INSTRUMENTATION EQUIPMENT

2.0 INTRODUCTION
Instrumentation is the art of measuring the value of some plant parameter,
pressure, flow, level or temperature to name a few and supplying a signal
that is proportional to the measured parameter. The output signals are
standard signal and can then be processed by other equipment to provide
indication, alarms or automatic control. There are a number of standard
signals; however, those most common in a CANDU plant are the 4-20 mA
electronic signal and the 20-100 kPa pneumatic signal.
This section of the course is going to deal with the instrumentation
equipment normal used to measure and provide signals. We will look at
the measurement of five parameters: pressure, flow, level, temperature,
and neutron flux.
2.1 PRESSURE MEASUREMENT
This module will examine the theory and operation of pressure detectors
(bourdon tubes, diaphragms, bellows, forced balance and variable
capacitance). It also covers the variables of an operating environment
(pressure, temperature) and the possible modes of failure.
2.1.1 General Theory
Pressure is probably one of the most commonly measured variables in the
power plant. It includes the measurement of steam pressure; feed water
pressure, condenser pressure, lubricating oil pressure and many more.
Pressure is actually the measurement of force acting on area of surface.
We could represent this as:
Pressure Force
Area
P F
or A
The units of measurement are either in pounds per square inch (PSI) in
British units or Pascals (Pa) in metric. As one PSI is approximately 7000
Pa, we often use kPa and MPa as units of pressure.
2.1.2 Pressure Scales
Before we go into how pressure is sensed and measured, we have to
establish a set of ground rules. Pressure varies depending on altitude above
sea level, weather pressure fronts and other conditions.
The measure of pressure is, therefore, relative and pressure measurements
are stated as either gauge or absolute.
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Gauge pressure is the unit we encounter in everyday work (e.g., tire
ratings are in gauge pressure).
A gauge pressure device will indicate zero pressure when bled down to
atmospheric pressure (i.e., gauge pressure is referenced to atmospheric
pressure). Gauge pressure is denoted by a (g) at the end of the pressure
unit [e.g., kPa (g)].
Absolute pressure includes the effect of atmospheric pressure with the
gauge pressure. It is denoted by an (a) at the end of the pressure unit [e.g.,
kPa (a)]. An absolute pressure indicator would indicate atmospheric
pressure when completely vented down to atmosphere - it would not
indicate scale zero.
Absolute Pressure = Gauge Pressure + Atmospheric Pressure
Figure 1 illustrates the relationship between absolute and gauge. Note
that the base point for gauge scale is [0 kPa (g)] or standard atmospheric
pressure 101.3 kPa (a).
The majority of pressure measurements in a plant are gauge. Absolute
measurements tend to be used where pressures are below atmosphere.
Typically this is around the condenser and vacuum building.
Absolute
Scale
Atmospheric
Pressure
Perfect
Vacuum
101.3 kPa(a)
0 kPa(a)
Gauge
Scale
0 kPa(g)
-101.3 kPa(g)
Figure 1
Relationship between Absolute and Gauge Pressures
2.1.3 Pressure Measurement
The object of pressure sensing is to produce a dial indication, control
operation or a standard (4 - 20 mA) electronic signal that represents the
pressure in a process.
To accomplish this, most pressure sensors translate pressure into physical
motion that is in proportion to the applied pressure. The most common
pressure sensors or primary pressure elements are described below.
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They include diaphragms, pressure bellows, bourdon tubes and pressure
capsules. With these pressure sensors, physical motion is proportional to
the applied pressure within the operating range.
You will notice that the term differential pressure is often used. This term
refers to the difference in pressure between two quantities, systems or
devices
2.1.4 Common Pressure Detectors
Bourdon Tubes
Bourdon tubes are circular-shaped tubes with oval cross sections (refer to
Figure 2). The pressure of the medium acts on the inside of the tube. The
outward pressure on the oval cross section forces it to become rounded.
Because of the curvature of the tube ring, the bourdon tube then bends as
indicated in the direction of the arrow.
Pressure
Motion
Cross
Section
Figure 2
Bourdon Tube
Due to their robust construction, bourdon are often used in harsh
environments and high pressures, but can also be used for very low
pressures; the response time however, is slower than the bellows or
diaphragm.
Bellows
Bellows type elements are constructed of tubular membranes that are
convoluted around the circumference (see Figure 3). The membrane is
attached at one end to the source and at the other end to an indicating
device or instrument. The bellows element can provide a long range of
motion (stroke) in the direction of the arrow when input pressure is
applied.
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Pressure
Motion
Flexible
Bellows
Figure 3
Bellows
Diaphragms
A diaphragm is a circular-shaped convoluted membrane that is attached to
the pressure fixture around the circumference (refer to Figure 4). The
pressure medium is on one side and the indication medium is on the other.
The deflection that is created by pressure in the vessel would be in the
direction of the arrow indicated.
. Pressure
Motion
Flexible
Membrane
Figure 4
Diaphragm
Diaphragms provide fast acting and accurate pressure indication.
However, the movement or stroke is not as large as the bellows
Capsules
There are two different devices that are referred to as capsule. The first is
shown in figure 5. The pressure is applied to the inside of the capsule and
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if it is fixed only at the air inlet it can expand like a balloon. This
arrangement is not much different from the diaphragm except that it
expands both ways.
Pressure
Motion
Flexible
Membranes
continuous
seam
seam
Figure 5
Capsule
The capsule consists of two circular shaped, convoluted membranes
(usually stainless steel) sealed tight around the circumference. The
pressure acts on the inside of the capsule and the generated stroke
movement is shown by the direction of the arrow.
The second type of capsule is like the one shown in the differential
pressure transmitter (DP transmitter) in figure 7. The capsule in the bottom
is constructed with two diaphragms forming an outer case and the interspace
is filled with viscous oil. Pressure is applied to both side of the
diaphragm and it will deflect towards the lower pressure.
To provide over-pressurized protection, a solid plate with diaphragmmatching
convolutions is usually mounted in the center of the capsule.
Silicone oil is then used to fill the cavity between the diaphragms for even
pressure transmission.
Most DP capsules can withstand high static pressure of up to 14 MPa
(2000 psi) on both sides of the capsule without any damaging effect.
However, the sensitive range for most DP capsules is quite low. Typically,
they are sensitive up to only a few hundred kPa of differential pressure.
Differential pressure that is significantly higher than the capsule range
may damage the capsule permanently.
2.1.5 Differential Pressure Transmitters
Most pressure transmitters are built around the pressure capsule concept.
They are usually capable of measuring differential pressure (that is, the
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difference between a high pressure input and a low pressure input) and
therefore, are usually called DP transmitters or DP cells.
Figure 6 illustrates a typical DP transmitter. A differential pressure
capsule is mounted inside a housing. One end of a force bar is connected
to the capsule assembly so that the motion of the capsule can be
transmitted to outside the housing. A sealing mechanism is used where the
force bar penetrates the housing and also acts as the pivot point for the
force bar. Provision is made in the housing for high- pressure fluid to be
applied on one side of the capsule and low-pressure fluid on the other.
Any difference in pressure will cause the capsule to deflect and create
motion in the force bar. The top end of the force bar is then connected to a
position detector, which via an electronic system will produce a 4 - 20 ma
signal that is proportional to the force bar movement.
Detector
4-20mA
Seal and Pivot
Force Bar
Silicone Oil Filling
High Pressure Low Pressure
D.P. Capsule
Metal Diaphragm
Housing
Backup Plate
Figure 6
Typical DP Transmitter Construction
This DP transmitter would be used in an installation as shown in
Figure 7.
Controlled Vessel
Pressure
(20 to 30 KPa)
Impulse Line
Isolation
Valve
H L
Pressure Transmitter
Vented
4-20mA
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Figure 7
DP Transmitter Application
A DP transmitter is used to measure the gas pressure (in gauge scale)
inside a vessel. In this case, the low-pressure side of the transmitter is
vented to atmosphere and the high-pressure side is connected to the vessel
through an isolating valve. The isolating valve facilitates the removal of
the transmitter.
The output of the DP transmitter is proportional to the gauge pressure of
the gas, i.e., 4 mA when pressure is 20 kPa and 20 mA when pressure is
30 kPa.
2.1.6 Strain Gauges
The strain gauge is a device that can be affixed to the surface of an object
to detect the force applied to the object. One form of the strain gauge is a
metal wire of very small diameter that is attached to the surface of a
device being monitored.
Force Force
AREA AREA AREA
Resistance Ω Increases
Cross Sectional Area Decreases
Length Increases
Figure 8
Strain Gauge
For a metal, the electrical resistance will increase as the length of the
metal increases or as the cross sectional diameter decreases.
When force is applied as indicated in Figure 8, the overall length of the
wire tends to increase while the cross-sectional area decreases.
The amount of increase in resistance is proportional to the force that
produced the change in length and area. The output of the strain gauge is a
change in resistance that can be measured by the input circuit of an
amplifier.
Strain gauges can be bonded to the surface of a pressure capsule or to a
force bar positioned by the measuring element. Shown in Figure 9 (next
page) is a strain gauge that is bonded to a force beam inside the DP
capsule. The change in the process pressure will cause a resistive change
in the strain gauges, which is then used to produce a 4-20 mA signal.
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Field Terminals
Electronics
Feedthrough
Electronics Enclosure
Electronic
Amplifier
Compensation
Circuit Board
Strain Gauge
Beam
Overpressure
Stop
Process Seal
Diaphragm
Sensing Capsular
Element
Liquid Fill
Figure 9
Resistive Pressure Transmitter
2.1.7 Capacitance Capsule
Similar to the strain gauge, a capacitance cell measures changes in
electrical characteristic. As the name implies the capacitance cell measures
changes in capacitance. The capacitor is a device that stores electrical
charge. It consists of metal plates separated by an electrical insulator. The
metal plates are connected to an external electrical circuit through which
electrical charge can be transferred from one metal plate to the other.
The capacitance of a capacitor is a measure of its ability to store charge.
The capacitance of the capacitance of a capacitor is directly proportional
to the area of the metal plates and inversely proportional to the distance
between them. It also depends on a characteristic of the insulating material
between them. This characteristic, called permittivity is a measure of how
well the insulating material increases the ability of the capacitor to store
charge.
d
C = ε A
C is the capacitance in Farads
A is the area of the plates
D is the distance of the plates
ε is the permittivity of the insulator
By building a DP cell capsule so there are capacitors inside the cell
capsule, differential pressures can be sensed by the changes in capacitance
of the capacitors as the pressure across the cell is varied.
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2.1.8 Impact of Operating Environment
All of the sensors described in this module are widely used in control and
instrumentation systems throughout the power station.
Their existence will not normally be evident because the physical
construction will be enclosed inside manufacturers’ packaging. However,
each is highly accurate when used to measure the right quantity and within
the rating of the device. The constraints are not limited to operating
pressure. Other factors include temperature, vapour content and vibration.
Vibration
The effect of vibration is obvious in the inconsistency of measurements,
but the more dangerous result is the stress on the sensitive membranes,
diaphragms and linkages that can cause the sensor to fail. Vibration can
come from many sources.
Some of the most common are the low level constant vibration of an
unbalanced pump impeller and the larger effects of steam hammer.
External vibration (loose support brackets and insecure mounting) can
have the same effect.
Temperature
The temperature effects on pressure sensing will occur in two main areas:
The volumetric expansion of vapour is of course temperature dependent.
Depending on the system, the increased pressure exerted is usually already
factored in.
The second effect of temperature is not so apparent. An operating
temperature outside the rating of the sensor will create significant error in
the readings. The bourdon tube will indicate a higher reading when
exposed to higher temperatures and lower readings when abnormally cold
- due to the strength and elasticity of the metal tube. This same effect
applies to the other forms of sensors listed.
Vapour Content
The content of the gas or fluid is usually controlled and known. However,
it is mentioned at this point because the purity of the substance whose
pressure is being monitored is of importance - whether gaseous or fluid –
especially, if the device is used as a differential pressure device in
measuring flow of a gas or fluid.
Higher than normal density can force a higher dynamic reading depending
on where the sensors are located and how they are used. Also, the vapour
density or ambient air density can affect the static pressure sensor readings
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and DP cell readings. Usually, lower readings are a result of the lower
available pressure of the substance. However, a DP sensor located in a hot
and very humid room will tend to read high.
2.1.9 Failures and Abnormalities
Over-Pressure
All of the pressure sensors we have analyzed are designed to operate over
a rated pressure range. Plant operating systems rely on these pressure
sensors to maintain high accuracy over that given range. Instrument
readings and control functions derived from these devices could place
plant operations in jeopardy if the equipment is subjected to over pressure
(over range) and subsequently damaged. If a pressure sensor is over
ranged, pressure is applied to the point where it can no longer return to its
original shape, thus the indication would return to some value greater than
the original.
Diaphragms and bellows are usually the most sensitive and fast-acting of
all pressure sensors.
They are also however, the most prone to fracture on over-pressuring.
Even a small fracture will cause them to read low and be less responsive to
pressure changes. Also, the linkages and internal movements of the
sensors often become distorted and can leave a permanent offset in the
measurement. Bourdon tubes are very robust and can handle extremely
high pressures although, when exposed to over-pressure, they become
slightly distended and will read high. Very high over-pressuring will of
course rupture the tube.