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The leakage can be through the valve or around the valve via the seat gasket.

Anytime that the internal cylinder pressure is less than the discharge cavity pressure, high temperature gas will leak back into the internal cylinder bore cavity. This will occur during the expansion, suction, and compression processes in the cylinder end.

The gas temperature in the discharge valve cavity will increase because hot gas is leaking into the cylinder, raising the temperature of the gas at the beginning of the compression phase. During compression this higher temperature gas is further raised in temperature.

As with any internal recycle leakage, the cylinder discharge temperature goes up as it is directly related to the gas temperature at the suction toe and the compression ratio. The cylinder measured discharge temperature will be a blend of the discharge temperatures for both ends. This affects the flow rate calculations of the healthy cylinder-end often resulting in its calculated flow balance being greater than 1.00. The actual discharge temperature will be greater than the theoretical discharge temperature. The theoretical discharge temperature of a healthy cylinder end can be reasonably calculated if there is a known gas k-value and a known suction cavity temperature.

If you have access to a PV card, the measured discharge volumetric efficiency (VE) will go up. In addition to the normal pressure rise in the cylinder during the compression process, high pressure gas is leaking into the cylinder causing the internal cylinder pressure to reach discharge pressure earlier. The discharge valve opens earlier; however, some of the gas now leaving the cylinder was previously compressed. Flow calculations based on the discharge conditions are therefore, erroneously high. 

During the expansion process, the measured suction VE decreases for the same reason. Since gas is leaking into the cylinder during the expansion process, a greater portion of the piston stroke is used to reach suction pressure and the suction valve opens later. Flow calculations will be lower than those made at discharge conditions, but since gas is leaking back into the cylinder during the expansion event, these calculations will also be too high.  

Some general observations for pure discharge valve leakage are as follows:

• The discharge valve caps on the end of the cylinder with the leaking valve will get the hottest, but this is not a good indication as to the actual severity of the leak or the predictability of future leakage progression.
• A leaking discharge valve by itself does not raise the cylinder suction cavity temperature.
• In a single stage, low ratio pipeline application, temperature differences between a healthy and a unhealthy cylinder may be small.
• In a multi-stage application, the unhealthy cylinder stage may only show a minor temperature increase since the stage compression ratio may drop because of the lower capacity; however, the discharge delta temperature will go up.
• The horsepower, rod load, and pin reversals in an unhealthy cylinder are still measured accurately by the analysis equipment, whereas the accuracy of theoretical models may suffer significantly. The actual horsepower load in the cylinder may go up, go down, or remain relatively constant; however, in all cases the horsepower/mmscfd of gas moved will increase.
• Complete sealing failure or an improperly installed discharge valve may totally unload the cylinder end at discharge pressure. If this is a head-end cylinder, insufficient XH pin reversals may occur. Cylinder discharge temperature may tend to return toward normal.

Happy monitoring,

Warren

Many areas of the country are experiencing temperatures near or above 100^o F for prolonged periods of time. Staying aware of heat watches and warnings is the best way to prevent heat related illnesses. Know what to do if you see someone experiencing heat exhaustion or heat stroke.

Heat Safety. NWS. Retrieved from https://www.weather.gov/media/safety/Heat-brochure17.pdf July 23, 2023.

Heat Index. NWS. Retrieved from https://weather.com/maps/current-heat-index July 23, 2023.