Instrumentation_and_Control presentaton.ppt

This topic focuses on the principles and applications of instrumentation and control systems used in electrical and industrial processes. It covers how various instruments measure physical quantities—such as temperature, pressure, flow, and level—and how control systems use these measurements to regulate processes automatically. Students learn about sensors, transducers, controllers, and feedback mechanisms that ensure systems operate safely, efficiently, and accurately. Emphasis is placed on understanding how instrumentation and control contribute to automation, process optimization, and modern smart technologies in engineering industries.

2. What are the reasons for instrumentation and control systems ?
• Plant economy and efficiency
• Consistency of product quality
• Plant and personnel safety
• Eliminate or minimise environmental pollution

3. What properties are measured by instrumentation ?
• Level
• Flow
• Temperature
• Pressure
There are other physical measurements used, but the above
form the basis of the majority of Process Control requirements.

4. What are the units of measurement ?
All systems of weights and measures, metric and non-metric, are linked through a network of
international agreements supporting the International System of Units. The International System is called
the SI, using the first two initials of its French name Système International d'Unités.
The International System of Units (SI)
At the heart of the SI is a short list of base units defined in an absolute way without referring to any other
units. The base units are consistent with the part of the metric system called the MKS system. In all
there are seven SI base units:
the meter for distance,
the kilogram for mass,
the second for time,
the ampere for electric current,
the kelvin for temperature,
the mole for amount of substance, and
the candela for intensity of light.

5. Other SI units, called SI derived units, are defined algebraically in terms of these fundamental units.
For example, the SI unit of force, the newton, is defined to be the force that accelerates a mass of
one kilogram at the rate of one meter per second per second. This means the newton is equal to
one kilogram meter per second squared, so the algebraic relationship is N = kg·m·s
-2
. Currently
there are 22 SI derived units. They include:
the newton for force and the pascal for pressure;
the joule for energy and the watt for power;
the degree Celsius for everyday measurement of temperature;
units for measurement of electricity: the coulomb (charge), volt (potential), farad
(capacitance), ohm (resistance), and siemens (conductance);
What are the units of measurement ?
The International System of Units (SI)

6. Units of Measurement on Site
The Units of Measurement that we concern ourselves with at the Woodside Onshore Gas Plant are
of course dependent on what physical properties we wish to measure.
The typical physical properties are as previously stated:
 Level
 Flow
 Temperature
 Pressure
Of these four only two of them can be measured directly into derived SI units, namely Temperature
and Pressure.
Temperature is measured in degrees Celsius whilst Pressure is measured in pascals. However
as the pressures encountered within the process are considerable the use of a metric prefix is
utilised.
An example is the supply pressure for domestic gas to the Epic pipeline, which could be
represented as:
8.4 Mpa (Megapascals)
8,400 kpa (Kilopascals)
8,400,000 pa (pascals)

7. Metric Prefixes
yotta- (Y-)10
24
1 septillion
zetta- (Z-)10
21
1 sextillion
exa- (E-)10
18
1 quintillion
peta- (P-)10
15
1 quadrillion
tera- (T-)10
12
1 trillion
giga- (G-)10
9
1 billion
mega- (M-)10
6
1 million
myria- (my-)10
4
10 thousand
kilo- (k-)10
3
1 thousand
hecto- (h-)10
2
1 hundred
deka- (da-)101 ten
deci- (d-)10
-1
1 tenth
centi- (c-)10
-2
1 hundredth
milli- (m-)10
-3
1 thousandth
micro- (µ-)10
-6
1 millionth
nano- (n-)10
-9
1 billionth
pico- (p-)10
-12
1 trillionth
femto- (f-)10
-15
1 quadrillionth
atto- (a-)10
-18
1 quintillionth
zepto- (z-)10
-21
1 sextillionth
yocto- (y-)10
-24
1 septillionth

8. How are these properties measured ?
Pressure Measurement
What is Pressure?
Pressure is defined as force per unit area.
The earth’s gravity exerts a force on all matter in, on or near the earth and exerts sufficient force on the
earth’s atmosphere to create a pressure of 101kpa @ sea level.
There are two reference points that pressure can be measured from, they are:
• Absolute pressure
• Gauge pressure
Absolute pressure uses absolute zero as it’s point of reference, that is no pressure at all, a perfect
vacuum so zero equals -101kpa.
Gauge pressure zero indicates atmospheric pressure ie: 101kpa
Both reference points are used in the measurement of pressure and are identified by a distinction
between the units of measurement. For gauge pressure kpag is used to indicate the reference point.

9. How are these properties measured ?
Pressure Measurement
Field Aux Room
Pressure measurement is performed using a pressure transmitter which senses the pressure via a
tapping point and converts it to a 4 to 20 mA 24 Vdc signal. This PV signal is compared to a setpoint
and an output signal is generated which moves the control valve.
SP 1200 kpa
PV 1150 kpa
Output
DCS in CCR
Field junction box
Control valve
Pressure
transmitter
Terminal panel Honeywell controller

10. How are these properties measured ?
Temperature Measurement
Temperature is the degree of ‘hotness’ of a body: more precisely it is the potential for heat transfer.
For measurement of temperature a thermometer is used. A thermometer is a device in which some physical
property of a substance changes with temperature in a reliable, reproducible and quantifiable way.
There are several different types of thermometers. Choosing which one to use depends on the
application, temperature range and required accuracy.
A resistance thermometer or RTD makes use of the
change of resistance in a metal wire with temperature.
The higher the temperature the greater the impedance
and the higher the resistance. This effect is very marked
in pure metals, and for a well-behaved material enables
measurements of temperature to be made to better than
0.001 °C.

11. How are these properties measured ?
Temperature Measurement
Thermocouples depend on the principle of the Seebeck effect: If two different metals, such as
copper and iron are joined together to form a closed loop, and if one junction is kept at a different
temperature from the other, electric current will flow in the closed loop.
The simplicity, ruggedness, low cost, small size and wide temperature range of thermocouples
make them the most common type of temperature sensor in industrial use although their
accuracy is much lower than an RTD.

12. Thermowell
Thermocouple or RTD sensor
Pipe X-section
Temperature Measurement
How are these properties measured ?
Typical RTD or Thermocouple arrangement where the sensing device is encased in a metal sheath
and inserted into the process stream via a thermowell.
The thermowell protects the sensing
device and isolates it physically from the
process. It enables the change out of the
RTD or Thermocouple without the need
to isolate the process.

13. How are these properties measured ?
Level Measurement
Level is measured as a percentage based on the range of the instrument. This is derived from
measuring the pressure caused by the height of the liquid level. This is called the hydrostatic head and
is dependent on the height of the liquid and the specific gravity of the liquid, so:
Pressure = Specific Gravity x Gravity x Head
The specific gravity of a substance is the weight of a volume of a substance relative to the same
volume of water. The specific gravity of water is 1. Specific gravity can be measured as kilograms per
cubic meter and represented as Kg/m
3
.

Gravity is the average acceleration produced by gravity at the Earth's surface (sea level) ie. 9.80665
meters per second per second (m/s
2
).
Head is the height of the liquid measured in meter (m).
So when the units of measurement for the above formula are applied we come up with;
Pressure = Kg/m
3
x m/s
2
x m which becomes
Pressure = Kg/m/s
2
= pascals (p)
The level has now been translated into a SI derived measurement, namely pascal. However for the
purpose of process control this value will be compared to the range of the calibrated instrument and
reported as a percentage.

14. Open Tank
Minimum Level
Maximum Level
Level Measurement
In an open tank a single tapping point is all that is required as the
pressure exerted on the sensing device is the head of the liquid +
atmospheric pressure.
As the sensing device is vented to atmosphere the effect of
atmospheric pressure is cancelled.
Sensing device
Working Range
Perhaps the most frequently used device for the measurement of level is a differential pressure
transmitter. Using DP for level is really an inferential measurement. A DP is used to transmit the head
pressure that the diaphragm senses due to the height of the material in the vessel multiplied by a
density variable.
The primary benefit of DP’s is that it can be externally installed or
retro-fitted to an existing vessel. It can also be isolated safely from
the process using block valves for maintenance and testing.
D/P transmitters are subject to errors due to changes in
liquid density. Density variations are caused by
temperature changes or change of product. These
variations must always be compensated for if accurate
measurements are to be made.

15. Closed Tank
Minimum Level
Maximum Level
Level Measurement in a Closed Tank
In a closed tank or vessel the reference to atmospheric pressure no longer applies and in fact the
internal pressure of closed vessels can vary considerably depending upon their application.
In order to negate the effect of internal pressure the D/P transmitter must be referenced back to vessel.
This is achieved by adding a tapping point to the low pressure side of the transmitter back to a point
above the maximum level expected in that vessel.

16. Closed Tank
Minimum Level
Maximum Level
Dry Outside Leg
Closed Tank
Minimum Level
Maximum Level
Wet Outside Leg
Wet Legs versus Dry Legs
If the outside leg operates in an environment where the vapour above the liquid will not condense under
normal process conditions then the leg is left dry; ie gas filled.
If the vapour above the liquid has a tendency to condense into a liquid then a dry leg is unsuitable as liquid
will accumulate in the dry leg and will offset the pressure being exerted by the liquid level. This will cause
an error in level measurement, which will vary as condensation continues to accumulate.
To overcome this problem a
liquid of higher specific gravity is
added to the outside leg, this is
then termed a wet leg. The
sensing device may then be
calibrated to compensate for the
wet leg and maintain accurate
measurement.

17. Bubblers
This simple level measurement has a dip tube installed with the open end close to the bottom
of the process vessel. A flow of gas, usually air or nitrogen passes through the tube and the
resultant air pressure in the tube corresponds to the hydraulic head of the liquid in the vessel.
The air pressure in the bubbler tube varies proportionally with the change in head pressure.
Advantages
Simplicity of design and low initial purchase cost are frequently given as advantages of
bubblers, but this is somewhat misleading. The system consists of a pipe, an air supply, a
pressure transmitter and a differential pressure regulator. The regulator produces the
constant gas flow required to prevent calibration changes.
Disadvantages
Calibration is directly affected by changes in product density. It is frequently also necessary
to periodically clean this device. The tip of the pipe can collect material from the process,
solidify, and plug the hole. Bubblers are not suitable for use in non-vented vessels.
Regulated air supply

18. How are these properties measured ?
Flow Measurement
Flow is generally reported in tonnes per day (tpd) but determining this value requires the
measuring of the differential pressure generated across a specific flow-measuring device. The
two most common devices utilised on site are orifice plates and venturis. Both these devices rely
on changing the flow velocity and using the associated pressure drop that occurs across the
device. This relationship is expressed in the following formula;
P = K D
ƒ
V
2

62.3
Where P = Measured pressure drop
V = Velocity of the fluid through the device
D
ƒ = Density of the fluid whether liquid or vapour
K = Orifice coefficient
As the differential pressure is easily measured via tapping points and the density of the fluid is
known from laboratory sampling and the orifice coefficient is known from the design of the device
the velocity can then be determined from the above formula. From determining the velocity you
can then using the density of the fluid ascertain the mass flow.

19. Flow Measurement using Orifice Plate
Simple U tube manometer used to measure differential pressure
High Pressure side Low Pressure side
P
Orifice Plate
Bevel edge faces downstream
Flow
Orifice
Tang stamped with orifice
information, hole drilled in tang
to identify as orifice plate or RO Drain hole
Pipe I.D.

20. Flow Measurement using Venturi
Throat tap
Lowest Pressure
Upstream tap
Differential Pressure transmitter senses pressure variation across venturi throat

21. The tapping points relay the pressure variation across the orifice plate to the
DP transmitter. The transmitter converts the dp to a 4 to 20 mA 24 V dc
signal to a field junction box and then to a terminal panel within a field
cabinet.
dp
4-20mA
4-20mA
Flow Measurement and control…how it happens

22. Flow Measurement and control…how it happens
4-20mA
The 4-20mA PV signal from the
terminal panel enters the honeywell
controller box, is compared to the
setpoint & an output signal is
generated
The PV, Setpoint & output
are viewed via the DCS by
the Operator in the CCR.
4-20mA
4-20mA
The 4-20mA output signal from
the honeywell controller returns
via the terminal panel to the
field junction box and then to
the control valve

23. Controller Theory
Control of a production process in a stable condition ensures maximum efficiency and safety.
Process control can be carried out in the following modes:
• Manual (open loop)
• Automatic (closed loop)
• Cascade
Various types of control action are used within these modes:
• Two step (on/off action)
• Proportional
• Integral
• Derivative

24. Controller Theory
Manual Control:
Manual control is also known as open loop control.
With this method the process condition is measured and continuously indicated.
The reading from the measurement element is manually checked by the operator who makes
adjustments to the process by hand.
Measuring element
Detecting element
Correcting element
(Hand valve)

25. Controller Theory
Automatic Control
Automatic or "closed loop" control allows the maintenance of a production process in a stable and
consistent manner with a minimum of operator intervention. The automatic control makes
adjustments to the process to correct any deviation from the pre-set operating condition.
The automatic control loop consists of:
•a measurement of the Process Variable (PV) being controlled,
•a reference or Set Point (SP) at which the process is maintained,
•an Output (OP) to the device controlling the process.
Instrument Air
Measuring element
Measured element
Instrument Air
Instrument Air
Pneumatic level
controller
Setpoint

26. Controller Theory
Automatic Control
Automatic control is also known as feedback control as the process condition is measured,
compared with the required set-point and corrected as necessary. It is a continuous and
automatic process of checking the feedback from the PV signal and adjusting the error signal
output.
Multiple automatic control loops can be installed to maintain conditions in a whole section of
the process.
Cascade Control
This is an automatic control system in which various control units are linked in sequence
with each control unit regulating the operation of the next control unit in line.

27. Controller Theory
Types Of Control
There are four basic types of control:
•two step action: responds by starting or stopping,
•proportional action: responds in proportion to the size of the error,
•integral action: responds to an average error (offset) over time,
•derivative action: responds to the rate of error change.
On/Off Action (Two Step)
The control element can only be switched on or off. An example of this type of control is a thermostat:
when it detects a pre-selected low temperature it switches a burner on; and then switches it off when a
pre-selected high temperature is reached.

28. Controller Theory
Proportional Action
Proportional action acts on the error between the PV and the SP. With proportional action the correcting
element, (usually a control valve), is adjusted in proportion to the deviation of the PV from the required
SP.
Proportional action is modified by adjusting the gain (K) of the controller. A gain setting of 1.0 gives an
output change equal to the change in error.
Figure 1 shows a gain setting of
approximately 0.3. It needs a larger
change in the temperature (i.e. a larger
error) to cause a proportionally smaller
change in the valve position.
Figure 2 shows a gain setting of approximately
1.3. It needs only a small change in the
temperature to cause a proportionally larger
change in the valve position.

29. Controller Theory
Unlike the two step control, proportional action immediately applies a varying amount of
control action depending on how far the process deviates from the required SP. This reduces
the high and low points as seen in the following figure.
The disadvantage of proportional
control is that it cannot respond
accurately to a significant
load change in the process which
causes it to stabilise at a point offset
from the required
setpoint.

30. Controller Theory
Integral Action
Integral action is applied when there is an offset due to a process load change which cannot be
compensated for by proportional action. Integral action progressively alters the output of the controller
until the measured value of the PV achieves the set point value.
Integral action is used when offset must be automatically eliminated.

31. Derivative Action
On a large and sudden load change the proportional action tends to cause a large overshoot
and undershoot on the process variable. This causes a longer recovery time before the process
stabilises. Derivative action reduces the size of the swing and decreases the recovery time by
adjusting the controller output according to the rate at which the deviation is changing.
Controller Theory

32. Basic Control Loops
Level
Transmitter
Minimum Level
Maximum Level
Pressure
Transmitter
Pressure
Control
valve
Level
Control
valve
Process
Flow

33. Basic Control Loops
Minimum Level
Maximum Level
Pressure
Transmitter
Pressure
Control
valve
Process
Flow
Cascade Loop
Level-Flow S.P.

34. Basic Control Loops
S.P.
Cascade Loop
Temperature-Flow
Heated
Water Flow
Process
Temperature
Distillation Column
Reboiler

35. Basic Control Loops
Cascade Loop
Level-Pressure
S.P.
Distillation
Column
Propane Kettle
Reflux
Drum
Pressure control loop
To other process
50%-100%
0%-50%
To Flare

36. Basic Control Loops
Distillation
Column
Reflux
Drum
Split Range Pressure control loop50%-100%
0%-50%
Reflux Flow
On-Off minimum flow
Level control
loop
To Flare
Side Product

37. THE ENDTHE END