By Grant Laidlaw
Even if the oil level is correct, the pressure may not be adequate to ensure lubrication of all the components in the compressor.
|Jackson asks: Hello Grant, we do work on servicing and sometimes see compressor failure. I noticed that somehow there seems to be a lubrication issue. I was looking at the article on lack of maintenance causing compressors to fail. But why would the oil pressure be too low if the oil level is correct? Thanks.|
Hi Jackson, with regard to oil pressure failure, let us examine the individual causes:
- Liquid refrigerant in the oil Poor practice
- Incorrectly adjusted expansion device
- Blocked strainers
- Incorrect oil viscosity
- Worn oil pump
- Weakened spring on oil pressure relief valve
- Worn bearings
- Excessively high temperature
- Oil pumped out into refrigeration system
- Leaking unloader mechanisms
Liquid refrigerant in the oil
Liquid refrigerant entering into the oil has the effect of diluting the oil. This will thin the oil and have a negative impact on oil pressure and may allow bearings to run metal on metal instead of running (floating) on a layer of oil.
In addition, the lubricating qualities of the oil will be reduced, increasing friction. When liquid refrigerant has migrated into the compressor, on start up the refrigerant will rapidly boil off causing the oil to foam at the oil pump inlet, affecting pump operation and may cause slugging with resultant damage.
This problem can come about in various ways. For example:
Crankcase can be too cold during shut down periods (no crankcase heater or crankcase heater not working).
A crankcase heater is incorporated to keep the oil warm, not to keep the oil thin but to discourage refrigerant migration.
The process of refrigerant migration is brought about by the natural tendency of refrigerant to evaporate from warm parts of a standing system, and to condense in cold parts. During the plant off-period, the compressor, which often is located outside, may well represent the colder part of the system (for example: at night).
Cold oil in the crankcase results in a situation of refrigerant migrating into the crankcase. It is a dangerous situation and can result in:
- Compressor damage
- Oil being lost from the sump into the system
- The oil pressure switch tripping
Moving on, we may find a technician charging the system with liquid refrigerant, directly into the compressor. As with migration, the liquid refrigerant entering into the oil will have the effect of diluting the oil. And again this will thin the oil and have a negative impact on oil pressure and may allow bearings to run metal on metal instead of running (floating) on a layer of oil.
When liquid refrigerant has been added to the oil in this way, again on start-up, the refrigerant will rapidly boil off, causing the oil to foam at the oil pump inlet. This will affect pump operation and may causing slugging with resultant damage.
Incorrectly adjusted expansion device
The next possible cause can be an expansion device that has been set for too low a superheat or is not operating correctly.
Many systems are controlled by expansion valves. The basis of control of an expansion valve is suction superheat. In a normal operating system, there should be roughly 6K of suction superheat when operating at full load.
At an average of 6K suction superheat, there should be no more than occasional traces of liquid in the suction line; if at all. However, if the valve superheat setting is too low, this can result in unacceptable amounts of liquid in the suction line moving toward the compressor.
An oversized value also causes the same problem. If the expansion valve is oversized, it will tend to hunt and this in turn may allow liquid refrigerant into the suction line.
If a refrigeration system is being forced to work at too low a load, this can effectively bring about a situation where the valve is oversized for the operation at that time. Again, liquid will intermittently be passed into the suction line, and possibly into the compressor.
Blocked oil strainer
Blocked oil strainers will gradually cut off the oil supply to the oil pump and in turn this will cause a reduction of oil pressure.
Strainers should be checked, and renewable filter cartridges should be renewed at least once a year. If a problem with contaminated oil has arisen, the checks, cleaning functions and cartridge renewals must be much more frequent. Keep a plant room logbook and record the relevant pressures as part of your servicing procedures.
Incorrect oil type or viscosity
It is very important that the correct oil is used. Should the oil be too thin, the oil pump will not be able to supply the oil at the correct pressure as the oil will leak back through the pump. In addition, the oil clearance gaps between components will be too great and allow the oil to move through too quickly, further lowering the gallery pressure.
It is therefore very important to follow the manufacturer’s recommendations regarding the oil type and viscosity.
Worn oil pump
A worn oil pump will produce low oil pressure as the pump will not be able to supply sufficient oil to keep the compressor lubricated. Oil pumps are not normally problematic but do fail and can be replaced on larger systems.
Many of the smaller compressors are built with an oil pump which includes the pump end bearing as part of its construction. These pumps are not built to be repaired. If such a compressor is overhauled, its oil pump is automatically replaced at the same time.
Oil pressure relief valve
Oil pressure relief valves are usually factory set. Refrigerant oil at pressure is fed to the relief valve, while the pressure of the feed is slowly increased. If the valve starts to open before the prescribed oil pressure for the system is reached, you will again observe insufficient oil pressure. In some cases this can be caused by a weakened spring in the valve.
Worn bearings allow excessive amounts of oil to be passed through the bearings, dropping system oil pressure. Once it has been confirmed that refrigerant in the oil is not the cause of low oil pressure, and the oil temperature is normal, worn bearings will become the next most likely reason for weak oil pressure.
High oil temperature
Overheated oil will thin out creating oil pressure issues as previously stated.
Normal operating temperature for crankcase oil is 55°C to 65°C. If it is an open drive compressor, the shaft seal housing will be somewhat hotter. It could operate in the range of 70°C to 75°C.
Note: The compressor should not be operated with the shaft seal housing being above 75°C, as this will be harmful to the seal.
Operation temperature of the oil tends to rise with high discharge temperatures which may be due to bad condensing conditions, or due to a high compression ratio in low temperature applications.
When condensing conditions are at fault, the problem should be rectified. If high oil temperature is due to a high compression ratio, the machine should have been fitted with an oil cooler.
Oil pumped out into the refrigeration system
Foaming of the oil at startup has already been given as a potential reason for the sudden pump-out of oil into the system by the compressor. All compressors pump oil out continuously with the hot gas. This could amount to about half a litre per hour for every kW of refrigeration. Under normal circumstances, this oil returns steadily to the compressor. When oil return problems do manifest themselves, they are usually at times of low load when refrigerant gas velocities will be low. The oil will be colder than usual, and therefore thick. If oil is periodically disappearing from the crankcase into the plant, then this situation requires investigation. Refrigerant piping alterations could be required.
Leaking unloader mechanisms
In machines fitted with cylinder unloaders, a power element is pressurised with oil from the force feed lubrication system in order to cause a cylinder to load. This arrangement is repeated for each of the unloading cylinder sets in the compressor. This means involvement of an extensive oil system that could suffer from an oil leak.
You will find that the first indication is a lubrication system that works well when the compressor is unloaded, but which suffers pressure loss when the machine begins to load.
Where three-way solenoid valves are used, these valves may be accessible. If this is the case, you can power the solenoid valves, one at a time. Each time a cylinder is loaded, a dip in oil pressure will be observed while the power assembly is filling with oil. In the case of a leak, the pressure dip will be sustained, identifying the issue and corrective action can be taken.
Jackson, as you can see there are many reasons why you may experience low oil pressures in your equipment, but generally this is not a common problem. I have seen this a few times. Blocked strainers and incorrect viscosity oils are what I experienced.
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