Why does a seal leak? The 5 most common technical causes

A leaking seal is almost never an isolated problem. In most cases, leakage is the result of a combination of technical factors: material selection, groove or housing geometry, operating conditions, installation method, fluid compatibility and long-term system stability.

For this reason, simply replacing the damaged seal with an apparently identical component may not solve the problem. If the technical cause is not identified, the leakage can occur again, leading to plant downtime, unplanned maintenance and reduced system reliability.

In the field of sealing solutions, understanding why a seal leaks means analysing the application as a whole. A seal never works alone: it interacts with materials, surfaces, pressures, temperatures, fluids and operating cycles. Each of these elements can influence its behaviour.

Leakage as a symptom, not the root cause

When a seal leaks, the first mistake is to consider the seal itself as the only cause of the problem. In reality, leakage is often the visible symptom of a wider technical issue.

A sealing system can fail because the material is not compatible with the fluid, because the groove or housing does not provide the correct compression, because the mating surfaces are damaged, or because the actual operating conditions differ from those considered during the design phase.

For this reason, the analysis should start from a fundamental question: was the seal selected and installed consistently with the real application?

1. Material not compatible with fluid, temperature or environment

One of the most frequent causes of leakage is the selection of a material that is not suitable for the operating conditions. Each elastomer or technical material has its own optimal field of use. NBR, HNBR, FKM, EPDM, VMQ, PTFE and other materials can behave very differently in the presence of oils, fuels, gases, water, steam, solvents, chemical agents or high temperatures.

An incompatible material may swell, harden, lose elasticity or progressively degrade. In some cases, the component may appear suitable at first, but lose performance after a certain period of operation. This often happens when the choice is based on a “usual” material without checking the specific operating conditions.

Temperature is another decisive factor. An elastomer exposed to temperatures above its recommended operating range may undergo accelerated ageing, loss of elasticity and permanent deformation. Likewise, very low temperatures can reduce the material’s ability to adapt correctly to the groove or housing.

Chemical compatibility should therefore not be assessed in a generic way, but in relation to the fluid, temperature, duration of contact and operating conditions.

2. Incorrect sizing, groove/housing or tolerances

A seal can leak even when the material is correct, if the system does not allow the seal to work under the intended conditions.

Groove or housing geometry, compression level, dimensional tolerances and surface finish are fundamental elements. If the seal is compressed too little, it does not generate enough contact pressure. If it is compressed too much, it may deform, extrude or wear prematurely.

This issue is particularly common in custom-designed applications, where small dimensional deviations can have a significant effect on sealing performance. A groove or housing that is not consistent with the seal profile can cause uneven compression, localised load points or micro-passages for the fluid.

The tolerances of the mating components must also be considered. A seal that is theoretically correct may not work properly if the system in which it is installed has excessive clearances, misalignment or dimensional variations beyond the expected range.

In these cases, leakage does not depend on the quality of the seal itself, but on the interaction between the seal, its groove or housing and the surrounding components.

3. Compression set and loss of elasticity over time

Compression set is one of the most important phenomena in sealing applications, but it is often underestimated. It refers to the permanent deformation retained by an elastomeric material after being compressed for a certain period of time.

A seal must retain enough elasticity to maintain contact pressure over time. If the material loses its elastic recovery, sealing performance can gradually decrease, eventually leading to micro-leakage or visible leaks.

Compression set is influenced by several factors: material, temperature, compression time, compression level and operating environment. In applications involving high temperatures or prolonged cycles, this phenomenon can accelerate significantly.

This means that a seal may work correctly at first, but lose effectiveness after a certain period of operation. In these cases, the problem is not always immediately visible: the component may appear intact, but no longer be able to provide the correct sealing pressure.

Material selection and correct groove or housing design are therefore essential to reduce the risk of leakage over time.

4. Incorrect installation or damage during assembly

A seal can also leak because of problems that occur during installation. Cuts, pinching, twisting, stretching and contamination can compromise sealing performance even before the system is started.

This often happens when the component is installed without proper lubrication, with unsuitable tools or in grooves with sharp edges. Even a correctly designed seal can be damaged if it is forced into place, dragged over rough surfaces or installed in the wrong orientation.

In dynamic applications, incorrect installation can cause premature wear. In static systems, it can create localised deformation or discontinuity in the contact surface.

Installation must therefore be considered an integral part of seal performance. Having the right material and correct geometry is not enough: installation must also be consistent with the type of component and the application conditions.

5. Operating conditions different from those expected

A very common cause of leakage is the difference between theoretical conditions and real working conditions. During selection, nominal data are often considered: pressure, temperature, main fluid, dimensions and component function. However, the actual system may involve pressure peaks, vibration, contaminants, thermal variations, unexpected movement or more intensive cycles than originally expected.

These variables can radically change the behaviour of the seal. A material suitable for constant pressure may not be sufficient in the presence of repeated pressure peaks. A seal designed for limited movement may wear quickly if the real duty cycle is more demanding. A fluid that appears compatible may become critical when combined with high temperatures or contaminants.

For this reason, seal selection should always be based on actual operating conditions, not only on nominal system data.

How to reduce the risk of leakage

Reducing the risk of leakage means treating the seal as part of a system, not as a simple component. The evaluation should consider material, geometry, groove or housing, mating surfaces, fluid, temperature, pressure, movement and expected service life.

Before selecting or replacing a seal, it is useful to check some essential technical aspects: material compatibility with the fluid and temperature, correct compression level, condition of the surfaces, installation method and actual operating conditions. This analysis helps avoid repeated replacements and identify more stable long-term solutions.

In many cases, leakage can be prevented already during the design or component review phase by correctly defining specifications, materials and tolerances.

Tecnonext’s approach to technical support and sealing solution selection

Tecnonext supports companies, technical departments and purchasing managers in selecting seals and sealing components that are consistent with the real application.

Our approach starts from the analysis of the operating context: material, fluid, temperature, pressure, movement, groove or housing geometry and service life requirements. The goal is not simply to supply a seal, but to identify a reliable technical solution that is sustainable over time.

Thanks to its knowledge of elastomeric materials, technopolymers and the different product families, Tecnonext supports customers in defining the most suitable solution, reducing the risk of non-optimised choices and improving operational continuity.

For a technical discussion, an application assessment or to schedule a visit, contact Tecnonext.

035 059 0010 info@tecnonext.it

Via Rudello, 11, 24067 Sarnico BG

FAQ – Why does a seal leak?

Does a leaking seal always need to be replaced with the same material?

Not necessarily. If the leakage is caused by chemical incompatibility, temperature or permanent deformation, it may be necessary to evaluate a different material.

Can the problem depend on the groove or housing rather than the seal?

Yes. An incorrect groove or housing, damaged surfaces or unsuitable tolerances can compromise sealing performance even when the seal itself is correct.

Can compression set cause leakage?

Yes. If the seal loses elasticity over time, contact pressure decreases and micro-leakage or progressive leakage may occur.

When is it useful to request a technical assessment?

It is advisable when leakage occurs repeatedly, when the application is critical or when the operating conditions are not completely clear.