If the suction service valve on a reciprocating machine is front seated and the low side pressure rises immediately after shutting off the can pump, what is likely the cause?

In the context of a reciprocating machine, when the suction service valve is front seated, it effectively seals off the suction line. If the low side pressure rises immediately after shutting off the can pump, it indicates that there is a pressure imbalance occurring. The primary role of the suction compressor valves is to allow refrigerant vapor to enter the compressor while preventing it from flowing back into the suction line when the compressor is off.

A rise in low side pressure suggests that refrigerant is being allowed back into the low side from the high side, potentially due to faulty suction compressor valves that are not sealing properly. This failure can cause the low side to equalize with high side pressure, resulting in an unwanted increase in pressure. The diagnosis is specifically tied to the malfunction of the compressor's internal valves in holding back refrigerant, making this the most plausible cause of the observed pressure rise.

The other factors, such as a blocked suction line, insufficient refrigerant, or excessive system pressure, might manifest in different ways, but the immediate rise in pressure after front seating the valve distinctly points to issues with the suction compressor valves.