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Tuesday, May 20, 2008

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A centrifugal pump feeding liquid to a heat exchanger located downstream. Check valve is provided between pump and heat exchanger to avoid back flow damaging the pump impeller. As the heat exchanger design pressure is lower than the pump shut-in pressure, A pressure relieve valve (PRV) is provided just upstream of the heat exchanger to avoid overpressure during pump shut-in condition. A shutdown valve (SDV) is located downstream of the heat exchanger for system isolation purpose. Both heat exchanger and SDV are located at one deck (~ 10m) higher than the pump. In the event of system shutdown, the Emergency Shutdown System (ESD) will initiate shut close of SDV and pump stop. Above arrangement is pretty standard in Chemical plant or Oil & gas plant.

When the system is commissioned and put under operation, everything run smooth. Nevertheless, the PRV open and slam close whenever the system shutdown. What was the problem ?


Why PRV passing during shutdown?
After some level of analysis, the following were postulated :

i) Whenever the system (pump & SDV) shutdown, the closure of SDV and lead to incompressible liquid back flow to check valve (in wave motion). The check valve slam (ordinary check valve) and lead to forword wave . The forward and backward wave would subsequently lead to pressure spike in the line and resulting opening of PRV. Read "How to predict Check Valve Slam ?" to get more insight into how ordinary check valve lead to pressure spike.

ii) As the heat exchanger and SDV is located ~ 10m above the pump & check valve, whenever the pump stop and check valve closure, liquid column would induce back flow and further increase the pressure spike.

iii) Pump rotate in forward direction would have substantial inertial and take longer time from full speed to zero speed. However, SDV would have much shorter closing time from full open to full close position. Apart from pressure spike caused by the ordinary check valve & SDV, the reducing forward flow would further increase the potential of pressure spike.

All aboves lead to pressure spike and potential lifting PRV.


How to avoid ?
Few approaches may be considered to minimize / avoid above situation :

a) Conduct surge analysis and check the potential of pressure spike

b) Use Non-Slam Check valve (if requried). Read "How to predict Check Valve Slam ?"

c) Increase SDV closing time. Read "Maximum Allowable Time from Shutdown Initiation to SDV Fully Closed" for maximum SDV closing time.

d) Use surge suppressor.

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posted by Webworm, 1:46 PM

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