Set Pressure of Spare PSV

Anyone can tell me what will be the set pressure for the spare PSV in the multiple PSV (3 X 50%) arrangement. ? Set pressure for 1st PSV is preset at 100% whilst set pressure for 2nd PSV is 105% of 1st PSV set pressure. Should we set 3rd PSV (spare) at 100% or 105% of 1st PSV set pressure ???

Considering a PSV systems with

• PSV-1 set at 100% set pressure
• PSV-2 set at 105% set pressure
• PSV-3 is spare for PSV-1 or PSV-2.

IF PSV-3 set at 100% set pressure

i) whenever PSV-1 is removed for inspection / maintenance, PSV-3 will be put in service. Then the situation will be

• PSV-1 out of service
• PSV-2 set at 105% set pressure
• PSV-3 set at 100% set pressure

Situation remain same as before. Healthy and proper setting.

ii) whenever PSV-2 is removed for inspection / maintenance, PSV-3 will be put in service. Then the situation will be

• PSV-1 set at 100% set pressure
• PSV-2 out of service
• PSV-3 set at 100% set pressure

Situation slightly different. There is potential of chattering in case relief. However, this situation is intermittent, short period, operator aware and attended and manageable. This is acceptable.

IF PSV-3 set at 105% set pressure

i) whenever PSV-1 is removed for inspection / maintenance, PSV-3 will be put in service. Then the situation will be

• PSV-1 out of service
• PSV-2 set at 105% set pressure
• PSV-3 set at 105% set pressure

PSV-2 or PSV-3 will only start to open at 105% set pressure. This is not acceptable as relieving pressure is exceeded 100% of set pressure..

ii) whenever PSV-2 is removed for inspection / maintenance, PSV-3 will be put in service. Then the situation will be

• PSV-1 set at 100% set pressure
• PSV-2 out of service
• PSV-3 set at 105% set pressure

Situation remain same as before. Healthy and proper setting.

Recommended :
Subscribes to FREE Hydrocarbon Processing
 
RECOMMENDATION
Therefore setting spare PSV other than 100% of set pressure will possibly lead to situation where the initial relieving pressure higher than 100% of set pressure. This is against code requirement and therefore the spare PSV is recommended set at 100% of set pressure.


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