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Acetylene cylinder procedure

Due to the risk of decomposition occurring within acetylene cylinders after any fire has been extinguished, specific operational procedures are required for acetylene incidents.

The flow chart below includes guidelines to assist in deciding if a cylinder has been involved in a fire that has caused decomposition. A common example, where a cylinder operator damages a hose, should not always be considered sufficient to initiate decomposition, provided any flames from the damaged hose are extinguished in a timely fashion.

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Diagram 4 Gas cylinder cooling process

Does the cylinder show signs of heat damage, flash-back or direct flame contact?

It is important to assess whether the cylinder has been sufficiently heat affected for decomposition to be initiated. It requires a significant temperature increase (i.e. above 3000C) and this is normally only achieved by direct flame contact on a cylinder.

Signs that can be used to detect possible heating:

  • A visible bulge in the shell: the cylinder should be treated with extreme caution as this indicates a greatly increased likelihood of catastrophic failure
  • Burnt cylinder labels
  • Melting of the plastic rings around the cylinder valve
  • Burnt or blistered cylinder paintwork
  • The cylinder surface steams or dries out quickly when water is applied
  • Pressure relief devices are operating, if fitted (the operation of a pressure relief disc or fusible plug with gas burning off or leaking indicates an increased likelihood of catastrophic failure and should not be regarded as a sign of safety)

Eye witnesses may be able to provide information to enable the incident commander to confirm:

  • Whether a cylinder has suffered direct flame contact, and if so, for how long
  • The severity and duration of any heating of a cylinder
  • Whether a flash back, and not a backfire (i.e. a single cracking or 'popping' sound) has occurred.

A flash back might be accompanied by a shrill hissing sound. A flash back occurs when the flame travels back through the hoses into the body of the cylinder. This may be caused by user error or poorly maintained or faulty equipment. Flash back arrestors fitted to the hoses will detect and stop reverse gas flow, preventing a flammable oxygen and acetylene mixture from forming in the hose.

Flashback arrestors are mandated under DSEAR in the UK and are designed to prevent a flashback. The arrestor is an automatic flame trap device designed not only to quench the flame but also to prevent the flame from reaching the regulator. N.B. extra care should be taken with illegally imported cylinders that may not have flashback arrestors.

Flash backs into acetylene cylinders, which may initiate decomposition, are generally due to the failure to fit a flame arrestor.

Where cylinders are in the proximity of a fire but show no signs of direct heating (see list above), they are likely to be safe to move. However, before doing so the temperature of the cylinder walls should be checked by spraying with water and seeing whether they remain wet or they should be examined using thermal imaging equipment. Personnel must be made aware of the manual handling problems associated with moving an unheated cylinder to a safe location. Acetylene cylinders are comparatively heavy in relation to other cylinders and are awkward to carry, especially when wet.

Do not move the cylinder in the cooling phase. Designate a hazard zone. Apply cooling water from a shielded location as soon as possible. Continue cooling for one hour after the fire has been extinguished.

If a decomposing cylinder is leaking or is moved, the rate of decomposition and heat generated may be increased to such an extent that the cylinder walls are weakened abnormally and rupture. Greater safety can be achieved if the decomposition process is slowed or arrested by water spray cooling.

Water cooling is currently the most effective method of preventing failure of an acetylene cylinder and should be used whenever it can be implemented without compromising the safety of firefighters (e.g. where protection is offered by suitable shielding).

The period of greatest risk is when the cylinder shell is hot, so every effort should be made to cool it comprehensively, taking full advantage of any available substantial shielding/cover and using ground monitors and/or lashed jets.

Applying water will result in the cylinder shell cooling quickly. This in turn will slow down any internal decomposition process inside the cylinder. As a consequence of this, and after carrying out a risk assessment, the incident commander (in liaison with the hazardous materials adviser (HMA), could reduce the initial cordon distance in favour of a risk-assessed hazard zone.

Considerations when determining the hazard zone include:

  • Size of cylinder(s)
  • Number of cylinders
  • Shielding provided by any buildings or structures
  • Type and extent of adjacent structures
  • Local topography (e.g. protection provided by slopes and gradients of ground levels, etc.)
  • Effect of the potential blast pressure wave
  • Effect of a potential fireball (of up to 25 metres)*
  • Effect of the cylinder being thrown (up to 150 metres)*
  • Flying fragments and other projectiles (e.g. valve assembly, which may be thrown up to 200 metres).* The steel used to make acetylene cylinders is heat-treated to ensure that when cylinders do fail, they do so in ductile mode rather than as brittle failure. Many merely split open, releasing the gas contents, but if they explode the typical result is into three or four large pieces that may have high looping trajectories
  • Flying glass and other structural material
  • Structural damage to buildings in the vicinity
  • Possible need for an exclusion zone within the hazard zone
  • Proximity and importance of adjacent occupancies and key infrastructure, such as major roads and railways

*Possible maximum travel distances for a cylinder in the open (i.e. not within a structure or building that would provide shielding and therefore reduce the distances projectiles could travel).

Consideration should always be given to using any substantial portable materials that might offer shielding between the cylinder(s) and associated risk points (e.g. a public highway, railway or other thoroughfare), to reduce the hazard zone. This might be possible where such materials are palleted and can easily be moved into place without subjecting the operative to any undue risk. It may be necessary when cylinders cannot be water cooled, for example, due to their location within a dangerous structure.

There may be circumstances where attempts to apply water would expose firefighters to unacceptable levels of risk that outweigh the benefit likely to be gained. At such incidents the alternative would be to leave the cylinder in place without applying water until it bursts or all sources of heating are removed. Remotely operated vehicles have been used at cylinder incidents to assist the incident commander in monitoring the condition, degree of heat damage and temperature of cylinders.

Consider contacting the gas supplier for advice

During an incident involving acetylene cylinders it is important to identify the gas supplier that owns the cylinder. This will enable the gas company to provide assistance in identifying the contents of the cylinder and to provide any help required to manage the incident effectively.

When contacting the relevant gas supplier the following information should be given:

  • The fire and rescue service that is attending and the name of the caller
  • The address of the incident and the premise name if applicable
  • The advice required. (i.e. telephone support or on-site support)
  • Number of cylinders involved and whether collection will be required
  • If on-site attendance is required:
    • Map references and directions
    • Confirmation that the police service at the outer cordon are aware that cylinder supplier’s representative is attending

The gas supplier will then pass the call on to the competent person. As all the companies are different in size, and therefore have different levels of resource, the way in which the first call is dealt with may vary but the technical advice offered will be standard across all companies.

If the incident commander would like on-site assistance this will be arranged by the competent person. However, before requesting a site visit certain questions need to be considered, such as:

  • Is acetylene definitely involved? If unsure, the incident commander should try to gather as much information as possible from the site occupier and from the competent person over the telephone. If still unable to determine whether acetylene is definitely involved, a site visit may be required.
  • Is the cylinder visible? If it is covered in debris or behind other shielding and cannot be seen, the attendance of the competent person will be unlikely to add value to the decision making process
  • Is there adequate lighting for the competent person to see the cylinder/s involved clearly? If not it would be best to advise the competent person not to arrive on site until after first light.

Attendance on site by competent persons should be managed in daylight hours where at all possible unless the incident involves severe disruption such as closing major arterial routes.

The competent person will be able to help with identification and if necessary assist with providing guidance for the wetting test (see below). They can also arrange for the cylinder(s) to be removed at the conclusion of the incident. The incident commander can discuss all relevant issues with the competent person, police service, local authority and any other relevant organisations in attendance, to make an informed decision on whether to maintain or reduce the initial hazard zone.

Applying the wetting test and/or using thermal imaging equipment to check that cooling has been effective

Decomposition of the acetylene contained within a cylinder may take place after the external heat source has been removed if there has been sufficient transfer of energy to start the reaction. Once started, decomposition will continue until all of the acetylene is consumed or until the cylinder is effectively cooled and made safe. Acetylene cylinders are designed and tested to withstand such decomposition and can cool naturally without any problem. The porous mass is designed to assist in this. Cooling will slow the reaction and allow it to self-extinguish.

It is therefore important to be able to identify if an acetylene cylinder is hot or becoming hotter by itself. This can be achieved by carrying out a wetting test and/or using thermal imaging equipment.

Testing may be performed immediately on discovering cylinders that may have been exposed to heat, or during the cooling process for cylinders known to have undergone heating.

Testing establishes and confirms:

  • Whether the cylinder shell is cool
  • That acetylene is not undergoing internal decomposition (a number of successful tests spread over at least one hour should be observed)

This procedure will ensure that, in the highly unlikely event of internal decomposition occurring deep within the cylinder’s filler, any heat build up will be noticed externally before it can reach temperatures likely to weaken the cylinder shell or cause dangerous internal pressures.

When externally cool, the cylinder should be safe to approach as long as no gas is leaking but it must not be moved in case there is a large internal cavity due to damage to the porous mass. Movement of the cylinder may accelerate decomposition and result in catastrophic failure, hence the importance of continuing to apply wetting tests for at least one hour after a perceived test pass has been observed.

The wetting test involves:

  • Getting a clear view of the cylinders from a shielded location
  • Briefly spraying water on to the cylinder surface
  • Stopping spraying and looking for signs of steam rising from the surface of the cylinder
  • If steam is not seen rising, checking to see whether the wetted cylinder surface dries out quickly (i.e. 1-2 minutes). During the combustion process, tars and oils may be released and deposited on the cylinder surfaces; this may make the drying out part of the test difficult to interpret due to the reaction between oil and water

If either check is failed then water cooling must be reapplied for one hour before testing again. If both tests are passed then the monitoring phase should start.

Using thermal imaging equipment and remote temperature measuring equipment gives additional confidence and should be used whenever possible.

Monitoring phase:

  • Stop water cooling
  • Do not move the cylinder
  • Maintain hazard zone
  • Reapply the wetting test and/or thermal image equipment tests at 15-minute intervals for one hour.

If any unexplainable re-heating is observed water cool continuously for one hour then start the monitoring phase again.

Water cooling must be completely stopped during the monitoring phase to allow any internal heating to show itself by raising the temperature of the exterior of the cylinder shell. Testing should be carried out at 15-minute intervals so that any heat build up through decomposition will be noticed before it can reach a dangerous temperature. A written record of the test results should be maintained throughout the monitoring phase.

Effective water cooling may reduce the cylinder temperature down to the temperature of the cooling water, which may be lower than the ambient temperature. This means that during the monitoring phase, when no water is being used to cool the cylinder its temperature may rise slowly and naturally to its ambient temperature. The incident commander should take care not to misinterpret this rise in temperature as the result of internal decomposition.

The natural heating effect of direct sunlight on dark coloured or blackened cylinders will also result in a rise in the cylinder shell temperature that is not attributable to decomposition. If in doubt the incident commander should extend the monitoring phase or, if the temperature rises significantly above ambient/expected levels (i.e. enough to fail the wetting test), recommence water cooling for at least one hour.

An appropriate, risk-assessed hazard zone should be maintained throughout the monitoring phase as in the unlikely event that re-heating of the cylinder takes place, it may be difficult to quickly reintroduce cooling phase cordons due to staffing levels and adverse public reaction.

If any re-heating above ambient cylinder temperature (either steaming or rapid drying out) is observed at any of the wetting tests then the cylinder must be continuously water cooled for a further hour. After this period the wetting test and/or testing with thermal image equipment should be carried out again. If no re-heating has occurred the full monitoring phase procedure should also be started again (i.e. stop water cooling and carry out wetting tests at 15-minute intervals for one hour).

Failure of the cylinder occurs because the cylinder has reached temperatures of over 300oC and the cylinder walls are losing their tensile strength. Cylinders at temperatures close to 300oC will cause water to violently boil off, as seen when red hot metal is plunged into cold water. The failure of the wetting test due to a hotspot does not imply that the cylinder is at immediate risk of failure unless there is an extremely violent reaction.

Is the cylinder leaking?

The monitoring phase will have established that the cylinder shell has been effectively cooled from its original temperature and, more importantly, that any decomposition reaction has stopped. However, If there is still any low-level decomposition within the cylinder then this will be fuelled and potentially accelerated if fresh acetylene passes through the area (i.e. if a leak pulls gas across the decomposition zone). The leak would need to be serious to stimulate rapid decomposition. A leak such as this would be seen from a melted fusible plug (if a fusible plug is present) or a massive release from the valve. The porous mass should be adequate to self-extinguish in the event of small leaks. If decomposition is fuelled, then the cylinder will heat up. This heating will then be detectable as the shell of the cylinder shows signs of heat once again. If there is a significant leak the incident commander should consider re-establishing the monitoring phase (i.e. cylinder shell temperature checks at 15-minute intervals for a further hour).

Leaking acetylene gas may also cause an additional fire and/or explosion hazard if it is confined around the leaking cylinder. This risk needs to be assessed and managed by the fire and rescue service.

No further action by fire and rescue service, hand over to owner or responsible person

Heat-damaged cylinders are not the responsibility of the fire and rescue service. Once the fire and explosion risk has been dealt with the site and cylinder(s) should be handed over to the responsible person, owner or operator. The incident commander should give a full brief to the responsible person, detailing the action taken and reasons why. If the cylinders are not at a premises but on public land, the incident commander should contact the local authority or Highways Agency as appropriate.

Multiple cylinders (or substantially concealed single cylinders)

Where multiple cylinders are encountered and they are very closely packed and/or concealed/buried by debris, there may be a risk that the cooling water may not come into contact with a substantial proportion of the cylinder shell, therefore limiting the effect of cooling.

If the incident commander considers that a significant area of any cylinder is dry then the cooling phase should extended (e.g. if 50% of a cylinder is not being touched by cooling water then consider increasing the cooling phase to three hours).

In extreme circumstances, where the vast majority of a cylinder is concealed or buried and is believed to be dry, the incident commander should consider increasing both the cooling phase and the monitoring phase