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Tolley’s Basic

Science and Practice

of Gas Service

Gas Service Technology Volume 1

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Tolley’s Basic

Science and Practice

of Gas Service

Gas Service Technology Volume 1

Fifth edition

John Hazlehurst

AMSTERDAM BOSTON HEIDELBERG LONDON NEW YORK OXFORD

PARIS SAN DIEGO SAN FRANCISCO SINGAPORE SYDNEY TOKYO

Newnes is an imprint of Elsevier

Newnes is an imprint of Elsevier

Linacre House, Jordan Hill, Oxford OX2 8DP, UK

30 Corporate Drive, Suite 400, Burlington, MA 01803, USA

First edition 1978

Second edition 1990

Third edition 1994

Fourth edition 2006

Fifth edition 2009

Copyright 2009 Elsevier Ltd. All rights reserved

No part of this publication may be reproduced, stored in a retrieval system

or transmitted in any form or by any means electronic, mechanical, photocopying,

recording or otherwise without the prior written permission of the publisher

Permissions may be sought directly from Elsevier’s Science & Technology Rights

Department in Oxford, UK: phone (+44) (0) 1865 843830; fax (+44) (0) 1865 853333;

email: [email protected]. Alternatively visit the Science & Technology website

at www.elsevierdirect.com/rights for further information

Notice

No responsibility is assumed by the publisher for any injury and/or damage to persons or

property as a matter of products liability, negligence or otherwise, or from any use or

operation of any methods, products, instructions or ideas contained in the

material herein

British Library Cataloguing in Publication Data

Hazlehurst, John.

Tolley’s basic science and practice of gas service. – 5th ed.

1. Gas appliances–Installation. 2. Gas appliances–Maintenance and repair.

I. Title II. Basic science and practice of gas service 683.80

8-dc22

Library of Congress Control Number: 2009925599

ISBN: 978-1-85617-671-2

For information on all Newnes publications visit

our website at elsevierdirect.com

Printed and bound in Great Britain

09 10 11 12 13 9 8 7 6 5 4 3 2 1

Table of Contents

Preface........................................................................................................... vi

1 Properties of Gases............................................................................ 1

2 Combustion ........................................................................................ 14

3 Liquefied Petroleum Gas .................................................................. 41

4 Burners ............................................................................................... 83

5 Energy................................................................................................. 111

6 Pressure and Gas Flow ..................................................................... 147

7 Control of Pressure ........................................................................... 181

8 Measurement of Gas ......................................................................... 207

9 Basic Electricity................................................................................. 240

10 Transfer of Heat ................................................................................ 288

11 Gas Controls ...................................................................................... 311

12 Materials and Processes.................................................................... 361

13 Tools .................................................................................................... 387

14 Measuring Devices............................................................................. 439

Appendix 1 SI Units..................................................................................... 461

Appendix 2 Conversion Factors ................................................................. 463

Index.............................................................................................................. 466

v

Preface

Following comprehensive updates and revision of the two other volumes in this

series ‘Domestic Gas Installation Practice’ and ‘Industrial and Commercial Gas

Installation Practice’ (formerly Gas Service Technology 2 and 3), it was clearly

essential that this, the first volume in the series, be brought up to date. ‘Basic

Science and Practice of Gas Service’ leads the reader through the knowledge

and understanding required to put into practice the safe installation and

servicing procedures described in Volumes 2 and 3.

Changes to standards and legislation have been included, in particular the

European gas directive relating to the prevention of products of combustion

being released into a room in which an open-flued appliance is installed.

Chapter 8 covers the devices used to ensure that these types of appliances

conform to this directive. New types of combustion analysers and appliance

testers which take advantage of the new technology available have also been

included. Since the release of British Standards 7967 Parts 1, 2 & 3, on the

8th December 2005 and 7967 Part 4 on 29th June 2007 the industry has once

again focused on Combustion Analysers. Compliance with the standards is of

no value without a full working understanding of using your chosen Elec￾tronic Combustion Analyser. Chapter 14 covers requirements of British

Standards 7967.

There have also been changes to the manner in which gas operatives are

required to prove their competence. It is now a legal requirement that all gas

operatives in the domestic field and most operatives working in industrial and

commercial sectors currently be registered with the Confederation of Registered

Gas Installers (CORGI). However, on 8th September 2008, HSE awarded

a 10-year contract to the Capita Group Plc to provide a new registration scheme

for gas installers from 1st April 2009. The current scheme has been in place for

more than 17 years. During this time the number of domestic gas related

fatalities has fallen significantly. However, a review in 2006 involving gas

industry stakeholders (including gas installers and their representatives) and

consumer groups identified no room for complacency and a strong case for

change. To achieve membership all operatives must be successful in a series of

initial gas safety assessments (Nationally Accredited Certification Scheme for

Individual Gas Operatives ACS) in the areas of work in which they operate. This

certification must be renewed every five years.

Scottish/National Vocational Qualifications are being amended to include

these ACS assessments as part of the qualification process. This volume and the

others in the series will prove invaluable to students studying for these

vi

qualifications and certificates, and for operatives wishing to improve their

knowledge and understanding of natural gas and Liquefied Petroleum Gas

(LPG) systems.

I would like to thank manufacturers for the use of photographs and

diagrams, in particular S.I.T. Gas Controls (ODS devices) and BW Technolo￾gies (flue gas analysers), and also Blackburn College for the use of their

facilities and resources.

Preface vii

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Chapter one

Properties of Gases

Chapter 1 is based on an original draft prepared by Mr E.W. Berry

INTRODUCTION

This first volume of the manual deals with the elementary science or ‘tech￾nology’ which forms the foundation of all gas service work. It outlines the

principles involved and explains how they work in actual practice.

To do this it has to use scientific terms to describe the principles of things

like ‘force’, ‘pressure’, ‘energy’, ‘heat’ and ‘combustion’. Do not be put off by

these words – they are simply part of the language of the technology which you

have to learn. Every activity from sport to music has its own special words and

gas service is no exception. While the football fan talks of ‘strikers’, ‘sweepers’

and ‘back fours’ the gas service man deals with ‘calorific values’, ‘standing

pressures’ and ‘secondary aeration’.

It is necessary for him to know about these things so that he can be sure that

he has adjusted appliances correctly. He must also know what actions to take to

avoid danger to himself or customers or damage to customers’ property.

GAS: WHAT IT IS?

Every substance is made up of tiny particles called ‘molecules’ (see Chapter 2).

In solid substances like wood or metal, there is very little space between the

molecules and they cannot move about (Fig. 1.1).

In liquids, there is a little more space between the particles, so that a liquid

always moves to fit the shape of its container. The molecules cannot get very far

without bumping into each other, however, so they do not move very quickly

and only a few get up enough speed to break out of the surface and form

a vapour above the liquid (Fig. 1.2).

A gas has a lot more space between its molecules. So they are able to move

about much more freely and quickly. They are continually colliding with each

other and bouncing on to the sides of their container. It is this bombardment that

creates the ‘pressure’ inside a pipe (Fig. 1.3).

Because the molecules are as likely to move in one direction as in any other,

the pressure on all of the walls of their container will be the same. Gases must, 1

therefore, be kept in completely sealed containers otherwise the particles would

fly out and mix, or ‘diffuse’, into the atmosphere (see section on Diffusion).

The word ‘gas’ is derived from a Greek word meaning ‘chaos’. This is

a good name for it, since the particles are indeed in a state of chaos, whizzing

about, colliding and rebounding with a great amount of energy.

KINETIC THEORY

‘Kinetic’ means movement or motion, so kinetic energy is energy possessed by

anything that is moving. A car has kinetic energy when it is moving along

a road. The effect of this can clearly be seen if it collides with another object!

Similarly gas molecules are in motion and possess kinetic energy at all normal

temperatures. The amount of energy increases as the temperature increases.

The Kinetic Theory states that:

1. The distance between the molecules of a gas is very great compared with

their size (about 400 times as great).

FIGURE 1.2 Molecules in a liquid.

FIGURE 1.1 Molecules in a solid.

FIGURE 1.3 Molecules in a gas.

2 Properties of Gases

2. The molecules are in continuous motion at all temperatures above

absolute zero, 273 C (see Chapter 5).

3. Although the molecules have an attraction for each other and tend to hold

together, in gases at low pressures the attraction is negligible compared

with their kinetic energy.

4. The amount of energy possessed by the molecules depends on their

temperature and is proportional to the absolute temperature (see Chapters

5 and 8).

5. The pressure exerted by a gas on the walls of the vessel containing it is due

to the perpetual bombardment by the molecules and is equal at all points.

DIFFUSION

If a small amount of gas is allowed to leak into the corner of an average-sized

room, the smell can be detected in all parts of the room after a few seconds. This

shows that the molecules of gas are in rapid motion and because of this, gases

mix or ‘diffuse’ into each other.

Graham’s Law of Diffusion

If two different gases at the same pressure were put into a container separated by

a wall down the centre and a small hole made in the wall as shown in Fig. 1.4

then, because the molecules are in continuous motion, some molecules of each

gas would pass through the hole into the gas on the other side. The faster and

lighter molecules would pass more quickly through the hole into the other gas.

After studying the rates at which gases diffuse into each other, Graham

discovered that the rates of diffusion varied inversely as the square root of the

densities of gases. Or,

Diffusion rate f 1 ffiffiffiffiffiffiffiffiffiffiffiffiffiffi

density p

Thus a light gas will diffuse twice as quickly as a gas of four times its density

(see section on Specific Gravity).

The effect can be demonstrated experimentally by filling a porous pot, made

from unglazed porcelain, fitted with a pressure gauge, with a dense gas such as

FIGURE 1.4 Diffusion of two gases.

Diffusion 3

carbon dioxide. Then place it in another vessel and fill that with a lighter gas

such as hydrogen (see Fig. 1.5). The pressure inside the inner pot will be seen to

rise, proving that the lighter gas is getting into the pot faster than the heavier gas

that is getting out.

CHEMICAL SYMBOLS

Symbols are often the initial letters of the name of the substance, like H for

hydrogen, O for oxygen. These symbols are not only a form of shorthand and

save a lot of writing, they also show the amount of the substance being

considered.

Each single symbol indicates one ‘atom’, which is the smallest chemical

particle of the substance (see Chapter 2). So H indicates one atom of hydrogen,

O is one atom of oxygen and so on.

It has previously been said that substances are made up of tiny particles

called ‘molecules’. This is true, but the molecules themselves consist of atoms.

Sometimes a molecule of a substance contains only one single atom, like

carbon, which is denoted by C. Often the molecules have more than one atom,

like those of hydrogen and oxygen which both have two atoms. So while an

atom is indicated by H, the smallest physical particle of hydrogen gas which can

exist is shown by H2. The ‘2’ in the subscript position indicates that the

molecule of hydrogen is made up of two atoms.

Some substances are made up of combinations of different kinds of atoms.

Water is an example. Water is composed of hydrogen and oxygen and its

formation is described in Chapter 2. The chemical formula for water is H2O.

This shows that a molecule of water has two atoms of hydrogen and one of

oxygen combined together. Similarly, methane gas, which forms the main part

of natural gas, is made up of one atoms of carbon and four atoms of hydrogen.

So its formula is CH4.

FIGURE 1.5 Experiment to demonstrate the effect of diffusion.

4 Properties of Gases

Table 1.1 shows the chemical symbols and formulae for some of the

substances met with in gas service work.

ODOUR

Gas can, of course, be dangerous. It can burn and it can explode. Some gases are

‘toxic’ or poisonous. But all fuels are potential killers if not treated properly.

Coke and oil both burn and can produce poisonous fumes. Electricity causes

more domestic fires than any other fuel and the first indication you get of its

presence could be your last!

Gas has the advantage of having a characteristic smell or ‘odour’ so it is

easily recognisable. Several of the combustible gases, including hydrogen,

carbon monoxide and methane, are colourless and odourless and could not

easily be detected without elaborate equipment. To make it possible for

customers to find out when they have a gas escape or have accidentally turned

on a tap and not lit the burner, a smell or odour, is added to the gas.

Gas manufactured from coal has its own smell, natural gas does not. But

suppliers of natural gas are required to add a smell to it before sending the gas

out to the customers. So an ‘odorant’ is used, originally a chemical called tet￾rahydrothiophene. Only a very small amount is added, something like 1/2 kg to

a million cubic feet of gas. Odorants now in use contain diethyl sulphide and

ethyl and butyl mercaptan.

Table 1.1 Chemical Symbols and Formulae for Substances Commonly

Used During Gas Service Work

Gases Metals

Substance

Chemical

Formulae Substance

Chemical

Formulae

Hydrogen H2 Iron Fe

Oxygen O2 Copper Cu

Nitrogen N2 Lead Pb

Carbon monoxide CO Tin Sn

Carbon dioxide CO2 Zinc Zn

Methane CH4 Antimony Sb

Ethane C2H6 Platinum Pt

Propane C3H8 Nickel Ni

Butane C4H10 Chromium Cr

Propylene C3H6 Tungsten W

Mercury Hg

Odour 5

TOXICITY

A number of gases are ‘toxic’ or poisonous and inhaling them can result in

death. Newspaper reports of people being ‘gassed’ are usually referring to

carbon monoxide poisoning.

Carbon monoxide, CO, is the ingredient which causes the problem. By

replacing oxygen in the bloodstream it prevents the blood from maintaining life

and so the organs of the body become poisoned.

CO is one of the constituents of gas made from coal or oil, and inhaling

the unburnt gas can prove fatal. Natural gas does not contain CO and so it is

‘non-toxic’.

This means, of course, that people can no longer commit suicide by gassing

themselves. There is another hazard, however. All fuels which contain carbon

can produce carbon monoxide in their flue gases if the carbon is not completely

burned. So people can still be gassed if the appliances are not flued or ventilated

correctly (see Chapter 2). There is always a risk of suffocation if the presence of

the gas reduces the amount of oxygen in the air.

CALORIFIC VALUE

All gases which burn give off heat (energy) and the ‘calorific value’, or CV,

indicates the heating power. It is the number of heat (energy) units which can be

obtained from a measured volume of the gas.

To measure CV in SI units, megajoules per cubic metre are used, written as

MJ/m3

. The CV of natural gas in the UK is about 39.3 MJ/m3 but does vary

slightly from district to district. Because customers pay for the gas they use

measured in heat units, The Gas supplier has to declare the calorific value of its

supply and this is printed on every gas bill. The actual CV is monitored at

official testing stations by gas examiners, appointed by the Department of Trade

and Industry.

Meters presently used by The Gas supplier measure gas in cubic feet

(100 ft3 ¼ 2.83 m3

) and until April 1992 customers were charged, based on the

number of ‘therms’ used (1 therm ¼ 105.506 MJ).

EC directive 80/181 required Britain to change their method of billing from

imperial to metric units and The Gas supplier implemented this change from

April 1992 using the metric kilowatt hour (kWh) as a basis for charge. It is the

common unit used in Europe and is of course the basis of charge for electricity.

The total amount of heat obtained from gas is, in fact, the Gross CV. If

however, the water vapour in the products of combustion is not allowed to

condense into water, the amount of heat obtained is the Net CV.

SPECIFIC GRAVITY (RELATIVE DENSITY)

Every substance has weight or ‘mass’, including gas. Some complicated scien￾tific equipment would be needed to do the weighing, but it can be weighed. It is

necessary, for various reasons, to compare weights of gases and to do this

6 Properties of Gases