Oil pressure switch- Low oil level

[aruiloba]

Besides checking the operation of the oil pump , may this switch protect a system from low oil level?
Would a low oil level result in a low (pump) discharge pressure soon enough for this swith to stop the system before it get damaged?

[Peter_1]

Besides checking the operation of the oil pump , may this switch protect a system from low oil level?
Would a low oil level result in a low (pump) discharge pressure soon enough for this swith to stop the system before it get damaged?

1st question: no, if you mean a oil pressure switch will not protect from low oil level.
2nd question: it will result in a low oil pressure but low oil pressure can have different reasons.

Oil pressure safety is the fastest and most reliable. Don't try to save money on a OP switch.

[aruiloba]

Peter:

1) So, according to your first answer, a compressor may be damaged due to lack of oil even if it is protected by an oil pressure switch ( like Danfoss MP 54)

2) In a central refrigeration system provided with mechanical oil level regulators (AC&R S 9010) , one of them failed, and the compressor associated with this regulator (protected by an oil pressure safety switch) get damaged by lack of oil.
Is there a way to protect a compressor of this type of failure?

Augusto

[S.M.Gokhale]

Theoratically low oil level should result in development of low oil pressure as the oil pump will have less quantity of oil (in the crankcase) to pump out from. I have seen oil safety switches bypassed due to nuisance tripping (perhaps the technicians do not have the patience to find out the cause of low oil pressure and rectify it -This is like replacing a fuse with a higher rated one if the correctly rated fuse blows often).

The oil pressure switch has a time delay mechanism (in some switches settable). The time delay prevents nuisance tripping in case of momentory oil pressure drop due to any system problem and also during start-up when it takes a few seconds for the oil pressure to develop.. It is expected that the compressor will not break down if the oil pressure is lower than required for a short time SAY 60 to 90 seconds. In the case you have mentioned would it be possible that the oil safety switch was bypassed? Oil level control along with oil safety switch is a double safety - in case one fails, the other safety is available as a back-up, But if one of the safeties is bypassed then no back-up is available.

[wesmax]

An oil safty switch should be tested at least twice a year, however if problems occure it should be tested more often also check for to much foaming or flooding durning start up. In colder climates check cch .
wesmax

[Gary]

A less common cause of nuisance tripping is compressor overload problems. The compressor trips on internal overload for whatever reason and shuts off. The oil pressure control continues to time out until it trips. The compressor overload eventually resets itself. You are left with a tripped OP control and wondering why.

[wesmax]

this sounds like a wireing problem I have worked on many systems with an oil failure sw and never had this problem.
wesmax

[S.M.Gokhale]

I agree with Wesmax. I too have rarely faced problems with Oil Safety Switches. I feel mainly it boils down to the system design. In well balanced systems where the condenser, evaporator and expansion valve are properly sized the problems of liquid returning to compressor or high discharge pressure resulting in motor overload rarely occur. Problems, whenever they occur, seldom lead to immediate breakdown of compressor.Usually there are indications that something has started to go wrong. When these are not noticed and therefore corrective action not taken, the compressor breakdown occurs. For this reasons users should also be trained to maintain a log book and note down all temperature and pressure readings, check oil level regularly every day SAY at least two or three times during the day. Practically many users (maintenance engineers at users' facilities) seldom look at the ref./a.c. system until a complaint of no-cooling is received.

Also oil level in the compressor does not suddenly drop. There could be a problem of oil return from the evaporator or it could be related to the way suction line is laid. In open type compressors, the shaft seal would leak if liquid returns to compressor for prolonged periods - in which case the evaporator should be cleaned. If compresors are overhauled regularly, the wear and tear of all parts including shaft seal can be seen and rectified.

[Peter_1]

A less common cause of nuisance tripping is compressor overload problems. The compressor trips on internal overload for whatever reason and shuts off. The oil pressure control continues to time out until it trips. The compressor overload eventually resets itself. You are left with a tripped OP control and wondering why.

We wire the OP in such a way that it's only energized when compressor runs. If overload trips, then supply to OP is also disconnected. It's of nouse measuring OP when compressor stands still for whatever reason.

[RogGoetsch]

A less common cause of nuisance tripping is compressor overload problems. The compressor trips on internal overload for whatever reason and shuts off. The oil pressure control continues to time out until it trips. The compressor overload eventually resets itself. You are left with a tripped OP control and wondering why.

In my experience, this is the most common cause of trips and usuallly due to a faulty contactor with failing contacts in one leg.

The unit overloads on the other two legs, trips the overload and the oil safety times out and trips. You reset it, the contacts make perfectly this time, and you stand there scratching your head with all values normal.

Usually the contactor, I find.

Rog

[aruiloba]

I asked directly to two compressor manufacturers if an OP switch may protect their compressors from a low oil situation , since I consider that the design and the dynamic behavior of a compressor under low oil conditions may be critical to the final result . Bock answered NO,an OP switch will not protect a Bock compressor from failure in a case of low oil level; Bitzer answered YES

[chemi-cool]

Hi aruiloba,

OP switch will be useful only if there is an oil pump present.

If not, there is no oil differential pressure.

It also protect against oil pump failure.

Chemi