safety valve blown out

sir, am experiencing an issue in my atlas copco compressor. some times the safety valve blown out. i had gone through the troubleshooting steps, its mentioned to check the pressure settings and the differential pressure. the pressure settings are correct.. changed new safety valve, changed all the kit etc.. some times its running for 15 hours, same times 4 hours etc i don't know how to check the differential pressure.. please help siju