Explosive decompression
in O-rings —
why the seal fails
from the inside
Under high pressure, gas permeates into the O-ring material. When pressure is released quickly, that gas cannot escape fast enough. It expands inside the elastomer, nucleating bubbles that blister, crack or rupture the material from within. The O-ring looks intact until it does not. The primary failure mode in this pattern is decompression — not normal wear, not chemical attack, not simple compression set. The cross-section confirms it.
Scope: This article explains the explosive decompression / rapid gas decompression failure mechanism. It is not a material qualification for high-pressure gas, sour gas, CO₂, hydrogen or oxygen service. Seal selection for those duties must be based on compound-specific RGD test data, the actual gas mixture, pressure, temperature, decompression rate and the relevant project documentation.
The mechanism — what happens inside the elastomer
Elastomers are not impermeable to gas. Under elevated pressure, gas molecules — particularly small molecules such as CO₂, H₂S, N₂, CH₄ and high-pressure hydrocarbon gases — permeate into the elastomer matrix over time. The amount absorbed is related to the gas type, the partial pressure, the temperature and the specific elastomer compound. This absorption is a normal physical process and does not itself damage the seal.
The problem occurs at depressurisation. When system pressure drops rapidly, the gas dissolved within the elastomer matrix is no longer in equilibrium with the external pressure. The dissolved gas attempts to leave the elastomer — but it can only escape by diffusing to the surface, which takes time. If the pressure drops faster than the gas can diffuse out, the excess gas forms bubbles within the elastomer matrix rather than migrating to the surface as a controlled outgassing.
Schematic illustration. The rate and severity of damage depend on gas type, pressure, decompression rate, elastomer compound, temperature and cross-section geometry.
The bubbles formed within the elastomer grow as the expanding gas continues to drive internal pressure. If the internal bubble pressure exceeds the tensile strength of the surrounding elastomer at that point, the material fails locally — producing blisters at the surface where bubbles break through, or internal voids and fractures where the matrix tears between adjacent bubble sites.
What makes explosive decompression damage recognisable
The damage pattern from explosive decompression has a characteristic appearance that, in most cases, distinguishes it from other O-ring failure modes.
Explosive decompression — typical signs
Blisters: dome-shaped protrusions on the O-ring surface, ranging from small pits to large raised bubbles depending on severity. May have burst, leaving craters or eruption points.
Internal voids: visible when the cross-section is cut — hollow cavities within the elastomer body, often with clean or slightly torn walls.
Porous or cratered cross-section: in severe cases, the cut cross-section looks Swiss cheese-like rather than solid rubber.
Irregular surface cracking: branching cracks emanating from burst blisters, not following the compression surfaces.
Other failure modes — how they differ
Chemical attack / swelling: uniform volume increase, softening, surface tackiness. No internal voids. The whole O-ring is affected, not localised blisters.
Compression set: flat face on the compressed side. No internal damage. Volume unchanged.
Extrusion: nibbling or tearing at the low-pressure edge. Damage at the groove exit, not internal.
Abrasion / wear: surface material removed progressively, smooth worn face. No internal voids.
Do not discard the O-ring before examining the cross-section. Surface blisters may be visible on the intact O-ring, but the internal damage — voids, porosity, internal fractures — is only visible when the cross-section is cut. If explosive decompression is suspected as the failure mode, cut the O-ring at the damage zone and examine the cross-section before drawing conclusions. An O-ring that looks externally similar to a compression set failure may have very different internal structure.
Factors that increase explosive decompression risk
Material properties and relative susceptibility
The following table provides general tendencies only — not design data — for how common O-ring elastomer families compare in properties relevant to explosive decompression resistance. It is not a design reference — actual performance depends on the specific compound grade, formulation, hardness, and the gas and conditions involved. Confirm from the elastomer manufacturer's technical data for any high-pressure gas application.
| Elastomer | Gas permeability tendency | Tensile strength | ED resistance tendency | Notes |
|---|---|---|---|---|
| EPDM | Higher | Moderate | Lower | Good general-purpose material; not typically selected for high-pressure gas ED service |
| NBR | Moderate | Moderate | Moderate | Depends strongly on compound and hardness; widely used in oil/gas but requires ED-specific grades at high pressure |
| HNBR | Lower | Higher | Better | Often specified for some high-pressure gas services; better ED resistance than standard NBR in many qualified compounds. Compound-specific — verify RGD qualification data. |
| FKM (Viton) | Lower–moderate | Higher | Better | Commonly used in high-pressure hydrocarbon gas service. Compound and grade specific — always verify qualification data for CO₂ and H₂S service. |
| FFKM | Low | Moderate–high | Better | High chemical resistance; used in aggressive high-pressure applications; cost is typically very high |
Relative tendencies only — not design data. Actual behaviour depends on specific compound formulation, hardness, cross-section size, gas type, pressure, temperature and decompression rate. Always confirm against the manufacturer's technical datasheet and, for critical applications, material qualification testing under service conditions.
Reducing the risk — what can be changed
- Select an elastomer compound specifically qualified for explosive decompression resistance: standard grades of a given elastomer family are not automatically suitable for high-pressure gas service. Many elastomer manufacturers offer specific ED-resistant compound grades, typically characterised by lower permeability and higher tensile strength. Look for grades qualified under NORSOK M-710, ISO 23936-2 or equivalent standards for rapid gas decompression resistance — these specify the test method and acceptance criteria.
- Control the depressurisation rate: a slower, staged depressurisation gives absorbed gas time to diffuse out without forming internal bubbles. Where process requirements allow, controlled slow depressurisation is one of the most effective risk reduction measures — it does not require any change to the O-ring material.
- Higher hardness within the compound family: higher durometer grades of the same elastomer — all else being equal within compounds of similar chemistry and formulation — tend to offer better resistance to internal bubble expansion than softer grades, because the stiffer matrix is better able to resist deformation under bubble pressure. The trade-off is that harder O-rings require higher seating stress and conform less readily to surface irregularities.
- Correct groove design to avoid under-compression: Correct groove design helps maintain the intended squeeze and support geometry, which supports seal stability under high-pressure gas service. Ensure groove dimensions produce the compression percentage specified for the application.
- Consider backup rings in high-pressure gas service: backup rings reduce O-ring extrusion risk under high pressure and support correct compression geometry — both of which may influence ED susceptibility indirectly.
Replacing a damaged O-ring with the same compound in the same service is likely to produce the same result. Explosive decompression damage is usually a compound-and-service mismatch or a decompression control problem — not a random failure. If the elastomer compound is not suited to the pressure, gas type and decompression rate, the next O-ring will absorb the same gas at the same rate and fail under the same decompression. Material selection must be reviewed before re-sealing the joint — not after the next failure.
The O-ring failed because the gas had nowhere to go fast enough.
Explosive decompression is a physical mechanism, not a material quality failure. The elastomer absorbed gas under pressure — as all elastomers do to varying degrees — and the gas expanded faster than it could escape when pressure dropped. The outcome depends on the gas type, the pressure, the decompression rate, and the elastomer compound's permeability and tensile strength. Identifying the failure mode from the cross-section pattern is the first step. Selecting a compound specifically qualified for rapid gas decompression resistance — and reviewing the depressurisation procedure — is what prevents the next one.
FAQ
What is explosive decompression in O-rings?
Explosive decompression — also called rapid gas decompression or RGD — is a failure mechanism in elastomeric seals caused by the rapid release of gas that has been absorbed into the elastomer under elevated pressure. Under sustained high pressure, gas molecules permeate into the elastomer matrix. When pressure is released quickly, the gas cannot escape fast enough through the elastomer surface, and the expanding gas nucleates into bubbles within the material. If the pressure differential and decompression rate are sufficient, these internal bubbles can cause the elastomer to blister, crack, or rupture from the inside — producing characteristic damage that looks very different from chemical attack, wear, or compression set failure.
What does explosive decompression damage look like on a removed O-ring?
Explosive decompression damage has a characteristic appearance that distinguishes it from other O-ring failure modes. The most distinctive signs are blisters — dome-shaped protrusions on the O-ring surface caused by internal gas bubbles — and internal voids or pits visible when the O-ring cross-section is examined. The surface may show multiple small eruption points where blisters have burst. In severe cases the O-ring cross-section is porous, cratered or partially fragmented. This damage pattern is distinct from chemical swelling, which produces a uniform volume increase, and from compression set, which produces a flat face on the compressed side without internal voids.
Which elastomer materials are most resistant to explosive decompression?
Resistance to explosive decompression is associated with several material properties. Lower gas permeability reduces the rate of gas absorption under pressure, meaning less gas is present to expand during decompression. Higher hardness (durometer) and lower compression set are also associated with better resistance, as a stiffer matrix is better able to resist bubble nucleation and growth. Among common O-ring materials, HNBR and some fluorocarbon grades are often cited as offering better explosive decompression resistance than standard EPDM or NBR in some high-pressure gas services. However, the specific grade, hardness, compound formulation and the gas involved all affect the outcome — material selection for explosive decompression risk should be confirmed with the elastomer manufacturer's technical data for the specific application.