External loading caused by a sudden great change of temperature, which in hard and brittle face materials can lead to cracks and fractures. A seal face or stationary seat damaged by thermal shock displays – usually radial – cracks separated by far larger spacings than those between thermal stress cracks (which have a different origin). In practice, thermal shock damage is observed practically only in mechanical seals that were heated to an unacceptable level by dry running prior to arrival of the process medium. Experimentally determined values of the sensitivity to thermal shock of some common face materials are given in the table. Ducted under the following conditions:

• hot/cold test

• heating of test specimen in a furnace at intervals of 50° up to a maximum 720 °C
• immersion in water at 20 °C

• cold/hot test

• temperature stabilization in water at 20 °C
• immersion in glycerine at 150 °C

Mechanical seal made of SiC with thermal shock cracks