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AUTHORISED GAS TESTER TRAINING COURSE NOTES1) Properties of Flammable and Toxic Gases 1.1 Flammable Gases KEY DATA Most natural gases are rich in methane. Most portable gas detectors are therefore calibrated for methane Other flammable gases / vapours found in the oil and gas industry include the LPG gases, propane, butane, hydrogen acetylene and methanol. Flammability of Methane:Lower Explosive Limit (LEL) of methane is 5% by volume i.e. 5 vols of methane, 95 vols of air. This is the minimum quantity of methane in a methane/air mixture that will just ignite to produce a self-propagating flame if exposed to a hot source i.e. cigarette, match or spark from electrical equipment The Upper Explosive Limit (UEL) of methane is 15% by volume, ie 15 vols methane, 85 vols air. This is the maximum quantity of methane in a methane/air mix that will ignite. Above 15% methane there is insufficient oxygen in the remaining air to support combustion. Flammable range for methane is therefore 5% 15% by volume methane in air Portable flammable gas detectors used in the oil industry have a working range of 0 - 5% methane in air scaled as 0 - 100% LEL on the meter. Gas detectors are usually calibrated with 1% or 2.5% methane in air - to give a reading of 20% or 50% LEL respectively ADDITIONAL INFORMATION Range of methane in North Sea NG is about 50 90%. In some areas of gas separation - greater quantities of the condensate gases eg. Pentane to octane gases and liquids are found Hydrogen from battery charging, acetylene from cutting equipment, methanol used in pipeline pigging The correct expression is now Lower Flammable Limit (LFL) but most gas detectors and paperwork still use the terms LEL and UEL Ignition occurs if the hot source has a temperature higher than 530 C. 5% methane in air produces a lean combustion. 15% methane in air produces a rich combustion. Above 15%, a methane gas cloud is potentially very hazardous as dilution with air will produce a flammable gas/air mixture The relationship between % gas vol. and % LEL is 1% methane in air = 20% LEL 2% methane in air = 40% LEL 3% methane in air = 60% LEL 4% methane in air = 80% LEL 5% methane in air = 100% LEL (the concentration at which the gas mixture will ignite) Note that these figures apply to normal temperatures, high temperatures lower the LEL and raise the UEL Density of Flammable Gases Pure gases are either:a) lighter than air b) neutral in density c) heavier than air Pure methane is lighter than air, about half the weight of air with a density of 0.55, relative to Air = 1 . Other examples:- (Relative to Air = 1) Hydrogen - much lighter, relative density = 0.07 Ethane - neutral, relative density = 1 Propane - heavier, relative density = 1.5 Butane - heavier, relative density = 2 Carbon dioxide - heavier relative density = 1.5
. High pressure gas releases into the atmosphere are Here is a calculation of a tenfold dilution accompanied by enormous turbulence which causes ie:CONCN DENSITY dilution of the gas cloud. At typical offshore Methane 10% or 0.1 x 0.5 = 0.05 pressures 80 -150 bar, tenfold dilution (or more) Air 90% or 0.9 x 1.0 = 0.9 will be common. This generates a gas cloud which Relative Density of gas cloud = 0.95 is neutrally buoyant. A neutrally buoyant gas cloud may be more influenced by effects such as ventilation and air movement than a very small density effect. This factor acquires importance when searching for gas leaks Any gas cloud with a density of 0.9 - 1.1 relative to air will show few density effects. When gas testing, you will need to allow for the importance of air movement when searching for gas leaks and the influence it has on a neutral density but highly flammable gas cloud Condensate Gases, Propane, Butane and Solvent Vapours These heavier-than-air materials produce dense vapours gas clouds that obey the rules of density. The vapours flow like liquids and accumulate in low-lying areas and plant.
The vapour density relative to air of all these materials is high, ie they are naturally heavier than air materials, in addition, the mechanism of changing from liquid to vapour is relatively slow compared to a H.P. gas release. Dilution due to turbulence is therefore a minor factor and vapour remains concentrated and dense.
An expression used with some of the less volatile liquids is the Flash Point: this is the lowest temperature at which a flammable liquid will form a flammable vapour cloud. In summary, a flammable gas cloud can therefore be: (a) Lighter-than-air (b) Neutral in density (c) Heavier-than-air and when gas testing full allowance should be made for density effects. 1.2) Oxygen and Toxic Gases Many detectors with digital displays just give a number. It is important to understand the units of measurement. For Oxygen - 20.9% is the amount of oxygen in the atmosphere. Remember that for Flammable Gases (like methane) %LEL is the unit most frequently used. 5% Methane = 100% LEL 4% Methane = 80% LEL 2
The atmosphere contains 20.95% Oxygen and 78% Nitrogen
3% Methane = 60% LEL 2% Methane = 40% LEL 1% Methane = 20% LEL For Toxic Gases (like hydrogen Sulphide) ppm (parts per million) is the unit used , for example: 10ppm = 0.001% 1000ppm = 0.1% 1.2.1) Hazards of Toxic Gases The permissible concentration of gases is defined in HSE document EH40/2002, this defines for gases like hydrogen sulphide and carbon monoxide the following Occupational Exposure Standards (OES) a) Long Term Exposure Limit - (LTEL) has an 8 hour time weighted average exposure period and for hydrogen sulphide = 5ppm; for carbon monoxide = 30ppm b) Short Term Exposure Limit - (STEL) has a 15 minute time weighted average and for hydrogen sulphide = 10ppm; for carbon monoxide = 200ppm Other toxic gases and vapours found offshore include paint strippers, solvents and degreasers 1.2.2) Oxygen Hazards The concentration of oxygen in the atmosphere needs to be 20.9% by volume. If it is less than 20.9% it is hazardous, the atmosphere is oxygen deficient and breathing apparatus is needed for less than 19%. For a thorough understanding read HSE EH40 / 2002 - it is reprinted each year. In broad terms:
The higher hydrocarbon gases, eg Butane, Pentane, Hexane - have OESs at concentrations well below flammable limits
If it is more than 20.9% it is also hazardous; oxygen enriched atmospheres can create a feeling of nauseous intoxication and euphoria which prevents rational thought and also represents a severe fire risk 1.2.3) Hydrogen Sulphide Highly toxic gas, which at low concentrations, smells of bad eggs. The important concentrations to remember:0.15ppm - threshold of smell. 5ppm - the Long Term Exposure Limit, which means that medical opinion (EH40) believes no harm will come to you during or after repeated 8hour daily exposures. 10 ppm the Short Term Exposure Limit 100ppm - you lose all sense of smell in 3 - 15 minutes. 200 - 300ppm - you lose your sense of smell 3
Any combustion process in a confined space will reduce atmospheric oxygen. Rusting of steel in an enclosed space can quickly create an oxygen deficient atmosphere. Purging of vessels and pipe freezing operations can also create oxygen deficient atmospheres. Leaking welding torches in confined spaces can cause oxygen-enriched atmospheres.
Hydrogen Sulphide is almost as poisonous as hydrogen cyanide.
Hydrogen sulphide can be produced from stagnant sea water
Note that although pure hydrogen sulphide is a heavier-than-air gas (density 1.2 relative to air), in
instantly, very hazardous and toxic concentration 1000 ppm - 1 or 2 deep breaths causes an almost instant loss of consciousness, immediate first aid is essential 1.2.4) Other Toxic Gases Carbon monoxide and carbon dioxide are both toxic gases that can be generated during combustion processes. Carbon dioxide is sometimes used as a deluge fire extinguishant for electrical fires - as a heavier- than-air gas it accumulates at low levels.
many natural gases it is very unlikely that concentrations will reach levels where they influence the overall gas density.
Incomplete combustion in a confined space is most likely to create carbon monoxide
2) GAS DETECTORS AND METHODS OF DETECTION Why is there a need for gas detectors: a) Most gases are invisible b) Most are odourless c) Hydrogen Sulphide, which smells, paralyses the sense of smell at hazardous concentrations. 2.1) Flammable Gas Detectors 2.1.1) Flammable Gas in Air Single range flammable gas in air detectors include:- Crowcon 84GA, Sieger 1601 and Gas Scout Multi range detectors suitable for measuring flammable gas in air include: Crowcon Custodian and Triple Plus Neotronics Minigas detectors; Sieger Gas Scout and Leader Principle of Operation - operates on a catalytic principle by burning the flammable gas on a specially treated hot catalytic filament and measuring the temperature rise of the filament caused by the combustion Limitations:a) needs oxygen to function and therefore cannot be used to detect flammable gases in inert atmospheres. The detector will not give any indication and therefore fails-to-danger. b) detects most flammable gases and vapours - but is more sensitive to some gases than others. For example, a Sieger Gas Scout calibrated for methane will only indicate about half scale if exposed to butane in air at its LEL c) can only detect flammable gases in air at low concentrations - up to the LEL. Detectors will give false - ambiguous readings at high gas concentration 4
Most catalytic flammable gas detectors are broadspectrum devices ie they detect all flammable gases, with each substance in a complex mixture of gases contributing to the eventual meter reading. For example the Crowcon 84GA will detect: - Methane, ethane, propane, butane hydrogen, methanol and most other flammable materials, however: it can only be calibrated to be correct for 1 material and offshore we usually choose methane.
d) there are a number of failure modes which cause the detector to fail-to danger:i) poisoning by silicones and other materials. ii) sinter blockage by water, paint, oil, mud etc. these are most serious limitations of catalytic gas detectors. As both i) and ii) can be almost instantaneous under the wrong conditions, a daily exposure to a standard gas mixture eg 1 or 2.5% methane in air is strongly recommended 2.1.2) Flammable Gases in Inert Gas Instruments used by the oil industry for this application are the MSA Tankscope, GMI Oxygas 2 and the Neotronics Digiflamm 2000. They are used when purging flammable gases from pipes or vessels Principle of Operation - operates by measuring the heat conducting properties of gases. Methane and most natural gases are better conductors of heat than air. Nitrogen, the usual inert gas, is a poor conductor of heat. Limitations:a) Usually low sensitivity detectors - therefore not suitable for gas testing prior to hot work and confined space entry b) Non-selective - suitable for detecting binary mixtures only ie. Methane in nitrogen 2.2) Oxygen and Hydrogen Sulphide Detectors Instruments used by oil companies capable of detecting oxygen and hydrogen sulphide include:Crowcon 84TR, Neotronics Minigas, Draeger Oxywarn (oxygen only) and Sieger Gas Leader Principle of Operation - operates using an electrochemical cell (or gas-battery) where hydrogen sulphide or oxygen combines with a highly reactive chemical jelly. During this reaction a minute electrical signal is generated, which when amplified displays the amount of oxygen or hydrogen sulphide in the atmosphere. Limitations:a) like a battery the cell goes flat after 12 - 18 months depending on type of cell being used. b) cell lifetime is reduced by high operating temperatures (greater than 40C). 5
These most serious failure mechanisms produce a requirement in most Safety Cases that portable gas detectors are recalibrated at 28 day intervals
The Neotronics Digiflamm 2000 is a 2 range detector:i) 0-5% methane in air using the catalytic principle. ii) 0-100% gas by volume using the thermal conductivity principle This heat conducting or thermal conductivity principle is a most mis-understood subject. Special calibration gases are needed which should include:a) Pure inert gas, usually nitrogen and the detector should be zeroed with this b) A mixture of methane in inert gas, 10%, 20% or 50% methane in nitrogen is used for calibrating the detector. 2 mixtures will confirm linearity.
Oxygen and toxic gas detectors are generally selective, unlike catalytic flammable gas detectors.
Like most other gas detectors this can be a fail-to-danger decay mode and cells need regular replacement If subjected to high temperatures, particularly if cycled from low to high temperatures there is a risk of cell leakage - and the
c) pressure pulses can cause false alarms, particularly with the Crowcon Triple +, take care not to restrict the sample tube inlet whilst aspirating a gas sample. d) portable radio transmitters can cause false alarms, cure by moving detector away from aerial, to the other side of your body. Calibration of Hydrogen Sulphide Detectors Monthly recalibration with the manufacturers recommended test gas containing ppm concentrations of hydrogen sulphide in nitrogen or air is required
cell jelly is highly acid or alkaline depending on type of cell This effect is sometimes observed when entering or leaving a pressurised area, but more frequently when aspirating a gas sample through the Triple +, the back pressure generated during aspiration triggers the alarm. This problem is caused by the very low output signal of the electrochemical cell, requiring a very high gain amplifier which is susceptible to interference Whilst flammable gas in air test mixtures are stable for prolonged periods of time, reactive toxic gases, like hydrogen sulphide rapidly react with their containing vessel, for example, a 20 ppm mixture of hydrogen sulphide in air contained in an aerosol type aluminium can will disappear in 3-4 months therefore do not hold large stock of reactive test gases
Oxygen detectors should indicate about 20.9% on their readout. Always confirm suspect readings by comparing with known fresh air. 3) USING STAIN TUBE GAS DETECTORS Principle of Operation Coloured crystals in a narrow glass tube marked with calibrated graduations, which chemically change colour when exposed to certain toxic (and non-toxic) gases. The amount of colour change indicates the approximate quantity of toxic gas present. Limitations:a) The detector monitors an atmosphere only while the bellows is expanding, it is not a continuous monitor like many gas detectors b)It is a selective gas detector like most toxic gas detectors, you therefore need to know the substance you seek and select the correct tube. c) Confirm that the concentration level you need to detect can be achieved with the tube you have chosen. d) The chemical tubes have a shelf life - about 2 or 3 years; the use-by date is printed on the box. e) You must not use the oxygen tube for permitto- enter gas tests, as the accuracy of the tube is not good enough. f) Hydrogen detector tubes must not be used in areas that could contain hydrogen !!- a sampling bladder method must be used, and the test done in a safe area - as the tube becomes very hot and 6
As a point of general interest Draeger were the manufacturers of the first alcohol-in-breath device to be used in the UK - the Alcotest.
The American expression is grab sampling.
Reference to HSE document EH/40 will tell you OESs to comply with the COSHH regulations Never use expired tubes and keep stock inventories to a sensible level. Refer to the Draeger, Kitagawa or Gastec Detector Tube Handbook, If there is more than 3% hydrogen in the atmosphere the catalytic chemical layer becomes red-hot!
could become a source of ignition 3.1) Testing Procedure a) Compress bellows fully to observe the overall well being of the bellows unit. b) Insert any tube - before breaking its sealed ends - into the bellows, compress the bellows and observe upon release that the bellows do not expand. Any expansion indicates leakage c) Break both fused ends of your chosen tube using a tube breaker and insert into bellows with sample flow arrow pointing towards the bellows d)Hold unit in atmosphere to be monitored and compress bellows fully, allow to expand naturally. Note that some tubes require one aspiration (n=1); other tubes require more than one, eg 10 aspirations (n=10).
A leaking bellows unit cannot be used at all and must be replaced . Caution, broken glass!
e)Take the whole unit to an area with good lighting - read and record the stain indication quickly, for some tubes the stain fades or diffuses making interpretation difficult f) Remove tube and dispose of in a safe manner. The Medics sharps bin is often used when offshore g) Purge acidic gases from the bellows by compressing a few times in clean air. Note The most frequent errors occur from:a) Inserting the tube wrong way round. b) Bellows leakage. 4) CERTIFICATION FOR HAZARDOUS ATMOSPHERES Principle All electrical equipment for use in potentially flammable industrial atmospheres needs to be certified as being safe to use such that it cannot become source of ignition should it be exposed to a flammable gas cloud. Electrically powered gas detectors are included. 4.1) Concepts of Safety Coding are used to define methods of making gas detectors safe to use in a flammable atmosphere. The most common are as follows:Ex ia and Ex ib = Intrinsic Safety Ex d = Flameproof Ex s = Special (a test for the sinter) A, B or C = The Gas Group T = Temperature rating These symbols are used together for the certification label of the gas detector. 7 Refer to BS EN 5501 for detailed information Acid vapours from the tubes will corrode the internal precision valve.
4.2) Important Points to Remember a) It is important to understand that the certification label is always on the outside surface of the certified product. Some gas detectors need a leather case around the instrument for certification, hence the label is on the outside surface of the leather case. If you take the detector out of the case, it becomes an uncertified product! b) Although certified gas detectors do not have to be sent back to their original manufacturer for repair (many are, of course) the user needs to ensure that only knowledgeable and competent technicians repair gas detectors. 5) USING PORTABLE AND TRANSPORTABLE GAS DETECTORS 5.1) Detector Choice Most detectors can be used as diffusion samplers or with an aspirating system. sampling systems are needed for leak seeking, Personal preference and availability frequently diffusion sampling is adequate for area dictates which detector is used for which monitoring application 5.2)Preparing the Detector for Use a) Refer to the Manufacturers booklet regarding switch-on b) If you are not familiar with a particular detector, dont use it until the Safety Department have given you instructions in its correct use. c) Following the manufacturers booklet, confirm that the calibration due date has not expired, that the batteries are OK and that the detector passes other diagnostic tests associated with switch-on. d) If the detector is to be used with a sampling system, confirm that the detector and flow system is dry and that:i) the aspirating bulb non- return valve does not leak. ii) the sample tubing is of adequate length and not perished or cracked iii) the overall system does not leak - a test with finger over the end of the sample tube, squeeze the aspirator and release, the bulb should not refill or refill only slowly 8 Use only the special gas detector sample tubing bought from a manufacturer. Some tubing strongly absorbs the higher hydrocarbons This will always activate the oxygen alarm on the Crowcon Triple Plus caused by the negative pressure pulse. Note that the sampling system with the Sieger 1650 is fairly leaky. BS EN 50073 1999 is a harmonised CENELEC Code of Practice about using fixed and portable flammable gas detectors An external case is needed for some detectors to prevent the danger of:a) The Thermite reaction, when a light alloy (aluminium) gas detector is dropped on rusty metal work - produces a white-hot spark (ignition source). b) a static charge developing on large plastic cased detectors under friction and producing a static spark ignition hazard. It is important to understand that a cowboy repair can infringe a certificate of intrinsic safety and turn a safe gas detector into a potential ignition source
Remember, no matter what the Manufacturers publicity says - all catalytic flammable gas detectors fail-to-danger, none are fail- safe; this is why the daily gas test is so important.
Remember that a leaking sampling system could cause you to under read a toxic or flammable gas atmosphere and over-read an oxygen deficient atmosphere. e) With the instrument switched on in clean air, confirm that the display reads correctly ie close to zero for flammable and hydrogen sulphide gas detectors and close to 20.9% for oxygen detectors. f) For catalytic flammable gas detectors confirm correct functioning by aspirating or subjecting the detector to standard test gas, either 1.0% or 2.5% methane in air. For 1.0% methane in air, the reading should be 20% LEL; for 2.5% Methane in air the reading should be 50% LEL Gas detector zeros tend to drift with time and ambient temperature. Detectors which have drifted a small amount - say up to 3 or4 digits may still be used but confirm by checking in clean air the absence of a hazardous atmosphere A tolerance is necessary to allow for overall system inaccuracies for example : 20% LEL - 18 to 22 % LEL is OK 50% LEL - 45 to 55 % LEL is OK - the 28 day re-calibration will correct any small changes in detector sensitivity Important! If a detector does not pass this test dont use it! Return it to the issuing authority g) If a detector does not respond correctly to any mechanical or electrical test, it should be returned to the issuing authority
6) GAS TESTING 6.1) Hot Work and Purge Gas Testing APPLICATION 1 - Hot Work Close To The Worksite Detecting small gas leaks on plant and equipment in the immediate vicinity of where hot work is to take place. APPLICATION 2 - Hot Work Around The Worksite General atmosphere monitoring searching for leaks generally around a planned site of hot work, particularly upwind of the work site. Upon completing the leak search, the monitor is correctly positioned upwind of the hot work site as a gas sentinel to monitor for an approaching gas cloud.
APPLICATION 3 - Purging Applications Monitoring methane in inert gas mixtures when purging operations are being undertaken. 9
6.1.1) Which Detector to Use APPLICATION 1 - Hot Work Close To The Worksite Inside the habitat or close to where hot work is to be undertaken even small gas leaks are important! The ignition of a small gas leak can lead to overheating and escalation. To undertake this gas test a gas detector fitted with an aspirating sampling system and sample tubing is ESSENTIAL. Flanges, glands, valves, compression fittings, pumps, compressors and other rotating machinery, which are frequent sources of leakage cannot be leak tested in an effective manner with a diffusion sampling system. Detectors used by the oil and gas industry include: (a) Crowcon 84TR Triple and Triple Plus with aspirating system fitted (b) Neotronics Minigas with aspirating system fitted (c) Sieger Gas Scout and Leader with aspirating system fitted (d) Crowcon 84 GA with aspirator system fitted APPLICATION 2 - Hot Work Around The Worksite Generally around the hot work site, ie outside the Habitat or distances greater than twice the distance sparks from hot work could travel, a more global approach to gas testing can be taken. At these distances small gas leaks have less significance, as they would be so diluted by ventilation air as to be undetectable at the hot work site. For this application, an aspirated sampling system is a disadvantage and cannot be used at all for gas sentinel duties. For this application use one of the following instruments without aspirating fitments eg: (a)Sieger Gas Scout or Leader without aspirating fitments (b) Crowcon 84TR Triple or Triple Plus without aspirating fitments (c) Sieger 1601 - the Dalek; or Crowcon Detective APPLICATION 3 - Purging Applications When hot work is to be undertaken on a pipe or vessel containing methane, the following procedure is adopted:(a) The pipe or vessel is depressurised. (b) The pipe or vessel is purged with low pressure inert gas, usually nitrogen, to remove the methane gas. (c) When the methane gas concentration in the pipe or vessel falls to a low level by volume in inert gas, it is safe to open up the pipe or vessel to the atmosphere. Note of caution - a concentration of less than 0.5% by volume will be necessary for some condensate gases containing higher hydrocarbons To undertake this gas test you CANNOT USE any of the catalytic detectors described in Applications 1 and 2. These detectors WILL NOT INDICATE CORRECTLY and under many circumstances WILL NOT INDICATE AT ALL! You must use the MSA Tankscope, Neotronics Digiflam 2000 or GMI Oxygas 2 on its % gas range (and for the Digiflamm - in its purge mode) 10
Remember that this instrument measures the heat conducting properties of the gas mixture thermal conductivity principle and it is desirable to ZERO the instruments with pure inert gas(usually nitrogen) and not air. The instrument should be calibrated with a mixture of methane in inert gas, suitable for the scaling of the chosen detector. REMEMBER - this principle CANNOT be used in Applications 1 and 2. Note that if air is used as zero gas, the detector will under-read the methane concentration by 1 - 2% vol 6.1.2) Practical Hot Work Gas Testing 1 Prior to issue all instruments used for Applications 1 and 2 will have been fitted with a freshly charged battery and given the daily gas test with 20% or 50% LEL methane in air - an instrument suitable for use will indicate between 18 - 22%LEL or 45 - 55% LEL respectively 2 Arrive at hot work site and determine the main direction of ventilation air, frequently difficult when in exposed external platform areas. REMEMBER - it is usually ventilation air which brings a flammable gas hazard to a hot work site. 3 Use a detector suitable for Application 1 within the Habitat and extend the area of search all around the hot work site on an item by item basis to a radius about twice the distance sparks or ignition sources from the hot work could penetrate - REMEMBER a platform is three dimensional test above and below floor, grating or mezzanine levels if appropriate 4 Having completed leak searching in the immediate vicinity of the proposed hot work extend the search as for Application 2 with an instrument suitable for that purpose. REMEMBER - in most cases it will be a gas leak upwind of the hot work site that will create a hazard at site, your prime area of search is therefore a cone-shaped volume of air upwind of the site. Whilst the prime search area is upwind of the hot work site DO NOT FORGET to check briefly downwind, PARTICULARLY if condensate gases or other heavier-than-air gases could be present. Finally, locate the detector as a gas sentinel a few metres upwind of the hot work site, directly in the ventilating airflow. Use your own common sense to locate the gas sentinel in a. location where it would detect an approaching gas cloud! In exposed areas of a platform with random winds, and when there is no obvious ventilation air, gas testing cannot concentrate on a cone-shaped prime search area and more general area testing is necessary. 6.1.3) Important Points in Hot Work Gas Testing 1 Although pure methane is lighter than air most gas clouds formed from high pressure leaks will be neutrally buoyant - the same density as air - due to dilution. 2 Condensate leaks form flammable gas clouds that are always heavier than air - think heavy, search low! - drains, ducts, bund areas and remember, condensate leaks can travel a long way and their vapours will flow like liquids!
3 Silicones poison/deactivate the catalytic sensors used for all Applications 1 and 2 almost instantly - the daily gas test with standard test gas is therefore ESSENTIAL. 4 Water can block the flame arrestor (often called the sinter) on the Crowcon 84GA and Sieger 1601 very easily and quickly. A blocked sinter will cause a detector to indicate zero, even if hazardous concentrations of gas are present. 11
6.2) Vessel Entry Testing Procedure 6.2.1) Detector Choice Instruments used by many that can be used for vessel entry gas testing include:- Crowcon 84TR and Triple Plus, Neotronics Minigas, Sieger Gas Leader and Draeger chemical tubes Note that the above electronic instruments can be used alone for confined space entry gas testing only if it known that other toxic materials ie. solvents, are not present; if other materials could be present then Draeger chemical tube tests will also be necessary 3.4.2) Testing Procedure Rule No. 1 - the gas tester remains outside the vessel at all times while performing the initial gas tests! You will therefore be using a Minigas, Leader Crowcon 84 TR or Triple Plus with an aspirating bulb and tube fitted to confirm that: The oxygen content is correct There is no flammable gas present There is no indication for whichever toxic gas(es) the detector can sense While testing also consider each of the following:a) In an enclosed vessel there is little or no ventilation and gases will therefore tend to display their density characteristics. You will therefore need to search at different levels of the vessel - a representative sample. b) What is the history of the vessel and what did it last contain and perhaps the time before? Could there be heavy residues that give off flammable or toxic gases when disturbed during entry? Will your gas detector measure these materials at toxic levels or do you need another detector, eg Draeger chemical stain Tube. c) Look inside the vessel! (With an approved torch if necessary) - are there any hidden areas where gases could be trapped, you may need to extend the detector sampling line. d) Are the internal walls of the vessel corroded, if so the corrosion, eg rust, could be trapping hydrocarbons which could be released into the vessel during its removal or hot work. e) Are there any sludges which when disturbed by your boots could evolve toxic vapours? heavy hydrocarbon vapours from oil based sludge
Vessel-entry gas testing is usually undertaken by Authorised Gas Testers with the gas test being a small part of a detailed procedure which comprises:1) - Pre Entry Preparation a) Isolation of the vessel by total isolation, blocking, blinding or spading. b) Cleansing to the satisfaction of the Area Authority 2) - Vessel Opening in the presence of the A.G.T. or Safety Tech. a)Atmosphere testing for oxygen, hydrocarbons and other known, (toxic) contaminants ie H2S, solvents, heavy hydrocarbon sludges - WITHOUT entering the vessel - all under a Preparation/Reinstatement Certificate 3) - Worksite Precautions
a) Workforce leader nominates the Safety watch or Buddy and informs him of his duties.
b) Worksite authority shall ensures ventilation aids are correctly installed, access is clear, entry tag system is correctly used, and that all precautions associated with welding equipment and hot work in confined spaces are in place.
H2S from stagnant water based sludges In summary, the confined space entry gas tester needs to be a detective looking for the clues that suggest a potentially hazardous atmosphere 6.2.2) When Do You Test? a) Whilst preliminary testing can be done at convenience, a final authorising test must be done immediately before entry is made b) The Work Permit will state how frequently re-testing needs to be undertaken but it is normal practice to have a gas detector acting as a continuously monitoring sentinel or Worksite monitor inside the confined space while the work progresses. c) Many factors will influence the frequency of re-testing eg: i) Change in atmospheric conditions or wind direction. ii) The nature of the work being undertaken iii) Changes in the area classification. iv) Stopping and re-starting of work.
Gas testing is a serious business, your life and the well-being of your workmates will be decided by your actions - do it right the 1st time - - you may not get a 2nd chance!
7. Confined Spaces - Definitions and Hazards Confined Spaces can be deadly On average, work in confined spaces kills 15 people every year in the U.K. across a wide range of industries (about 5% of industrial deaths), from those involving complex plant through to simple storage vessels. In addition, a number of people are seriously injured. Those killed include not only people working in confined spaces but those who try to rescue them without proper training and equipment. What is a confined space? It can be any space of an enclosed nature where there is a risk of death or serious injury from hazardous substances or dangerous conditions e.g. lack of oxygen. Some confined spaces are fairly easy to identify, e.g. enclosures with limited openings: Storage tanks Vessels Pits, wells and voids Enclosed drains Sewers. Others may be less obvious, but can be equally dangerous, for example: Open-topped chambers Vats Combustion chambers in furnaces etc. Duct work Unventilated or poorly ventilated spaces
It is not possible to provide a comprehensive list of confined spaces. Some spaces may become confined spaces when work is carried out, or during their construction, fabrication or subsequent modification. What are the dangers from confined spaces? Dangers can arise in confined spaces because of: 14
A lack of oxygen This can occur: Where there is a reaction between metalwork and the oxygen in the atmosphere Following the a nitrogen purging operation In ships holds etc as a result of the cargo reacting with oxygen inside the space Inside steel tanks and vessels when rust forms.
Poisonous gas, fume or vapour These can: Build-up in sewers and manholes and in pits connected to the system Enter tanks or vessels from connecting pipes Leak into vessels and drains
Liquids and solids which can suddenly fill the space, or release gases into it, when disturbed. Free flowing solids which can also partially solidify or 'bridge' in silos causing blockages, which can collapse unexpectedly. Fire and explosions e.g. from flammable vapours, excess oxygen etc. Residues left in tanks, vessels etc, or remaining on internal surfaces, which can give of gas, fume or vapour. Dust may be present in high concentrations. Hot conditions leading to a dangerous increase in body temperature.
Some of the above conditions may already be present in the confined space. However, some may arise through the work being carried out, or because of ineffective isolation of nearby plant, e.g. leakage from a pipe connected to the confined space. The enclosure and working space may increase other dangers arising through the work being carried out, for example: Machinery being used may require special precautions, such as provision of dust extraction for a portable grinder, or special precautions against electric shock Gas, fume or vapour can arise from welding, or by use of volatile and often flammable solvents, adhesives etc If access to the space is through a restricted entrance, such as a manhole, escape or rescue in an emergency will be more difficult.
What the law says You must carry out a suitable and sufficient assessment of the risks for all work activities for the purpose of deciding what measures are necessary for safety (The Management of Health and Safety at Work Regulations 1999). For work in confined spaces this means identifying the hazards present, assessing 15
the risks and determining what precautions to take. In most cases the assessment will include consideration of: The task The working environment Working materials and tools The suitability of those carrying out the task Arrangements for emergency rescue. If your assessment identifies risks of serious injury from work in confined spaces, such as the dangers highlighted above, the Confined Spaces Regulations 1997 apply. These regulations contain the following special duties: Avoid entry to confined spaces, if at all possible, e.g. by doing the work from outside If entry to a confined space is unavoidable, follow a safe system of work; and Put in place adequate emergency arrangements before the work starts
Avoid entering confined spaces You need to check if the work can be done another way so that entry or work in confined spaces is avoided. Better work planning or a different approach can reduce the need for confined space working. Ask yourself if the intended work is really necessary, or could you: Modify the confined space itself so that entry is not necessary; Have the work done from outside, for example: Blockages can be cleared in silos by use of remotely operated rotating flail devices, vibrators or air movers; Inspection, sampling and cleaning operations can often be done from outside the space using appropriate equipment and tools; Remote cameras can be used for internal inspection of vessels.
Safe systems of work If you cannot avoid entry into a confined space make sure you have a safe system for working inside the space. Use the results of your risk assessment to help identify the necessary precautions to reduce the risk of injury. These will depend on the nature of the confined space, the associated risk and the work involved. Make sure that the safe system of work, including the precautions identified, is developed and put into practice. Everyone involved will need to be properly trained and instructed to make sure they know what to do and how to do it safely. The following checklist is not intended to be exhaustive but includes many of the essential elements to help prepare a safe system of work. Appointment of a supervisor Supervisors should be given responsibility to ensure that the necessary precautions are taken, to check safety at each stage and may need to remain present while work is underway. 16
Are persons suitable for the work? Do they have sufficient experience of the type of work to be carried out, and what training have they received? Where risk assessment highlights exceptional constraints as a result of the physical layout, are individuals of suitable build? The competent person may need to consider other factors, e.g. concerning claustrophobia or fitness to wear breathing apparatus, and medical advice on an individual's suitability may be needed. Isolation Mechanical and electrical isolation of equipment is essential if it could otherwise operate, or be operated, inadvertently. If gas, fume or vapour could enter the confined space, physical isolation of the pipework etc needs to be made. In all cases a check should be made to ensure isolation is effective. Cleaning before entry This may be necessary to ensure fumes do not develop from residues etc while the work is being done. Check the size of the entrance Is it big enough to allow workers wearing all the necessary equipment to climb in and out easily, and provide ready access and egress in an emergency? For example, the size of the opening may mean choosing air-line breathing apparatus in place of self-contained equipment which is more bulky and therefore likely to restrict ready passage. Provision of Ventilation You may be able to increase the number of openings and therefore improve ventilation. Mechanical ventilation may be necessary to ensure an adequate supply of fresh air. This is essential where portable gas cylinders and diesel-fuelled equipment are used inside the space because of the dangers from buildup of engine exhaust. Warning: carbon monoxide in the exhaust from petrol-fuelled engines is so dangerous that use of such equipment in confined spaces should never be allowed. Testing the air This may be necessary to check that it is free from both toxic and flammable vapours and that it is fit to breathe. Testing should be carried out by a competent person using a suitable gas detector which is correctly calibrated. Where the risk assessment indicates that conditions may change, or as a further precaution, continuous monitoring of the air may be necessary. Provision of special tools and lighting Non-sparking tools and specially protected lighting are essential where flammable or potentially explosive atmospheres are likely. In certain confined spaces (e.g. inside metal tanks) suitable precautions to prevent electric shock include use of extra low voltage equipment (typically less than 25 V) and, where necessary, residual current devices. Provision of breathing apparatus This is essential if the air inside the space cannot be made fit to breathe because of gas, fume or vapour present, or lack of oxygen. Never try to sweeten the air in a confined space with oxygen as this can greatly increase the risk of fire or explosion. Preparation of emergency arrangements This will need to cover the necessary equipment, training and practice drills. Provision of rescue harnesses Lifelines attached to harnesses should run back to a point outside the confined space. Communications 17
An adequate communications system is needed to enable communication between people inside and outside the confined space and to summon help in an emergency. Check how the alarm is raised It is necessary to station someone outside to keep watch and to communicate with anyone inside, raise the alarm quickly in an emergency, and take charge of the rescue procedures Is a 'permit-to-work' necessary? A permit-to-work requires that a formal check is undertaken to ensure all the elements of a safe system of work are in place before people are allowed to enter or work in the confined space. It is also a means of communication between site management, supervisors, and those carrying out the hazardous work. Essential features of a permit-to-work are: Clear identification of who may authorise particular jobs and any limits to their authority and who is responsible for specifying the necessary precautions (e.g. isolation, and testing, emergency arrangements etc;) Provision for ensuring that contractors engaged to carry out work are included; Training and instruction in the use of permits; Monitoring and auditing to ensure that the system works as intended.
Emergency procedures When things go wrong, people may be exposed to serious and immediate danger. Effective arrangements for raising the alarm and carrying out rescue operations in an emergency are essential. Contingency plans will depend on the nature of the confined space, the risks identified and consequently the likely nature of an emergency rescue. Emergency arrangements will depend on the risks. You should consider: Communications How can an emergency be communicated from inside the confined space to people outside so that rescue procedures can start? Also, consider what might happen and how the alarm can be raised. Rescue and resuscitation equipment Provision of suitable rescue and resuscitation equipment will depend on the likely emergencies identified. Where such equipment is provided for use by rescuers, training in correct operation is essential. Capabilities of rescuers There need to be properly trained people, sufficiently fit to carry out their task, ready at hand, and capable of using any equipment provided for rescue, e.g. breathing apparatus, lifelines and fire-fighting equipment. Rescuers also need to be protected against the cause of the emergency. Shut down It may be necessary to shut down adjacent plant before attempting emergency rescue. First-aid procedures Trained personnel need to be available to make proper use of any necessary first-aid equipment provided. Relevant law The Confined Spaces Regulations 1997 The Management of Health and Safety at Works Regulations 1999 18
The Control of Substances Hazardous to Health Regulations 1999 The Personal Protective Equipment at Work Regulations 1992 The Provision and Use of Work Equipment Regulations 1998 Electricity at Work Regulations 1989 Workplace (Health, Safety and Welfare) Regulations 1992 Hot-Work Practical Gas Testing and Skill Assessment Conducted in an area containing valves, flanges, and other typical plant items - area should not be too noisy to facilitate giving instructions and discussion. 1. Assessor explains location and nature of hot work - e.g. Cutting, welding, etc. 2. Candidate chooses suitable detector which has a catalytic flammable gas-in-air range and is fitted with an aspirator and sampling tube assembly
3. Candidate inspects well-being of detector with confidence and with particular emphasis on: 3.1 sampling system leak-test - (both valves, whole system and tubing) 3.2 battery checks 3.3 reading in air 3.4 reading in Standard Test Gas 3.5 ..- and an understanding of what the readings should be 3.6 recognition of a faulty or otherwise non-conforming detector 3.7 reporting any fault condition to the Assessor 4. Assessor confirms that Candidate switches-on detector to the correct range and confirms zero reading in clean air before approaching hot-work site 5. Candidate shows confidence and competence in detailed gas testing of plant items eg. Flanges, valves, governors, cladding etc. immediately around the hot work site 6. Candidate becomes more general in search pattern and area covered when testing further away. Candidate shows awareness that a flammable gas cloud from another area of the plant will probably arrive at the hot work site in ventilation air masses and the detector is used to check these volumes of approaching air 7. If test site is in an area where condensate or other heavy vapours could be present, Candidate demonstrates awareness that vapours will be found in low-lying voids, spaces and drains 8. Candidate walks around perimeter of worksite gas testing drains, voids and localised air masses, noting any potential hazards near hot worksite, e.g. other hot worksites nearby 9. Candidate demonstrates to the Assessor where to locate detector as a site monitor for the duration of the hot- work, Assessor confirms that : 9.1 ..Candidate removes sampling system or selects another diffusion sampling gas detector 9.2 ..Candidate positions detector just away from immediate area of hot-work, upwind if there is a steady wind direction or between hot-work site and most probable source of gas leakage, if there is no steady direction to the site ventilating air masses 10. Assessor confirms by questioning that the Candidate is aware that there is no acceptable level of flammable gas concentration in the atmosphere for this type of gas testing and that the only acceptable reading is zero ! 19
11. Assessor confirms by questioning that the Candidate is aware that a check with the Standard Test Gas (usually 1 or 2.5 % methane in air) is the ONLY way of confirming that a gas detector really Works JMS Consultants 1995 Confined Space Entry Practical Gas Testing and Skill Assessment Conducted at the entrance to a confined space in an area that is not too noisy, to facilitate giving instructions and discussion. 1. Candidate chooses suitable detector(s) to measure firstly oxygen and then the flammable gas content of the vessel 1.1 The detector(s) chosen shall be fitted with an aspirator and sampling tube assembly long enough to obtain a representative atmosphere sample from the vessel. 1.2. Assessor confirms the correct choice of equipment and sequence of testing 2. Candidate inspects the well being of the detector with confidence and with particular emphasis on: 2.1.. sampling system leak-test (both valves, whole system and tubing) 2.2.. battery checks 2.3...readings in air 2.4 ...readings in standard flammable test gas 2.5- and the Assessor confirms that the Candidate has an understanding of what the readings should be 2.6 ..recognition of a faulty or otherwise non-conforming detector 2.7 ..reporting any fault condition to the Assessor Assessor checks that Candidate switches-on detector and confirms correct readings in clean air 3.1 Candidate approaches vessel from an up-wind direction3.
4. Candidate makes the initial test at the entrance to the vessel, sampling from within as far as possible without leaning over entrance or inserting head into the vessel. Candidate confirms that the oxygen is correct, then notes the flammable gas reading. 5. Assessor confirms by observation and questioning the Candidates ability to use the equipment correctly and interpret the results - and also that for any abnormal reading, Candidate knows to withdraw from vessel area upwind and reports the gas readings 6. If appropriate, the Candidate then samples the atmosphere in the vessel for the presence of any other possible toxic gases from a knowledge of past usage of the vessel, e.g. heavy condensate hydrocarbons or benzene - choosing an appropriate chemical stain tube gas detector. 6.1 Candidate checks for aspirator leakage, correct stain tube, correct use-by date and correct number of aspirations 7. Assessor confirms by questioning correct equipment choice and technique in performing these initial atmosphere tests 8. Assessor will confirm by questioning that the Candidate knows that a satisfactory atmosphere is zero concentrations of flammable and toxic gases and the correct quantity of oxygen. 9. Having confirmed that the atmosphere at the entrance to the confined space is satisfactory, with a torch, the Candidate close to the entrance of the confined space then examines the internal structure of the vessel to determine : 9.1 .is vessel one or more separate confined spaces? - Assessor confirms by questioning that the 20
Candidate understands the need for further testing if a complex multi-compartment vessel or one with blind spaces is being tested 9.2is there evidence of rusting or sludges? - Assessor confirms that Candidate is aware of the hazards each condition can create 9.3has the Candidate obtained a representative sample of the vessel atmosphere, fully taking into account vapour densities and any compartmenting within the vessel? 9.4Assessor confirms by questioning that the Candidate is aware of these 3 fundamentally important observations 10. Assessor confirms that Candidate is aware of the importance of using breathing apparatus if sludges, corrosion or multi-compartments or blind spaces exist and further testing inside the vessel is required. Candidate must be aware of the hazard that any one of these 3 conditions can generate and until a representative sample of the vessel atmosphere has been obtained the vessel atmosphere cannot be certified as safe 11. Whilst preparative gas tests can be done at any convenient time, Vessel Entry Permit gas testing shall be done immediately before entry is made. Assessor shall confirm by questioning that the Candidate is aware of when gas testing shall be undertaken 12. Upon completing a satisfactory atmosphere test, results shall be recorded on the companies paperwork - the Assessor shall confirm by observation, inspection of Permit form or questioning that Candidate is knowledgeable of company standing instructions regarding the issue of vessel entry work permits and their completion from a gas testing viewpoint 13. Assessor uses 'Flash Cards' for Atmosphere Conditions 1 - 8. Assessor will prompt the Candidate to identify the nature of the hazard and possible causes, Candidate will show knowledge in recognising the extent of each of these likely gas hazards which can all be met whilst conducting vessel entry gas tests. JMS Consultants 1995