FPASA BULLETIN SF 08
AIR COMPRESSORS
Fires caused by the overheating or exploding of compressors are not a very common
occurrence and as a result the associated hazards of air compressors are not always fully
appreciated.
Any fire can, however, have serious consequences as was the case in a textile mill in the
Cape when combustible fluff was ignited by hot pipes on the discharge side of an air
compressor. The resultant fire spread to cause R7 million damage.
Types of compressor
Of the two common types of compressors, ie: rotary, and reciprocating, the latter is
considered to be the most hazardous. This bulletin deals mainly with reciprocating
compressors though many of the recommendations are applicable to other compressors.
Ignition sources
Leak in final stage of compression or discharge
A defective non-return valve in the final stage of compression in a reciprocating
compressor may allow hot compressed air to leak back into the cylinder on the suction
stroke and subsequent re-compression could result in a rapid temperature increase which
could heat up lubricating oil under pressure in the form of mist or vapour to its auto-ignition
temperature.
Explosive limits
The lubricating oil feed-rate to the compressor is controlled to ensure that the oil vapour
mist concentration remains well below its lower explosive limit. However, should there by a
drop in air flow caused by a blocked inlet, the oil vapour concentration could increase to
within the explosive range.
Carbon deposits
The ignition of carbon deposits is virtually the prime cause of compressor fires or
explosions. Carbon is deposited in the inter-cooler system, valve chests, discharge
passages and interconnecting pipes as a result of oil mist and vapour being readily
oxidised in the hot zones of the compressor and subsequently being carried away to the
cooler zones. This presupposes the normal minimum level of lubrication is maintained. If
the lubrication level is increased, or the air outlet temperature becomes abnormally high,
carbon deposition may be initiated in the hot zones.
Once the oxidation process is started the build-up of deposits will accelerate. However,
these carbon deposits are normally wetted by a thin film of oil which cools the heat
generated during oxidation.
If the air temperature increases or if the running of the compressor is interrupted or
continued during an off-load period, the oil film may dry out causing heat to be evolved at a
greater rate than it is dissipated, resulting in a fire.
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Design and arrangement
Location
Compressors should be placed out of doors protected against the elements or in a special
room separated from other areas by fire resisting walls and doors.
Air inlets
Should be positioned high up on structures or outside the building where the air will be
cool, clean and free from any flammable gases or vapours.
Cooling
Air-coolers should be fitted vertically with the air-flow downwards and installed as close to
the discharge side of the compression chamber as possible. Water and oil traps should be
fitted at the bottom of the cooling unit to afford drainage.
Oil circulating through an oil-flooded system should be immediately cooled before injection
into the compression chamber.
Valves
Safety valves and off-loader connections should be fitted on the compressor side of
delivery valve to ensure safety even if the compressor is accidentally run with the delivery
valves closed.
Very high pressures can develop within reciprocating compressors, since electric motors
are capable of running at large overloads for short periods.
Discharge pipework
Pipework should be protected by a guard which allows for the free circulation of cooling air
and the pipes should not be insulated. Vertical bends should be avoided as they act as oil
traps, unless they are fitted with drainage taps.
Safeguards

Detailed operation and maintenance instructions, including emergency procedures
based on manufacturers recommendations, should be drawn up and posted at the
compressor
building.

Maintain good housekeeping in compressor areas.

All mechanical parts should be regularly examined and overhauled.

Monitoring facilities such as temperature pressure and flow gauges should be regularly
tested and recalibrated were necessary.

The compressor should be bonded and earthed to prevent any static electricity buildup.

Discharge pipework should be cleaned using soap detergents and not petroleum based
products and wire brushes. Carbon deposits must not exceed 3 mm in thickness.
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
Oil collection points should be drained and all receivers blown down at least once per
shift.

Air and oil filters should be cleaned or replaced frequently.

Cooling systems should be flushed when necessary, removing rust or scale which may
interfere with heat dissipation.
With acknowledgement to:
1.
“Fire and Explosion Hazards in Compressors:, FPA(UK) Journal ‘Fire Prevention’
No. 122.
2.
“Explosions in Compressed Air Plant”, C C Kruger, ‘The Certificated Engineer’
October 1976
Published by
Fire Protection Association of Southern Africa
(Incorporated Association not for Gain)
(Reg.No. 73/00022/08)
P O Box 15467
Impala Park
1472
4/2000