Hi Nadavran,
Its good practice to use the discharge pressure for tripping the pump. But in some cases, if the discharge valve kept closed, the above said condition will not safeguard the pump. It better to take motor current as an input in addition to the discharge pressure for trip.
If the pump is running dry/ at shut off pressure, the current drawn by the motor will be less. If the motor takes very less current (Minimum shaft input), sudden trip to be actuated. Because the pump is running dry. If the motor takes current more than minimum shaft power but less than ordinary condition, you can give little time to operate the discharge valve. If the time exceeds valve opening time, pump need to be tripped to avoid from overheating.
Hope this is clear to you.
#Samayaraj
Edited by samayaraj, 17 November 2014 - 01:37 PM.