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[dutru]

Dear Gents,

 

I'm an junior integrity engineer, my boss lets me involved in an actual case described below.

 

We have 2 parallel centrifugal compressors, which discharge gas into a fiscal metering skid. Downstream of the skid there are 2 dissimilar check valves then a 130 m 16" pipeline leading to a riser, 120 m vertical height, connecting to a 20" subsea pipeline. The compressor skid operates at 158 barg, maximum discharge pressure on the provided compressur curve is 225 barg. The design pressure for the compressor skid and the metering skid is 174 barg. Downstream of the skid, the piping is designed under ASME B31.1 and 160 barg. The riser ESDV is known to be spurious (fail to close position) and thus a closed discharge case is prominent.

 

The 130m 16" segment is protected by a PZHH set at 156 barg. The activation of PZHH would simply put the compressor skid to idle mode; since it is driven by gas turbine, the fuel gas supply does not stop but gradually decreases. Between the metering skid and the check valves, there is a PSV set at 160 barg.

 

Now due to production pressure, the operation team wants to increase the setpoint of PZHH, to 157-158 barg for example.

 

My problem are:

- As far as I can see, the first protection layer of overpressure for the 130m 16" segment, is the PZHH. The fact that compressor is only put to idle mode, with unavoidable delay of several seconds, means that this method might not be fast enough to prevent pressure rise.

- The PSV does not seem to be well positioned. With the set point of 160 barg, it may lift as late as 176 barg (10%). When it finally lifts, the pressure already built up downstream of the check valves (02) can not be relieved.

 

Do I need additional PSV downstream of check valves? What should I verify before approving the change of PZHH setpoint?

 

Thank you all in advance for any discussion.