Explosion Proof Air Compressors for Hazardous Environments

Explosion-proof compressors are a niche category in the whole compressor industry. Most compressor salespeople work five or six years without ever selling a single explosion-proof unit. But run into customers from petrochemical, pharmaceutical, or grain processing industries, and you can't avoid the topic.

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.

Dust environments correspond to Zone 20, 21, 22. Flour mill grinding rooms, feed mill crushing rooms, when dust concentration is high you can't see your hand in front of your face, that's Zone 21. Warehouse where dust occasionally kicks up, that's Zone 22.

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.

But some old plant sites weren't planned well during construction, compressor room and process units ended up close together, later when hazardous areas were re-designated, compressor room got circled in. This situation means either relocating the compressor room or replacing all equipment with explosion-proof, both options cost money.

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. Transport is also troublesome, needs helicopter lift or supply vessel transfer.

Gas station compressors often get overlooked. Gas stations need to inflate tires, where's that small compressor installed? Many stations put it under the canopy, not far from fuel dispensers. Per code, within 4.5 meters around fuel dispensers is Zone 1, if that small compressor falls in this circle, it needs to be explosion-proof. In reality lots of gas stations use ordinary household small compressors, pushing the boundaries.

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.

Solvent vapor is invisible and intangible, by the time your nose smells it concentration is already not low. Paint shop compressor selection, either put it in an independent compressor room, keep distance from spray booth; or go explosion-proof configuration.

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.

Grain processing, wood processing, feed mills, all high-incidence areas for dust explosions. Compressors in these places also need to consider explosion-proof issues.

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 are heavy. 55kW ones are 300-400 kg heavier than ordinary motors, handling and installation are laborious. Price is also expensive, over double is common.

Know how to read explosion-proof markings. Ex d IIB T4 Gb: Ex is explosion-proof marking, d is flameproof type, IIB is gas group, T4 is temperature class, Gb is equipment protection level. Take site gas parameters and match item by item, doesn't match then change equipment or change installation location.

Flammable gases are divided into IIA, IIB, IIC groups based on minimum ignition energy needed. Propane, butane belong to IIA, ethylene belongs to IIB, hydrogen and acetylene belong to IIC. IIC is hardest to deal with, just a tiny bit of energy can ignite it. Equipment rating must be greater than or equal to site gas rating, IIC equipment can go in any environment, IIA can only stay in IIA environments. Some owners buy IIC across the whole plant, spend more money to buy peace of mind.

Temperature class is easy to confuse. T4 doesn't mean equipment can work in 135°C environment, it means equipment surface temperature won't exceed 135°C. Compare against site gas auto-ignition temperature. Gasoline vapor auto-ignition temp is around 280°C, T4 is enough. Carbon disulfide is only 90°C, need T6 equipment. Compressor discharge end is a hot spot, normal operation 75 to 95°C, cooling system problems will spike higher. Most explosion-proof screw machines get T3 or T4 certification.

Inside control cabinets there's contactors, relays, VFDs, PLCs, buttons, switches. Contactors arc when engaging and breaking, that blue-white arc in Zone 1 is an ignition source. Explosion-proof control cabinets come in two types, flameproof and pressurized. Flameproof enclosure is thick cast iron or cast aluminum, joint surfaces machined per flameproof gap requirements. Pressurized type enclosure doesn't need to be as thick, but needs instrument air blown in to maintain positive pressure, flammable gas can't get in. Pressurized type control logic is complex, purging, pressure monitoring, loss-of-pressure interlock, whole system cost isn't low either.

Sensors can be explosion-proof type mounted directly on equipment, or ordinary type mounted in safe area with capillary tube or extension cable carrying signal. The latter is used more in retrofit projects.

Where cables enter explosion-proof equipment use cable glands with seals, compression nut tightened to compress the seal. During inspection wiggle the cable by hand, if it moves the nut isn't tight.

Domestically they recognize explosion-proof certificates, Nanyang Institute, Shanghai Institute, Tianjin Institute are the most common issuers. Certificate has explosion-proof marking, applicable location, gas group, temperature class, plus serial number you can verify online. Valid for five years, expires if not renewed.

Imported equipment comes with ATEX or IECEx certificates, some places safety bureau recognizes them some don't, to be safe do another type test domestically and swap for a domestic certificate. This process takes three to four months, arrange in advance if urgent.

Later maintenance, check flameproof joint surfaces annually, if rusted sand it down and apply anti-rust oil. Cable gland rubber seals replace in batches every three to five years. Replacement parts match original part numbers, spare parts list on housing isn't decoration.

If you can install in safe area don't move toward hazardous area. Build compressor room on perimeter of process area, run piping over, couple hundred meters pressure drop is a few tenths of a bar, acceptable. Piping use carbon steel seamless pipe, flanges use spiral wound gaskets, supports grounded, bonding jumpers every 30-50 meters. Do it this way and compressor can use ordinary screw machine.

Must install in hazardous area, honestly go full explosion-proof configuration, motor, control cabinet, sensors, junction boxes, buttons, full set of certificates. Get qualified contractor to do the work, do explosion-proof completion acceptance, keep reports on file.

There's also positive pressure ventilation solution, put ordinary equipment in enclosure and blow instrument air to maintain positive pressure. Before startup purge first, usually over five times enclosure volume of air, finish purging before energizing. During operation monitor enclosure pressure, lose positive pressure immediately de-energize. This system has lots of parts, complex logic, high maintenance, suits old equipment retrofits, new projects don't really use it.

Zone 2 can't use ordinary equipment. Zone 2 risk is lower than Zone 1, but Gc level equipment is still explosion-proof equipment, not the same as ordinary stuff. Money saved pushing boundaries, one incident and you lose it all back, plus criminal liability.

After repair must restore explosion-proof status. Flameproof enclosure opened for repair, putting back together all bolts must be fully tightened, can't skip a few to save trouble. Joint surfaces cleaned, can't have oil sludge or metal shavings scratched on. Cable glands re-threading cables must be compressed tight. Some workers don't have this awareness, repair done and looks about right so they think it's fine, explosion-proof performance is actually already gone.