3.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.

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.

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.

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.

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

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.

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.

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.

Common Pressure Detectors

1. 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.

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.

2.Bellows

Bellows type elements are constructed of tubular membranes that are

convoluted around the circumference . 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.

3.Diaphragms

A diaphragm is a circular-shaped convoluted membrane that is attached to

the pressure fixture around the circumference. 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.

Diaphragms provide fast acting and accurate pressure indication.

However, the movement or stroke is not as large as the bellows

4.Capsules

There are two different devices that are referred to as capsule.

The pressure is applied to the inside of the capsule and 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 expands both ways.

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 in the differential

pressure transmitter (DP transmitter). The capsule in the bottom

is constructed with two diaphragms forming an outer case and the inter-

space 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 diaphragm-

matching 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.

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

difference between a high pressure input and a low pressure input) and

therefore, are usually called DP transmitters or DP cells.

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 theforce 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.

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.

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.

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.

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.

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.

7. 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 œ

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

8. 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.

Faulty Sensing Lines

Faulty sensing lines create inaccurate readings and totally misrepresent the

actual pressure

When the pressure lines become partially blocked, the dynamic response

of the sensor is naturally reduced and it will have a slow response to

change in pressure. Depending on the severity of the blockage, the sensor

could even retain an incorrect zero or low reading, long after the change in

vessel pressure.

A cracked or punctured sensing line has the characteristic of consistently

low readings. Sometimes, there can be detectable down-swings of pressure

followed by slow increases.

As with any instrument that relies on AC power, the output of the D/P

transmitters will drop to zero or become irrational with a loss of power