Heat is one of the most common reasons for short seal life. It rarely acts alone: it combines with the fluid, the speed and the pressure, and the temperature that matters is seldom the one on the gauge. This guide covers how to assess a hot duty and which options to consider. It does not quote temperature limits, because those depend on the compound, the seal design and the application and should be confirmed against the supplier's data for the specific product.
What heat does to a seal
- Hardening and cracking: most elastomers gradually harden when kept above the temperature they are suited to. The lip loses flexibility and eventually cracks.
- Compression set: the seal takes a permanent set and no longer presses against the mating surface.
- Faster chemical attack: a fluid that is compatible at moderate temperature can become aggressive when hot.
- Thinner lubricant film: hot oil is thinner, so the film under a dynamic lip is weaker, friction rises and the lip runs hotter still.
- Softening: some materials soften rather than harden, and lose wear and extrusion resistance.
Establish the temperature the seal really sees
Before choosing a material, find out what the seal actually experiences.
- Bulk temperature and lip temperature. Friction makes a dynamic sealing lip run hotter than the surrounding fluid. The difference grows with surface speed, pressure and poor lubrication.
- Continuous and peak values. Record the normal running temperature, the peaks, and how long and how often the peaks occur.
- External heat. Furnaces, exhausts, steam lines and hot product can heat the seal from the air side.
- Heat soak. After shutdown, cooling flow stops and the temperature near the seal may rise for a time.
- The cold end. Equipment that runs hot may still start cold, and the seal has to cope with both conditions.
Material options as temperature rises
In qualitative terms, the common options step up as follows:
- NBR: the general-purpose choice for mineral oils and greases at moderate temperature.
- HNBR: generally improves on NBR for heat, ozone and wear resistance.
- FKM: generally offers better heat and chemical resistance than NBR, and is a common choice for hot oil duties. The trade-offs are covered in NBR vs FKM seals.
- Silicone: has a wide temperature range but limited abrasion and tear resistance, so it is mostly used for static or lightly loaded duty.
- PTFE: combines low friction with broad chemical resistance and a wide temperature range. Because it is not elastic, it is used in specially designed lip seals or as PTFE spring energized seals.
The material has to suit the fluid at the operating temperature, not only the temperature itself. Remember the other components as well: springs, metal cases and back-up rings also have to tolerate the heat and the media.
Design measures that reduce heat at the seal
Changing the material is not the only option, and it is not always the most effective one. Consider whether the heat at the seal can be reduced:
- make sure lubricant actually reaches the sealing lip;
- check that the shaft or rod finish is as the seal supplier recommends, since a rough or damaged surface raises friction;
- relieve unnecessary pressure on the lip, for example by clearing blocked vents;
- shield the seal from radiant heat, or move it away from the heat source where the design allows;
- improve cooling or oil circulation around the seal;
- consider a lower-friction lip design or material.
Signs that a seal is running too hot
A seal removed from a hot duty often shows the evidence: a hard, shiny or cracked lip, a cross-section that has taken a permanent set, or dark, baked oil deposits around the contact zone. If the same position fails in the same way repeatedly, the duty has moved beyond what the present material or design can accept. Our article on the common causes of seal failure describes these and other appearances.
Share the details of your hot application
Our sales engineers help customers select seals for demanding temperature conditions and confirm the choice against the supplier's data. Send the shaft, housing and width dimensions, the media, the continuous and peak temperatures, the pressure and speed, and a drawing or sample if you have one, through the enquiry form.
