the signal may be either pneumatic or
electrical)
A controller which keeps the temperature of the hot
water at a position set by the Technician (set point).

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The controller adjusts the correcting unit (automatic
control valve) using an output signal line similar to the
input line from the transmitter.

The controller may provide alarm signals to alert the
Technician if the system fails. It may also shut off the
gas if the water starts to boil.

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A group of instruments that are used to control a
particular process is a ‘Control Loop’
Control loops can be very simple like the one illustrated
in the following video clip which is a simple temperature
control on the steam into a heater
Other systems can be quite complex and involve a wide
range of instruments that all ‘talk’ to one another to
control a system
A good example of this is the level control system on
the CPF Inlet Separators - these will be dealt with later.

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Control Loop:
A Control Loop is an active system that keeps a
process variable within maximum and minimum
values by continuous measurement and continuous
corrective actions.

A level control loop
controlling the level in a
vessel by restricting the
pump discharge

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 A process Control Loop may contain many control
Instruments and can be very complex.

 However, if each part or device that is in the complex
unit is taken one by one the system becomes much
easier to understand.

The following series of video clips describe a very
simple type of process control and will give an
understanding of how changes made to a process take
effect

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The previous video clips were simplified and described
a very basic system – the control of steam being used
to heat water.

Some of the terms used in the video are terms in
common use in this type of industry

The following is a list of a few of these terms and their
meanings

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Basic Definitions
Instrument Any device for measuring, indicating,
controlling, recording and adjusting a physical
or chemical property e.g. flow pressure,
acidity, weight, gas concentration, etc
Instrumentation A complete set of instruments used to control
a process e.g. refining, oil/gas production,
LNG, LPG, etc

Indicator A device which shows a measured value to
the operator

Lag The time taken between an adjustment being
made and the process responding to it
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Basic Definitions
Recorder A device which continuously records
measurements, either electronically or on an
ink chart. It is used to show production
figures, etc.
Process Loop A group of instruments used to control a single
process variable e.g. pressure, flow, level, etc.

Process The word for a manufacturing unit e.g.
refining, liquefying gas, etc.

Measured The value of the property being controlled by a
Variable or single process loop e.g. pressure, flow, level,
Process etc.
Variable (MV)
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Basic Definitions
Desired Value The value required by the operator.
or Set Point
(SP)
Error Signal The difference between the measured
(ES) variable and the set point - should be zero for
good control.
Controller A device, either pneumatic or electrical /
electronic, which adjusts the error signal to
zero.
Correcting Unit A device which works on the command of the
(Final Control controller. It is used to adjust the measured
Element) value to obtain a zero error signal e,g, control
valve etc.
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Basic Definitions
Transmission A method of standardising signals sent
from various parts of the plant.

Transmitter A device which takes a measurement and
changes it into a standard signal.
Transducer A device which changes one form of
energy to another; particularly from
electrical to pneumatic.

Disturbance A change in the process that is not
anticipated
Load A planned change in throughput of a unit
Change
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Note: The instruments use in a processing facility can
vary greatly, depending on the age of the installation.
They may be air (pneumatic), liquid (hydraulic) or
electric/electronic in operation.
The way the information is shown or recorded may be
simple, like a clock or thermometer
In other cases it may be by the latest information
technology displaying the information on a personal
computer screen (video display unit).

36


Process Variables
A process variable is a process that we can measure and
change. There are many process variables in a Processing
Facility.

However there are only four main variables in a facility:
 Pressure
 Flow
 Level
 Temperature
We will concentrate on these four process variables.
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Pressure

It is pressure that pushes fluids through pipes
and equipment.

It can be considered that pressure is the most important
process variable.

It is therefore important that you have a very good
understanding of pressure.

Without that understanding you will find it difficult to follow
courses that come later in your training.
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Pressure
Pressure (P) is defined as the Force (F) applied divided by Area (A).

Pressure and Gases

The diagram shows a force (F)
applied to a piston pressing on
a liquid in a cylinder.

The liquid is considered
incompressible and the pressure of the liquid on the walls of the cylinder
is the same in all directions. This gives the formula
P=F
A


Pressure on a Gas
This diagram shows a force (F)
applied to a piston pressing on
gas in the cylinder. The gas is
compressible.

The volume of the gas will
decrease until the pressure
of the gas on the walls of the cylinder equals the pressure
applied by the piston.
This gives the formula F = P
A


Pressure Units
There is no agreed standard for pressure
measurement in the petrochemical industry.

Some companies use Imperial Units (USA), some
use International Standard Metric Units (ISO)

Some use both.

The Technician needs to understand both systems and
be able to convert from one to another.

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Pressure Units
Imperial ISO

Force Pound Newton

Area Square Inch Square Metre

Pressure Pounds per square inch Newtons per square
called PSI metre called Pascal

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Pressure Units

There are tables for changing from one system to another. An
example is the conversion of psi to kPa.

Conversion: 1 psi = 6.89 kPa

The Pascal is a very small unit so the KILOPASCAL (kPa) is
often used. The bigger unit is the BAR. The bar is the most
common ISO unit.

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Pressure Conversions:
100 kPa = 1 bar
Note: On very old installations the kilogram per centimetre
square is still used.

For all general purposes. 1 kg/cm2 = 1 bar

Very small pressures are measured using the height of a
column of liquid. The liquids used most are water (H20) and
mercury (Hg).

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Pressure
Absolute, Gauge and Atmospheric Pressure
The price of oil or gas depends on the quantity (mass) of
the product.
The quantity of oil or gas in a given volume depends on
the pressure.
For this measurement, absolute pressure must be used.

Absolute Pressure
This is the pressure above a total vacuum (there are no
particles of matter in a total vacuum).

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Pressure
Gauge Pressure
This is the pressure measured by a gauge. Gauge
pressure is the pressure above that of the surrounding
atmosphere.

Atmospheric Pressure
The pressure of the air all around you.
This is not constant, it depends on things like the
weather and the altitude of the plant.

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Pressure
The equation linking the above pressures together is
Absolute Pressure = Gauge Pressure + Atmospheric
Pressure.

Because atmospheric pressure can vary, a standard
atmospheric pressure of 1.013 Bar or 14.70 psi. is used

Gauge pressure is written as psig.
Absolute pressure is written as psia.

48

Pressure
Example
A pressure gauge indicates 11.4 psi.
To find the absolute pressure if the atmospheric pressure
14.65 psi.
Solution
The absolute pressure is equal to atmospheric pressure
plus the gauge pressure (AP = GP + Atmospheric Pressure)
AP = 11.4 + 14.65 = 26.05

Therefore the Absolute Pressure is 26.05 psi

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Units of Flow

This simplified diagram shows a tanker being loaded from a
storage tank. The amount of oil loaded must be accurately
measured to know how much it costs. The total flow
(quantity) of oil into the tanker can be measured in two
ways.
By volume, in barrels or cubic metres.
By mass, in metric or imperial tonnes.


Units of Flow
For control purposes the rate of flow (how fast the ship is
loaded) is also measured. Rate of flow units can also be
given in either volumetric or mass units.
Rate of flow by Volume (Volumetric)
Barrel / Hour
Cubic Feet / Minute
Cubic Metres / Second
Rate of flow by Mass
Tonnes / Hour
Kilograms / Second
Pounds / Minute

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Units of Flow
The petrochemical industry uses many different units and
there is no common standard. The following list gives
some of the units and their conversion

VOLUME
Barrel (bbl) = 42 US gallons = 34.97 Imperial gallons
Cubic foot (ft3) = 0.0929 m3
Cubic metres (m3) = 10.76 ft3
Cubic metres (m3) = 1000 litres
1 litre = 1000 cubic centimetres (millilitres)

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Units of Flow

MASS
Pound (lb) = 0.454 kg
Kilogram (kg) = 2.2 lb
Imperial Tonne = 2240 lb (long tonne)
Metric Tonne = 1000 kg
American Tonne = 2000 lb (short tonne)
Long Tonne = 1.016 Metric tonne
Metric Tonne = 0.984 long tonne
Note: It is not necessary to memorise conversions.
Conversion tables will be available at your facility.
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Units of Flow
VELOCITY
ft/sec ft/min metre/sec metre/min
1 60 0.3048 18.29
0.01667 1 0.005080 0.3048
0.03281 1.9685 0.01 0.600
3.281 196.85 1 60
0.547 3.281 0.01667 1

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Units of Flow
ABBREVIATIONS
Bopd = Barrels oil per day
Blpd = Barrels liquid per day
Bcpd = Barrels condensate per day
Scfpd = Standard cubic feet per day
MScdpf = Thousand standard cubic feet per day
MMScfpd = Million standard cubic feet per day
Nm3pd = Normalised cubic metres per day
1/m or Llt/m = Litres per minute
5000 ml/s = 500 millilitres per second

Note: Sometimes the p (per) is omitted in the abbreviation. Standard
and normalised refer to a standard temperature and pressure. Common
standards are: 14.7 psi at 680F and 1.013 Bar at 150C. 55


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Temperature

There are different scales for measuring temperatures. The diagram
compares the two common temperature scales; Fahrenheit (Imperial) and
Celsius (ISO).


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Temperature
The fixed points for both scales are the temperature at
which ice melts and water boils at standard pressure.

A temperature in Fahrenheit can easily be changed to
Celsius and vice versa. The conversion equations
depend on the number of divisions in each scale.
Fahrenheit has 180 divisions between the freezing and
boiling points of water but Celsius has only 100 divisions.
Therefore, the ratio is 180/100 or 9:5. This gives:
0
C = 5/9 (0F – 32) or 0F = 9/5 0C + 32

There are tables available for Fahrenheit – Celsius
conversions.
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Pressure Measurement

Introduction

The object of this unit is to describe the common
devices used to measure pressure.

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The Bourdon Tube Pressure Gauge
The Bourdon tube gauge is the most common pressure
indicator in the petrochemical industry. It shows the
pressure in a clear, simple way.

The previous diagram showed a typical Bourdon gauge.
It consists of the following parts:

 The Bourdon tube itself. This is a metal tube shaped
like a ‘C’. It has an oval cross sectional area. It is
sealed at one end. The sealed end is free to move.

 A linkage and pinion to turn the pointer.

 A scale to indicate the pressure.

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The Operation of a Bourdon Tube Pressure Gauge
When a pressure is applied to the inside of the tube it will
try to straighten. The closed end (the tip) will move and
the linkage moves the pinion which moves the pointer.
The movement of the pointer shows how much pressure is
applied to the Bourdon tube.

The Bourdon gauges come in all shapes and sizes and
can measure from about 0-15 psig (0-1 bar) to 0-10,000
psig (0-700 bar) depending on the stiffness of the material
used.

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There are also other types of Bourdon tubes:

 Spiral Bourdon tubes

 Helical Bourdon tubes

These perform the same function as the simple ‘C’ type
Bourdon tube except that they provide more movement
and are more accurate.

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Bourdon Tube Pressure Gauge - Spiral Bourdon Tube
This diagram shows a spiral Bourdon tube. It is used to
indicate low pressures. When pressure is applied the spiral
unwinds and the free end moves to indicate the pressure.

Bourdon Tube Pressure Gauge - Helical Bourdon Tube
This diagram shows a Helical Bourdon tube. This is usually
used to indicate high pressures. When pressure is applied
the helix unwinds and the free end moves to indicate the
pressure applied.


A helix coil is used for low pressure applications. They
expand to a greater degree than the Bourdon tube

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Bellows
Bellows are tubes with thin walls
made of brass, stainless steel,
etc. The thin walls are
corrugated. This improves their
ability to expand and contract.
When pressure is applied (either
to the outside or the inside), the
corrugated walls expand or
contract. This movement is used
to indicate pressure. Bellows
units are used in various ways.
These are the three most
common methods

Diaphragms
A diaphragm is a stiff corrugated disc which is flexible under
pressure. A single diaphragm is often used as a seal to
protect a gauge from corrosive liquids. A typical example is
given in the illustration.

Diaphragms
Diaphragms are also used to make high pressure bellows
(a diaphragm stack). A typical example is shown

Capsules
Capsules are made of two
diaphragms welded onto a
metal ring and filled with a
fluid. Different mechanical
and electrical methods are
used to show the differential
pressure across the capsule.
The diagram shows a capsule
used in a pneumatic
differential pressure
transmitter.

The Strain Gauge
The strain gauge is a resistor which has been deposited into a
flexible bar. As the bar is bent the resistor will change in length
and thus its resistance. The changes in resistance are detected
and electronically changed to a pressure signal. The method is
used in electrical transmitters.

Vibrating (Resonant) Wire
The vibrating wire is the operating method used in some
pressure transmitters. The diagram shows the basic
construction.

Vibrating (Resonant) Wire - Operation

The frequency of vibration of a wire depends on its
tension. The tension of the vibrating wire is changing by
the pressure applied to the diaphragm. The electronics
unit vibrates the wire and measures the change in
vibration frequency caused by pressure moving the
diaphragm.

The electronics unit changes the pressure applied to the
diaphragm into an electrical output signal.

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Electrical Pressure Sensing Methods
The old mechanical methods of detecting pressure are
slowly being replaced by electrical methods.
Electrical methods are more accurate and cheaper.
The following gives a simple explanation of the principle
involved.

The Piezo Electric Effect
Certain crystals, such as quartz, produce a voltage
across them when a pressure is applied. This voltage is
simplified electronically and displayed digitally on a
multimeter.

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Differential Pressure
A differential pressure is the difference in pressure
between two measuring points
The differential pressure is used in a process to
measure the pressure drop across a resistance to a flow
This resistance could be an orifice of a known size and
the pressure differential can be used to calculate a flow
rate – this principle is used in an orifice plate.
A pressure differential across a filter is used to
determine the fouling across the filter and is used to
know when to change an element

84

Differential Pressure

Not all Differential
Pressure Indicators
have a dial face.
This is the Differential
Pressure Indicator
across the diesel filter in
the Kutubu Refinery.
The pressure is read on
a linear scale.

85

Pressure Switch

Pressure switches are devices that open or close
electrical circuits when they sense a pre-set pressure.

The electrical circuits can then be used to open or
close valves to relieve pressure in a system.

The switches can be used to switch on pumps or
compressors to maintain pressure in a system.

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Level Measurement

Introduction

This unit will describe the common methods
and devices used to measure liquid levels in
process equipment.

90

Level Measurement

Types of Level Measuring Devices

There are two main types of level measuring devices.

 Direct level measuring devices.

 Indirect level measuring devices

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Level Measurement
Direct Level Measuring Devices
Direct methods allow the operator to actually ‘see’ the
liquid level or to take a direct measurement of the levels of
liquid in a vessel.
You can see how much liquid you have in your windscreen
washer tank by looking at the level through the wall of the
tank.
You can see how much acid you have in your car battery
by looking at the level through the wall of the battery.
You can measure the level of oil in your car engine by
looking at the dipstick. You physically measure the oil level.
All of the above are direct level measuring devices.

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Level Measurement
Indirect Level Measuring Devices
You cannot measure or see how much petrol you have
in the tank of your car.
An instrument measures the level and shows you how
much petrol there is on a indicator on the dashboard (the
petrol gauge).
This is an example of an indirect level measuring
device.

93

Level Measurement
Direct Level Measuring Devices - The Dip Stick
The Dip Stick is the only true measurement of level.
It is still used by operators and ships captains to check that
the instrumentation which measures the level of a liquid in a
tank is correct.

Level Measurement

Direct Level Measuring Devices - The Dip Stick
The Dip Stick is a long calibrated ruler.
The depth of the liquid in the tank is indicated by a WET mark
when the stick is removed. It is the same principle as checking
the oil level of a car.
Because there may be rubbish at the bottom of the tank the
level may be taken from a bottom level datum line.
A datum line is a base line from which things can be
measured.
There is also a top datum line which is used to measure the
space between the liquid and the top of the tank.

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Direct Level Measurement

The Dip Tape
The Dip tape (see Figure 3.2) is a
development of the dip stick.
It is used to find the level in large tanks.
The tape is calibrated like the dip stick. The tape is run
out until the weight touches the bottom of the tank.
It is then pulled up. The wet mark of the tape indicates
the level of the liquid.
By using a special water finding paste on the bottom of
the tape you can detect the level of water that could be
below the oil in the tank.

96

Level Measurement

Direct Level Measuring Devices - The Dip Tape

HANDLE

WINDER

Level Measurement

The Sight Glass
This is the indicator used by
operators in the plant to ‘see’
inside of a vessel.
The sight glass is connected
to the side of a vessel and
the level is seen by looking
through the glass.
A high pressure sight glass
is illustrated

Indirect Level Measurement

Level Transmitter Bridle
Vent

Transmitter /
Control Box
vessel
Displacer

Liquid level
Level Column

Drain

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Level Measurement
Sight Glasses

B

A

Magnetic Sight-glasses. A on the Inlet Separator, CPF and B is on a
Liquid KO pot on Gobe South Compressors

100

Level Measurement
Typical Level Control system in a process area

Level Transmitter
Level Switches
Level Column

Bridle

Sight Glass

101

Level Measurement

Indirect methods product mechanical or electrical output
signals which indicate changes in level.

Simple Floats
Figure 3.4 shows a simple float level indicator. It is still
used by water departments and on chemical tanks on
older oil platforms.
It is cheap to install and easy to operate.

102

Level Measurement


Simple Floats Operation
The float and counter weight are connected together
by a wire on pulleys.
The system is in balance with the float on the surface
of the liquid.
If the level rises, the float rises and the counter weight
falls to the new balance point. If the level falls the
counter weight rises.

103

Level Measurement
The counter weight has a
pointer which indicates the
level scale on the outside of
the tank.
This scale is the reverse to FLOAT

normal.
The pointer shows ‘full’
when the counter weight is at
the bottom of the scale and
‘empty’ when it is at the top.
The scale can be made very
large so that it can be seen
from the ground by the
operator.
104

Level Measurement
Simple Float Operation (cont)

The simple float is not very accurate and can be very
difficult to read.
If the surface of the liquid is moving then the float starts
to swing.
This problem is solved by fitting special devices inside
the tank as shown in the following slide.

105

Level Measurement
Simple Floats
Operation
“A” is a guided wire
system.
C
“B” is a Still Pipe A
system where the
float is in a slotted
pipe and connects
with the ground
level display
“C” is another Still
B
Pipe but the float
connects with a
transmitter which
sends a signal to
the control room

Level Measurement
Simple Float Operation - Guide Wire System (Figure A)

This is the cheapest system. The float is held in place by
wires which are are fixed to the bottom by a concrete block.
The wires are kept tight by a spring.

The float is connected by a wire. The wire runs through a
pulley system and through a pipe to the indicating unit The
pipe is supported on brackets fixed to the tank. The
indicating unit is the counterweight and the level is indicated
by a mechanical counter.

107

Level Measurement
Simple Floats Operation - Still Pipe System (B and C)

 This is a more expensive but more accurate method.
 The float is contained inside a still pipe (a steel pipe with
holes in it). The level inside the pipe does not move so it
gives very accurate measurements of level.
 Figure B shows the older mechanical indication method.
 Figure C shows the modern method where the system is
electronically controlled and the level measurement is sent
as an electronic signal to the control room.

108

Level Measurement

Hydrostatic Tank Gauging
(HTG)
Pressure indicator
Many of the modern oil
storage tank facilities (tank
farms) use hydrostatic tank
gauging to indicate the level Transmitter
in a tank.
HTG is good because there is
no equipment inside the tank.
A Hydrostatic Tank Gauging
installation on the Skim Tank at
It is cheaper to install and the CPF
maintain than the float
installations.

Level Measurement

110

Level Measurement

Hydrostatic Tank Gauging (HTG)
The higher the level of a liquid in a tank, the higher the
pressure on the bottom of the tank.
An outlet near the bottom of the tank is under more
pressure than an outlet near the top of the tank.
The greater the pressure the further the outflow
stream will reach.

111

Level Measurement
Hydrostatic Tank Gauging (HTG)
The pressure on the bottom of the tank only depends
on the level of the liquid in the tank; not the volume or the
shape of the tank.
No matter what the shape of the tank, the pressure at
the bottom of the tank is the same.
Using this principle, a pressure sensor at the bottom of
the tank can gauge the level of the liquid in the tank.
The higher the pressure, the higher the level of the
liquid in the tank

112

Level Measurement
Displacers and Local Level Control
The displacer is a locally mounted device which
controls the level in a vessel.
It is used on remote sites where it is too expensive to
return signals to the control room.
The most common types in use are manufactured by
Fisher or Masoneilan.
The diagram Figure 3.7 shows a Fisher device (The
Level-Trol).

113

Level Measurement

114

Level Measurement
Displacers and Local Connecting Rod
Level Control

The Displacer unit is
connected to both the Torque Tube
vessel and the control valve.

Displacer

Level Measurement
Operation
 The weight of the displacer changes as the level of the
liquid rises or falls in the displacer housing.
 The displacer hangs on the torque tube via the
connecting rod. The changing weight of the displacer
makes the torque tube twist or untwist.
 The twisting motion of the torque tube moves a flapper
against a nozzle. This sends a control signal to the
pneumatic control valve.
 The pneumatic control valve opens and closes to control
the flow of liquid into the tank. This keeps the level of
liquid in the tank constant at the set point.
116

Level Measurement
This forms a self contained local control loop as shown in the
figure below

Level Measurement
Air Bubble Method
The Air Bubble method is one of the oldest and simplest
means used to indicate level and/or transmit a signal. The
diagram shows a simplified layout of the method.

Level Measurement

119

Level Measurement
Air Bubble Method - Operation
 An inert gas (air or nitrogen) is passed down the bubbler
tube. There is just enough gas pressure to push the
bubbles out of the bottom of the tube when the liquid is at
the maximum level in the vessel.
 When the vessel is full the pressure gauge or transmitter
will read a maximum back pressure. This back pressure is
equal to the hydrostatic head (H), (the pressure of the
liquid above the zero level).
 At zero level there will be no back pressure so the gauge
or transmitter will read zero. No back pressure means the
liquid level is at zero; the tank is nearly empty.

120

Level Measurement
Air Bubble Method - Operation (cont)
 The back pressure between zero and maximum levels is
proportional to the liquid level in the vessel. The
pressure gauge or transmitter can be calibrated to
indicate the liquid level.
 The gas pressure is adjusted by the regulator to give a
steady flow of gas down the bubbler tube. The gas flow
is indicated on the Rotamater.
 This is a very accurate method of showing liquid level
using modern instrument systems.

121

Level Measurement
Level Switches
A level switch is the last safety device when controlling level.
If the level controller stops working the vessel can overfill. This
can be dangerous.
A level switch uses a float to operate a switch to shut down
filling pumps in an emergency. A typical example is shown.

Level Measurement
Level Switches
Figure 3.10 shows a pneumatic level switch.
When the level of liquid is low the float hangs down. The
operating screw on the end of the flexible shaft holds the
flapper tight against the nozzle.
The output signal is a maximum so the pumps continue to
fill the vessel.
If the level rises and lifts the float the screw on the end of
the flexible shaft moves down. The flapper moves away form
the nozzle and the output signal falls to zero.
This shuts down the pumps so no more liquid comes into
the vessel.

123

Level Measurement
Other Methods of Level Measurement
This unit has introduced some common methods of measuring
levels used on most installations.
There are many other methods using various types of high
technology.
These will be special for only one or two installations.
They will have to be learnt on the job. A few examples are:
a) Radar, ultrasonic, gamma and infrared detectors
b) Capacitive sensors
c) Resistive sensors

124

Temperature Measurement
Introduction

This unit will describe the common methods and devices
used to measure temperature.

It will also describe when and where these devices are
used and how they are protected.

125

Filled Thermal Elements
Thermal filled elements operate by the expansion and contraction of
fluids or vapours in a closed tube.
The simplest of these devices is the mercury filled thermometer.

‘Liquid in glass’
thermometers are
not strong enough
for use on the plant.
Stronger systems
have to be used.

Filled Systems
One common kind of temperature measuring device
used in industry is the filled system. However, it is not
made of glass like a hospital thermometer. These
systems use steel bulbs and stems.

The stem has a bourdon tube at the end. The liquid or
gas in the device expands and contracts as the
temperature changes. The expansion and contraction of
the fluid in the system is changed to pressure.

An increase in pressure expands the bourdon tube which
moves the pointer to the scale.

127

Filled Systems

The liquid filled system is normally used in
process plant applications.

Bi-Metal Strip Thermometers
Liquid and gas filled systems use the expansion of fluids to
measure temperature. Some temperature measuring
devices use the expansion of solids to measure
temperature. One kind of solid expansion thermometer is
the bi-metal strip illustrated in the diagram

Cold

Hot

Bi-Metal Strip Thermometers
Two strips of metal, brass and invar, are tightly bonded
together and fixed at one end. When the strip is heated the
brass expands much more than the invar and the strip bends
as shown. This action is used to make a dial thermometer as
shown. The most common type is the Rototherm.

Bi-Metal Strip Thermometers - Operation

The bi-metal strip is shaped into helix.
The helix is fixed at one end.
The other end of the helix is free to rotate the shaft
which is fixed to it.
The heat applied to the bi-metal strip at the fixed end
causes the helix to unwind and turn the pointer on the
scale.

131

Thermocouple
When two different metals are welded together at their
ends a junction is formed. This is called a thermocouple.
If this junction is heated a small electrical emf
(electromotive force) is produced that causes a current to
flow.
This current can be measured by attaching a meter to
the free ends of the metal strips as seen in figure 4.5
(next slide).
The strength of the current can be used to show
changed in temperature on the thermocouple.

132


Thermocouple

Temperature Measurement Devices
The thermocouple is used to sense the process
variable and transmit the signal to the controller
electrically.

Filled thermal bulb
and capillary tubing

Resistance bulb

Thermocouple and protective well

134

Radiation Temperature Detectors (Pyrometers)
Temperature measuring devices such as a bi-metallic
strip or a thermometer must be in contact with the
substance or thing which they are measuring.
Radiation temperature detectors (pyrometers) are
non-contact devices.
They are used to measure the temperature of
something which is difficult to reach, eg gas turbine
combustion chambers.
They are also used to measure very high temperatures
(above 15000C).
All the other devices would melt at these temperatures.

135

Radiation Temperature Detectors (Pyrometers)
The heat from the objects is focused by lenses onto a sensor.
It’s the same as when you use a magnifying glass to focus the
heat from the sun in order to start a fire. The output from the
sensor is electronically processed by the amplifier to give a
reading in degrees. This device can also transmit a signal to the
control room if required.

Resistance Temperature Detector (RTD)
The device indicates temperature by measuring the
change in the electrical resistance of a metal.
When metals get hotter their resistance increases.
This increase in resistance is almost linear. In other
words, the resistance increases at the same rate as
the temperature.
When the resistance is measured it gives an
accurate indication of temperature.
There are other temperature sensors in use but
these are of more interest to instrument technicians.

137

Resistance Temperature Detector (RTD)


139


140

Thermowells
The thermowell is a device
fitted into a flow line so that Thermocouple
the temperature of a fluid can
be measured without shutting
down the process.
A thermowell is placed in a
flow line when the line is
built.
The thermometer or
thermocouple is fitted into the
thermowell. Thermowell

Thermowells
Most vessels and pipes in process and production
plants contain liquids or gases under pressure.
The thermowell protects the temperature sensor from
damage from pressure and also from fluid flow.
The heat in a fluid takes longer to transfer through a
thermowell, so changes in temperature take longer to
show.
Different methods are used to speed up heat transfer.
Sometimes the space between the probe and the
thermowell is filled with a liquid which conducts heat well.

142

Thermowells
Sometimes the probe is placed in a corrugated
aluminum cover to give a direct metal contact between the
probe and the thermowell.
When a thermowell is filled with heat conducting liquid a
small amount of air has to be left as a gas cap at the top
of the well to allow for thermal expansion of the liquid.
As the conducting liquid expands with the increase in
temperature the liquid compresses the gas cap.
This prevents the pressure generated by the expansion
from damaging the instruments in the well.

143

Flow Measurements
Introduction

This unit will describe how the flow in a process is
used to control the other process variables.

It will also describe how flow is measured.

144

Flow Measurement
Flow Measurement
Flow measurement means measuring how much
material moves past a given point in a given time.

For example, the petrol in the pump at the service
station flows at about 20 litres per minute.

Therefore the rate of flow is 20 litres per minute.

In a process system it’s very important to know the
rate of flow through different process equipment.

145

Flow Measurement
The rate of flow affects how well the process works.

When we know how much is flowing we can decide
if it is too much or too little.

We can then change the flow to what we want it to
be, in other words set it at the desired value.

146

Flow Measurements
Flow Measurement
The flow must be controlled to Control the
Process

We use flow control to control other variables in a
Process such as:
 Pressure
 Temperature
 Level

147

Flow Measurements
Rate of Flow Measurement
Devices for measuring the rate of flow do not have to
be very accurate.
It is the change in the rate of flow that is important to
a Technician.
Flow measurement devices are often used to give a
flow signal directly to a controller. In this case they are
called Flow Indicator Controllers (FIC’s)
If they sent a signal to the Control Room or a locally
mounted recorder they would be called Flow Recorder
Controllers (FRC’s)

148

Flow Measurements
Rate of Flow Measurement - Flow Basics
It is pressure that pushes fluids through a pipe.
For a flow to occur there must be a pressure drop (decrease
in pressure) between the ends of the pipe.
The downstream pressure is less than the upstream pressure
therefore the direction of the flow is from upstream (high
pressure) to downstream (low pressure).
Upstream means where the fluid is coming from.
Downstream means where the fluid is going to.

149

Flow Measurement
Flow Basics
The flow is produced by the difference in pressure
across the ends of the pipe.
If there is a big difference in pressure then the rate of
flow will be fast.
If there is a small difference in pressure than the rate
of flow will be slow.
The difference in pressure is called the ‘Differential
pressure’.
The term ‘Differential Pressure’ is a common
expression and one that you need to understand

150

Flow Measurements
Flow Basics – Friction and Types of Flow

The walls of pipes are not perfectly smooth.

The frictional force at the walls will cause the fluid to
go slower at the edge than at the centre.

There are two types of flow:
 Laminar Flow
 Turbulent Flow

151

Flow Measurements
Rate of Flow Measurement - Flow Basics

Flow Measurements

Laminar Flow
Laminar flow occurs when the fluid flow rate is slow.
The velocity (speed) of the fluid through the pipe is much
higher in the centre of the pipe than at the edges.
The fluid next to the walls of the pipe flows more slowly
because the fluid is rubbing against the pipe.
The liquid is slowed down by friction.

153

Flow Measurements

Turbulent Flow
Turbulent flow occurs when the fluid flow rate is high.
The velocity of the fluid through the pipe is nearly the
same across the pipe.
The flow is a little slower at the edges because of the
friction between the fluid and the wall of the pipe.

154

Flow Measurements
Flow Basics – Calculating the Rate of Flow

There must be a differential pressure across the ends
of the pipe in order for fluid to flow.
If the differential pressure and the size of the pipe is
known, we can calculate how fast the fluid is flowing
through the pipe (the rate of flow).
The calculation is complicated. An easier method had
to be found to calculate the flow through the pipe.
The easier method is to put a restriction in the pipe. A
restriction is something which blocks part of the flow.

155

Flow Measurement
There are three main devices used to make restrictions
in a pipe:
 Orifice Plate
 Venturi Tube
 Flow Nozzle

The restriction produces a differential pressure across it.
In other words the pressure downstream of the restriction
is lower than the pressure upstream of the restriction.

156

Flow Measurement


The pressure difference is due to the increase in
velocity as the process fluid flows through the restriction.
When the velocity of the flow increases, the pressure at
that point in the line decreases.
By knowing the differential pressure, the internal
diameter of the pipe and the size of the hole in
restriction, we can calculate the rate of flow.
 The instruments do the calculation for us.

157

Flow Measurement

158

Flow measurement

159

Flow Measurement
Orifice Plate Restrictions
The illustration shows a side
view of an orifice plate fitted
into a pipe.
The pressure downstream of
the orifice is lower than the
pressure upstream. The
Instrument measures this
differential pressure.
The measurement can be used
to calculate the rate of flow at
that point in the pipe.

Flow Measurement

Orifice Plate Restrictions

All orifice plates are marked
with the orifice size.
The side of the plate which
goes upstream (inlet) is also
clearly marked.

Flow Measurement
Differential Pressure Cell
The upstream side of the plate is
at higher pressure than the flow
on the downstream side of the
plate.

The difference between the two
pressures is called the
Differential Pressure or the DP.

The Orifice plate is held between
two Orifice flanges – learn to
DPC - Differential Pressure Cell recognize these

162

Flow Measurement

Orifice plates installed at the Agogo/Moran facility.
This type of orifice plate can be removed while the line is in use. A normal
orifice plate requires that the process be shut down as the line is separated
when the plate is removed
163

Flow Measurements
Venturi Tube Restrictions
Another type of restriction device is the venturi
tube.
If the fluid in a pipe is flowing under very low
pressure the restriction by an orifice plate could stop
the flow.
In these cases a venturi tube is used.
These devices are very expensive.

164

Flow Measurement
Venturi Tube Restrictions
A venturi tube works on the same principle as an orifice
plate. Instruments measure the pressure differential
across the restriction. However, the shape of the venturi
tube allows the fluid to pass through it easily.

Flow Measurement
Flow Nozzle Restrictions

A third type of restriction device is the flow nozzle.
The flow nozzle is a combination of the orifice plate and
the venturi tube.
The pressure loss across the nozzle is more than
across the venturi, but it is less than across an orifice
plate.
The flow nozzle is less expensive than a venturi, but
more expensive than an orifice plate.

166

Flow Measurement
Flow Nozzle Restrictions
Flow nozzles are good for liquids with high flow rates.
Orifice plates are usually used for gases with high flow rates.

Flow Measurement
Flow Straighteners (Straightening Vanes)
All flow measuring devices which use a restriction need
a stream-lined flow.

Flow measuring devices must not be placed in pipes
near things that disturb the flow; elbows, valves, etc.

If this is not possible then the flow has to be stream-
lined (made to flow smoothly).

The flow is stream-lined with flow straighteners.

168

Flow Measurements
Flow Straighteners (Straightening Vanes)
A flow straightener is a cylinder filled with many small
pipes. This device is place in the pipeline upstream of
the flow measuring device. It causes the fluid to flow
smoothly and evenly which means the measuring device
can get a more accurate measurement.

Flow Measurement
Calibration of Differential Devices

Calibrating a differential-pressure, flow measuring
device is a skilled job. An instrument technician will use
figures given by the design engineer to do this.
For control purposes the actual measurement of flow
need not be exact.
It’s the changes in the rate of flow which are
important.

170

Flow Measurements
Variable Area Meters
These are simple devices used to indicate small rates of
flow. They are used by a Technician in the field.
Typical uses are:
 In seal-oil and lubrication-oil flow
lines on large rotating machines
e.g. diesel engines and gas C
compressors.
B
 In cooling water lines for
machines and processes. A
A
A
 The diagram shows a variable
area meter or Rotameter.

Flow Measurements
Variable Area Meters - Operation
The Rotameter is fitted vertically into the flow line. The
flow of the fluid is from the bottom to the top of the cylinder.
The cylinder is bigger at the top than at the bottom.
When there is no flow, the float is at the bottom of the
cylinder (position A)
When the flow increases, the increased pressure makes
the float rise.
It will rise to a position where the flow pressure on the
float equals the weight of the float, (position B).

172

Flow Measurements
Variable Area Meters - Operation

If the flow gets faster there is more pressure on the float
and it will rise higher (position C).

The flow rate indicated depends on the size of the
device. It is pre-calibrated by the manufacturer

The Technician reads the flow rate from the transparent
scale using the top of the float as a marker.

173

Flow Measurement
Positive Displacement Meters
Positive displacement flow-measurement meters are
very accurate. They are also called quantity meters.

Basic Principle
The meter traps a known fixed volume of fluid and
transfers it from the inlet to the outlet.

The number of fixed volumes of fluid transferred (or
moved) is a measure of flow.

174

Flow Measurement
Positive Displacement Meter
Look at the drawing
The bucket holds 12 litres.
The man moves 10 buckets
of water from the inlet tank to
the outlet tank in one minute.
Therefore, we can say the
rate of flow is 10 x 12 litre
buckets a minute or 120 litres
per minute.

175

Flow Measurements
Positive Displacement Meters - Basic Principle
A positive displacement metre works on the same
principle as the man with the bucket. However, a positive
displacement meter is much faster and more reliable than
a man with a bucket.

Flow meters that use this
basic principle are:
Reciprocating piston meters
Rotating vane meters
Lobed impeller meters

Flow Measurements
Reciprocating Piston Meter
Each time the piston moves up and down in the cylinder a
fixed amount of fluid is pushed out of the outlet.
The valves are arranged to work in time with the piston so
one side of the cylinder is filled as the other side is emptied.

Flow Measurements
Positive Displacement Meters - Rotating Vane Meter
The rotating vane meter is another type of positive
displacement meter.
Each time a vane moves past the outlet it pushes out a
measured volume of liquid, e.g. 2 deciliters.
This type of meter is used on petrol pumps at service
stations.
The meter counts how many times the vanes go around
on the cam and you pay for this amount of petrol.

178

Flow Measurements
Rotating Vane Meter

Flow Measurements
Positive Displacement Meters - Lobed Impeller
Another kind of positive displacement meter is a lobed
impeller meter. Each rotation of the impeller pushes a
measured quantity of fluid through the meter.

Flow Measurements
Velocity Meters (Semi-Positive Displacement)
The velocity meter measures the speed of flow.
 then calculates the volume of flow using calibration figures.
It
The calibration figures are placed in the electronics unit’s memory by the operator or instrument technician.
These calibration figures depend on the type of liquid flowing through the meter.

181

Flow Measurements
Velocity Meters
(Semi-Positive Displacement)
The magnet rotates with the rotor.
The pickup coil gets a signal from the
magnet (pulse) each time the rotor
completes a rotation.
The number of pulses is counted by an
electronics unit.
The electronic unit then displays the
total quantity of flow.
Note: If the type of fluid flowing
through this meter changes, then the
calibration figures in the meter's
electronic memory have to be
changed.

Flow Measurement
Vortex Meters

Vortex Meters are
used on fuel gas
systems such as
gas from the Test
Separator

183

Flow Measurement

Micro-motion Flow Transmitter

A micro-motion meter
measures a flow by
sensing the vibrations
between two parallel
loops that the flow
causes.
The higher the flow the
greater the vibration
and distortion between
the loops.
This type of meter is
very accurate and
reliable

184

Flow Measurement

185

Increased Density Density changes Vibration action Decreased Density

Increased
Increased Density
Density

186

Flow Measurement
Turbine Meter

Turbine meters are
used extensively in
a process facility.
This one is at the
CPF Export Pump
Station. Others are
at the valve stations
along the export
pipeline and at the
Marine Terminal -
Kumul

187

Flow Measurement
The following video clips have been included to let the
participant gain a knowledge of the components of a
Turbine meter.
Although the video is aimed at the maintenance of the
equipment the information is applicable to a Process
Technician as well
The rotating element is much smaller than you would
expect.
Even in large meters such as those that are used on the
pipeline an appreciation can be gained of how easily the
blades of the rotor could become damaged and worn.

188

Flow Measurement

189

Flow Measurement

190

Flow Measurement

191

Flow Measurement

192

Instrumentation
This completes this first package on Instrumentation

The second package deals with the Control Valves and
the ancillary equipment which complements them.

Sophisticated control systems such as PLC’s and
TEC’s are also dealt with in that package.

For comments and suggestions contact Len Dallow or Peter Cannell on
ldallow@picknowl.com.au or petercannell1943@bigpond.com

193

Instrumentation tech 1

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