Fire, gases and confined spaces

Welding is hot work, and hot work starts fires. Sparks and spatter travel many metres, fall through gaps in floors and walls, lodge in dust and rubbish, and can smoulder for hours before bursting into flame after the welders have gone home. The gases welders use add their own hazards: fuel gases that burn and explode, oxygen that makes everything burn more fiercely, and shielding gases that are harmless in the open but suffocate in an enclosed space. And welding inside tanks, vessels, pipes and pits, confined spaces, concentrates every hazard in this book in a place where escape and rescue are difficult. The deaths in welding are disproportionately in confined spaces.

This chapter covers hot work fire prevention, the safe handling of gas cylinders, the arithmetic of a gas leak in a confined space, both oxygen depletion and enrichment, and the controls the confined spaces code requires.

How welding fires start

Most welding fires start from sparks, spatter or slag reaching something combustible: dust, sawdust, rags, packaging, paper, oil, plastic, timber, insulation or flammable liquids and vapours. Sparks bounce and roll across floors, fall through openings into rooms below, and pass through gaps around pipes and cables into wall cavities. Heat conducted along a pipe or beam can ignite material on the other side of a wall. Many fires begin not during the welding but in the following hour or so, from smouldering material that was not seen.

Figure 1Hot work fire controls

	What it does	Notes
Dedicated welding areas	keep routine welding where combustibles are excluded	non-combustible floors and walls; no storage of flammables
Hot work permit	authorises hot work outside dedicated areas after checks	lists the hazards, precautions, fire watch and sign-off
Clearing and covering	removes combustibles or covers them with fire blankets	including below and behind the work
Fire watch	a person watching for fires during and after the work	for the period the permit sets after work stops
Extinguishers and hoses	suited to the materials present, at hand	people trained to use them
Isolation of flammables	lines, tanks and drums emptied, cleaned and tested	never weld on a container that has held flammables until it is proved safe

The hot work permit

Routine welding in the shop’s dedicated bays, where combustibles are kept away and the floors and walls are non-combustible, does not usually need a permit. Hot work anywhere else does: in other parts of the building, in customers’ premises, on site, on plant and structures, and anywhere combustibles cannot be removed. A permit is a written check, done by the person in charge before work starts, that the area has been inspected, combustibles removed or protected, openings covered, flammable atmospheres tested where they may exist, extinguishers in place and a fire watch arranged, and then signed off after the fire watch has finished and the area has been checked. It is valuable not for the paper itself but for the careful walk round the area, above, below and behind the work, that it forces someone to make before the first spark is struck.

Containers that held flammables

Welding or cutting on a drum, tank or pipe that has held fuel, oil, solvent or any flammable material is one of the oldest causes of fatal explosions in welding. Even a container that looks empty can hold enough residue or vapour to explode when heated. Such containers are welded only after they have been emptied, cleaned by a proper method, tested with a gas detector to show that no flammable vapour remains, and, for some work, filled with water or inert gas, under a procedure for the purpose. The shop’s rule is that it does not weld on any container that has held flammables unless a competent person has cleaned and certified it.

Gas cylinders

Welding gases are stored under high pressure in cylinders, and a damaged cylinder valve can turn a cylinder into a projectile. Cylinders are secured upright with chains or brackets so they cannot fall; stored in ventilated areas away from heat, ignition sources and electrical equipment; kept with their valve guards or caps fitted when not in use; moved on trolleys, not rolled or dragged; and kept apart by type, with oxygen separated from fuel gases. Acetylene cylinders are always used upright, because their acetylene is dissolved in a solvent that must not reach the valve, and a cylinder found lying down is stood up and left for the time the supplier advises before use. Regulators, hoses and torches are kept in good condition, leak-tested, and fitted with flashback arrestors as required. Oxy-Fuel by the Numbers and Industrial Gases by the Numbers cover gas equipment in detail.

Oxygen: never for comfort

Oxygen is not flammable, but everything burns far more fiercely in it. Air enriched with oxygen makes clothing, hair and grease ignite from a spark and burn with intense heat. Oxygen must never be used to blow dust off clothing, cool a person, ventilate a confined space or power air tools, and oily rags and grease are kept away from oxygen equipment, because oil can ignite spontaneously in high-pressure oxygen. Several fatal fires have begun with someone “freshening” a confined space with oxygen.

Gases in a confined space

A confined space, under the work health and safety regulations, is an enclosed or partly enclosed space not designed to be occupied by people, with a risk from its atmosphere, contaminants, engulfment or fire. Tanks, vessels, silos, pits, pipes, ducts and the interiors of large weldments can all be confined spaces. In such a space, gases from welding cannot disperse, and a leak changes the atmosphere quickly.

Figure 2Oxygen in an unventilated confined space with an argon leak

oxygen percentage ≈ 20.9% × e^(−leak rate × time ÷ volume)

Space volume

4 m³

Argon leak

15 L/min, 0.015 m³/min

1. after 60 minutes: 20.9% × e^(−0.015 × 60 ÷ 4) ≈ 20.9% × 0.80 ≈ 16.7%
2. the oxygen falls to about 18% in about 40 minutes and to about 16% in about 71 minutes
3. a torch left running in a tank over a lunch break makes the air dangerous, and at the bottom much sooner

The calculation is for a perfectly mixed space, which overstates how long it takes. Argon and carbon dioxide are heavier than air and collect at the bottom of a tank or pit, so a welder lying at the bottom of a vessel can be in an oxygen-deficient layer while the air at the manhole is normal. Low oxygen gives little warning: judgement and coordination fail first, then consciousness, and a person can collapse after a few breaths in severely oxygen-deficient air. Rescuers who climb in without breathing apparatus often become victims too.

Oxygen enrichment in a confined space

The opposite danger arises when oxygen leaks into a confined space, from a leaking hose, a torch left on, or cutting with excess oxygen. In an unventilated 4 m³ space, an oxygen leak of 10 litres a minute raises the oxygen from 20.9% to about 26.6% in half an hour, by the same kind of calculation. At such levels, a spark that would barely mark clothing in normal air can set it ablaze. Gas hoses and torches are therefore never left inside a confined space during breaks, and the atmosphere is tested for oxygen at both extremes before and during work.

Confined space controls

Figure 3Controls for welding in a confined space

• Avoid: Permitconfined space entry permit by a competent person; hazards and controls listed
• Avoid: Isolationthe space isolated from all lines, energy and moving parts
• Avoid: Atmospheretested for oxygen, flammable gases and toxic contaminants before entry and continuously during work
• Avoid: Ventilationforced ventilation with clean air; never oxygen; extraction at the arc
• Avoid: Gas equipmentcylinders outside; hoses and torches removed during breaks
• Avoid: Electricaldirect-current machine with tested voltage-reduction device, outside the space
• Avoid: Standby personoutside the space, in contact, trained, able to raise the alarm and stop the work
• Do: ? Rescuea planned rescue method and equipment, tested before work starts

The standby person and the rescue plan matter most when things go wrong. A standby person who is in contact with the welder throughout, can switch off the power and gases, and can summon rescue without entering, prevents the commonest tragedy, the would-be rescuer who goes in and is overcome. The rescue plan says how an unconscious welder will be brought out of the space, with what equipment, by whom, and how quickly, and it is practised.

The fire watch in practice

A fire watch is a person whose only job, during hot work and for a period after it, is to watch for fires and deal with them. They stand where they can see where sparks and slag are landing, including below and behind the work, have an extinguisher at hand and know how to raise the alarm. After the work stops, they stay for the period the permit sets, checking the area, especially the places sparks could have reached unseen, such as the floor below, wall cavities and the far side of a wall that was heated. The fire watch is easy to cut short when a job runs late, and many fires have begun in exactly that gap. The shop’s permits record the fire watch’s finish time and the final check, signed by the person who did it.

Building fabric

Modern buildings contain materials that hot work can ignite in ways the welder may not see: insulated sandwich panels with foam cores, roof insulation, plastic cladding, and cable trays packed with insulation. Heat from welding or cutting on steel attached to such panels can ignite the core inside the panel, where the fire spreads unseen. Hot work near or on such materials needs particular care in the permit: identifying the materials, protecting them, keeping the heat away, and extending the fire watch. On the running case’s site jobs in customers’ premises, the supervisor asks about panel construction before quoting any hot work.

Fuel gas leaks

Fuel gases behave differently when they leak. Acetylene and natural gas are lighter than air and rise, collecting under roofs and in ceiling spaces; propane and other liquefied petroleum gases are heavier than air and flow along the ground into pits, drains and basements, where they can collect unseen until an ignition source reaches them. Leaks are found by smell where the gas is odorised, by leak detection solution on connections, and by gas detectors, never by a flame. Hoses are checked for cuts and burns, and cylinders are closed at the valve when work stops.

Testing for flammable atmospheres

Where flammable gases or vapours may be present, in a tank that held fuel, near a fuel line, in a pit that may have collected gas, or around equipment that uses solvents, the atmosphere is tested with a calibrated gas detector before hot work starts and monitored during it. Detectors measure flammable gas as a percentage of its lower explosive limit, the lowest concentration in air at which it can ignite; the permit sets the reading below which hot work may proceed. The person testing is trained in the detector, its calibration and its limits, including the gases it does and does not detect.

The emergency plan

Every workplace has an emergency plan, and for a welding shop it covers fire, serious burns, electric shock, gas leaks, and rescue from confined spaces. People know where the extinguishers, fire blankets, first-aid kits, eye-wash stations and defibrillator are, how to raise the alarm, how to shut off gas and power, and where to assemble. The shop runs an emergency drill twice a year, including once a confined space rescue exercise with the equipment it would use, because a plan that has never been practised usually fails at its first real test.

The running case: the hopper repair

This is an illustration. A customer asked the shop to reline the inside of a steel hopper of about 6 m³, with access through a single manway. The coordinator’s plan treated it as a confined space: isolation of the hopper’s feed and discharge, atmospheric testing before entry and continuously by a monitor on the welder, forced ventilation through the manway and an exhaust at the bottom outlet, gas cylinders outside with hoses removed at each break, a direct-current machine with a voltage-reduction device outside the hopper, supplied-air respiratory protection for the welder, a trained standby person at the manway, and a tripod and winch for rescue, set up and tested before entry. The job took two days longer than an unplanned job would have, and nobody was hurt.

A failure: the argon in the pit

This is an illustration. At another workshop, a welder used gas tungsten arc welding to repair a pipe in a pit about 1.5 m deep, and left the torch on a ledge in the pit with the gas still flowing during a break. Returning, the welder climbed down into the pit and collapsed within a minute. A workmate who climbed in to help collapsed too. Both were pulled out by others using a rope and survived, one with lasting harm.

The pit had filled with argon from the bottom up. No atmospheric test was made before re-entry, there was no standby person and no rescue plan, and the pit had not been recognised as a confined space. The workshop now treats all pits as confined spaces, tests their atmosphere before every entry, and shuts off gas at the cylinder at every break.

Exercise: a carbon dioxide leak in a tank

A welder inside a 3 m³ tank uses a gas metal arc torch with carbon dioxide shielding at 18 litres a minute, without ventilation. Treating the space as well mixed and the gas as displacing air, what is the oxygen percentage after 30 minutes and after 60 minutes of continuous gas flow? What else does this suggest?

The exponent after 30 minutes is 0.018 × 30 ÷ 3 = 0.18, so the oxygen is about 20.9% × e^(−0.18) ≈ 20.9% × 0.835 ≈ 17.5%. After 60 minutes, the exponent is 0.36 and the oxygen about 20.9% × 0.70 ≈ 14.6%.

Both are well below normal, and carbon dioxide is itself toxic at high concentrations, besides displacing oxygen; it also collects at the bottom of the tank. The tank needs forced ventilation and continuous atmospheric monitoring before and during welding, and the shielding gas flow should be no more than the process needs.