For machines operating in parallel, avoid allowing one machine to take more than its share of the load. Constant-speed, identical reciprocating compressors may be well matched, but deterioration can occur on one machine due to a broken valve, or worn rings. This would cause the other machine to take on more load in situations where the suction pressure varies with flow rate.
For example, three 50% compressors are piped in parallel to a large receiver vessel feeding fuel gas to a combustion gas turbine, with fuel gas flow varying between 70 and 100% of the rated flow. Two compressors would be running, with one on standby. One unit would be on “base load”, and the other would be the “swing” unit taking all the load variation. This load variation could be accommodated by automatically actuating suction-valve unloaders to maintain the receiver pressure within a set range. Clearance pockets could be used in combination with the unloaders to reduce the control fluctuations.
When the system has no receiver, a bypass with control valve is required if it is necessary to regulate the flow to quantities between those provided by the capacity steps. Otherwise, the control systems set-up could be the same as described above.
There are numerous possibilities for control system arrangements for parallel operation which are beyond the scope of this manual. Consult a control systems engineer to optimize the system and work out the details.