Where seals are used on a compressor
A typical oil-injected screw compressor contains dozens of static and dynamic seals, and a reciprocating compressor has its own set. Each one sees a different combination of fluid, heat and movement.
- Airend and gearbox – shaft seals, end-cover O-rings and gaskets, bearing housing seals.
- Oil circuit – oil filter seals, separator element and cover gaskets, thermostatic valve O-rings, oil cooler connections, hose fittings.
- Air circuit – inlet valve and unloader diaphragms and O-rings, minimum-pressure valve seals, discharge flange gaskets, aftercooler connections.
- Reciprocating machines – cylinder head and valve cover gaskets, valve seat gaskets, crankcase cover and breather seals, piston rod packing on crosshead machines.
- Dryers, filters and drains – filter housing O-rings, drain valve diaphragms, desiccant tower seals.
- Cooling water – water-cooled heat exchanger gaskets and end-cover seals.
A leak at any of these points wastes oil or air, and some failures, such as a minimum-pressure valve seal or a separator gasket, change how the machine behaves.
Common elastomer and gasket materials compared
The table gives general, widely published behaviour of each material family. Individual compounds vary a great deal, so the maker’s part specification and the compound supplier’s data always take priority.
| Material | General strengths | General limits | Typical compressor uses |
|---|---|---|---|
| NBR (nitrile) | Good with mineral oils; tough; economical | Limited high-temperature life; can harden with heat and some synthetic oils; weak to ozone | General O-rings and gaskets on mineral-oil machines; piston compressors |
| HNBR | Better heat and oil resistance than NBR | Higher cost; still compound-specific with some fluids | Higher-temperature oil-circuit seals |
| FKM (fluoroelastomer) | Good high-temperature and broad oil resistance | Some compounds are attacked by certain ester-based fluids; poorer at low temperature | Screw airend, thermostatic valve and hot oil-circuit seals |
| EPDM | Good with water, steam and weathering | Swells badly in mineral and many synthetic oils | Cooling water circuits only, never in contact with compressor oil |
| Silicone | Wide temperature range; flexible | Low tear strength; poor with many oils | Some oil-free and food-grade air applications, static seals |
| PTFE and filled PTFE | Very wide chemical and temperature resistance; low friction | Creeps under load; not elastic, needs correct design | Shaft lip seals, piston rod packing, back-up rings, oil-free machines |
| Compressed fibre and graphite sheet | Stable flat gasket for flanges and covers | Needs correct bolt load; can relax with heat cycling | Flange and cover gaskets |
The most common mistake is fitting an EPDM O-ring somewhere it meets oil. It looks identical to a black nitrile ring, but it can swell and soften quickly and then leak or extrude.
What decides whether a seal will last
- Fluid. The lubricant type matters as much as the seal material. Mineral, PAO synthetic, ester, polyglycol and food-grade fluids each affect elastomers differently. A seal that is fine in one oil can swell, shrink or harden in another.
- Temperature. An oil-injected screw compressor normally discharges at about 75–95 °C, and seals near the airend and separator see this continuously, with peaks on hot Indian summer afternoons. Long periods at high temperature age elastomers faster.
- Pressure and cycling. Load and unload cycles flex seals and can extrude soft O-rings into clearance gaps. Higher pressures may need harder compounds or back-up rings.
- Geometry. Groove depth, width, surface finish and flange flatness must match the seal. An O-ring of the right inside diameter but wrong cross-section will not seal reliably.
- Movement. Static seals, rotating shaft seals and reciprocating rod seals each need different designs.
- Installation. Nicked, twisted or dry-fitted seals fail early regardless of material.
Changing oil can change seal behaviour
Changing from one compressor lubricant to another, particularly from mineral to synthetic or from one synthetic family to another, can affect seals that have been in service for years. Old nitrile seals that have adjusted to one oil may shrink when a different fluid removes the plasticisers they had absorbed, and leaks appear a few weeks after the changeover.
Before changing oil type, check with the compressor and lubricant makers whether the fitted seals are compatible, plan a flush where needed, and be ready to replace seals in the oil circuit at the same time. Our lubricant changeover page covers the full process.
Why seals fail: reading the evidence
| What you see on the old seal | Likely cause |
|---|---|
| Swollen, soft or sticky | Incompatible fluid, such as EPDM in oil or the wrong compound for the lubricant |
| Hard, cracked, flat on one side | Heat ageing and compression set; machine running hot for long periods |
| Nibbled or torn edge | Extrusion into a gap from high pressure or excess clearance |
| Cut or spiral marks | Damage during fitting; sharp edges; twisting in the groove |
| Worn flat on a moving face | Abrasion, dirt, poor surface finish or lack of lubrication |
| Flat gasket leaking with no visible damage | Low or uneven bolt load, flange not flat, gasket relaxed after heat cycles |
Keeping a failed seal and photographing it before it is thrown away often tells the engineer more than the leak itself.
Good practice when replacing seals
- Identify the machine from its nameplate and the assembly from the parts book or the old part before ordering.
- Match material, hardness, cross-section and size, not just the inside diameter.
- Clean grooves and flanges; remove old gasket material without scoring the surface.
- Lubricate O-rings with a fluid compatible with both the seal and the compressor oil.
- Use protective sleeves over threads and keyways when fitting shaft seals.
- Tighten flanges and covers evenly to the maker’s torque in the correct sequence.
- Do not reuse gaskets or O-rings that have been compressed in service.
- Run the machine to temperature and check for leaks before handing it back.
- Store spare seals cool, dark and sealed, away from ozone sources such as electric motors and welding plant.
Flat gaskets and bolted joints
Flat gaskets on discharge flanges, cylinder heads, valve covers and cooler end covers depend on the joint as much as the gasket. The gasket only seals when the bolts apply enough even load to make it fill the surface irregularities, and that load must survive heating, cooling and vibration. Too little load and the joint weeps; too much and a soft gasket is crushed or extruded and a cast cover can crack.
Check that both faces are flat and clean, that the gasket thickness and material match the original (a thicker gasket can change clearances on some valve and head joints), and that bolts are tightened in a cross pattern in stages. On hot joints, a recheck after the first heat cycle is often recommended by gasket makers.
Special duties
Oil-free compressors supplying pharmaceutical, food and beverage plants may require seals made from materials suitable for those industries, and the maker’s specification should be followed exactly. Gas compressors handling CNG, nitrogen or process gases need seal materials chosen for that gas, its pressure and any decompression effects; oxygen, hydrogen and toxic gas duties follow their own gas-specific codes and the maker’s design rules, and generic air-compressor seals must never be substituted there.
How Reddy Compressors helps
Reddy Compressors supplies spare parts, including gasket sets, O-ring kits, shaft seals and service kits, for ELGi, Atlas Copco, Ingersoll Rand, KAESER, Chicago Pneumatic, Sullair, Doosan, Kirloskar and other makes, as an independent multi-brand specialist rather than an authorised dealer. We identify parts from the machine’s make, model, serial, nameplate photo and the old part, and confirm the fit before quoting. Parts are supplied across India.
Our service team of more than 20 service engineers and technicians, based at Tarnaka, Secunderabad and covering Telangana and Andhra Pradesh, traces recurring leaks to their cause, whether that is material, temperature, lubricant or fitting, and replaces seals during service, overhaul and oil changeovers.
What to send us
- Compressor make, model and serial number, with a nameplate photo.
- Photo of the leaking area and, if removed, the old seal or gasket.
- The oil currently in use, and any recent oil change.
- Parts book reference or old part number, if available.
- Operating pressure and normal discharge temperature.
Frequently asked questions
Can I use any nitrile O-ring on my screw compressor?
Not safely. Nitrile is a family of compounds with very different heat and oil resistance. Seals near the airend, separator and thermostatic valve run hot continuously, and many makers specify fluoroelastomer or a high-temperature compound there. The O-ring must also match the groove’s cross-section and the lubricant in use. Identify the original part from the parts book or nameplate before choosing a replacement.
Why did my compressor start leaking oil after an oil change?
A change of lubricant type can affect seals that have been in service for a long time. Older nitrile seals may shrink or harden when a different oil removes the plasticisers they had absorbed, and leaks then appear within weeks. Check that the new oil is approved for the machine and its seals, and plan seal replacement in the oil circuit as part of any changeover.
What is the difference between NBR and Viton seals for compressors?
NBR (nitrile) works well with mineral oils at moderate temperatures and is economical. Viton is a trade name for a type of FKM fluoroelastomer, which tolerates higher temperatures and many more fluids, so it is often used in hot oil circuits of screw compressors. FKM is not universally compatible either, and some compounds react badly with certain fluids, so the compressor maker’s specification decides.
Can an EPDM O-ring be used in a compressor oil system?
No. EPDM resists water, steam and weathering well, but it swells and softens in mineral oils and most compressor lubricants. In an oil circuit it can lose its shape within a short time, leak or extrude out of the groove. EPDM belongs in cooling-water circuits and some air-only duties. Because it looks like nitrile, label and store seals carefully to avoid mix-ups.
Can I reuse a compressor gasket or O-ring?
It is not good practice. A gasket or O-ring that has been compressed and heated in service takes a permanent set and will not reseal reliably when refitted, especially on hot oil-circuit and discharge joints. The small saving is not worth the risk of a leak or an oil spray inside the enclosure. Replace seals whenever a joint is opened, using parts matched to the assembly.
How should spare O-rings and gaskets be stored?
Store them in sealed bags in a cool, dry and dark place, away from direct sunlight, ozone sources such as motors and welding sets, and solvents. Keep them flat and free of tension or kinks, and label each pack with the material, size and machine it belongs to. Rotate stock so older items are used first, and inspect for cracking or stickiness before fitting.
Related pages
Technical content reviewed by D. Srinivas Reddy, Managing Partner and service engineer, Reddy Compressors. Last updated 17 October 2026. How our engineering content is reviewed.