instrumentation engineering

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:
or
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.

Revision 1 ñ January 2003

Science and Reactor Fundamentals ñ Instrumentation & Control 8 CNSC Technical Training Group
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
Atmospheric Pressure

Perfect Vacuum

101.3 kPa(a)
0 kPa(a)

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.

Revision 1 ñ January 2003

Note

Science and Reactor Fundamentals ñ Instrumentation & Control 9 CNSC Technical Training Group
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

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.