Instrumentacion Industrial

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

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