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4.8.1 INTRODUCTION
Compressed gases on Das Island are used in a variety of work applications. This section deals with the portable, refillable type of cylinders used for welding and cutting, inerting, fuel and some special purposes.
4.8.2 HAZARD CLASSIFICATION 4.8.2.1 DANGEROUS GOODS
All compressed or liquefied gases are classified dangerous goods. There are nine classes of dangerous goods in all including Class 6 poisonous, Class 8 corrosive, etc.
A class diamond (label) is used on all dangerous goods packages to indicate the class to which it belongs.
(Refer Section 4.7 - Labelling Hazardous Substances.
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All gases are Class 2 and a major hazard is the pressure under which gases are stored.
Class 2 is divided into three sub classes to indicate the chemical hazard:
2.1 Flammable Gas (red diamond) 2.2 Non-flammable gas (green diamond) 2.3 Poisonous (white diamond) 4.8.2.2 SUBSIDIARY RISK
Some dangerous goods have hazards in more than one category. These are given a second class or 'subsidiary risk' and are labelled with a second diamond. Oxygen is an exception, having a single 'oxidising gas' yellow diamond.
Some typical gases with subsidiary risk are:
Carbon monoxide (2.1, 2.3) - red, white diamonds Chlorine (2.3, 5.1) - white, yellow diamonds Hydrogen chloride (2.3, 8) - white/black diamonds Liquid oxygen (2.2, 5.1) - green, yellow diamonds Exceptions are:
Compressed oxygen (2.2, 5.1) - yellow diamond only Compressed nitrous oxide (2.2, 5.1) - yellow diamond only.
4.8.3 PROPERTIES OF GASES 4.8.3.1 PRESSURE
Non Liquifiable gas can only be stored under pressure, which can result in two major hazards:
· Over pressure - leading to rupture of the container with the resultant shock wave and fragmentation damage.
· Leaking into the atmosphere - creating a hazard dependent on the sub-class of the gas involved.
Over pressure precautions are:
· Check that gas pressure will not exceed the pressure rating of the equipment.
· Do not tamper with pressure safety devices
· Do not weaken containers through damage - ensure they are secured in vertical position, etc.
· Shield cylinders from the heat of the sun and flares (the surface temperature of gas cylinders should never exceed 45°C
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Leaking precautions are:
· Use care and cleanliness in connections and valve operation
· Check for leaks regularly
· Use/store/transport in well ventilated conditions preventing build-up and concentration
4.8.3.2 FLAMMABILITY (CLASS 2.1)
Gases such as acetylene, propane and hydrogen, when mixed with air or oxygen can explode violently in the presence of an ignition source. Explosions, depending on size, can lead to serious damage and injury. The flammable range of different gases varies and is defined by the Lower Explosive Limit (LEL) below which concentration in air an explosion cannot occur due to lack of fuel and the Upper Explosive Limit (UEL) above which an explosion cannot occur due to lack of oxygen.
The precautions to be taken with flammable gases, in addition to those precautions of pressure are:
· Avoid sources of ignition
· Avoid mixtures of flammable gas and air/oxygen/oxidising agents
· Segregate flammable gases from oxidising gases, flammable/ combustible materials.
· Have fire fighting equipment on hand
4.8.3.3 TOXICITY (CLASS 2.3)
Poison gases react adversely on human beings. They usually enter through the lungs but some may also be absorbed through the skin.
These effects may be quick or delayed. They are termed acute if they kill in minutes or days. Their effects may also be long term and be termed chronic. Survivors may be left with lingering disability. Some toxic gases may also be carcinogenic.
The ways by which gases poison humans are varied. One group of poisons are simply irritants, i.e. the acid gases such as sulphur dioxide, hydrogen chloride and others, and the caustic gases, e.g. ammonia. Irritant gases can be quite deadly but do have the virtue of making their presence felt. Other poisons are more insidious.
The common measure of toxicity is the Threshold Limit Value Time Weighted Average or TLV (TWA) for short. This is the concentration in parts per million (ppm) to which you may be exposed eight hours a day, five days a week without harm. As a general rule, any gas with a TLV (TWA) of less than 100 ppm is classed 2.3 (white diamond). Another measure of toxicity is the Short Term Exposure Limit (STEL) which is the concentration (ppm) which will not harm you with a 15 minute exposure.
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Peak limitation values are also quoted for some substances where short term (i.e. a few minutes) exposure may cause intolerable irritation, chronic tissue damage or narcosis leading to accidents.
Yet another is the Immediate Danger to Life and Health (IDLH) which is the concentration (ppm) which will permanently disable in 30 minutes.
An example is carbon monoxide which has a TLV of 50, a STEL of 400 and an IDLH of 1500.
4.8.4 OXYGEN
4.8.4.1 OXYGEN DEFICIENT ATMOSPHERE
While only some gases are poisonous, only oxygen is positively beneficial (and even then only at 21% in a breathing atmosphere)
Any gas entering the atmosphere can create a hazard if it lowers the oxygen content to 18%. Any gas at about 17% concentration in the atmosphere will displace oxygen to this hazardous level.
Gases which create a hazard simply by displacing oxygen are termed simple asphyxiants.
Asphyxiation can cause death in seconds if the oxygen content is zero, or in minutes if it is less than 8%.
All simple asphyxiants are Class 2.2 with no subsidiary risk. Common asphyxiant gases are nitrogen, argon, carbon dioxide and some refrigerant gases.
The hazard is increased because there is no warning (no smell, no flames). The victim is unaware of anything wrong and rescuers are often trapped (and become victims) in oxygen deficient atmospheres.
The universal precautions of leak avoidance and ventilation apply. There is however a further precaution which is "beware of confined spaces".
The two significant asphyxiant gases found in quantity on Das Island are nitrogen used in purging and carbon dioxide in fire fighting systems.
Further Information
Chapter 7 Confined Space Entry
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4.8.4.2 OXYGEN ENRICHED ATMOSPHERE
An oxygen content above 23% (i.e. 2% additional oxygen) will increase the flammability of any fuels in the atmosphere, even those that would not normally burn in air.
A particular hazard is oxygen saturated clothing. This will burn vigorously if ignited. A grinding spark, which would normally just scorch cotton overalls, will cause them to ignite in an oxygen enriched atmosphere with serious to fatal consequences for the wearer.
Warning
Never attempt to blow dust etc. off work clothing with the oxygen stream from a welding or cutting torch.
Again the precautions of leak avoidance and ventilation apply.
Additionally:
· Follow the rules in 7 CONFINED SPACE ENTRY.
· Never use oxygen as a substitute for compressed air for any purpose.
4.8.4.3 OXYGEN NON COMPATIBILITY
Flammable gases and other fuels will burn because they react with oxygen.
Burning is rapid oxidation.
An important property of oxygen is that it is very reactive. Almost everything will oxidise if enough heat and energy (i.e. pressure) are applied. These conditions can be achieved by rapid pressurisation of closed oxygen systems.
Oxygen systems made up of oxygen compatible components may be contaminated with non compatible materials. Oil is one, but any contamination, e.g. dust, grit, swarf and lint will be non compatible. They will either burn readily or provide a source of ignition for compatible components.
Once a fire starts, the abundance of oxygen will fan it into a major burn-out with risk of burn injury and property damage.
Caution
· Keep oxygen systems clean.
· Never use lubricants or sealants.
· Use only equipment designed for oxygen.
· Open valves slowly, 'cracking' first to pressure up.
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4.8.5 LIQUIFIABLE GASES
All gases are stored under pressure, but some gases are liquefied at storage pressure and ambient temperature. This increases the gas contents of the cylinder, but creates some new hazards. The common liquifiable gases are carbon dioxide, LPG, nitrous oxide and refrigerants.
The hazards with liquefied fuel gases are:
· Depending on the orientation of the cylinder, liquid may come out of the cylinder valve and into the user system. This will give a higher than expected flowrate (an LPG torch could act as a flame thrower).
· Similarly liquid leaks have a much greater effect on the working atmosphere contamination level than dry gas leaks.
· If the safety device of the cylinder is submerged in the liquid phase of the contents, its operation will not lower the pressure as quickly as with gas venting. This may lead to over pressure and rupture in a fire.
· Venting liquid flashes to cold gas and liquid which may cause cold burns.
· If cool liquid is trapped in a system and subsequently warmed up, the pressure may rise to exceed the bursting pressure of that system.
The precautions to be followed are:
· Only use cylinders in the upright position.
· Only store and transport cylinders in the upright position.
· Wear gloves when handling any liquefied gas, consider further protection, e.g. eye and face protection.
· If a liquid leak cannot be stopped, move the cylinder to convert it to a gas leak.
4.8.5.1 GAS NAME IDENTIFICATION
The gas name label is the primary identification of any cylinder contents and is usually located on the shoulder of the cylinder. This identifies the product by its correct technical name and classification. On some cylinders the gas name is labelled or stencilled on the body of the cylinder.
4.8.5.2 UNITED NATIONS NUMBER (UNNO.)
The number may appear near the gas name on cylinders, either on the label or as stencil.
4.8.5.3 COLOUR CODE
Table 4-4 shows typical cylinder colours and characteristics for gases in common used on Das Island
GAS CHARACTERISTICS CYLINDER
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BodyColour Neck Bands
Oxygen (O2)
No smell. Non-toxic. Will not burn but supports and accelerates combustion. Materials not normally considered combustible may be ignited by sparks in oxygen rich atmosphere.
Black
Nitrogen (N2)
No smell. Does not burn. Inert except at high temperatures. Non-toxic but does not support life so could cause asphyxiation in high concentrations.
asphyxiation in absence of sufficient oxygen to support life.
Peacock
Blue Black
Acetylene (C2 H2)
Distinctive garlic-like smell. Fire and explosion hazards are similar to those for propane. However, it is lighter than air and less likely to collect in ducts and drains.
Maroon
Propane (C3H8)
Distinctive fish-like smell. Will ignite and burn instantly from a spark or piece of hot metal. Is heavier than air and will collect in ducts, drains, etc.
No smell. Non-toxic. Much lighter than air. Will collect at the highest point in any enclosed space unless ventilated there. Fire hazard.
Signal Red
Air No smell, Non-toxic. Essential for life not burn. Will cause asphyxiation in absence of sufficient oxygen to support life
Black White
Table 4-4: Gas Characteristics/Cylinder Colours
Caution
If the contents of a gas cylinder cannot be positively identified, the cylinder should not be used. It must be identified with a tag, manifested as dangerous
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goods and returned to the gas agent onshore. Never use an unidentified cylinder and always assume the worst, i.e. It is flammable and poisonous.
4.8.6 SAFE STORAGE Storage Design
Gas cylinder stores must be:
· Constructed of fire rated materials and have firm level floors.
· Away from escape routes and emergency equipment.
· Well ventilated and preferably provided with basic weather protection.
· Free from fire risk.
· Away from sources of artificial heat and ignition.
· Free from contamination (dust, fumes, spray or chemicals).
· Designated and marked as a 'no hot work' area.
· Clearly marked as a gas store with appropriate hazard diamond signs, e.g.
flammable, poison, corrosive, etc.
· Kept clean and with clear access restricted to authorised personnel.
· Provided with appropriate safety/emergency equipment and fire and gas detection/protection systems.
· Provided with sign "DANGER" in red Arabic and English letters at least 50mm high with an adequate description of the substances.
· Provided with appropriate racks or chains to secure cylinders.
4.8.7 SAFE HANDLING
Personnel who regularly move and use gas cylinders must follow the precautions for this work:
· Wear personal protective wear free of grease, oil and grit including gloves, and eye protection along with minimum worksite safety wear requirements.
· Use recommended mechanical handling aids, e.g. quads, racks, trolleys, etc. and secure cylinders at all times.
· Follow the approved manual handling procedures and techniques to prevent back injury.
· Make sure valve caps, plugs and other devices are fitted when provided.
4.8.8 LEAK TESTING
In the case of an oxy/acetylene torch assembly a simple leak test of the complete system is recommended:
· 'Crack' the cylinder valve to remove dirt
· Attach and tighten the regulator, hose and equipment
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· Shut off the control valve on the blowpipe
· Slowly open cylinder valve, then screw in regulator control knob to pressurise the system
· Close the cylinder valves and watch the regulator pressure gauges.
If pressure is retained there is no leak in the system. If pressure drops, a leak is present and should be located with the leak detection solution then de pressurised and repaired.
Further leak testing using leak detection solution will include:
· Cylinder valve gland.
· Cylinder safety device.
· Cylinder neck thread.
4.8.9 CARE OF CYLINDERS
· Do not drop cylinders from a height or allow them to fall over.
· Secure cylinders against falling while in use, storage or transport.
· Do not allow naked flames to impinge on cylinders.
· Do not allow any form of heating which would raise the temperature of the cylinder above 45°C.
· Do not use cylinders as rollers or supports.
· Protect cylinders from impact damage from passing vehicles or crane loads.
· Protect cylinders from paint spray, acids, alkalis and prolonged water contact.
· Ensure no electrical power can flow through a cylinder, e.g. arc welders.
· Do not attempt to repair cylinder valves.
· Keep cylinders and in particular the valves, clean and oil free.
· Do not use cylinders with dirty valves
· Do not attempt to clean valves other than 'cracking' before fitting regulators
· Close cylinder valves when the cylinder is not in use and especially when the cylinder is empty
Note
Discharged cylinders must be tagged and/or prominently marked "MT" and stored in a separate dedicated area for return to the supplier.
4.8.10 EMERGENCY SITUATIONS 4.8.10.1 GENERAL
Emergencies involving compressed gases are unlikely provided the recommendations are followed for their correct storage, handling and use.
When major hazards do arise they are usually due to:
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· Fire threatening the cylinder. Compressed gas cylinders if in contact with excessive heat for prolonged periods may rupture or explode.
· Flammable gas leak. All flammable gases will form explosive mixtures with air.
When ignited, significant explosive energy can be released from flammable gas/air mixtures.
· Toxic gas leak.
· Non-flammable compressed (inert) gas leak. This is usually considered to be
harmless, however, if in a confined or poorly ventilated space, asphyxiation and death of unprotected personnel can occur.
4.8.10.2 FIRE EMERGENCIES
General procedures and precautions are:
· Isolate supplies so they do not flow into the affected area. Safely release gas
pressure in affected pipelines and equipment. Ventilate the area to prevent explosive atmosphere build-up, i.e. keep it below the lower explosive limit (LEL).
· When fire threatens compressed gas cylinders there is a risk of rupture/explosion of cylinders subject to prolonged heating. These should be moved to a protected safe place before they become too hot or if this is not possible, cooled with a fire hose at a safe distance.
· Any cylinder that has been involved in a fire must be segregated and clearly marked as such and the supplier must be notified accordingly to prevent hazards to people who refill cylinders.
· Visibility of flames. Not all fuel gases burn with a clearly identifiable flame. Burning hydrogen for example is very difficult to see
Warning
With a burning flammable gas leak isolate the gas supply, if this is not possible, do not extinguish the flame. Ensure the flammable gas burns in a controlled manner, does not ignite anything else and does not impinge on any pressurised gas containers, equipment or pipelines. keep surrounding area cool by spraying with water from a protected safe distance.
4.8.10.3 GAS LEAKS
In general, gas leaks are dealt with by:
· Establishing the name and type of gas involved.
· Isolating the cylinder valve.
· Evacuating the area if necessary.
· Instituting fire prevention measures if appropriate.
· Ventilating if possible and moving faulty cylinders to an open area.
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Note
Dirt ingress in cylinder valves is a common cause of leakage. In this case 'crack' the cylinder valve to open air wearing hearing and eye protection. This should be a standard procedure before fitting regulators together with
examination for oil or other contamination in the area of connection.