Seal Extrusion in Pressurised Systems: Causes, Signs and Solutions
and initially provide effective sealing, yet deteriorate rapidly when pressure and system geometry create conditions that favour extrusion.
The phenomenon occurs when part of the elastomer is forced into the clearance between mating components. If the stress continues or is repeated over time, the material may deform, become damaged and progressively lose its ability to maintain an effective seal.
In O-Rings, extrusion can also appear in the form of so-called nibbling: the edge of the seal, normally on the lower-pressure side, shows torn, chipped or apparently “nibbled” areas. Parker identifies this morphology as one of the characteristic signs of the phenomenon.
High pressure is therefore an important factor, but it is not the only cause. Clearance, tolerances, hardness, temperature, chemical compatibility, component deformation and dynamic conditions must be analysed together.
What seal extrusion means
In a pressurised system, there is normally a pressure differential between the two sides of the seal.
This difference generates a force that tends to push the elastomeric material towards the available gaps between the components. If the clearance is sufficiently large in relation to pressure, hardness and material characteristics, part of the seal may progressively enter the gap.
The problem is not necessarily immediate.
During the first operating cycles, the component may continue to provide an effective seal. As the stress is repeated, however, the portion of material entering the gap may become deformed, cut or torn.
In dynamic applications, the phenomenon can be further aggravated by relative movement between the components. Even in static applications, however, pressure pulsations and structural deformation can repeatedly open and close the clearance, subjecting the edge of the seal to significant stress.
Pressure and clearance must be evaluated together
Referring generically to “excessive pressure” can lead to an incomplete diagnosis.
A seal’s ability to operate under pressure also depends on the size of the extrusion gap, meaning the space into which the elastomer may be forced.
With the same material and pressure, reducing the clearance can significantly change sealing conditions. Conversely, wide tolerances, eccentricity or component deformation can locally create gaps larger than those expected during the design phase.
This means that two apparently similar systems operating at the same pressure may behave very differently.
Sizing should therefore consider at least:
continuous pressure and possible peaks;
maximum clearance resulting from tolerances;
deformation of components under load;
material hardness and behaviour;
temperature;
possible movement;
compatibility with the fluid.
Nominal pressure alone does not describe the complete problem.
Tolerances and eccentricity can create a weak point
The groove is normally designed using nominal dimensions, but the actual system operates within a range of tolerances.
A clearance that appears suitable according to the nominal dimensions may become larger under the most unfavourable combination of piston, rod, cylinder, groove or other mating-component tolerances.
Eccentricity can also significantly affect behaviour.
If two components do not remain perfectly concentric, the clearance may decrease on one side and increase on the other. The O-Ring will therefore be more exposed to extrusion precisely in the area where the available gap is greater.
Parker identifies both excessive clearances and irregular clearances caused by eccentricity as possible causes of extrusion, together with an increase in clearance resulting from system deformation under pressure.
For this reason, during a failure analysis it is not sufficient to measure only the removed seal. The geometry and condition of the surrounding components must also be checked.
When components “breathe” under pressure
Pressure and load can deform even apparently rigid metal components.
Flanges, covers, cylinders and other parts of the system may undergo small dimensional changes during the operating cycle. When these deformations cause a temporary increase in clearance, the seal may be pushed into the available space.
As pressure subsequently decreases, the gap may close again.
This phenomenon, often referred to as component breathing, can repeatedly subject the elastomer to pinching and cutting. It is one of the reasons why even a static configuration can develop extrusion damage in the presence of pulsating pressure.
System rigidity must therefore be considered together with nominal tolerances.
Seal hardness affects resistance to extrusion
The hardness of the elastomer influences its ability to deform under load.
Under the same conditions, a softer compound generally tends to penetrate a clearance more easily than a harder one. Increasing hardness may therefore be one possible corrective action when an extrusion problem has been identified.
However, this does not mean that automatically selecting the hardest O-Ring is the correct solution.
As hardness increases, deformability, installation behaviour and the ability to conform to surfaces also change. Furthermore, if the main cause is excessive clearance, eccentricity or incorrect geometry, changing only the compound may not eliminate the problem.
Hardness, groove and pressure must be evaluated as elements of the same system.
Temperature: a compound can become more vulnerable during operation
The nominal hardness specified for an elastomer does not necessarily describe its behaviour under all operating conditions.
Increasing temperature can alter the mechanical response of the compound. A material that provides adequate resistance to extrusion at room temperature may become more deformable when operating at higher temperatures.
Parker also identifies materials that soften at operating temperature among the conditions that can promote extrusion.
Temperature can also accelerate the interaction between the elastomer and the fluid, further changing hardness, volume and mechanical properties.
For this reason, pressure and clearance must be evaluated at the actual operating temperature, not only under the conditions present during installation.
The fluid can also increase the risk
An elastomer that is incompatible with the fluid may swell, soften, shrink or degrade.
If the material loses part of its mechanical strength, it may become more vulnerable to being forced into the clearances within the groove.
A seal showing signs of extrusion should therefore not be analysed solely from a geometrical perspective.
The causes identified in Parker’s technical documentation also include material degradation caused by the fluid, with effects such as swelling, softening, shrinkage or cracking.
Verifying chemical compatibility therefore remains an integral part of the analysis.
How to recognise extrusion damage
The appearance of the removed seal can provide important information.
An O-Ring affected by extrusion frequently shows damage concentrated on the low-pressure side, meaning the area towards which the material is pushed.
The edge may appear:
cut, frayed, irregular, chipped or with small portions missing.
In the case of nibbling, very small pieces may appear to have been progressively removed from the edge of the O-Ring. Parker notes that deterioration can become extensive before catastrophic leakage occurs.
The position of the damage is therefore particularly useful.
If wear is evenly distributed over the entire surface, or has a different appearance, other causes such as abrasion, twisting, chemical incompatibility or installation damage may need to be considered.
The morphology of the damage must always be assessed in relation to the actual groove configuration.
Extrusion and compression set are not the same thing
Both phenomena can compromise sealing performance, but they have different causes.
Compression set refers to the loss of elastic recovery after prolonged deformation. A seal progressively remains more compressed and may no longer exert sufficient contact pressure.
Extrusion, on the other hand, concerns the penetration of the material into the clearance between components under the action of pressure.
In one case, the predominant issue is therefore loss of elastic recovery; in the other, it is the combination of pressure, available space and the mechanical strength of the material.
The two phenomena can, however, interact.
A material that has degraded, softened or been subjected to conditions inconsistent with the application may become simultaneously more vulnerable to different failure modes.
For this reason, a diagnosis based exclusively on visual appearance should be confirmed by checking the operating conditions.
Dynamic applications: movement and clearance increase complexity
Extrusion is particularly important in piston and rod seals.
In these systems, pressure is combined with relative movement between the seal and the surface. If part of the elastomer enters the clearance, movement can subject it repeatedly to shearing.
For this reason, extrusion and nibbling represent relevant failure modes in dynamic hydraulic applications.
The following factors must therefore be considered together:
pressure, speed, direction of movement, clearance, hardness, lubrication and surface finish.
Pressure variations during the operating cycle may also continuously change the load applied to the seal.
Groove geometry can promote the problem
An incorrectly sized groove can increase the risk of extrusion.
Unsuitable edges, inconsistent dimensions, excessive groove fill or out-of-tolerance machining can alter the behaviour of the seal.
Parker also identifies incorrect groove machining, critical edges and the installation of incorrectly sized O-Rings resulting in excessive groove fill among the possible causes of extrusion.
Checking the material alone is therefore not sufficient.
Even when the removed component shows clear extrusion damage, the groove should be dimensionally inspected and compared with the actual operating conditions.
Back-up rings: when the seal needs additional support
One solution used to limit extrusion is to install an anti-extrusion ring, commonly known as a back-up ring, alongside the O-Ring.
Its purpose is to reduce the space into which the elastomer can be forced and provide support on the side affected by pressure.
ISO 3601-4:2008, confirmed in 2023 and still current, defines dimensions and tolerances for five types of anti-extrusion rings intended for use with O-Rings and the corresponding grooves covered by the ISO 3601 series.
Back-up rings can be manufactured in different configurations and materials.
Trelleborg, for example, offers versions made from PTFE, elastomers and thermoplastic materials, with solid, split or spiral geometries.
Their presence does not, however, make correct groove sizing irrelevant.
An anti-extrusion ring must be selected according to the direction of pressure, type of movement, O-Ring dimensions and available space.
A back-up ring does not automatically solve every pressure-related problem
Adding an anti-extrusion ring may be appropriate in certain configurations, but it should not become an automatic response to every pressure-related leakage problem.
The first step is to understand why the elastomer is entering the clearance.
If the problem is caused by out-of-specification tolerances, abnormal deformation of a component, chemical incompatibility or incorrect installation, a back-up ring may not eliminate the primary cause.
The pressure values used to recommend their use also vary according to the configuration and the manufacturer’s guidelines. This confirms that there is no universal threshold applicable to every O-Ring and every groove.
The assessment must remain application-specific.
What to check when a seal shows signs of extrusion
When damage consistent with extrusion is detected, the analysis should reconstruct the behaviour of the complete system.
The first step is to determine where the damage is located in relation to the direction of pressure. The maximum clearance, tolerances, possible eccentricity and groove condition should then be checked.
Nominal pressure and actual pressure peaks should also be compared, together with temperature, compound hardness and fluid compatibility.
If the system operates dynamically, speed, movement and surface conditions must also be considered.
Only after this assessment does it make sense to determine whether the solution should involve:
reducing the clearance;
reviewing tolerances;
increasing component rigidity;
modifying hardness or compound;
correcting groove geometry;
using a back-up ring;
adopting a different sealing solution.
Problem solving therefore does not simply mean replacing the damaged O-Ring.
Why replacing the seal with an identical one may not be enough
When an extruded O-Ring is removed, installing a new component restores the initial geometry but does not change the conditions that caused the failure.
If clearance, pressure, temperature and material remain unchanged, the new component may be subjected to the same deterioration mechanism.
Failure recurrence is therefore important information.
If several seals show the same damage in the same position, the analysis should focus on the system before attributing the problem to an individual defective component.
This principle is directly linked to the broader analysis of the technical causes of leakage: the seal is part of a system and can rarely be evaluated in isolation.
Material, clearance and pressure must be evaluated as a single problem
Extrusion clearly demonstrates why seal selection cannot be based on a single parameter.
A suitable material may fail if the clearance is too large.
A correctly designed groove may become critical if pressure exceeds the expected level.
A compound that is sufficiently hard at room temperature may behave differently during operation.
A correctly sized system may ultimately lose stability if components deform, become eccentric or are subjected to pressure peaks that were not considered.
The question to ask is therefore not simply:
“Which seal can withstand this pressure?”
but:
“What conditions must the seal withstand when the system reaches this pressure?”
It is this second question that makes it possible to properly analyse the risk of extrusion.
Tecnonext’s approach to extrusion-related issues
Tecnonext supports companies, technical departments and maintenance teams in selecting O-Rings and seals for pressurised systems and in analysing recurring sealing problems.
The assessment begins with the actual application conditions: nominal pressure and pressure peaks, temperature, fluid, type of movement, material currently in use, hardness, seal dimensions, groove geometry, tolerances and the way in which the damage occurs.
Depending on the application, different compounds, dimensional modifications, standard or custom O-Rings and, where necessary, anti-extrusion solutions can be evaluated.
The objective is not simply to replace the damaged component, but to understand why the material is being forced into the clearance and act on the parameters causing the problem.
For a technical discussion, seal selection or an analysis of a critical issue in a pressurised system, contact Tecnonext.
+39 035 059 0010
Via Rudello, 11, 24067 Sarnico BG, Italy
FAQ – Seal extrusion
What is O-Ring extrusion?
It is the phenomenon in which part of the elastomer is forced by pressure into the clearance between components. Over time, the material may deform, become cut or be progressively removed.
How can an extruded O-Ring be recognised?
Damage frequently appears on the low-pressure side and may take the form of an irregular, cut or frayed edge, with small portions of material missing. This form of deterioration is often referred to as nibbling.
Does using a harder seal eliminate the risk of extrusion?
Not necessarily. Greater hardness may increase resistance under certain conditions, but clearance, pressure, temperature, fluid, geometry and tolerances must also be checked.
What is a back-up ring used for?
A back-up ring supports the O-Ring and limits the possibility of the elastomer being forced into the clearance between components. ISO 3601-4 defines several types of anti-extrusion rings for use with O-Rings.
Can extrusion also occur in a static seal?
Yes. It can occur in static systems subjected to high pressures, pulsations or component deformation that temporarily changes the available clearance.
Are extrusion and compression set the same thing?
No. Compression set concerns the loss of elastic recovery after prolonged compression. Extrusion, on the other hand, concerns the penetration of the material into the clearance under the action of pressure.
Comments