For reciprocating compressors, what do “three returns per valve, two returns per valve, one return per valve” mean? And from which point does each return take place?

The three-stage one refers to the control valve that allows fluid to flow back from the three outlet stages to the compressor inlet; it is used for pressure regulation and to prevent overpressure and excessive current during startup.

Return 1 refers to the flow from the first three sections back to the upstream of the control valve at the compressor inlet; it is used to regulate the inlet pressure and to prevent overpressure and overcurrent during startup.

For reciprocating compressors, there are modes such as one cycle per step, one or two cycles per step, one or three cycles per step; depending on the process requirements, there may also be six cycles per four steps, etc. These modes are used when starting or stopping the compressor or when the production process is not operating properly. The principle behind them is as follows: for example, with one cycle per step, the pressure at the first-stage outlet is sent back to the first-stage inlet through a bypass pipeline, thereby reducing the load on the entire compressor unit. It is well known that it is best not to start a reciprocating compressor under pressure, as this can cause excessive stress on the unit. When starting without pressure, it is advisable to open all the circuits first; once the unit is operating normally, pressure can be increased gradually according to the compression ratio. Once the pressure in the unit is slightly higher than the system pressure, air can be supplied to the system. This is the normal procedure for starting and supplying air. When the system’s operation is not normal and less air volume is required, the circuits can be opened to maintain pressure while the system continues to operate. With the circuits open, the amount of air supplied by the compressor can be adjusted to meet the system’s needs. It is important to consult experienced technicians and compressor maintenance workers, as they must know how to operate compressors properly.