Oil safety control trip

Question: The trip of the oil pressure safety switch is a warning that a system has been without proper lubrication for a period of two minutes.

On the system in question, which serves a walk-in cooler and produce showcase, this tripping has been going on for some time. I have changed to a new compressor contractor, oil pump, and oil protector, as well as using new compressor oil up to slight more than ½ the mark of the sight glass. There is a slight bubble when the compressor is running r when the compressor starts up.

The system has been running for more than three years without any oil failure problem. What causes the failure of the oil pressure safety switch?

-John Leong, Edmonton, AB

Answer: I agree that any trip of the lube oil safety control is a warning that there is a problem that needs attention. However, all trips are not necessarily an indication of lubrication problems.

Bubbles at start up may be an indication of some refrigerant in the crankcase. Your comment that there are slight bubbles normally does not indicate that this is a potential problem.

Your letter doesn't tell whether the lube oil safety control trips at start up or during the run period.

If the trip is at start up, it is generally an indication of excess refrigeration in the crankcase. You indicate that only a few bubbles appear at start up. Based on this, I cannot believe that to be the problem.

If the trip is during the run cycle, there are several items to be checked. You state that the system ran for three years with out a problem. During some time before the problem started, something changed in the system such as:

•  The evaporator or evaporators are logging oil because of a TXV losing its charge.

•  Compressor discharge temperature has risen because of high superheat. High discharge temperature causes piston ring/cylinder wall wear. This will lead to severe blow by and will cause pressurized crankcase and this will cause a lube oil safety control trip.

•  High superheat can cause the motor to overheat and will cause the overload to open. Two minutes after the internal overload opens, the lube oil safety control trips.

My personal bet is that there has been piston ring/cylinder wall wear causing a pressurized crankcase. To verify this, connect one service gauge directly to the compressor crankcase. Connect a second gauge to the suction service valve gauge connection. Slowly front seat the suction service valve. Both gauges should show lowered pressure and track together. If there is a pressurized crankcase, at some point during the front seating operations, the crankcase gauge will stop dropping but the suction service valve gauge will continue to go lower. It is at this time that the oil check valve will close and will not permit oil to return to the crankcase.

It would be nice to know what refrigerant is being used and what is the actual superheat at the suction service valve.

What causes the lube oil safety control to trip? Many items, some of which are not true loss of lubrication as when the compressor is running.

-George Story, Copeland