FAQ sealing behaviour of elastomer seals in diaphragm valves
Causes of leaks after prolonged operation
Here you will find answers to questions we are frequently asked about the sealing behaviour of diaphragm valves.
If you have a specific question, please send us an e-mail and we will be happy to assist you.
Why do diaphragm valves lose their sealing function over time?
Diaphragm valves are key sealing elements in aseptic and industrial applications. Their functional reliability is largely based on the properties of the elastomer seal used.
However, under continuous thermal and mechanical stress, for example due to repeated SIP cycles, the material-specific properties of this material change. These changes occur over time and are characteristic of elastomers.
Leaks are therefore not necessarily caused by installation errors or faulty sizing, but can be the result of material-specific relaxation and deformation processes.
Which material-specific processes influence sealing behaviour?
If an elastomer seal is subjected to stress over a long period of time, its deformation does not remain constant. Instead, it gradually increases. This behaviour is described by the effects of creep and stress relaxation. The cause lies in the interaction of physical and chemical influences, in particular in the viscoelastic behaviour and molecular structure of the material.
Viscoelasticity and molecular rearrangement
Elastomers are viscoelastic. When deformed, part of the energy applied is stored, while another part is dissipated.
The long polymer chains are cross-linked and partially intertwined. This structure influences how the material behaves under stress. The speed and type of molecular rearrangement depend in particular on:
- chemical structure
- duration of stress
- temperature
- deformation rate
Compression Set
The compression set describes the extent to which an elastomer seal returns to its original shape after permanent deformation. A low value indicates good resilience.
A low compression set is often accompanied by a higher degree of cross-linking. This material structure can reduce crack resistance under high mechanical stress.
What are the implications in practical operation?
Stress relaxation and loss of sealing force
Stress relaxation refers to the time-dependent reduction of mechanical stress stored in the elastomer seal. In a diaphragm valve, this effect manifests itself as a continuous decrease in contact sealing force. With increasing operating time, the diaphragm loses its original preload, which can reduce the sealing effect.
Tightening as a short-term measure
After the first thermal cycle, retightening can partially compensate for the lost sealing force. This reduces the risk of external leakage in the short term. However, the fundamental problem remains. Stress relaxation continues, meaning that higher readjustment forces are required as operating time increases.
Mechanical overload and crack formation
Repeated or excessive tightening significantly increases the mechanical stress. High local stresses occur particularly at structural transitions and clamping areas. If the load limit is exceeded, microcracks initially form, which spread over time and can eventually lead to visible cracks and failure of the diaphagm.
What are the design implications – and how has GOETZE addressed this?
The typical properties of elastomers indicate that a permanently safe sealing function cannot be guaranteed solely by retightening or optimising a single material characteristic. The decisive factor is the sizing of the sealing system in the diaphragm valve. This is the only way to prevent leaks and premature diaphragm damage in the long term.
GOETZE diaphragm valves feature a patented design advantage in their sealing system. A stainless steel pressure sleeve integrated into the actuator acts as a mechanical compensating element and ensures constant compression of the diaphragm.
This means that rather than optimising a single characteristic value in isolation, the interaction between elastomer behaviour, preload and mechanical stress is taken into account in the design. The goal is to achieve a permanently stable sealing function even under thermal and mechanical stress.
The most important questions and answers about the sealing behaviour of elastomer seals in diaphragm valves
Diaphragm valves perform a key sealing function in aseptic and industrial applications. This is based primarily on the properties of the elastomer seal used. However, under continuous thermal and mechanical stress – for example, due to repeated SIP cycles – the material-specific properties of this material change.
These time-dependent changes can lead to a loss of sealing function in the long term. Leaks therefore do not necessarily occur due to faulty installation, but can be the result of material-typical ageing and relaxation effects.
If an elastomer seal is subjected to mechanical stress over a long period of time, its deformation does not remain constant. Instead, it gradually increases. This behaviour is described by the effects of creep and stress relaxation. The cause lies in the interaction of physical and chemical influences. The viscoelastic behaviour of the material and its molecular structure are decisive factors here. Another relevant characteristic is the so-called compression set, which indicates the extent to which an elastomer seal returns to its original shape after permanent deformation.
Elastomers are fundamentally viscoelastic. This means that when they are deformed, part of the energy applied is stored in the material, while another part is dissipated.
The polymer chains of an elastomer are cross-linked and partially intertwined. This structure allows for elastic behaviour, but at the same time limits the mobility of the chains. The speed and type of molecular rearrangement depend in particular on:
- the chemical structure of the elastomer seal
- the duration of the load
- the temperature
- the deformation rate
These influencing factors determine how strongly and how quickly the mechanical properties change over time.
Stress relaxation describes the time-dependent reduction of mechanical stress stored in the elastomer seal. In a diaphragm valve, this effect causes the original preload of the diaphragm to decrease continuously. The contact sealing force decreases with increasing operating time, which can reduce the sealing effect. Leaks can therefore occur even if the valve was correctly installed and sized at the outset.
After the first thermal cycle, retightening the connection can partially compensate for the loss of sealing force. This reduces the risk of external leakage in the short term. However, this does not solve the underlying problem. The stress relaxation of the elastomer seal continues. With increasing operating time, therefore, ever higher readjustment forces are required to maintain the sealing effect.
Repeated or excessive tightening significantly increases the mechanical stress on the diaphragm. High local stresses occur particularly at structural transitions and clamping areas. If the load limit of the elastomer seal is exceeded, material fatigue begins. Initially, microcracks form, which spread over time. This results in visible cracks, which can ultimately lead to diaphragm failure.
The compression set indicates the extent to which an elastomer seal returns to its original shape after permanent deformation. A low value indicates good resilience. Low compression set is often associated with a higher degree of cross-linking in the material. However, this structure can reduce crack resistance under high mechanical stress. The material then reacts less by elastic yielding and more by crack formation.
A material with very good resilience can be both stiffer and less damping. This reduces its ability to dissipate local stress peaks. Localised high stresses occur particularly during retightening. If an elastomer seal cannot absorb and distribute this energy sufficiently, the stress concentrates in the material. This can result in cracking. A low compression set therefore does not automatically mean higher long-term resistance under all load conditions.
A permanently safe sealing function cannot be guaranteed solely by retightening or by selecting a single material characteristic value. The decisive factor is the sizing of the sealing system in the diaphragm valve, such as the patented solution used in GOETZE diaphragm valves. This is the only way to prevent leaks and premature diaphragm damage in the long term.