Ask a plant electrician what it costs to put a 5 HP electric motor on a stirrer inside a Class I Division 1 area and watch the face they make. Explosion proof enclosure, certified glands, conduit seals, the grounding scheme, the inspection, the paperwork. Then ask what it costs to hang an air motor on the same shaft and run a hose to it.
That gap is why compressed air for oil and gas is not a support utility the way it is in a machine shop. In a classified area, air is frequently the cheaper and safer way to move something, and the whole facility gets designed around that.
Where the air actually goes
Three distinct jobs, and they want different things from the system.
- Instrument air. Control valve actuators, positioners, on/off valves, analyzer purge, transmitters on older installations. Small flow, absolutely cannot fail, and needs to be very dry.
- Pneumatic drives and tools. Mixers, agitators, hoists, winches, pumps, grinders, impacts, drills. Larger flow, intermittent, and this is where the classified area argument lives.
- Utility and plant air. Blow down, purging, the shop, the truck bay, general maintenance.
The mistake we see most often is running all three off one system dried to the loosest requirement on it. Instrument air sets the dew point for everything it touches, and if you have not separated the branches you have effectively promised instrument air quality to the truck bay too.
Why pneumatic drives win in a classified area
This is the part worth being specific about, because the reasoning is not just tradition.
No ignition source by design. An air motor has no windings, no brushes, no commutator and no electrical connection. There is nothing in it to arc when a connection loosens or a brush wears. You are not managing an ignition risk, you have removed the category.
It runs cool. Expanding compressed air absorbs heat, so an air motor actively cools itself in operation rather than heating up. In an enclosed space around flammable vapor, surface temperature is a real design constraint, and this is a genuine advantage rather than a marketing line.
Stalling is a non-event. Stall an electric motor and you are into locked rotor current, heat, and a protection scheme that has to act correctly. Stall an air motor and it simply stops, holds torque, and starts again when the load clears. For a hoist, a winch or an agitator that can jam, that behavior is worth a great deal.
The install is simpler and cheaper. No explosion proof enclosure, no conduit seals, no certified glands, no grounding scheme for that device. A hose and a fitting.
Infinitely variable speed, no drive. Throttle the air and the motor changes speed. No VFD, no drive enclosure, no harmonics.
The honest trade-offs, because there are some:
| Factor | Air motor | Electric motor |
|---|---|---|
| Energy efficiency | Poor. Compressed air is expensive energy. | Much better |
| Install cost in Class I Div 1 | Low | High |
| Behavior at stall | Stops, holds torque, self recovers | Locked rotor, needs protection |
| Speed control | Throttle the air | Needs a drive |
| Noise | Loud, exhaust needs a muffler | Quieter |
| Power density | High for the size and weight | Lower |
So the rule of thumb: continuous duty in a safe area, use the electric motor. Intermittent duty, classified area, stall risk, or somewhere you do not want to run conduit, use the air motor. Our vane versus piston air motor guide covers picking between the two designs, which comes down to whether you need starting torque or speed.
One thing people miss on air motors: most of them want lubricated air. Starving one of lubrication does not just wear it out, it raises internal friction heat, and in a classified area that can compromise the temperature class the motor was assessed at. Fit the lubricator, keep it full, and check it. Our FRL guide covers the setup.
Instrument air, and the standard behind it
Instrument air is the branch that cannot fail, and it has a defined quality target rather than a vibe.
The working standard is ISA-7.0.01, and the dew point requirement is expressed usefully: pressure dew point at least 18 degrees F below the lowest ambient temperature any part of the system is exposed to. On a wellsite in North Dakota that is a genuinely low number, and it rules out a refrigerated dryer immediately.
What that means in practice:
- Desiccant drying, not refrigerated. A refrigerated dryer holds about a 38 to 40 degree pressure dew point and cannot go below freezing by design. Outdoor instrument air needs a desiccant dryer. Our comparison of the two covers where the line falls.
- Oil free, or filtered like you mean it. Oil on a positioner seat is a stuck valve. Particulate matters too, because the orifices in instrument air are small.
- An afterfilter behind the desiccant dryer. Beads shed fines and those fines plug instrumentation. Non negotiable.
- Redundancy and backup volume. Instrument air failure means valves go to their fail position across the facility. Size dedicated receiver volume for a controlled shutdown, not for convenience.
If the dryer stops holding its number, work it in order rather than swapping parts. Our desiccant dryer troubleshooting guide walks the sequence.
Sizing and specifying the plant
Being straight about scope first. A fully engineered API instrument air package for a refinery, or a gas driven compressor for a remote wellsite with no grid power, is specialty equipment and comes from a package house. What we can do well is the rest: the plant air and utility compressors, the drying and filtration train, receivers, piping, and the air motors and tools that run in the classified areas.
How to approach it:
- Separate the branches. Instrument air gets its own treatment and its own reserve. Do not dry the truck bay to minus 40 to serve a positioner.
- Count air motors honestly. They are efficient users of your money and inefficient users of air. A handful of continuously running air motors will dominate your plant demand, and that is exactly the case where you should ask whether an electric motor in a safe area with a shaft or a hydraulic drive is cheaper over ten years.
- Size storage for the swing. Hoists, winches and blowdowns are step loads. See our note on air receivers.
- Specify for the environment. Outdoor packages, sour service areas, dust, heat, cold starts, and cooler cores that survive the local air. Intake placement matters more here than almost anywhere, because the ambient air may be the thing you are trying to keep out.
- Ask for the CAGI data sheet. These machines run continuously and energy dwarfs purchase price. See how to read a CAGI data sheet.
- Pipe it in something that survives. Black iron outdoors in a wet, corrosive environment rusts from both sides. See pipe materials compared.
What we see go wrong
- Treating air as free because it is not metered. Compressed air is the most expensive utility per unit of work in most facilities, and a bank of air motors running continuously is a large power bill wearing a disguise.
- One dryer for everything. Either the instrument air is underdried or the whole plant is overdried. Both cost money, in different ways.
- No lubricator on the air motors. Then wondering why they fail early, and not realising the temperature class assessment assumed lubrication.
- Ignoring leaks. A facility running 8,760 hours with hundreds of outdoor connections that thermally cycle has a leak rate worth finding. See leak detection.
- Unmuffled air motor exhaust. It is loud, and in an enclosed space it is a hearing conservation problem you created for free.
- No instrument air reserve. Finding out how long you have to safe the unit during the event is the wrong time to find out.
Frequently Asked Questions
Why does oil and gas use air motors instead of electric motors?
Because in a classified area an air motor has no ignition source at all. No windings, no brushes, nothing to arc. It also runs cool since expanding air absorbs heat, it stalls safely without locked rotor current, and it avoids the cost of explosion proof enclosures, conduit seals and certified glands. The trade is energy efficiency, so it is the right answer for intermittent duty in hazardous areas and the wrong answer for continuous duty in a safe one.
What dew point does instrument air need?
The ISA instrument air standard puts it at 18 degrees F below the lowest ambient temperature any part of the system sees. For outdoor installations in a cold climate that is well below zero, which means a desiccant dryer. A refrigerated dryer cannot go below freezing and does not qualify for outdoor instrument air.
Do air motors need lubricated air?
Most do, and it matters more in a hazardous area than elsewhere. Running one dry wears it out and raises internal friction heat, which can push it past the surface temperature class it was assessed at. Fit a lubricator, keep it filled, and put it on the maintenance schedule.
Should instrument air and plant air be the same system?
They can share a compressor, but they should not share treatment or reserve. Instrument air needs its own drying to its own dew point target, its own filtration including an afterfilter, and dedicated receiver volume sized to let you safe the facility if the compressor stops. Drying the entire plant to instrument air spec is expensive and unnecessary.
Are air motors expensive to run?
Yes, on energy. Compressed air is an expensive way to deliver mechanical work, and an air motor running continuously costs far more in power than an equivalent electric motor. The economics work when you count what you avoid: explosion proof enclosures, conduit and seals, drives, and the downtime from a stall event. Count the hours honestly before deciding.
Where to start
Draw your three branches separately and be honest about which loads genuinely need instrument air quality. Set the dew point from the coldest exposure, not the equipment room. Then count your air motors and their run hours, because that number tells you whether your air system is sized for tools or for production, and the answer changes what you buy.
Browse our air motors, or send us your classified area equipment list and we will work through what should be pneumatic and what should not.
