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Tolley’s Basic
Science and Practice
of Gas Service
Gas Service Technology Volume 1
This page intentionally left blank
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 Ó 2009 Elsevier Ltd. All rights reserved
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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: Alternatively visit the Science & Technology website
at www.elsevierdirect.com/rights for further information

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
8thDecember2005and7967Part4on29thJune2007theindustryhasonce
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

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 aeratio n’.
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 side s 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).

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

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 H
2
Iron Fe
Oxygen O

6
Tungsten W
Mercury Hg
Odour 5
TOXICITY
A number of gases are ‘toxic’ or poisonous and inhaling them can resu lt 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/m
3
. The CV of natural gas in the UK is about 39.3 MJ/m
3
but does vary

).
Densities of substances vary very considerably. Lead is heavier than wood
and wood is lighter than water. In order to compare densities they are related to
a standard substance. For solids and liquid s the standard is water. For gases the
standard is air.
This relationship between the density of a substance and the density of the
standard is known as the ‘relative density’ or ‘specific gravity’. Let us take the
example of mercury. The specific gravity (or SG) of liquid mercury is 13.57. So
it is about 13 V2 times as heavy as the same bulk of water, or 1 l would weigh
13.57 kg.
The specific gravity of natural gas is in the region of 0.5. So it is about half
the weight of the same volume of air. The specific gravity of liquefied petroleum
gas (LPG) is greater than that of air.
WOBBE NUMBER
The Wobbe number of Wobbe ‘index’ gives an indication of the heat output
from a burner when using a particular gas (the terms Wobbe number and Wobbe
index are used interchangeably in this book).
The amount of heat which a burner will give depends on the following factors:
1. The amount of heat in the gas as given by its calorific value.
2. The rate at which the gas is being burned. This rate depends on the
following.
2.1 The size of the ‘jet’ or ‘injector’.
2.2 The pressure in the gas, pushing it out of the injector.
2.3 The relative weight of the gas. This affects how easily the pressure
can push it out and is indicated by the specific gravity.
Looking at these factors it can be seen that they divide up into two groups.
1. Factors depending on the gas:
– calorific value (1),
– specific gravity (2.3).
2. Factors depending on the appliance:

Sea reserves, with excess gas being supplied to Continental Europe. However,
more recently the UK have become a net importer of gas. It is estimated that
additional supplies will be required by 2010; the UK is projected to import up to
half of its gas demand and that by 2020 imports may be as high as 90%.
LIQUEFIED NATURAL GAS (LNG)
The reasoning behind storing and transporting LNG as opposed to gaseous
natural gas is the physical property of a reduction in volume of 600 times.
Although there may be some modifications of the make up of the revapourised
gas which change its combustion characteristics.
Liquefied petroleum gases include propane, butane and mixtures. They will
be dealt with in more detail in Chapter 3.
Table 1.2 Families of Gases
Family Approximate Wobbe Number Range Type of Gas
1 22.5–30 Manufactured
(inc. LPG/air)
2

L 39.1–45 Natural
H 45.5–55
3 73.5–87.5 LPG
Appliances are designed to operate on gas of a particular family.
8 Properties of Gases
AIR REQUIREMENTS
Everything that burns must have oxygen in order to do so. Fuel gases contain
carbon and hydrogen compounds which burn when they are lit and allowed to
combine with oxygen (see Chapter 2).
Fortunately the atmosphere consists of about 21% oxygen and 79% nitrogen
(with a tiny amount of other gases). This means that if a gas flame is allowed to
burn freely in the open, it can get the oxygen it needs from the surrounding air.
For each cubic metre (m

Wobbe index. Thus,
Gas modulus ¼
ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
pressure
p
Wobbe index
Using the modulus shows that to change from a manufactured gas with a Wobbe
index of 27.2 supplied at a pressure of 6.23 mbar to a natural gas with a Wobbe
index of 49.6 required the pressure to be increased to 20.62 mbar to maintain the
same operating conditions (see Chapter 4, section on Modifying Appliances to
Burn Other Gases).
Gas modulus 9
FLAMMABILITY LIMITS
Mixtures of gas and air will burn, but only within limits. If there is either too
much gas or too much air, the mixture will not burn. The ‘flammability limits’
are those air and gas mixtures at each end of the range which will just burn. For
natural gas the range is from 5% up to 15% of gas in the mixture. So the limits
are 5% gas for the lower explosive limit (LEL) and 15% gas for the higher
explosive limit (HEL). For commercial propane the limits are 2.0–10.0% and
for butane/air the limits are narrower being from 1.6 to 7.75% gas.
FLAME SPEED
All flames burn at particular rates. You can watch a flame burning its way along
a match or taper. Similarly a gas flame is burning along the mixture ( Table 1.3)
as it comes out of the jet or burner at a particular speed. The speed at which the
mixture is coming out has to be adjusted so that the flame will stay on the tip of
the burner. If it came out too fast it would blow the flame off and if it was too
slow the flame could burn its way back inside the tube!
The flame speed of gas is measured in metres per second. Typical flame
speeds are:
 natural gas: 0.36 m/s,

12.0
Propane C
3
H
8
1.0 85.9 2.5
Butane C
4
H
10
0.4 0.6 16.5
100 100 100
10 Properties of Gases
FIGURE 1.6 Substitute natural gas (SNG) plant. A catalytic rich gas producer with a double methanation process.
Flame speed 11
IGNITION TEMPERATURES
Gases need to be lit or ‘ignited’ before they will burn. This is done by heating
the gas until it reaches a sufficiently high temperature to burst into flame and
keep burning.
Heating the gas to the required temperature may be done with a match,
a small gas flame or ‘pilot’, an electric spark, or a coil of wire or ‘filament’ made
red hot by an electric current. Ignition temperatures of the common gases are:
 natural gas: 704

C,
 butane/air: 500

C,
 commercial propane: 530


limits
% gas
in air
5–15 2–10 1.6–7.75
Flame speed m/s 0.36 0.46 0.38
Ignition
temperature

C 704 530 500
12 Properties of Gases
The characteristics of SNG from the ‘double methanation process’ are
shown in Table 1.5. The gas produced is a non-toxic, high-methane, low-inert
gas interchangeable with natural gas, and it can be supplied directly into
pipelines.
Table 1.5 Characteristics of Typical SNG
Constituent Formulae
Percentage by
Volume
Methane CH
4
98.5
Hydrogen H
2
0.9
Carbon monoxide CO 0.1
Caloric value 38 MJ/m
3
Specic gravity 0.555 (air ¼ 1.0)
Wobbe number 51 MJ/m
3

The weight or mass of an atom depends on the number of protons and
neutrons in the nucleus. The electrons are so tiny, by comparison, that they do14
not affect the overall mass of the atom. So the three components of an atom
are:
1. electrons,
2. protons,
3. neutrons.
Electrons move around the nucleus of an atom in fixed orbits. There can be up to
seven orbits of electrons in the more complex atoms. The inner orbit or ‘shell’
can contain up to two electrons. The next holds up to eight electrons. Each shell
holds a fixed number of electrons.
The chemical behaviour of an atom depends largely on the number of
electrons in its outer shell. This particularly affects the ability of the atom to
combine with others to form molecules. Stable substances are those whose outer
shell contains its full complement of electrons. Other substances, with outer
shells which are not completely full, are less stable and so combine more readily
together to form more stable substances. Carbon, for example, has two electron
shells. With two electrons in its inner shell, but only four in its outer shell, it
combines readily with other substances.
MOLECULES
A molecule is the smallest particle of a substance which can exist independently
and still retain the properties of that substance.
A molecule of methane, CH
4
, consists of one atom of carbon and four atoms
of hydrogen (Fig. 2.3). You can see that, by sharing electrons with the hydrogen
atoms, the carbon atom can fill up its outer shell to the full eight electrons and so
form a stable substance.
FIGURE 2.1 Structure of the hydrogen atom.
FIGURE 2.2 Structure of the deuterium (or ‘heavy hydrogen’) atom.

Produced by a chemical reaction usually
associated with heat
Made by adding constituents
together in some container
Cannot be separated by physical means Can be separated by physical means
Has properties often very different from
those of its constituents
Has properties related directly to its
constituents, each contributing its
own particular property to the whole
16 Combustion