Gaskets for compressed
air systems —
why standard EPDM
fails and NBR works
In many lubricated compressor systems, some oil carryover remains relevant at certain connection points unless confirmed otherwise by system design and filtration performance. EPDM swells and degrades in contact with petroleum-based oils. A standard EPDM gasket that seals correctly at installation may swell, lose its geometry, and fail within months in oil-contaminated compressed air service.
Scope note: This article is an application-selection reference. Final gasket choice depends on the exact medium, oil or glycol chemistry, concentration, pressure, temperature, flange condition and any site or regulatory requirement.
The first question: what is actually in the air at this connection?
Before selecting a gasket material for a compressed air connection, the relevant question is not "is this a compressed air system" — it is "what does the air at this specific connection contain?" The answer is not the same for every position in every compressed air system.
Contamination levels are indicative. Actual oil carryover depends on compressor type, age, maintenance condition and downstream filtration. Confirm from system documentation or air quality measurement before specifying gasket material.
A gasket on the discharge line of an unfiltered lubricated piston compressor and a gasket on an instrument air distribution point in the same building may both be described as "compressed air" connections — but the medium at each is significantly different. The selection rationale follows the medium, not the system name.
Why EPDM fails in oil-contaminated compressed air
EPDM — strong with water, weak with oils
EPDM has excellent resistance to water, steam, ketones, and many polar solvents. It is widely used in water supply, heating, and outdoor service. This is why it is the default elastomer in many plumbing and HVAC gasket products.
EPDM has limited resistance to petroleum-based lubricating oils. When exposed to oil, EPDM absorbs it — the elastomer swells as oil penetrates the polymer matrix. The degree of swelling depends on the oil type, concentration, temperature and exposure time. Minor swelling may initially improve the seal; significant swelling changes the gasket dimensions, reduces its mechanical strength, and eventually compromises the joint.
In a compressed air system with oil carryover, this process is slow but cumulative — the gasket may seal correctly for months before swelling becomes significant enough to produce a visible leak or a gasket that cannot be removed intact.
NBR — designed for oil and fuel service
NBR (nitrile butadiene rubber) was developed specifically for oil and fuel resistance. It is a widely used elastomer in hydraulic fittings, pneumatic components, and fuel system seals because of this oil resistance — it resists absorption of petroleum-based lubricating oils that would degrade EPDM.
The trade-off is that NBR has limited resistance to ozone, UV, and weathering — and poor compatibility with water-steam service at elevated temperature and with some polar solvents. An NBR gasket in a heating system or outdoor water service would be the wrong choice for the same reason EPDM is wrong in oil-contaminated air service.
For flat face BSP connections in compressed air systems where oil carryover is expected, NBR's oil resistance makes it a more appropriate base elastomer than EPDM — subject to confirmation of the specific oil type and operating conditions from the manufacturer's compatibility data.
What the failure looks like
EPDM degradation in oil-contaminated compressed air is typically a gradual process. The early stages may not produce an obvious leak — the swollen gasket may maintain its seal while changing dimensions. Signs that oil exposure may be occurring include:
- Gasket difficult to remove intact at maintenance: significant oil absorption makes the elastomer softer and stickier than a new gasket. A gasket that tears or deforms on removal — where the original installation was correctly executed — may have been affected by oil absorption.
- Visible swelling or change in profile: if the gasket OD or cross-section is visibly larger than a new gasket of the same specification, oil absorption has occurred. Compare against a new gasket of the same grade.
- Softened or tacky surface texture: the medium-contact face of the gasket may be soft or slightly sticky, while the back face retains its original texture. This differential degradation indicates the medium has been penetrating the material from the contact face inward.
- Progressive leak developing over months: a joint that was dry at commissioning and begins to weep after six to twelve months in service, with no change in operating conditions, can be more consistent with gradual medium incompatibility than with simple bolt load loss or face damage.
The failure mode tells you what to change. A gasket that shows material degradation — softening, swelling, changed texture — points to medium incompatibility. Replacing it with the same EPDM grade in the same compressed air service produces the same failure. The grade must change to match the actual medium. A gasket that is structurally intact but shows a light compression mark — no material degradation — points to load loss or face condition, not medium incompatibility.
When EPDM is still appropriate
Not all compressed air connections have the same medium. Where the air quality at the connection has been confirmed to contain negligible oil — oil-free compressor types, or lubricated compressors with high-specification coalescing filtration with confirmed residual oil content below the relevant threshold — EPDM may be appropriate for the connection gasket.
Condensate composition depends on the compressor and system. Oil-injected or lubricated systems can produce condensate containing oil, while verified oil-free systems have a different contamination profile. Where the primary concern at a connection is condensate handling — drain valves, moisture separators, after-cooler connections — the medium at that connection may be predominantly water, for which EPDM is well-suited. The relevant question is always what is actually present at the specific connection, not what is generally present in the system.
Do not assume the compressed air system is oil-free because it is a modern installation. The majority of workshop, industrial, and commercial compressed air systems use lubricated piston or screw compressors, which produce air with oil carryover regardless of age. Oil-free compressor types are typically specified explicitly for food, pharmaceutical, or laboratory applications. If the compressor type is not documented and the air quality has not been measured, a conservative assumption is that some oil carryover may still be relevant at the connection, unless the system is explicitly specified as oil-free.
Flat face BSP connections — what this means in practice
In compressed air distribution systems, the most common flat face threaded connections are BSP unions, compression fittings at pressure gauges and safety valves, and threaded blanking plugs. These are the positions where flat gaskets are used — at flat face unions and fittings rather than at tapered threads.
For these positions in oil-contaminated compressed air service:
- An NBR flat gasket — or an NBR O-ring in a union fitting designed for O-ring sealing — is generally more appropriate than EPDM for the reasons above. Confirm compatibility with the specific oil type and operating temperature from the manufacturer's data.
- The operating temperature in most compressed air systems is relatively modest — typically ambient to around 60–80°C in the pipework downstream of the aftercooler. This is within the temperature rating of standard NBR compounds for pneumatic service.
- Where the system uses synthetic compressor oils rather than petroleum-based lubricants, compatibility should be confirmed separately — some synthetic lubricants affect NBR differently from mineral oils.
The medium at the fitting is the selection input — not the system label.
Compressed air from a lubricated compressor contains oil. EPDM swells in oil. NBR resists oil. The selection logic follows from these three facts — but only at the connections where oil-contaminated air is actually the medium. For oil-free air, condensate drains, or instrument air with confirmed quality, the rationale may be different. Assess the actual contamination profile of the air at each connection position before specifying. A gasket grade that is correct for one position in a compressed air system may not be correct for another.
Practical FAQ
Why can EPDM fail in some compressed air systems?
EPDM rubber has excellent compatibility with water, steam, and many polar solvents — but it has limited resistance to petroleum-based oils and lubricants. Most piston and lubricated screw compressors produce air that contains some level of oil carryover — oil aerosol and vapour from the lubrication system that is not fully removed by aftercoolers or filters. When an EPDM gasket is exposed to oil-contaminated air over time, the elastomer can absorb oil and swell, depending on compound formulation, oil type, temperature and exposure duration. This can alter gasket dimensions and reduce mechanical stability over time. In flat face joints, this can produce a gasket that initially seals and progressively loses its geometry and structural integrity as the oil absorption continues — eventually producing a leak or a gasket that is no longer removable intact.
Is all compressed air the same from a gasket selection perspective?
No. The relevant question is what the compressed air actually contains at the point of the connection. Compressed air from a lubricated piston or screw compressor typically contains some oil carryover, even after filtration — the degree depends on the compressor type, age, maintenance condition, and the specification of any oil separation and filtration equipment downstream. Oil-free compressors — scroll, dry screw, or centrifugal types — produce air without intentional lubrication oil, though condensate may still be present. Instrument air and clean dry air systems with high-specification filtration produce air with very low contamination levels. The gasket selection should reflect the actual contamination profile of the air at that connection position — not the general label of 'compressed air'.
What gasket material is most appropriate for connections in compressed air systems with oil carryover?
For flat face threaded connections — such as BSP unions and compression fittings — in compressed air systems where oil carryover from the compressor is expected, NBR (nitrile rubber) is generally a more appropriate base elastomer than EPDM. NBR has good resistance to petroleum-based lubricating oils and is widely used in pneumatic and hydraulic fittings for this reason. The specific compound formulation, hardness, and operating temperature also affect the outcome — confirm from the gasket manufacturer's compatibility data for the specific oil type and temperature. For oil-free air systems or filtered instrument air with confirmed low oil content, EPDM may be acceptable — the contamination profile of the actual air determines which is appropriate.