Oil pressure trip on Bitzer 2N.2

[Bandana]

I am marine engineer on one of the ships. We use Bitzer 2N.2 fridge compressor. Recently having problems with oil pressure safety device which trips every time after compressor is stopped by LP switch. It appears after about 2 min time. Oil level is correct, refrigerant level correct. We have already replaced oil differential pressure switch. No joy. Compressor running time - 5880hrs.
Would it be possible that oil filter in the compressor is clogged and causes failure? Please advice.

[PaulZ]

Hi Bandana
If the oil failure switch trips every time the LP trips the oil failure switch is not wired up last on the safety circuit.
If it is wired like this there will still be power to the OFS and with no compressor running there is no oil pressure so the OFS trips. If so change the wiring so the OFS is last in the circuit.
Paul

[passandscore]

It is also possible that the heater element within the oil failure control is wired to the line side of the compressor contactor, not the load side as it should be. If the heater circuit has power after the compressor is offline then the bi-metal will open the switch contact after 2 mins every time.

Oil Failure Control - Schematic.jpg

[hookster]

Hi Bandana

Has the LP cut out been adjusted below the minimal setting for the oil pressure switch?
2 min delay on oil switch is for start up circuit when oil pressure is rising, if you pump down too hard via LP setting the differential switch will activate as there will be imbalance between oil pressure line and compressor sump LP.

[CONROD]

Not familiar with marine refrigeration plant - but I did have a similar fault once which traced back to a faulty auxiliary on the starting contractor. It meant the comp would run great, with good oil pressure but as soon as the comp staged down & stopped, 2 minutes later it would trip out on oil - the oil timer was only being brought into control circuit when the contractor dropped out!! Check your wiring connections to the contractors & auxiliary blocks.

[Bandana]

Thanks for reply PaulZ. It is very unlikely what you suggested because there is a St.By compressor that had been running before for a long time without any problems. All wiring and connections are exactly the same.

[Bandana]

It is also possible that the heater element within the oil failure control is wired to the line side of the compressor contactor, not the load side as it should be. If the heater circuit has power after the compressor is offline then the bi-metal will open the switch contact after 2 mins every time.

Oil Failure Control - Schematic.jpg

Passandscore OFS is danfoss MP55 without any heater element.

[Bandana]

Hi Bandana

Has the LP cut out been adjusted below the minimal setting for the oil pressure switch?
2 min delay on oil switch is for start up circuit when oil pressure is rising, if you pump down too hard via LP setting the differential switch will activate as there will be imbalance between oil pressure line and compressor sump LP.

hookster OFS has 0.7b diff. press setting and we had LP cutout value on 0.5b. Unfortunately nothing changed

[Bandana]

Check your wiring connections to the contractors & auxiliary blocks.

CONROD Electrician is going to check that tomorrow. Thanks for a quick reply.

[Bandana]

I am also considering clogged filter on the oil pump suction but need to find out what should be the proper working oil pressure. At the moment there is 5.5b while comp is running with full capacity. Isn't that to low? I could see marker sign on the oil pressure gauge made by somebody in the past on 8.0b.

[joe-ice]

Oil Pressure differential should be more than 1.4 bar .If its an mp55 switch it will break the circuit if the differential falls below 0.7 bar for 90 seconds .If its tripping on oil pressure after compressor has stopped then definitely wired wrong.

[Bandana]

Hi there. Problem solved. The problem was solved by electrician. Wrong electrical connection on one of the relays. Thank you all for help.