Low Discharge pressure of Screw compressor

[gonzalet]

Hi Everyone,
My name is Cuong and now i am working on a FPSO. Our facility has Dewpoint control system for process gas.
Frick Crew compressor PDSH 163L is used now:
- Lube oil: Frick #12b (regular check and analysis)
- Compressor maintenance is carried out regularly and follow maker schedule.
- Refrigerant: Propane (yearly charging and weekly leaktest)
- Operating parameters:+suction pressure 4 barg
+ Suction temp 1-2oC
+discharge pressure 11-11.5 barg (Normal operating)
+ Discharge temp 55oC
Recently, compressor discharge pressure was dropped to 9.5 barg and approached to low alarm limit (9 barg). We already charged 02 bottles propane for this system but discharge pressure just increase a little bit to 10.5 barg.
Could you guys explain or help me to find problems?
Best regards,
Cuong LM

[RANGER1]

gonzalet,
Welcome, need a little more information.
What type of condenser do you have?

air cooled with fan or
shell & tube heat exchanger.

Condenser must have some sort of control metering flow of water in heat exchanger or cycling fans to maintain correct discharge pressure.

Refrigerant levels must also be correct for system to operate correctly.

Also is the compressor slide valve in normal position like 100% load?

[gonzalet]

Dear Ranger1,
First of all, i want to send my thanks for your recommend.
Some additional formation:
- Condenser: Type fixed tube sheet and seawater was used. Have 1 LCV, which utilises Joule-thompson effect and normally open 80%.
- Air cooler: radiator for lube oil.
Propane is compressed and mixed with lube oil. Then, this mix will be routed to oil separator for separation. Condenser using seawater to liquefy propane. The liquefied propane goes to chiller for heat exchange between process gas and propane.
The slide valve position depends on suction and discharge pressure and variability (logic set up in PLC) to maintain discharge pressure approximate 11barg (normal).
This problem caused by propane shortage?
please find attached file for P&ID of DPC system
https://drive.google.com/open?id=0Bz...EF2amFUU2pRZEk

[gonzalet]

Here are some related document for our DPC system.
https://drive.google.com/open?id=0Bz...EF2amFUU2pRZEk

[RANGER1]

Gonzalet,
Sorry out of town only with iPhone so maybe someone
can comment.
Impressed with information you have sent, as usually hard to get enough good information.