Large reciprocating compressors have high power requirements, sometimes running in the range of megawatts. Integrating them into complex processes with side streams or multi-stream compression requires precise control and operational flexibility. Advanced technologies for compressor capacity control can provide flexibility and save massive amounts of energy, but it can be difficult to achieve precise control. Large reciprocating compressors are a poor match for variable-speed drives because the large motors make the drives costly and the high torque requirements require large flywheels. Traditional capacity control methods waste electric power by using recycle valves or recycle valves in combination with step control.
Today, with increasing capacities and power ratings of new reciprocating compressors, advanced methods such as stepless capacity control and split-range control systems can help to eliminate use of recycle valves, save energy, improve controllability and save costs. This article explains the basic theories of traditional and advanced capacity control methods for reciprocating compressors.
Traditional methods used to work
In recent years, many refineries were upgraded to produce clean fuels or process feedstock with higher sulphur content. The sole capacity of one compressor running at full load was sufficient to meet the hydrocracker or hydrotreater hydrogen demand. A second compressor was kept in standby mode only to ensure equipment redundancy. Traditional methods were well-suited for these conditions: