Explosion Proof Air Compressors for Hazardous Environments

IEC and ATEX divide hazardous areas into Zone 0, 1, and 2. Zone 0 basically never gets equipment installed. Inside storage tanks, inside reactors, people don’t even go in there, let alone compressors. What you actually deal with most is Zone 1 and Zone 2.

Zone 1 is places like pump houses, compressor buildings. During normal operation, valve packing might leak, mechanical seals might seep, probability of flammable gas in the air isn’t low. Zone 2 is more relaxed, inside well-ventilated buildings, normally fine, gas only accumulates if something goes wrong.

Design institutes mark hazardous area boundaries on the floor plan when drawing up plans, circled in red lines. One meter outside the boundary is Zone 1, further out might be Zone 2, further still is safe area. Whichever circle your compressor sits in, that’s the rating you select.

Refinery and chemical plant compressor rooms are usually built on the perimeter of the process area. There’s reasoning behind this: the process area is running oil and gas, put the compressor room far away, designate it safe area, equipment selection is simple, maintenance is convenient too. Piping from compressor room to use points, 200-300 meters is common.

Offshore platforms have no choice. Wellheads, separators, metering skids all on deck, entire platform except living quarters is basically Zone 1. Compressors can only use full explosion-proof configuration, a 30 cubic meter screw machine, price on platform is two to three times same-spec equipment on land.

Solvents evaporate in spray booths, xylene, butyl acetate, butanol, these substances have low lower explosive limit concentrations. Once spraying starts, shop concentration shoots up. Many factories install compressors in the room next to the spray booth, separated by one door, thinking that’s enough. Door doesn’t close tight, or workers leave it open for convenience, gas drifts over, compressor motor starts up, done.

Historical Incident — Harford Attachments: There was a company in Norwich, UK called Harford Attachments making excavator parts, had an incident in 2015. Two workers in the spray booth painting buckets, fireball engulfed them, one 56 years old, one 28. HSE went to investigate, found this company had moved to new premises, spray booth was newly purchased, equipment itself was fine, but nobody did a risk assessment, nobody wrote safety procedures, spray booth safety “got forgotten” during the move.

Gas leaks at least you can smell, dust accumulation is chronic, one layer today another layer tomorrow, by the time you notice the problem it’s everywhere.

Historical Incident — Imperial Sugar: There was a sugar factory in Georgia, USA, Imperial Sugar, in Port Wentworth, built in 1917, operated for almost a hundred years. Evening of February 7, 2008 at 7:15 PM, conveyor area below the sugar silo exploded first, shockwave kicked up sugar dust deposited throughout the facility, formed a dust cloud, then secondary explosion, third explosion, one after another. 14 people died, 36 seriously injured. CSB investigation report was very detailed. Sugar factory management knew sugar dust had explosion risk since 1958, issued internal documents, but fifty years passed with no substantive improvement.

Motors are compressors’ biggest ignition source. Ordinary motor housing is just a sheet metal shell, windings and terminals inside might spark during operation, sparks escape and ignite flammable gas outside. Flameproof motors take a different approach. Housing uses thick cast iron or cast steel, wall thickness can withstand internal explosion. If it explodes inside, flame wanting to escape has to travel through this narrow gap, metal absorbs the heat, by the time flame reaches outside it’s cooled down, can’t ignite external gas. Flameproof motors are heavy. 55kW ones are 300-400 kg heavier than ordinary motors. Price is also expensive, over double is common.

Increased safety motors take a different approach, preventing sparks from the root. Windings thoroughly varnish-impregnated, terminal distances increased, good bearings used, entire motor won’t produce sparks or dangerous high temps throughout design life.