How air compressors are built
A piston compressor has a cast-iron crankcase and cylinders, aluminium or cast-iron heads, valve plates, a crankshaft and connecting rods, driven by an electric motor or engine through a belt or directly. It usually sits on a welded steel receiver, the tank that stores air and smooths pressure. A rotary screw compressor uses a pair of rotors in an air-end, often bought from a specialist maker, packaged in a cabinet with a motor, oil separator, cooler, controller and sometimes a dryer.
Two parts carry most of the risk. The receiver is a pressure vessel: it is rolled and welded from plate, fitted with bosses, and hydrostatically tested. And the delivery figure on the brochure, the free air delivery (FAD), is often quoted generously. A factory that sells compressors for export should have a test bed with an orifice or flow meter, pressure and temperature measurement, and a power meter.
What we check on air compressors
| Check | How it is checked | What fails |
|---|---|---|
| Free air delivery and power | Units chosen at random; factory's ISO 1217 test witnessed at rated pressure; inlet conditions, speed, flow and shaft or electrical power recorded | FAD or specific power outside the tolerance agreed in the order |
| Pressure controls | Pressure switch cut-in and cut-out, or loading and unloading pressures on screw units, read from a calibrated gauge | Unit runs above its rated pressure or will not reach it |
| Safety valve | Set pressure on the valve and its certificate compared with the receiver's maximum allowable pressure; valve lifts during the run where it can be tested | Set above the receiver rating; no certificate; valve missing or sealed shut |
| Receiver | Nameplate, maximum pressure, volume, serial number, CE or other marking and hydro test record matched to the certificate; welds and drain checked | Nameplate missing or not traceable; weld defects; no drain |
| Guards | Belt guard, fan guard and hot-surface protection fixed and needing a tool to remove | Belt or fan reachable by hand |
| Running checks | Oil level and leaks, air leaks at joints (soap test), discharge temperature, vibration, direction of rotation | Oil or air leaks; overheating; wrong rotation |
| Electrical | Factory's earth continuity and insulation tests witnessed; overload protection, emergency stop, cable glands | Earth missing; no overload protection |
| Noise label | Guaranteed sound power level label present on units in scope; factory's sound measurement witnessed where the order asks | No label on a unit sold in the EU |
Free air delivery tests and receiver certificates
ISO 1217 specifies acceptance tests for the volume flow rate and power requirements of displacement compressors, which include piston and screw machines, and it has a simplified test for complete electrically driven packaged compressors. The flow is measured at the discharge and converted to the conditions at the compressor's inlet: that converted figure is the free air delivery. Piston compressor brochures sometimes quote displacement, the swept volume of the cylinders, which is always higher than FAD. The order should say "FAD to ISO 1217 at rated pressure" so there is no room for that confusion.
The inspector picks units, checks the flow meter or orifice and gauges are calibrated, and records inlet temperature and pressure, speed, discharge pressure, flow and power. Clear To Load witnesses the factory's test; it does not own a test bed. For an independent result, we arrange an accredited lab through lab testing coordination.
For the receiver, we review documents rather than design. In the EU, series-made simple welded vessels for air or nitrogen within set pressure and volume limits fall under the Simple Pressure Vessels Directive 2014/29/EU; receivers outside those limits fall under the Pressure Equipment Directive 2014/68/EU. Both lead to CE marking with a conformity route that may involve a notified body. US buyers often specify receivers built and stamped to the ASME Boiler and Pressure Vessel Code. We check the nameplate, stamp and certificates against the vessel, witness the factory's hydro test where the order asks, and match plate thickness to the drawing. Weld radiography, if required, is done by the factory's certified technicians or an outside agency and reviewed by us.
Common defects in air compressors
| Defect | Why it matters | Class |
|---|---|---|
| Safety valve missing, set too high or not certified | Receiver can be over-pressurised and burst | Critical |
| Receiver without traceable nameplate or certificate | Vessel cannot legally be sold or put in service | Critical |
| Belt or fan unguarded | Entanglement injury | Critical |
| No protective earth | Electric shock | Critical |
| FAD below the agreed tolerance | User needs a larger compressor than they paid for | Major |
| Oil or air leaks during the run | Lost air, oil carry-over, fire risk in screw units | Major |
| Noise label missing on an EU unit in scope | Unit cannot be placed on the EU market | Major |
| Paint damage, untidy wiring inside the cabinet | Appearance; can hide later damage | Minor |
Market rules for compressors
- Outdoor Noise Directive 2000/14/EC. Still in force in the EU, amended in 2024. Compressors below 350 kW are among the equipment with noise limits and must carry the CE mark and a guaranteed sound power level label, with an EC declaration of conformity; a notified body takes part in the conformity procedure. We check the label and declaration against the unit.
- Machinery rules. A compressor is machinery: Directive 2006/42/EC applies until 19 January 2027 and Regulation (EU) 2023/1230 from 20 January 2027. See CE and UKCA marking.
- Electrical. The EMC Directive and the Low Voltage Directive's safety objectives apply to the electrical equipment. See LVD and EMC. Motors bought separately are covered on the electric motor page.
- Pressure. Receivers under the Simple Pressure Vessels Directive or the PED, as above; in North America, ASME-stamped receivers are a common requirement.
Packing, transit and your purchase order
Compressors are top-heavy on their receivers and can tip in a crate. They should be bolted to a skid or pallet, with gauges and filters protected, and receivers drained of test water and dried to stop internal rust. Follow the maker's instructions on whether oil stays in the crankcase or air-end for transit. Crates must be ISPM 15 treated; see ISPM 15. The inspector checks fixings, drained receivers, manuals and certificates in each crate.
Put these in your purchase order:
- FAD to ISO 1217 at rated pressure, specific power and the tolerance you accept.
- How many units per model are delivery-tested in your presence.
- Receiver standard, maximum pressure, volume and the certificates you expect.
- Safety valve set pressure and certificate.
- Motor rating, voltage, frequency and plug or terminal arrangement for your market.
- Noise label, CE or UKCA documents and manual language.
- Packing, skid design and crate marking.
Book an inspection anywhere in India.
Sources
- ISO 1217:2009, Displacement compressors: Acceptance tests (BSI Knowledge)
- Directive 2014/29/EU on simple pressure vessels (EUR-Lex)
- Directive 2014/68/EU on pressure equipment (EUR-Lex)
- Directive 2000/14/EC on noise emission by equipment for use outdoors (EUR-Lex)
- European Commission: Machinery (Directive 2006/42/EC and Regulation (EU) 2023/1230)