unit 2 pressure measurement

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UNITS IN THIS COURSE

UNIT 1 INTRODUCTION

UNIT 2 PRESSURE MEASUREMENT

UNIT 3 LEVEL MEASUREMENT

UNIT 4 TEMPERATURE MEASUREMENT

UNIT 5 LOW MEASUREMENTS

UNIT 6 ANCILLARY CONTROL EQUIPMENT

UNIT 7 CONTROLLERS

UNIT 8 CONTROLLOOPS

UNIT 9 OVERVIEW OF

- DISTRIBUTIVE CONTROL SYSTEMS - DCS

- PROGRAMMABLE LOGIC CONTROLLERS - PLC

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TABLE OF CONTENTS

Para Page

2.0 COURSE OBJECTIVE 3

2.1 INTRODUCTION 4

2.2 THE BOURDON TUBE PRESSURE GAUGE 4

2.2.1 THE SPIRAL BOURDON TUBE 5

2.2.2 THE HELICAL BOURDON TUBE 6

2.3 BELLOWS 7

2.4 DIAPHRAGMS 8

2.5 CAPSULES 9

2.6 THE STRAIN GAUGE 9

2.7 VIBRATING (RESONANT) WIRE 10

2.8 ELECTRICAL PRESSURE SENSING METHODS 11

2.8.1 THE PIEZO ELECTRIC EFFECT 11

2.9 PRESSURE SWITCH 12

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2.0 COURSE OBJECTIVE

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The trainee will be able to demonstrate on understanding of the following pressuresensors.

Bourdon Tube

Bellows

Diaphragm

Capsules

Strain gauge

Vibrating wire

Electrical

Pressure switch

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2.1 INTRODUCTION

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The object of this unit is to describe the common devices used to measurepressure.

2.2 THE BOURDON TUBE PRESSURE GAUGE

The Bourdon tube gauge is the most common pressure indicator in thepetrochemical industry. It shows the pressure in a clear, simple way.

Figure 2.1 The Bourdon Gauge

The diagram (see Figure 2.1 above shows a typical Bourdon gauge. It consists ofthe following parts:

The Bourdon tube itself. This is a metal tube shaped like a "C". It has an oval

cross sectional area. It is sealed at one end. The sealed end is free to move.

A linkage and pinion to turn the pointer.

A scale to indicate the pressure.

Operation

When a pressure is applied to the inside of the tube it will try to straighten. Theclosed-end (the tip) will move and the linkage moves

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The pinion which moves the pointer. The movement of the pointer shows how much

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pressure is applied to the Bourdon tube.

Bourdon gauges come in all shapes and sizes and can measure from about 0-15psig (0-1 bar) to 0-10,000 psig (0-700 bar) depending on the stiffness of thematerial used.

There are also other types of bourdon tubes.

spiral bourden tubes

helical bourdon tubes

These perform the same function as the simple "C" type bourdon tube except thatthey provide more movement and are more accurate.

2.2.1 THE SPIRAL BOURDON TUBE

Figure 2-2 Spiral Bourdon Tube

The diagram above (see Figure 2-2) shows a spiral Bourdon tube. It is used toindicate low pressures. When pressure is applied the spiral unwinds and the freeend moves to indicate the pressure.

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2.2.2 THE HELICAL BOURDON TUBE

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Figure 2-3 Helical Bourdon Tube

The diagram above (see Figure 2-3) shows a helical Bourdon tube. This is usuallyused to indicate high pressures. When pressure is applied the helix unwinds andthe free end moves to indicate the pressure applied.

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2.3 BELLOWS

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Bellows are tubes with thin walls, made of brass, stainless steel, etc. The thin wallsare corrugated. This improves their ability to expand and contract. When pressure isapplied (either to the outside or the inside), the corrugated walls expand or contract.This movement is used to indicate pressure. Bellows units are used in various ways.The three most common methods are shown below:

Figure 2-4 Differential Bellows Unit (Foxboro)

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2.4 DIAPHRAGMS

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A diaphragm is a stiff corrugated disc which is flexible under pressure. A singlediaphragm is often used as a seal to protect a gauge from corrosive liquids. Atypical example is given below (see Figure 2-5).

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Figure 2-5 Metallic Diaphragm Gauge

Diaphragms are also used to make high pressure bellows (a diaphragm stack). Atypical example is shown below (see Figure 2-6).

Figure 2-6 A Diaphragm Stack

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2.5 CAPSULES

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2.6 THE STRAIN GAUGE

Capsules are made of two diaphragms welded onto a metal ring and filled with afluid. Different mechanical and electrical methods are used to show the differential

pressure across the capsule. The diagram below (see Figure 2-7) shows a capsuleused in a pneumatic differential pressure transmitter.

Figure 2-7 A Capsule

The strain gauge is a resistor which has been deposited onto a flexible bar. As thebar is bent the resistor will change in length and thus its resistance. The changes inresistance are detected and electronically changed to a pressure signal. A straingauge is shown below (see Figure 2-8). This method is used in electricaltransmitters.

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Figure 2-8 Simplified Strain Gauge Capsule

2.7 VIBRATING (RESONANT) WIRE

The vibrating wire is the operating method used in some pressure transmitters. Thediagram below (see Figure 2-9) shows the basic construction.

Figure 2-9 The Vibrating Wire

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Operation:

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The frequency of vibration of a wire depends on its tension. The tension of thevibrating wire is changed by the pressure applied to the diaphragm. The electronicsunit vibrates the wire and measures the change in vibration frequency caused bypressure moving the diaphragm.

The electronics unit changes the pressure applied to the diaphragm into an

electrical output signal.

2.8 ELECTRICAL PRESSURE SENSING METHODS

The old mechanical methods of detecting pressure are slowly being replaced byelectrical methods. Electrical methods are more accurate and cheaper. Thefollowing gives a simple explanation of the principle involved.

2.8.1 THE PIEZO ELECTRIC EFFECT

Certain crystals, such as quartz, produce a voltage across them when a pressure isapplied. This voltage is simplified electronically and displayed digitally on amultimeter.

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Figure 2-10 The Piezo Electric Effect

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2.9 PRESSURE SWITCH

Pressure switches are devices that open or close electrical circuits when they sensea pre-set pressure.

The electrical circuits can then be used to open or close valves to relieve excess

pressure in a system.

The switches can be used to switch on pumps or compressors to maintain pressurein a system.

unit 2 pressure measurement

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