How do I avoid heat damage to O-rings?
#1 Damage pattern: Too much heat
If an O-ring is exposed to excessive heat, it can rapidly lose its sealing properties, depending on the material. The sealing elements only achieve a fraction of the expected durability. This is not always due to the fact that a material with insufficient heat resistance is used from the outset.
Changing operating parameters and contact media also play an important role in practice. Here you can find out more about the damage pattern and common causes. Even more important in practice: Effective strategies for preventing heat-related seal failures.
#2 Temperature resistance of O-rings: Pay attention to interactions
O-rings are often subject to numerous influences at the point of use. These include the operating parameters at the sealing point, such as pressure or media. At the same time, the ambient air also acts on the sealing elements. Highly heated air can cause considerable damage to an O-ring. Other hot media can significantly change the behavior of the material once again.
#3 Damage mechanisms at a glance
Experts divide the stresses during the operating phase into these categories:
- Temperature and ageing: This group of causes of damage primarily includes the consequences of excessive continuous temperatures. Excessive heat often leads to superficial cracks on the O-ring. Brittleness can also occur. The O-ring then retains a high permanent deformation. This is also the case when suitable O-rings have exceeded their service life.
- Exposure to media: Aggressive media can cause unsuitable materials to swell considerably. This causes the sealing gap to overfill, resulting in mechanical damage to the O-ring. Conversely, process media can also dissolve substances out of the seal (extraction). This leads to a loss of elastic material properties and permanent deformation.
- Mechanical load: Preliminary damage to the O-ring can occur during installation. This risk exists in particular if no suitable assembly aids are available. This results in stretching or twisting. Other damage mechanisms in this category are the effect of pressurized media and sharp-edged installation spaces.
If an O-ring fails, in many cases this cannot be attributed to a single cause. The factors are much more complementary. The effect of heat should also always be considered in connection with the respective contact media.
#4 How heat affects O-rings
Excessive heat causes the polymer chains of elastomers to post-crosslink. This makes the O-ring harder and less elastic. In the laboratory, such a sealing element shows a correspondingly higher compression set. For the sealing point, this means that the O-ring can no longer compensate for small changes in the positioning of the components in relation to each other, for example.
At high temperatures, the ambient media also have a more aggressive effect on elastomers. Swelling and extraction often take place simultaneously. This results in volume changes, which can also lead to a loss of the sealing effect.
In contrast to heat, the consequences of low temperatures are reversible. O-rings lose their elasticity when the temperature falls below the glass transition temperature. However, the sealing effect is restored when it is heated up again.
#5 Influence of the base rubber
By selecting the base rubber, the designer determines the essential resistance properties of an O-ring. Each rubber has its own profile with regard to temperature limits and interaction with chemical media. Formulation-dependent additives can also influence the temperature suitability of an O-ring. This means that different temperature limits are also possible within a base polymer.
One example: Long-term tests by the Richter O-ring testing laboratory have shown that NBR O-rings can achieve up to 2.5 times longer operating times in heat than O-rings made from less suitable NBR compounds.
Here is an overview of the continuous temperatures of standard sealing materials:
Maximum temperature for an operating time of 1000 hours
The decisive factor for the temperature specifications is that the O-rings can be used for at least 1,000 hours. The values only apply to applications without the influence of aggressive media and without pressure load. The suitability of certain materials must therefore be checked depending on the conditions at the sealing point.
#6 Damage analysis uncovers heat damage
If an O-ring fails during operation, this can lead to considerable damage. It is therefore all the more important to draw the right conclusions from the failure of an O-ring.
#7 Documentation in the event of damage
It is often necessary to replace the defective seal quickly. This is because the affected machine should be ready for productive use again as quickly as possible. Nevertheless, maintenance staff should take enough time to analyze the damage. This is the only way they can take targeted measures to increase the durability of the seal. In particular, the following information should be recorded:
- What damage to the O-ring is caused by disassembly? They can be ruled out as a causal damage pattern.
- What do the mating surfaces and grooves look like? Sharp-edged components or deposits provide important information.
- What products and media does the O-ring come into contact with?
- What are the maximum pressures in operation and what is the profile of the pressure conditions?
- What are the temperatures at the sealing point?
- Are there special cleaning processes with specific temperatures and media in addition to the regular operating conditions?
#8 Recognize heat damage pattern
In addition to the operating situation, the O-ring itself also gives an indication of the cause of the damage. Heat damage to O-rings can be recognized by the following features:
- Superficial cracks, especially on the side facing the heat
- Embrittlement over the entire cross-section
- Permanent deformation
Various elastomers also show individual indications of excessive heat:
- NBR: Glossy surface
- FKM: Bonding with metallic sealing surfaces
- EPDM: Sooty coating on the surface
#9 These are the main areas of error
Failures due to heat occur primarily when the actual operating conditions deviate from the assumptions made during design. The picture cannot be reduced to the conditions during standard operation alone. High temperatures also play an important role in connection with cleaning processes and the respective media.
CIP and SIP procedure
Take food production, for example: O-rings come into contact with vegetable and animal oils and fats or flavorings. They already represent a high level of stress in themselves. At the same time, this industry often uses superheated steam processes to combat the growth of microorganisms in the plant. These cleaning and sterilization processes are also used in beverage production and the pharmaceutical industry.
These CIP (Cleaning in Place) and SIP (Sterilization in Place) processes combine chemically effective cleaning media with hot steam. Both put O-ring materials to the test. Hygienic design requirements for such production systems also stipulate a special design for the installation spaces.
The reduction of dead spaces leads to grooves with a higher degree of filling. If the seals contract again after heat-induced expansion, parts may be sheared off.
Changed operating conditions
When designing a seal, engineers ideally take all operating conditions into account. They weigh up temperatures, media and pressures and select a suitable seal. This includes not only the selection of a suitable O-ring material, but also the dimensions of the seal. They also define the appropriate design of the installation groove.
However, this care is only effective as long as the assumptions are confirmed in real operation. Although this is often the case initially, the operating parameters of machines and systems often change later on.
Adjustments in process control can result not only in higher temperatures, but also in the use of chemically more aggressive media. This means that previously adequate O-rings often reach their load limit.
#10 Solution approaches for practice
To prevent heat-related damage to O-rings, you should take two measures: If a seal fails, the analysis shows which damage mechanisms are acting on the O-ring. This allows valuable conclusions to be drawn for improved material selection. Other elements of the sealing system, such as the groove design, can also be improved.
When designing a new sealing system, temperatures and media should be considered in their interaction. If the operating parameters change, the suitability of the seals used should be checked proactively. The right choice of sealing material can extend the service life of an O-ring many times over.
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