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. 

We could represent this as: 

Pressure = Force/Area

or

P=F/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.

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

Figure 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 uses where pressures are below atmosphere. Typically this is around the condenser and vacuum building.

Relationship between Absolute and Gauge Pressures

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.

Theyinclude 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 

Bourdon Tubes

Bourdon tubes are circular-shaped tubes with oval cross sections. 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.  >>>

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.

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

Capsules 

There are two different devices that are referred to as capsule. The first is shown in figure. 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 it 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 shown in the differential pressure transmitter (DP transmitter) in figure.

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.D DifferentialPressure 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. Figure 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. 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.

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. 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 , 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

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.

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. 

C=ε A/d

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.

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 manufacturersí 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 ñ 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 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.

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.

Loss of Loop Electrical Power

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 supply.