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    March 2007 Authorised Gas Testing Manual Version 7.5 1

    CONTENTS PAGE

    1. INTRODUCTION 3

    1.1 What is Gas Testing? ............................................................................................................................. 3

    1.2 Why is Gas Testing carried out?............................................................................................................. 3

    1.3 Who Carries out Gas Testing?................................................................................................................ 3

    1.4 When is Gas Testing Carried out?.......................................................................................................... 3

    2. FLAMMABLE GASES 4

    2.1 The Chemistry of Fire ............................................................................................................................ 4

    2.2 Fuel ........................................................................................................................................................ 4

    2.3 Oxygen................................................................................................................................................... 4

    2.4 Ignition Source / Heat ........................................................................................................................... 4

    2.5 Lower Explosive Limit............................................................................................................................. 4

    2.6 Upper Explosive Limits .......................................................................................................................... 4

    2.7 Explosive Range...................................................................................................................................... 5

    2.8 Common Gases ......................................................................................................................................5

    2.9 Flash Point.............................................................................................................................................. 5

    3. TOXIC GASES 6

    3.1

    Toxic Gases and Vapours ....................................................................................................................... 63.2 Workplace Exposure Limit (WEL) .......................................................................................................... 6

    3.3 Toxic Gases............................................................................................................................................. 6

    3.4 Other Toxic Gases...................................................................................................................................7

    3.5 Detecting Toxic Gases............................................................................................................................ 7

    3.6 Hydrogen Sulphide (H2S)....................................................................................................................... 7

    3.7 Characteristics of H2S.............................................................................................................................7

    3.8 Odour .................................................................................................................................................... 7

    3.9 Exposure Limits ..................................................................................................................................... 7

    3.10 Measurement of H2S............................................................................................................................. 7

    3.11 Concentrations of H2S............................................................................................................................7

    3.12 Effects of H2S on Personnel .................................................................................................................. 8

    3.13

    Effects of H2S on Equipment ................................................................................................................. 83.14 Asphyxiants ........................................................................................................................................... 8

    3.15 Long Term Health Impacts..................................................................................................................... 8

    3.16 Personal Protective Equipment - Regulatory Requirements ................................................................. 8

    3.17 Selecting PPE - Controlling Risk from Hazardous Gases ........................................................................9

    3.18 Selecting PPE - Fitting and Ergonomic Factors ...................................................................................... 9

    3.19 Use and Maintenance of Personal Protective Equipment .....................................................................10

    4. PROPERTIES OF GASES 11

    4.1 Gas Cloud Movement............................................................................................................................. 11

    4.2 Gas Behaviour........................................................................................................................................ 11

    4.3 Physical Properties................................................................................................................................. 11

    4.4 Dispersion .............................................................................................................................................. 12

    4.5

    Outdoor Areas and Open Structures ..................................................................................................... 12

    5. CONFINED SPACE ENTRY 13

    5.1 What are Confined Spaces? ...................................................................................................................13

    5.2 Build up of Gases in Confined Spaces.....................................................................................................13

    5.3 Testing Confined Spaces ........................................................................................................................ 13

    5.4 Further Considerations .......................................................................................................................... 13

    5.5 Oxygen................................................................................................................................................... 14

    5.6 Oxygentoo Little ................................................................................................................................ 14

    5.7 Oxygen too Much................................................................................................................................ 14

    5.8 Work Involving Air or Gas Lines..............................................................................................................14

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    6. PORTABLE GAS DETECTION EQUIPMENT 15

    6.1 Introduction .......................................................................................................................................... 15

    6.2 Types of Portable Gas Detectors ........................................................................................................... 15

    6.3 Gas Detector Principles of Operation..................................................................................................... 15

    6.4 Catalytic Detectors - Disadvantages....................................................................................................... 16

    6.5 Infrared (IR) Detectors - Principles of Operation ................................................................................... 16

    6.6

    Infrared (IR) DetectorsAdvantages and Disadvantages...................................................................... 176.7 Inert Atmospheres................................................................................................................................. 17

    6.8 Portable Gas Detectors - Sampling ........................................................................................................ 17

    6.9 When to Use An Aspirated Detector ..................................................................................................... 18

    6.10 Portable Gas Detectors - Basic Checks................................................................................................... 18

    6.11 Portable Gas Detectors - general considerations................................................................................... 18

    6.12 Pre-Use Check........................................................................................................................................ 18

    6.13 Temperature Effects .............................................................................................................................. 19

    6.14 Limitations of Portable Gas Detectors ...................................................................................................19

    6.15 Erratic Indications...................................................................................................................................19

    6.16 Defective Equipment ............................................................................................................................. 19

    6.17 Environments that Affect Readings ....................................................................................................... 20

    6.18 Off-Scale Readings.................................................................................................................................. 20

    6.19

    Aspirated Detector Tubes...................................................................................................................... 206.20 Warning Systems ................................................................................................................................... 20

    6.21 Gas Alarm Limits - Flammable................................................................................................................ 20

    6.22 Gas Alarm Limits - Toxic ........................................................................................................................ 20

    6.23 Gas Alarm Limits - Oxygen......................................................................................................................21

    6.24 Personal Gas Detectors.......................................................................................................................... 21

    6.25 Fixed Gas Detectors ............................................................................................................................... 21

    6.26 Fixed Gas Detectors - Positioning .......................................................................................................... 21

    7. GAS TESTING PROCEDURES 23

    7.1 Process Plant ......................................................................................................................................... 23

    7.2 Hazardous Areas.................................................................................................................................... 23

    7.3

    Zone 0.................................................................................................................................................... 237.4 Zone 1.................................................................................................................................................... 23

    7.5 Zone 2.................................................................................................................................................... 23

    7.6 Gas Testing in Support of Work Activities.............................................................................................. 23

    7.7 The Permit to Work System .................................................................................................................. 23

    7.8 Continuous Gas Monitoring .................................................................................................................. 24

    7.9 Practical Gas Testing ............................................................................................................................. 24

    7.10 Air Movement........................................................................................................................................ 25

    7.11 Evaluation.............................................................................................................................................. 25

    7.12 HVAC Flow Paths ................................................................................................................................... 25

    7.13 Remember, The Main Points Of Gas Testing Are: ................................................................................. 25

    8. NITROGEN PURGING 26

    8.1

    Purging...................................................................................................................... ............................. 268.2 Direct Purging......................................................................................................................................... 26

    8.3 Indirect Purging ..................................................................................................................................... 26

    8.4 Nitrogen Hazards ................................................................................................................................... 26

    9. CASE STUDIES 28

    9.1 The Wrong Way to do it! ....................................................................................................................... 28

    9.2 The Right Way to do it............................................................................................................................ 28

    10. GLOSSARY 29

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    1. INTRODUCTION

    This handbook is intended to support the Authorised Gas Testing e-

    Learning Course and provides an easy reference to the requirements for

    gas testing, as well as defining some of the key terms and abbreviations

    used.

    1.1 What is Gas Testing?

    Gas testing involves testing for toxic and flammable gases using portable gasdetection equipment and is an integral part of establishing a Safe System of Work in the oil and gas industry.

    Gas tests are performed to confirm that the working environment is safe from the hazards of combustible or

    toxic gases; and to confirm that oxygen levels are within specified tolerances and safe to breath.

    The words flammable and combustible have a similar meaning, that is capable of igniting and burning.

    1.2 Why is Gas Testing Carried out?

    Oil and gas production by its very nature, presents many hazards including the release of flammable or toxic

    gases. It is the aim of companies and operators to ensure that gas testing is carried out by competent personnel

    to enable an area to be declared free from toxic or flammable gases, therefore reducing the risk of fire,

    explosion, or asphyxiation of personnel.

    1.3 Who Carries out Gas Testing?

    Authorised Gas Testers (AGTs) are responsible for carrying out gas testing duties in liaison with the other

    supervisory roles and in accordance with specified precautions.

    An Authorised Gas Tester must have completed the necessary training, have been certified as competent and:

    Be able to demonstrate the ability to survey potentially hazardous areas using the detection equipment

    available and be familiar with plant and process areas

    Be aware of the capabilities and limitations of gas testing equipment

    Be aware of and demonstrate knowledge of, the requirements of the Permit to Work Procedure relating to

    gas testing

    1.4

    When is Gas Testing carried out?Gas testing should be carried out wherever there is a risk of flammable or toxic gases being present or when

    oxygen enrichment or deficiency is a likely hazard. Typical circumstances requiring gas testing are:

    Hot work of any type where heat is used or generated, for example welding, flame cutting, grinding, etc.

    Work which can generate sparks or other sources of ignition

    Work which can cause an uncontrolled release of hydrocarbons or other flammable or toxic materials

    Entry into confined spaces

    Gas alarm investigation

    Monitoring purging operations

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    2. FLAMMABLE GASES

    2.1 The Chemistry of Fire

    For a fire or explosion to occur, three components must be present:

    Fuel

    Oxygen

    Ignition Source / Heat

    If any of these components are not present then fire is impossible. The components are often represented as the

    sides of a triangle and this is known as the fire triangle.

    2.2 Fuel

    As any fire burns it consumes fuel and converts this into heat energy and other by-products such as smoke.

    Almost any substance can be considered as fuel and burn under the right conditions. Common fuels in the oil and

    gas industry are: methane, oil and solid waste.

    When a fire consumes all of its fuel or the fuel is removed it is called starving the fire.

    All fuel gases and vapours are characterised by explosive limits between which the gas or vapour mixed with air

    is capable of sustaining the spread of flame. These can be referred to as explosive or flammable limits. Whilethese two terms are interchangeable, within this handbook we will use the term explosive limits.

    2.3 Oxygen

    Oxygen in air normally represents 20.9%.

    To prevent the risk of fire or explosion it is important to minimise the opportunities for flammable gases and

    oxygen (air) being present in such proportions that they can be ignited, this can be achieved by:

    Controlling the presence of combustible substances and ignition sources

    Minimising air entry into production equipment

    2.4 Ignition Source / Heat

    The basis for ignition can come a wide variety of sources. Some examples include, but are not limited to: The discharge of static electricity

    Switching of electrical contacts

    Pump bearings running hot, or

    Diesel engines taking in gas through their air intakes

    They all have the potential to produce enough heat energy to ignite a surrounding gas. Care must be taken to

    eliminate or protect all potential sources of ignition from coming into contact with a fuel/oxygen mix.

    A fire can be extinguished by applying a coolant to reduce and maintain its temperature below its ignition

    temperature.

    2.5 Lower Explosive Limit

    The Lower Explosive Limit (LEL) refers to the lowest concentration of a gas in the atmosphere that will result in a

    flammable mixture. For example, the LEL of methane is five percent by volume. This means that if there is less

    than five percent by volume of methane in air, the mixture is too lean (weak) to support combustion.

    2.6 Upper Explosive Limits

    The Upper Explosive Limit (UEL) refers to the highest concentration of a gas in the atmosphere, which results in a

    flammable mixture. For example the UEL of methane in air is 15% by volume. This means that if there is more

    than 15% by volume of methane in air, then the mixture is too rich (concentrated) to support combustion.

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    A rich gas mixture would typically occur in a confined area such as an oil storage tank where the methane cannot

    disperse. From the table shown, we can see that concentrations of methane in air between 5 and 15% are

    combustible.

    2.7 Explosive Range

    The region between the Lower Explosive Limit and the Upper Explosive Limit is known as the Flammable or

    Explosive Range.

    2.8

    Common GasesThe LEL and UEL of some common gases found in the oil and gas industry are shown in the table.

    For most practical gas testing purposes it is the LEL which is significant. The Authorised Gas Tester is responsible

    for recording the percentage of LEL for the specific flammable gas being tested.

    2.9 Flash Point

    The flash point for a liquid is the lowest temperature at which it produces sufficient vapour to form an ignitable

    mixture with air. This means that the concentration of flammable vapour above the liquid is close to the Lower

    Explosive Limit (LEL).

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    3. TOXIC GASES

    3.1 Toxic Gases and Vapours

    Many gases in the oil and gas industry may not be flammable but may still present

    a health hazard due to their toxicity; however, you should be aware that many

    gases may be both flammable and toxic.

    As previously stated, flammable gases are expressed as the LEL percent by volume. The concentration of toxic

    gases is expressed in parts per million (ppm), where one percent by volume equals 10,000 ppm. Many toxic gases

    present health hazards in concentrations of less than 100 ppm.

    3.2 Workplace Exposure Limit (WEL)

    The occupational exposure limits for many toxic and hazardous substances are controlled by a Workplace

    Exposure Limit (WEL), which is defined as the approved exposure limit for any hazardous substance in relation to

    a specified reference period, when calculated by an approved method.

    The limits for each substance are given in parts per million (ppm) and milligrammes per metre-cubed for:

    Long Term Exposure Limits (LTELs), that is for an eight hour reference period, and

    Short Term Exposure Limits (STELs), that is for a fifteen minute reference period

    3.3 Toxic Gases

    There are many toxic substances produced or processed in the petrochemical industry, such as:

    Sulphur dioxide

    Chlorine

    Benzene, and

    1,3 Butadiene

    Sulphur Dioxide is a toxic, corrosive, liquefied gas, which is produced as a by-product of many industrial

    processes.

    In appearance it is a colourless, inflammable gas with a strong suffocating odour.

    The physiological effects of sulphur dioxide include eye, nose, throat and upper respiratory tract irritation atlevels

    as low as 2 ppm.

    Chlorine is a toxic, corrosive and heavy gas, which is greenish-yellow in appearance and has a suffocating odour,

    Chlorine is a skin and lung irritant:

    Low concentrations can cause burning eyes, coughing, sneezing and hoarseness

    High concentrations can cause pulmonary oedemaa condition in which fluid is accumulated in the lungs

    On its own chlorine is non-flammable in air, but most combustible materials will burn in chlorine as they do in

    oxygen.

    Benzene is a highly flammable substance, toxic by inhalation, absorption through the skin and if ingested.

    It is a known carcinogen and is also potentially hazardous to health.

    1,3 Butadiene, another known carcinogen that is also flammable, polymerises readily and can cause frostbite if

    contact is made in its liquid form.

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    3.4 Other Toxic Gases

    Great care must also be taken with the fumes and gases emitted by certain substances such as degreasants,

    paint, paint strippers and adhesives. You should always be aware that certain work activities can also produce

    toxic gases such as welding, burning and the use of chemicals.

    Under certain conditions entry may be permitted into areas where there are toxic gases present in excess of the

    WEL, provided breathing apparatus is worn and precautions are taken.

    3.5 Detecting Toxic Gases

    A wide variety of instruments, including portable and fixed detectors, exist for

    the measurement of toxic gases. Ranging from simple chemical filled detector tubes

    to complicated electrochemical systems, they are relatively specific to individual gases

    or families of gases.

    3.6 Hydrogen Sulphide (H2S)

    When testing for toxic gases at the installations, our primary concern is hydrogen sulphide (H2S).

    Hydrogen sulphide is one of the most dangerous gases found in the oil and gas industry. It is possible that a field

    can start producing H2S at any time; therefore caution must be exercised at all times, particularly in confined

    spaces.

    3.7 Characteristics of H2S

    Hydrogen sulphide:

    Is often referred to as sour gas

    Has a distinct odour of rotten eggs at low concentrations

    Can deaden human sense of smell at high concentrations

    Is a colourless, flammable gas which may be liquefied under pressure

    Is soluble in water, crude oil or petroleum fractions

    Extremely corrosive

    Burns with a blue flame producing sulphur dioxide (also a toxic gas)

    Slightly heavier than air and may accumulate in low lying areas and confined spaces

    Easily dispersed by wind movements or air currents

    EXTREMELY hazardous to health - deadly

    3.8 Odour

    H2S is generally recognised by its characteristic foul odour of rotten eggs at

    concentrations under 10 ppm. Concentrations of Less than 1 ppm can be detected

    by this odour, although prolonged exposure will deaden the sense of smell.

    3.9 Exposure Limits

    Occupational Exposure Limits for H2S are as follows:

    Long-term exposure limit of 5 ppm (0.0005% by volume) over an 8 hour reference period

    Short-term exposure limit of 10 ppm (0.0010% by volume) over a 15 minute reference period

    3.10

    Measurement of H2SToxic gas detectors are calibrated to measure H2S in parts per million (ppm) of H2S in air, by volume ratio.

    3.11 Concentrations of H2S

    There is a significant difference between H2S concentration in air and H2S in liquid.

    The actual concentration measured in air by volume ratio can be 10-100 times

    higher than the equivalent measurement in liquid by weight ratio.

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    For example, crude oil being discharged into a storage tank may contain only 70 ppm H2S in the liquid by weight.

    However, the concentration of H2S in the vapour space above the crude oil could exceed 7000 ppm by volume.

    3.12 Effects of H2S on Personnel

    When H2S is inhaled by an individual, it passes directly through the lungs into the bloodstream. To protect itself,

    the body oxidizes (breaks down) the gases as rapidly as possible into a harmless compound.

    If the individual breathes in so much H2S that the body cannot oxidize it all, it builds up almost instantly in the

    blood and the individual is quickly overcome.

    The areas of the brain that control breathing become paralysed, the lungs stop working and the person becomes

    asphyxiated.

    The way in which H2S affects a person depends on the following:

    Intensity - the concentration of exposure

    Duration - the length of time the individual is exposed

    Frequency - how often the individual has been exposed

    Susceptibility - the individuals physiological make-up

    3.13 Effects of H2S on Equipment

    H2S is highly corrosive to steel and, at high stress levels extreme metal embrittlement

    may occur in a very short time.

    Equipment that may be subject to exposure should be specified accordingly

    and an appropriate schedule maintained to ensure detection of metal deterioration.

    3.14 Asphyxiants

    Gases such as nitrogen, hydrogen, methane etc., all act simply by diluting the air and so reducing the level of

    oxygen available. These gases are known as Simple Asphyxiants.

    Substances that affect the bodys assimilation of inspired oxygen, such as carbon monoxide, prevent the uptake

    of oxygen in the blood. These gases are known as chemical asphyxiants.

    More toxic asphyxiants, such as hydrogen sulphide (H2S) directly affect the respiratory centre of the brain,

    causing breathing to stop.

    3.15 Long Term Health Impacts

    Oxygen deprivation as a result of exposure to asphyxiants can cause permanent brain damage after only a short

    period of time.

    Repeated exposure to even small amounts of toxic or flammable gases can also have long-term health impacts.

    3.16 Personal Protective EquipmentRegulatory Requirements

    The Control of Substances Hazardous to Health (COSHH) Regulations 2002 require that where employers cannotprevent exposure to hazardous substances such as toxic and flammable gases, exposure should be adequately

    controlled.

    The preferred methods of control are to minimise the amounts of the hazardous material and/or the exposure of

    the workforce to them. The last resort is the provision of Personal Protective Equipment (PPE). The reason PPE is

    regarded as a last resort is because it is only effective if it is correctly fitted, maintained and properly used.

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    Employers are therefore required to ensure that PPE is appropriate for the risk or risks

    Involved and are aware of the conditions at the place where exposure to the risk may

    occur.

    Examples of PPE include:

    Safety footwear

    Safety helmet Hearing protection

    Eye protection

    Fire retardant/chemical resistant coveralls

    Gloves, and

    Respiratory Protective Equipment (RPE)

    3.17 Selecting PPEControlling Risks from Hazardous Gases

    Before selecting the correct type of PPE to use, it is necessary to assess the hazards and

    risks that may be present and determine the characteristics that the PPE must have.

    For example, if the hazard is nitrogen then there is a risk of asphyxiation if the nitrogen displaces oxygen in the

    atmosphere.

    Therefore, an atmosphere-supplying respirator (that is a respirator which provides breathing air form a source

    independent of the surrounding atmosphere) is required rather than air-purifying respirator (which does not

    supply oxygen).

    If the hazards are from toxic or flammable gases then it is important to understand the risks each gas poses to

    health.

    For example: Benzene is known to irritate the skin and eyes and to be carcinogenic if swallowed or inhaled,

    therefore PPE should include:

    An atmosphere-supplying respirator

    Eye protection

    Chemical resistant gloves, and Chemical resistant coveralls

    3.18 Selecting PPEFitting and Ergonomic Factors

    PPE will not be effective unless it fits the wearer, and this is not just a case of determining the right size of boots,

    gloves or coveralls.

    For example, if Respiratory Protective Equipment depends on a face seal, then it will be ineffective if the worker

    has facial hair or stubble.

    Furthermore, where two or more items of PPE are to be worn together they should be compatible so that

    wearing one item does not compromise the fit of another or, make it too uncomfortable to be worn.

    For example, ear protectors and safety glasses or visors should be designed to fit together.

    No matter how well fitted, PPE is likely to restrict what the wearer can do by limiting mobility and visibility, or by

    requiring them to carry extra weight.

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    When selecting PPE, the nature of the job and the demands it places on the worker should be taken into account

    including:

    The physical effort to do the job

    The methods of work

    How long the worker will need to wear the PPE

    The requirements for visibility and communication

    3.19 Use and Maintenance of Personal Protective Equipment

    In general, PPE should be examined to ensure that it is in good working order before being issued to the wearer.

    Properly trained staff should also examine PPE before it is put on and it should not be worn if it is found to be

    defective.

    Similarly the wearer should check the fit of each item of PPE and consult trained staff if necessary.

    COSHH regulations require that there should be an effective system of maintenance of PPE to ensure that it

    remains in good working order.

    This includes where appropriate:

    Cleaning Disinfection

    Examination

    Replacement

    Repair, and

    Testing

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    4. PROPERTIES OF GASES

    4.1. Gas Cloud Movement

    Vapours in air move from place to place under two influences:

    Gravity, where gases are heavier than air they flow in a similar way to

    that of liquids.

    Normal turbulence and ventilation, where gases are lighter than air

    4.2. Gas Behaviour

    In the event of a leak, combinations of gases remain mixed until each component is separated. This separation

    can take several hours and may be caused by many factors including condensation, gravity or air movement,

    depending on local ambient conditions. Hydrocarbon gases are often at high pressure, and have constituents

    that are both lighter and heavier than air. When gas is under pressure a relatively small leak can result in very

    rapid and large gas concentrations forming. These are referred to as plumes.

    Plant, equipment, pipework and vessels, from which gas could leak, may be situated outside in well ventilated

    spaces, inside louvred modules or in enclosed areas where there is forced ventilation. An appreciation of gas

    cloud movement is therefore critical.

    4.3.

    Physical Properties

    To anticipate gas cloud movement and behaviour, we must have a basic understanding

    of the physical properties of gases.

    Relative Density:

    The ratio of the density of a gas compared to that of air is known as the relative density.

    Gases that have a low relative density are lighter than air. For example, methane under normal conditions will

    rise. They will tend to collect beneath objects or surfaces, which prevent them from rising upwards.

    Gases that have a high relative density such as hydrogen sulphide or pentane will fall to the ground and tend to

    gather in low lying areas or in drains.

    In practice, other factors such as temperature and pressure can affect relative density.

    Velocity:

    The velocity of a gas is the speed at which it travels. The velocity of a gas escape or

    leak into the surrounding air will lead to a disturbance called turbulence, which causes

    the gas to mix with the surrounding air and increases the potential for an explosive

    mixture to develop.

    Temperature:

    In general, heating of gases will lead to a reduction in density causing the gas to rise.

    Cooling will have the opposite effect, resulting in an increase in density and a tendency

    for the gas to condense and fall.

    The main point to note is that a rise in temperature can alter the nature of the hazards

    posed by flammable gases and vapours. Usually the dangers are increased when the

    temperature rises.

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    Pressure:

    Some gases are kept in a liquid state by being pressurised, such as liquefied petroleum gases containing butane

    and propane. As a pressurised liquefied gas escapes into the surrounding atmosphere its pressure decreases and

    its temperature drops. This causes its relative density to increase and the gas to fall.

    Evaporation:

    Liquefied Petroleum Gas (LPG) contains propane and butane, which will evaporate when exposed to theatmosphere. As liquids evaporate their temperature drops, this causes their density to increase. The cold and

    therefore heavy gas released during evaporation behaves like a slow -motion liquid and will flow at a low level

    along floors.

    The reverse effect can occur with hot, heavier than air gases, which may be buoyant for several minutes after

    release.

    4.4. Dispersion

    The nature of the initial dispersion will affect the behaviour of the escaping gas. In the absence of air movement

    or any confining structure, the dispersion of gas from a source of release will initially be determined by the

    momentum of the released gas, its density relative to air, or both.

    Gas escaping with high velocity, for instance a leak from a pressurised line or container, will behave initially as ajet directed away from the source of release. As the distance from the source of release increases, the

    momentum of the jet will decay until, eventually, the dispersion of the gas will be controlled by buoyancy effects.

    4.5. Outdoor Areas and Open Structures

    In outdoor areas or on open structures, wind speed and direction will affect the dispersion of gas following a

    release.

    In open areas, the spread of gas upwind of the release will be reduced, whilst downwind of the release it will be

    increased. This effect will be greater at high wind speeds.

    More complex airflow patterns will occur around items of plant and other structures.

    Low lying areas, partially enclosed spaces or areas with restricted air movement may have a significant ef fect on

    gas movement and dispersion.

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    5. CONFINED SPACE ENTRY

    5.1 What are Confined Spaces?

    The term, confined space, is widely used throughout industry, but is also sometimes

    misunderstood. The following definition is recognised by various companies:

    Any area which has limited access / egress or which is sufficiently confined to permitthe accumulation of flammable / toxic gases or vapours; or where an oxygen deficiency

    or enrichment could occur.

    or:

    Any enclosure where the presence of air contaminants may be harmful to personnel and prevent their ability to

    escape unaided.

    5.2 Build up of Gases in Confined Spaces

    Hazardous concentrations of gases or vapours can arise from sources both inside and outside confined spaces,

    for example from:

    An operation performed inside the confined space, eg. welding Oxygen enrichment of the atmosphere caused by leaks of oxygen into the confined space

    A process which has previously been carried out in the confined space

    Sludge deposits that are disturbed during inspection / cleaning

    Adjoining plant due to ineffective isolation, or

    Migration from another area

    5.3 Testing Confined Spaces

    Gas testing of a confined space must be carried out before it can be certified as being

    safe to enter, or before safety precautions to be taken upon entry are implemented.

    Tests should check for the presence of gas or toxic fumes, and the adequacy of

    oxygen and air supply. An acceptable result must be obtained before work in any

    area proceeds.

    Where possible, all tests should be conducted from outside the vessel or confined space. When this is impractical

    the following basic rules should be adhered to when entering the confined space to carry out gas testing:

    Wear approved breathing apparatus

    Know what type of gas or vapours are to be expected

    Ensure all isolations to the confined space have been implemented

    Provide ready exit / entry routes for rescue team (side openings or manholes should be used in preference to

    top openings)

    Wear an approved safety harness, with lifeline attached, before entering a confined space

    Ensure there is at least one Standby Person on the outside, ready to raise the alarm in the event of an

    emergency.

    The Standby Man should always be in sight and call, of the Authorised Gas Tester

    5.4 Further Considerations

    Other considerations that the Authorised Gas Tester should be aware of are

    chimney stacks that may still contain flue gases, and the oxidation process

    which may cause rust. Rust can reduce the content of oxygen present in the

    atmosphere.

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    Where purging activities have been carried out, the atmosphere in the vessel may still contain inert gases that

    have displaced the original contents and the atmosphere within could be oxygen deficient. Remember that the

    gas may not be uniform inside a confined space and may be present in different concentrations at various levels

    with the heaviest gases falling to the lowest point and the lightest gases collecting at the highest point. Gas may

    also collect behind obstructions such as baffle plates or bulkheads, or may be trapped in sludge deposits in the

    vessel.

    After any test, the Authorised Gas Tester should record the maximum and minimum readings on the Entry

    Certificate.

    5.5 Oxygen

    Oxygen is a non toxic gas which represents 20.9% of the air we breathe. However, if the Oxygen content of the

    atmosphere falls below or rises above this level, as may be the case in a storage tank, or vessel, the human body

    will suffer from oxygen starvation or intoxication.

    5.6 Oxygentoo little

    Too little oxygen could result from purging with inert gas to remove flammable or toxic gas or vapour, or the

    formation of oxidant products such as rust on the inside surface of a vessel.

    5.7 Oxygentoo much

    Too much oxygen has a poisonous effect on the body as well as representing an increased fire hazard, this could

    for example occur in the vicinity of leaking gas welding equipment.

    5.8 Work involving air or gas lines

    It is important to isolate and remove all air / gas lines that are not being used, for example oxy-acetylene lines, to

    reduce potential leakage of gases into the confined space.

    Note; no gas / air cylinders other than breathing apparatus sets are allowed in confined spaces.

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    6. PORTABLE GAS DETECTION EQUIPMENT

    6.1 Introduction

    Portable and personal gas detectors are a convenient means of detecting the presence

    of flammable or toxic gases and vapours; and ensuring that oxygen levels in the

    atmosphere are safe.

    They have the advantage that they can be taken to the site where the sampling is to

    take place, and will give a clearly defined reading or signal.

    Portable gas detectors are preferred:

    For testing an atmosphere in a confined space for toxic gases

    For tracing leaks, and

    To give early warning of the presence of flammable gases when hot work is being carried out

    Portable gas detectors however, only monitor a small area around the operator and rely on the operator to take

    remedial action such as alerting other personnel to any danger or, send for the emergency services.

    In some circumstances a fixed detection system will be more appropriate.

    Fixed detectors provide continuous monitoring of plant and equipment over wide areas and can be configured to

    provide a range of automatic actions in the even of an emergency.

    Such detectors are particularly useful where there is the possibility of a leak of toxic or flammable gas into an

    enclosed space where it could accumulate.

    All gas monitors have specific operating instructions and limitations, these are documented in instruction

    manuals. These instructions should be read and clearly understood before using the gas detector.

    6.2 Types of Portable Gas Detectors

    Types of detector commonly used in the oil and gas industry are:

    Combustible, catalytic, thermal conductivity or electronic

    Toxic (H2S), electrochemical

    Oxygen, electrochemical

    Manufacturers supply electronic instruments to customer requirements; they may

    detect one specific gas or a combination of gases.

    Although the manufacturer and model may vary from site to site, all combination detectors are designed to

    measure oxygen, toxic (H2S) and flammable gases.

    6.3 Gas Detector Principles of Operation

    Mixtures of flammable gases and air cannot be ignited to cause a self-sustaining flame unless the concentration

    of gas exceeds the LEL.

    However, mixtures containing much lower concentrations will burn on the surface of a catalyst, even those

    approaching zero percent.

    If a flammable gas is passed over a heated platinum wire, the gas will burn on its surface, yielding heat in direct

    proportion to the gas or vapour concentration.

    This process is called surface or catalytic combustion.

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    When an electrically heated platinum wire is connected in an appropriate circuit, the heat released by catalytic

    combustion further increases the temperature of the wire, resulting in a change in its electrical resistance.

    The change in resistance can be measured on an electric meter and displayed in analogue or in digital format.

    Many portable gas detectors utilise the principle of catalytic combustion and the hot-wire type of flammable

    gas detectors operate in this way, though, you should be aware that such detectors will not indicate thepresence of either flammable dusts or fibres.

    Other, similar devices may employ a solid or porous catalytic mass instead of a wire and can sense the

    temperature by means other than the increase in electrical resistance.

    A more recent development is a type of instrument using solid-state sensors (semi-conductors).

    These solid-state sensors are generally less specific to flammable gases and can respond to changes in

    atmosphere moisture content and temperature.

    Such instruments do not commonly provide a meter or digital read-out but produce an audible or visual alarm or

    both, at one or more pre-selected alarm levels.

    Another recent development is the infrared sensor that operates on optical wavelength principles to detect

    hydrocarbons and other gases.

    6.4 Catalytic Detectors - Disadvantages

    Many portable gas detectors utilise the principle of catalytic combustion and the hot -wire type of flammable

    gas detectors operate in this manner.

    Catalytic detectors have a number of disadvantages including:

    They will not indicate the presence of either flammable dusts or fibres

    They require a level of more than 10 percent oxygen to work correctly

    They can give false readings in gas rich atmospheres, that is, above the Upper Explosive Limit (UEL), and

    The catalyst can be poisoned by trace gases such as silicones and hydrogen sulphide

    An additional problem is that the metal screen can be blocked, which can result in drift of the zero point, and loss

    of sensitivity; therefore it needs regular calibration and replacement.

    6.5 Infrared (IR) Detectors - Principles of Operation

    The operating principle is based on the absorption of infrared light by hydrocarbon molecules.

    If a volume of gas between an IR source and detector contains hydrocarbon molecules, then these molecules will

    absorb some of the infrared light decreasing the total IR radiation detected.

    The amount of absorption indicates the concentration of hydrocarbon in the gas.

    Infrared detectors can be either point or open-path.

    For point detectors a short beam is used to illuminate a volume of gas that

    has suffused into a measurement chamber.

    For open-path sensors the source of infrared light is a powerful narrow beam that illuminates the space between

    source and detector.

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    Alternatively, a mirror is positioned at the end of the path, and this reflects the beam back to the detector.

    6.6 Infrared (IR) DetectorsAdvantages and Disadvantages

    The advantage of IR detectors is that they:

    Do not require oxygen to operate

    Cannot be poisoned by trace gases such as silicones and hydrogen sulphide, and

    Are not ambiguous above the LEL

    The disadvantages are that they:

    Cannot detect hydrogen, and

    Are inherently pressure-sensitive

    Open-path detectors enable large spaces to be easily monitored but the alignment of source and detector

    requires great care and, objects in the beam can give false readings.

    Be aware that if the sun is low in the sky, stray radiation can enter the detector and can cause interference with

    the beam. If the beam is uncompensated this can result in high readings.

    6.7

    Inert AtmospheresMany gas detectors are designed for use in an atmosphere that contains an air / gas mixture, but detecting and

    measuring gases in an inert atmosphere requires a detector that does not need oxygen to operate.

    For this reason, detectors that use technologies such as thermal conductivity are used.

    The results from these types of detector are commonly expressed as a percentage by volume of the total

    atmosphere.

    Remember, even though expressed as percentages, gas testing results in % by volume are completely different

    to % LEL.

    For example, the diagram shows 8% LEL methane would not be considered hazardous as it equates to only 0.4%

    by volume.

    However 8% methane by volume is very hazardous as it is inside the explosive range. Always be aware of the

    scale that instrument readings are displayed in.

    When testing the effectiveness of inert atmospheres, it is important to check for low levels of oxygen.

    Sealed systems, such as a storage tank containing a nitrogen blanket, will typically require an atmosphere

    containing less than 2% oxygen and less than 0.5% hydrocarbon content by volume.

    6.8 Portable Gas Detectors - Sampling

    Portable gas detectors may have their capabilities extended by utilising additional sampling attachments such as

    tubes and probes.

    Only sample probes or sensors recommended by the manufacturer should be used.

    When manual aspirators and sample probes are used they should be checked to

    ensure that they are fitted correctly and are leak free.

    This can be achieved by squeezing the aspirator and, while pinching the probe tube closed, wait and ensure that

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    the aspirator bulb does not re-inflate. Where gases or vapours may be stratified rather than uniformly mixed,

    spot checks should be carried out at different levels using a sample probe, with an aspirator if required.

    Sample probes allow detection to be carried out from a safe distance and are also ideal for testing flanges or

    inaccessible areas for leaks.

    6.9

    When to Use An Aspirated DetectorNon-aspirating detectors rely on normal air movements to carry samples through vents to the sensors.

    Aspirated detectors use motorised or manual pumps to actively draw in samples of the air to be tested.

    Aspirated detectors should be used to test for the presence of gases in remote or inaccessible places such as:

    Drains and other low-lying voids where heavier than air vapour clouds may collect, and

    The lagging of the clad pipeline

    They should also be used where the air is very still, such as in a confined space, or very turbulent, such as when

    testing for a leak from a flange on a windy day.

    6.10

    Portable Gas Detectors - Basic ChecksBefore using detection equipment some basic checks should be carried out:

    Check the calibration date

    Check for visual signs of damage

    Check the battery is sufficiently charged

    Check the aspirator bulb and sample probe for leaks

    Check the diffuser heads for dirt or blockages

    Check that the readings in a clean air environment are within tolerance before starting tests

    Ensure that you are familiar with the operation of the detector to be used and the manufacturers instructions. If

    you are in any way unsure then you should not use the instrument.

    6.11 Portable Gas Detectorsgeneral considerations

    Re-calibration and checking of portable gas detection equipment shall only be carried out by competentpersonnel.

    Under no circumstances should you carry out repairs or make adjustments yourself if you are not competent and

    authorised to do so.

    Remember; only use the instrument for its designed purpose, making sure that the manufacturers instructions

    are fully adhered to.

    6.12 Pre-Use Check

    Before using any gas detector, Authorised Gas Testers must ensure that they are fully conversant with their

    operation and are responsible for completing a number of pre-use checks.

    Pre-use checks, include the following:

    Select the correct gas detector for the job

    Check that the detector is within the next calibration date

    Ensure that the detector is in a good state of repair, and that the casing is not

    damaged

    Check that the battery life is sufficient for the job

    Check that the sensor head membranes are clean and are not blocked

    Check that the detector zeros in a clean air environment as follows:

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    % LEL = 0%

    ppm hydrogen sulphide = 0

    % oxygen = 20.9%

    If using an aspirator check that the aspirator is in a good condition and leak free

    If required test the detector with a sample gas of known concentration

    It is important that care is exercised with portable detectors, not only to ensure that they are properly used, butalso to ensure that the correct interpretation is made of the readings and that their limitations are realised.

    Calibration and checking of portable gas detection equipment shall only be

    Carried out by competent personnel.

    In areas being surveyed with portable gas detection equipment, where gases

    or vapours may be stratified rather than uniformly mixed, spot checks should

    be made at different levels using an extension probe and aspirator (if required).

    When sampling vapour above a liquid, care should be taken to avoid the sample

    line or sensor from coming into contact with the liquid, since this may block

    the entry of gas or vapour to the apparatus.

    Only sample lines or sensors recommended by the manufacturer should be used with the detector.

    6.13 Temperature Effects

    When taking portable gas detection equipment from a warm to a cool environment, it is important to allow the

    equipment temperature to stabilise to avoid condensation (the formation of vapour) which may otherwise

    interfere with the operation of the gas detector.

    6.14 Limitations of Portable Gas Detectors

    Gas detection equipment may not be sensitive to a specific gas, for example H2S detectors may not detect

    methane and furthermore, adverse readings may be generated by the presence of gases other than those for

    which the detector is calibrated.

    It is important to note that some substances such as solvents or silicones may also adversely affect detectors and

    you should check the manufacturers specifications before use.

    If contamination is suspected, the detector must be returned for checking and re-calibration.

    6.15 Erratic Indications

    Erratic indications on detection equipment may point to contamination, an equipment malfunction or some

    atmospheric disturbance.

    In such cases carry out the test again.

    Where there are doubts a check should be made with another gas detector of the

    same type. Suspect equipment should be returned for checking and re-calibration.

    6.16 Defective equipment

    Erratic indications may point to equipment malfunction or some atmospheric disturbance. Where doubt exists a

    check should be made with another gas detector of the same type and / or the suspect equipment should be

    checked under controlled conditions before its continued use.

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    6.17 Environments that affect readings

    The presence of very low concentrations of combustible gas can produce indications that may be mistaken for

    zero drift. In such circumstances, the equipment should be removed to a clean air environment and re-checked.

    Dust or saturated steam may physically block the flame arresters of certain types of gas detection equipment,

    rendering them inoperative, and care should be exercised accordingly.

    6.18 Off-Scale Readings

    Where off-scale indications occur (in either direction), this may indicate the presence of a potentially explosive

    atmosphere. It will then be necessary to flush the detection equipment with clean air and to cross-check for the

    presence of gas by taking the reading again, or by using another type of gas detection apparatus.

    Under such circumstances, assume the presence of a potentially explosive atmosphere until otherwise proven.

    When using portable gas detection equipment, it is necessary to be aware that some flammable gases and

    vapours are also toxic.

    6.19 Aspirated Detector Tubes

    In addition to electronic gas detectors, aspirated detector tubes may be used.

    These consist of either a manual or battery operated suction pump, the in

    let of which is fixed to a reactive chemical tube. The tube and its chemical

    contents are selected to detect a known type of gas.

    Tubes exist for detecting a variety of gases, such as benzene, carbon

    monoxide, hydrogen, chlorine, H2Setc. Each type of tube is supplied with instructions. These must be checked to

    determine the tube limitations and precautions for use.

    The pump draws in a metered amount of the atmosphere to be tested through the chemical tube. The tube is

    normally calibrated such that, in the event of gas being present, the reaction between gas and chemical shows

    up as a distinct colour on the scale, indicating the concentration of gas present.

    6.20 Warning Systems

    All types of portable gas detectors, except chemical tubes, have visual and / or audible warnings to alert the

    operator to the presence of unwanted gases.

    The equipment must provide a method by which the quantity and type of gas can be

    determined, and at pre-determined levels initiate alarms on the portable gas detector.

    6.21 Gas Alarm Limits - Flammable

    Portable gas detectors are set to alarm at a percentage of the LEL based on the

    calibration gas. For example: the alarm may be set at 5% of the LEL for methane, and

    this would equate to 0.25% of methane by volume in air.

    6.22 Gas Alarm Limits - Toxic

    The Occupational Exposure Limit of H2S is 5 ppm and it is at this point that portable gas

    detector alarms are set.

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    6.23 Gas Alarm Limits - Oxygen

    The atmosphere normally contains approximately 20.9% oxygen and the portable detector is calibrated at this

    figure.

    The detector incorporates high and low oxygen alarms.

    6.24

    Personal Gas DetectorsPersonal gas detectors are small portable devices worn on the outside of the coverall and

    typically contain a single sensor for a specific gas.

    They are ideal for protecting workers in situations where a risk assessment has identified

    only one foreseeable hazard.

    They are not suitable for situations were multiple hazards are known or considered possible.

    6.25 Fixed Gas Detectors

    Fixed gas detectors have two principal parts:

    A sensor, and

    A wall/ceiling-mounted junction box

    The junction box contains the components necessary to process the output

    from the sensor before it is sent to a Central Control Unit.

    If a sensor detects a dangerous gas level at any time, the control unit raises

    The alarm.

    Fixed gas detectors can be point detectors or open-path detectors.

    Systems can be designed with path lengths of 100m or more.

    It is impossible however, to distinguish whether a reading is due to a high concentration along a small part of the

    beam or a lower concentration distributed over a longer length.

    Additionally, they are not specific to a particular gas, for example steam or water vapour can produce false

    readings and alarms.

    6.26 Fixed Gas Detectors - Positioning

    A small-scale system may have only one detector, but wide-area networks can include

    many sensors, all connected to the same central controller.

    The sensors should be positioned so that the entire area at risk is covered.

    Where there is the possibility of gas entering a confined space, the sensors should be

    positioned close to entry points.

    When deciding on the height at which the sensors are installed the density of the gas to

    be detected should be considered:

    Position the sensors:

    High if the gas is lighter than air,

    Low if the gas is heavier than air

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    Other factors affecting gas movement, such as temperature, pressure, wind direction and the impact of any

    forced ventilation must also be considered.

    Decisions regarding detector positioning and installation should be made only by experienced personnel familiar

    both with the process concerned and with the gases to be detected.

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    7. GAS TESTING PROCEDURES

    7.1. Process Plant

    A very large number and variety of different types of connections occur in a process plant and it is possible that

    at some time hydrocarbon gases will leak from these. Particular attention must be given to flanges, screwed

    connections, gaskets, drains and vents, valve glands and pump seals, when performing tests.

    The Authorised Gas Tester should be aware of the types of fluid that run through the process equipment, or areadjacent to the proposed area of work.

    7.2. Hazardous Areas

    A hazardous area is an area in which explosive gas/air mixtures are, or could be, expected to be present in

    quantities. These areas require special precautions to limit the sparking potential of electrical equipment. This

    can be achieved by design considerations for the equipment or pressurising the area to exclude the hazardous

    atmosphere.

    Hazardous areas are classified into Zones as follows:

    7.3. Zone 0

    An area in which an explosive gas/air mixture is continuously present or presentfor long periods.

    It is the most potentially hazardous area of any plant and is usually restricted to

    very small areas of the plant, for example within the void space of tanks containing

    volatile flammable liquids.

    7.4. Zone 1

    An area in which an explosive gas/air mixture is likely to occur in normal operation

    for example. where gas may be vented to atmosphere.

    7.5. Zone 2

    An area in which an explosive gas/air mixture is not likely to occur in normal

    operation but if it does occur it will be for a short duration.

    It must be realised that ventilation, specifically natural ventilation, can cause

    these potentially dangerous gas/air mixtures to migrate into adjacent safe areas,

    i.e., those not pressurised.

    Therefore, the Authorised Gas Tester must be aware of this and should consider

    testing these adjacent areas.

    In general the majority of the total area of an installation will be designated Zone 2.

    7.6. Gas Testing in Support of Work Activities

    The function of gas testing is to ensure that the site is clear of toxic/flammable gases and to reduce the risks

    associated with work activities.

    In any potentially flammable or toxic area or whenever a gas risk may exist at the worksite, the applicable

    signatory in accordance with the Permit to Work System will indicate that gas testing is required.

    7.7. The Permit to Work System

    The applicable signatory will advise a nominated Authorised Gas Tester on the Work Permits where a gas test is

    required.

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    In some cases before a Work Permit is issued an initial gas test MUST be made at the worksite. This test must be

    carried out by an Authorised Gas Tester. The results of this gas test must be recorded on the Work Permit or

    Entry Certificate. The Authorised Gas Tester will sign to signify that all gas tests made are within specified limits.

    Subsequent gas tests may be required at intervals after the initial gas tests. The applicable signatory will identify

    any additional requirements related to the frequency of the gas tests. The results of these tests are also recorded

    on the Work Permit or Entry Certificate.

    Gas tests where specified, must be repeated at the time of Work Permit re-validation and when the site is

    reoccupied after a major work break.

    7.8. Continuous Gas Monitoring

    The normal requirement following the initial gas test will be continuously monitored

    during the work activity.

    These tests are additional to the requirements for initial, re-validation and

    subsequent gas tests involving an Authorised Gas Tester.

    Where continuous monitoring is required, this shall be carried out using a portableor personal gas detector positioned adjacent to the job.

    These detectors will normally be left with the Person in Charge of the Worksite, who can appoint a Competent

    Person to continuously check the gas detector readings during the work.

    7.9. Practical Gas Testing

    The area for gas testing must be as wide as is necessary to identify any possible hydrocarbons or other process

    gases that may be present. Take as many readings from as many places as possible until confidence is achieved

    that the samples taken are truly representative of the whole area. When evaluating the conditions for gas testing

    the Authorised Gas Tester needs to be aware of the hazardous area conditions, which may prevail during normal

    operations.

    The Authorised Gas Tester should also be aware of the effects caused by:

    Open doors to the module

    Temperature / heat barriers at roof level

    Stagnant or flowing air patterns around the work site

    Movement of sands or sludge which may contain trapped pockets of gas

    In any area containing a potential source of gas release, fixed and portable detectors should, as a minimum,

    monitor work locations that are detailed on the Work Permit.

    The Authorised Gas Tester should also consider testing the following areas:

    In spaces near walls and large vessels

    In spaces where circulating air currents can pick up gas Close to potential sources of gas release for example, near vessels, hydrocarbon systems, voids and vents

    At the end of exhausts, in flues and service ducts

    Near drains and liquid surfaces

    And, where air movements may be negligible and gas could collect in clouds, for example in very congested

    areas

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    7.10 Air Movement

    In any process module there may be stagnant areas of little or no air movement and the Authorised Gas Tester

    should be aware that build up of gas in these areas may be possible.

    When carrying out gas testing, the Authorised Gas Tester should commence tests upwind of the work area to a

    minimum radius of 5m from the work site, working in towards the risk spot and around all potential sources of

    release.

    This test pattern will account for air movements around the area caused by a HVAC system, any prevailing wind

    etc. A thorough gas test of the complete area will involve testing above head height and at floor level to give the

    best possible chance of detecting gases with different relative densities.

    7.11 Evaluation

    Awareness of the hazardous area conditions which may prevail during normal operations assists the Authorised

    Gas Tester in pre-test site evaluation.

    The Authorised Gas Tester should also be aware of the effects caused by:

    open doors to modules

    temperature/heat barriers at roof level stagnant or flowing air patterns around the work site

    7.12 HVAC Flow Paths

    Openings from modules may provide differing air patterns from the HVAC flow paths and may give the

    impression of positive pressure within a module.

    The Authorised Gas Tester must be aware of air movement around the area of any potential gas hazard and

    ensure work site precautions specified and implemented address potential gas hazards.

    Within roof void areas and at roof level, a heat barrier is created which can prevent migration of gas or vapours

    to the fixed gas detectors at that level.

    The Authorised Gas Tester should also be aware that no alarm signals from these detectors does not necessarily

    indicate a no gas situation.

    If the HVAC system is operating correctly, detectors at roof level may not reach alarm situation due to dilution

    and dispersion of any release of gas plumes.

    7.13 Remember, the main points of gas testing are:

    Test area - 5m radius around work site

    Commence upwind of work site

    Take nothing for granted

    If in doubt restart test

    Identify potential hazards within the area and inform the applicable signatory of these hazards

    Never sign the authorised paperwork until you are 100% certain of full compliance with Work Permit

    requirements

    Always look, listen, advise and report

    Where any indication of gas is detected, do NOT assume it is harmless just because it is within the acceptable

    limit, check further

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    8. NITROGEN PURGING

    8.1. Purging

    Purging is required when there is a need to replace an active atmosphere with an inert one - for example, to

    prevent fire or explosion where flammable materials, such as hydrocarbon gases, come into contact with air

    during process operations.

    In the majority of cases the purging gas used is nitrogen, which is chemically un-reactive and inert towards a

    wide range of materials

    Purging may be direct or indirect.

    8.2. Direct Purging

    Direct purging involves purging directly from gas to air or air to gas.

    Indirect purging is the displacement of a flammable gas, for example natural

    gas, by an inert gas, followed by displacement by air or vice versa. In

    other words, an inert gas is sandwiched between the flammable gas and

    the air, preventing them from mixing and forming a flammable gas mixture.

    8.3. Indirect Purging

    Methods of indirect purging include:

    Displacement Purging:

    This involves purging the vessel or pipeline with nitrogen to a specified end point. The procedure is similar to

    direct purging, the difference being that the nitrogen separates the air from the natural gas, preventing the

    formation of a flammable-gas-in-air mixture.

    Slug Purging:

    Slug purging of linear or nearly linear structures usually uses less purge gas. A slug of purge gas is inserted as a

    barrier between the flammable gas and air. The purge should be continuous and the purge velocity shouldremain high to prevent stratification or layering of gas.

    Pressure Purging:

    Pressure purging involves successive pressurisation and de-pressurisation of a vessel until an acceptable end

    point is reached. For successful pressure purging, thorough mixing of the gases in the vessel is necessary.

    The process of purging when decommissioning and re-commissioning a vessel can be summarised as follows.

    When decommissioning, the vessel containing hydrocarbons is flushed and then purged using nitrogen. Air is

    then blown through the vessel and the nitrogen vented out to atmosphere or flared off.

    When re-commissioning the vessel, nitrogen is flushed through before the hydrocarbon is re-introduced,preventing the formation of a flammable-gas-in-air mixture.

    8.4. Nitrogen Hazards

    While nitrogen itself is not toxic, its use can present certain hazards:

    In liquid form, nitrogen can cause frostbite and crack equipment

    A nitrogen-rich atmosphere can disrupt equipment designed to operate using the oxygen in the air, such as

    explosimetres or gas detectors

    The greatest hazard however, is that in a confined space, nitrogen displaces oxygen, which reduces the

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    amount of oxygen available for breathing

    While reduced concentrations of oxygen are not immediately life threatening, they can affect behaviour and

    judgement, thereby inhibiting good decision making

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    9. CASE STUDIES

    9.1. The Wrong Way to do it!

    A tank that had contained light hydrocarbons was to be worked on.

    It had not been properly cleaned before work started.

    It had been filled with water and then emptied.

    Some hydrocarbons remained in the crevices.

    The results were as follows:

    No tests were made for combustible gas

    A welder working near the vent accidentally ignited the vapour

    6 people were killed and 29 injured

    The correct procedures for tank cleaning were not in place or had not been followed.

    The Permit to Work Procedure should have initiated gas testing before welding started.

    Continuous gas monitoring should have occurred around the tank outlets (vents).

    This accident was totally avoidable.

    9.2. The Right Way to do it

    An old gas line had been out of use for 12 years.

    It had to be modified for re-use.

    For the previous two years it had been blanked at one end and open at the

    other.

    A flange was welded on the open end without incident.

    The next job was to fit a 1" branch 60 metres from the open end. A hole was drilled in the pipe and a gas test

    proved negative.

    A few hours later, immediately before welding, a repeat gas check indicated the presence of flammable gas. It is

    believed that the gas was in the pipe for 12 years and started migrating when the hot work caused a rise in

    temperature.

    It may seem reasonable to assume that a line, redundant for 12 years, would not require a gas test. Fortunately

    the people involved here took a different view.

    They followed a procedure applying to all pipes. They assumed nothing - hence the repeat gas test after the

    delay.

    Note:

    You should also be aware of other risks, which may be present. For instance you should be aware of LSA scale and / or

    pyrophoric scale in susceptible areas.

    REMEMBER NEVER MAKE ASSUMPTIONS - ALWAYS CHECK IF IN DOUBT-THEN CHECK AGAIN

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    10. GLOSSARY

    Gas

    A fluid substance, which is neither a solid nor a liquid at ordinary temperatures, and has the tendency to disperse

    when not contained.

    VapourGaseous form of a normally liquid or solid substance (above boiling point)

    Liquid

    Any substance with the tendency to flow which is neither a gas nor a solid

    AGT

    Authorised Gas Tester

    CH4

    Methane

    H2SHydrogen sulphide

    HVAC

    Heating, Ventilating and Air-conditioning

    LEL

    Lower Explosive Limit

    WEL

    Workplace Exposure Limit

    UELUpper Explosive Limit

    ppm

    parts per million