An air starting system contains compressed air (up to 30 bar). An explosion can occur if the air becomes contaminated with oil/unsaturated hydrocarbons and the mixture is ignited. Contributing factors:

1. Oil contamination: worn oil scraper/compressor rings or faulty lubrication allow lubricating oil to pass into the air and condense as an oil mist, forming a flammable hydrocarbon/air mixture.
2. Carbon deposits and tarry products from a compressor, which are fuel for a fire/explosion.
3. Ignition source: a spark from static electricity, friction, or a hot spot, or on modern engines the auto-ignition temperature being exceeded locally.
4. Overheating of the delivered air causing auto-ignition of the oily mixture; or back-leakage of hot gas from the engine.
5. Accumulation of flammable gas (methane) from a defective fuel/air mixing, or decomposition products.
6. A high compression ratio and long residence time allowing spontaneous ignition at raised pressure and temperature.

Propagation: once ignited, the oil mist flame travels at high speed through the pipelines, the pressure builds rapidly and, because the mixture is confined, an explosion over-pressurises the pipe; the flame can pass through the valves into the engine room or back into the receivers.

• Ensure the starting air is clean and dry by maintaining proper oil-free compressors (correct piston ring condition and lubrication) and installing efficient filters and a coalescer/dryer so the air delivered is free of oil mist and moisture.
• Drain the air receivers and lines regularly of any accumulated water/oil.
• Maintain the system temperature within safe limits; avoid high-temperature delivery.
• Use of drains/drips to prevent oil accumulation at low points.
• Regular inspection, replacement of degrading hoses and testing - the system is surveyed and pressure tested as required by class.
• Keep the system free of ignition sources by using non-sparking valves/piping material, appropriate earthing (no static build-up).
• Independent shutdown/thermal protection of the compressor (high-temperature cut-out) and the use of safe design pressure and relief valves on the receiver.

These safety additions relieve the pressure if an explosion occurs:

1. A bursting disc / rupture disc fitted in a relief branch of the air receiver or pipeline: it ruptures at a set overpressure, venting the gas quickly.
2. A spring-loaded safety (relief) valve on the receiver which opens at a preset pressure to vent.
3. Fusible links / relief plugs.
4. A venting (safety) flap or deflector fitted so that any gas or flame is directed away from personnel and hazardous locations.
5. The general relief arrangement ensures that if burning occurs, the pressure is relieved before the line can fail catastrophically, and so that the flame jet is diverted to a safe area.

A bursting disc or relief valve that is correctly rated may still not prevent a serious explosion because:

1. The explosion (deflagration) develops extremely rapidly - the pressure rise can outpace the relief flow, so even a fully open relief valve has insufficient area or response speed to reduce the peak pressure; the disc is sized for slow pressure rise and the venting occurs too late.
2. The energy released depends on the volume of the receiver/line - if several receivers and a long pipe are coupled, a large mass burns and the expanding flame and pressure wave can exceed venting capability.
3. On many systems a flame arrester is not installed in the vent, so the vented jet of hot gas/oil can be ignited by the same source or the vent itself can discharge flames into the machinery space.
4. Personnel may be exposed to the venting jet; and the vent system may not direct the blast safely. Furthermore a bursting disc correctly rated for constant service may still be ruptured at a pressure that is too high relative to the explosive rise, or the flame can travel unarrested along the pipes to other parts of the machinery space, causing secondary ignition. So a single relief device is not enough without explosion-relief vents, flame arresters and keeping the system oil free.