PxT stands for pressure times temperature. It’s a factor used more and more to characterize the performance of gasket material. The advantage is that it addresses how elastomers like silicone, EPDM and neoprene lose strength at higher temperatures.
An Inverse Relationship
Specifications for elastomers call out temperature limits. What’s rarely appreciated is that these values may not be good in all applications. Unlike metals where hardness changes very little as temperature rises, elastomers soften. For example, neoprene sheet material may have a quoted upper limit of 250°F but at this temperature it’s strength will be far lower than at say 68°F.
In many sealing applications pressure isn’t a concern. If the neoprene gasket only seals against warm air, that reduction in strength may not affect performance. However, if it’s sealing a pipe flange in a steam system reduced strength could be a problem. Even though the temperature might not exceed the upper limit, sustained pressure could extrude the material out through the flange.
Pressure AND Temperature
PxT shows how a gasket material performs under varying pressures and temperatures. Here are two examples.
Specs for a neoprene gasket material show the upper-temperature limit as 250°F and the maximum pressure as 250 PSI. However, the maximum PxT factor for the material is only 20,000 (°F x PSI). If used in an application with a continuous operating temperature of 250°F peak pressure is just 80 PSI. (20,000/250 = 80)
A sheet of EDPM gasket material has a maximum temperature of 300°F, a pressure limit of 250 PSI and a PxT factor of 30,000. In this case, the material manufacturer is saying that if sealing against media with a pressure of 200 PSI, the temperature should not exceed 150°F. (30,000/200 = 150)
If In Doubt …
Every gasket material has an upper-temperature limit. It may fail if taken beyond that, but it could also fail at a lower temperature if the pressure is high. The best approach is always to consult a material specialist like those at Hennig Gasket.
