CONTROL AND PROTECTION OF ROTATING EQUIPMENT

Compressor controls and safety are almost always intertwined. While the  main purpose of a control function is to maintain compressor operation  within the safe limits, the role of a safety function is taking  compressor and the surrounding process to a safe state should such  control function fail to prevent an unsafe operational condition. 

Usually controls failure manifests itself via a safety system action.  For example, if surge control cannot keep compressor away from surge, a  surge protection safety function will have to activate by quickly  putting compressor on full bypass and likely shutting it down  completely. A well-designed and diagnosed control function warrants high availability of the compressor for uninterrupted operation and thus reduces the demand for a safety function. 

Surge control system one of the key components of the compressor control  system. It includes several field instruments, signal conditioning, a usually sophisticated anti-surge control function, and one or more  anti-surge valves. Some process equipment, such as heat exchangers,  knockout drums, check valves affect reliability and performance of anti-surge control system. 

Surge protection system must be capable of detecting and counting the  surge cycles when compressor surge cannot be avoided. Improper counting  of such cycles may result either in premature safety action or in not  meeting the maximum safe number of cycles specified by the compressor  OEM. 

Compressor control system usually includes several controllers, each  having its own manipulated variable. Each such manipulated variable  affects the efficiency of the compressor and the adjacent process, The  control system design should account for optimizing the efficiency by  coordinating control actions between those controllers. Good operational  example requiring such coordination are energy management of multiple networked compressors, and multi-valve controls in a multi-stage  refrigerant compressor.