Troubleshooting Worn Rings and Leaky Valves

A refrigeration or air conditioning system that is experiencing leaky valves or worn rings may be the most challenging troubleshooting scenario a technician will experience. That's because a varying degree of severity can occur with valve or ring problems. However, the low head pressure, while simultaneously having high suction pressure, is a dead giveaway that something is wrong with the compressor's valve or ring structure.

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• Troubleshooting

• Refrigerant Leaks

• Compressors

In this scenario, the service technician will also notice lower amperage draws from the compressor, normal-to-a-bit-high evaporator superheats and condenser subcooling, and often a higher-than-normal compressor discharge temperature. Otherwise, the system will be running but cooling inefficiently. Depending on the severity of the compressor's valve or ring damage, the service technician must perform a complete service checklist to properly troubleshoot these challenging situations.

When the compressor rings are worn, some of the high-side discharge gases from the top of the piston will leak through the rings during the compression stroke, giving the system a lower head pressure. Because discharge gases have leaked through the rings and into the compressor's crankcase, the suction pressure will also be higher than normal. The resulting symptoms will be a lower head pressure with a higher suction pressure.

The symptoms for worn compressor rings are very similar to leaky valves. Pistons and rings are only found on reciprocating compressors, so when servicing systems incorporating reciprocating or piston-type compressors, worn compressor rings and/or leaky compressor suction or discharge valves will cause lack of cooling in both refrigeration and air conditioning applications.

Below are some reasons why a compressor's valves or rings may become inefficient from being damaged:

Here are some of the symptoms a system may experience when rings or valves are damaged:

Once the compressor's valves start to leak and some of the discharge gases are being short-cycled in and out of the compressor's cylinder, there will be a low refrigerant flow rate through the condenser. This will make for a reduced heat-rejection load on the condenser, thus reduced condensing (head) pressures and temperatures. This will cause a lower condenser split. As mentioned earlier, when the compressor rings are worn, some of the high-side discharge gases from the top of the piston will leak through the rings during the compression stroke, giving the system a lower head pressure.

Refrigerant vapor will be drawn from the suction line into the compressor's cylinder during the down stroke of the compressor. However, during the upstroke, this same refrigerant may sneak back into the suction line because of the suction valve not seating properly from oil sludge or other oil breakdown byproducts adhering to its surface. The results are high suction pressures. Either suction or discharge valves may also be warped from a compressor overheating problem.

Low amp draw is caused from the reduced refrigerant flow rate through the compressor. During the compression stroke, some of the refrigerant will leak through the suction valve and back into the suction line, reducing the refrigerant flow. During the suction stroke, some of the refrigerant will sneak through the discharge valve because of it not seating properly and get back into the compressor's cylinder. In both situations, there is a reduced refrigerant flow rate causing the amp draw to be lower. The low head pressure that the compressor has to pump against will also reduce the amp draw.

There will be a reduced refrigerant flow through the condenser, thus through the entire system because of system components being in series. Most of the refrigerant will be in the condenser and receiver, which may give the condenser a bit higher subcooling.

Because of the reduced refrigerant flow through the system, the TXV may not be getting the refrigerant flow rate it needs. High superheat in the evaporator and coming into the compressor may be the result; however, the superheat may be somewhat normal if the valve problem in not severe.

A discharge valve that isn't seating properly because it has been damaged or sludged will cause the head pressure to be low. The reason is that refrigerant vapor will be forced out of the cylinder and into the discharge line during the upstroke of the compressor. On the down stroke, this same refrigerant that is now in the discharge line and compressed will be drawn back into the cylinder because of the discharge valve not seating properly. This short-cycling of refrigerant will cause heating of the discharge gases over and over again, causing higher-than-normal discharge temperatures. However, if the valve problem has progressed to where there is hardly any refrigerant flow rate through the system, there will be a lower discharge temperature from the extreme low flow rate. Remember, the temperature limit is 250°F for any discharge line temperature measured 2 inches from the compressor.

Learning how to troubleshoot a system that is experiencing leaky valves or worn rings can be challenging, so technicians need to take careful measurements in order to isolate the problem.

John Tomczyk is HVACR professor emeritus, Ferris State University, Big Rapids, Michigan, and coauthor of Refrigeration & Air Conditioning Technology, published by Cengage Learning. Contact him at [email protected].