Hi, for these compressors, you need to unload them and open the discharge flare before stopping. I believe the compressor will be able to run with recycle fully open and the suction and discharge fully closed. I shall detail how it should be done for both cases to prevent popping of first stage PSV.
For normal stop:
First place recycle valve on manual and open slowly. This will dump gas into the first stage, therefore close the suction slowly at the same time to maintain first stage pressure. At the same time, slowly close the discharge valve also. The reason is that the compressor has lesser load when the suction is being close and the recycle is being opened and so closing the discharge valve keeps the pressure stable.
Continuing doing this until the suction and discharge is fully closed. The compressor is now running on its own gas. The recycle valve may or may not be fully open (depends on sizing of valve and line but it most likely will not be otherwise its undersized). At this time start to completely unload the compressor by opening the flare valve at the discharge line and at the same time opening the recycle valve fully. The purpose of this is so that once the compressor stops, the gas at the discharge (higher pressure) will not all go the suction but will go to the flare (atm pressure).
In the event of an emergency stop, its actually almost the same steps. Only this time, first open the discharge flare fully immediately to prevent the gas from getting dumped into the suction and popping the PSV. After the flare is open, close the suction and discharge valves.
I didnt really scrutinize your P&ID to see if it can be done, but I think you get the idea how.
Also, recycle valves are fail open because if it fails closed it can overpressure. For example, if the plant IA fails but the electricity is still ok, the compressor is therefore still running but without IA, the recycle valve fail close will cause the discharge pressure to fly to the sky along with the rest of the compressor. Therefore they are fail open. Better to overpressure the low pressure suction than to overpressure the already high pressure discharge.
Edited by thorium90, 22 January 2013 - 10:44 PM.