Should it be possible to overpressurise the machanical seal?
The manufacturer should have given you proper start up procedures to make sure that that can not happen.
What type of seal configuration do you have? Tandem and you do not have buffer pressure ???
In any case, if you follow manufacturers recomended operation procedure it should not be possible to overpressurize. but to dtermine what is the problem we need to know the type of pump, the type of installation, the type of seal, the seal configuration and the circumstances under which the seal failed.
Best regards,
Scalleke