Compressor Surge Control: Design And Modeling For Performance Verification
Figure 3: Recycle Valve Tested During ESD Event At SwRI MRF.
When a compressor reaches its surge condition, it loses the ability to maintain peak head and the entire system becomes unstable.
Surge is most accurately described as a system phenomenon - not a localized instability. Under normal conditions, the compressor operates to the right of the surge line. Compressor surge is sometimes viewed as a common occurrence but design of a proper surge control system should be regarded as both a necessary design practice and an effective risk-mitigation measure.
The surge-control system is an important element in the compressor system because it protects the compressor from surge over the range of compressor operations. Protection of the compressor through the surge-control system will help to avoid considerably more costly repairs or overhauls due to damaging surge conditions. Control systems may be implemented using a variety of methods and philosophies. However, the primary objective of any surge-control system should be to predict and prevent the occurrence of surge so as to reduce possible damage to the compressor and ensure a safe working environment for all station personnel. In order to ensure the system is designed properly for its various and often competing requirements, operators will choose to verify component selection, response time and behavior through a dynamic compressor surge model.
The principle of a centrifugal compressor surge-control system is based on ensuring that the flow through the compressor is not reduced below a minimum flow limit at a specific head. The majority of surge-control techniques restrict the operation of the compressor to flow rates above a defined surge-control line based on the surge margin for a particular compressor. Restriction of the operating window of the compressor in order to avoid surge because of mistakes in the surge control system design should be avoided. A properly designed surge-control system can allow the operational range of the compressor to be extended based on the response of the surge-control system.
At a minimum, the control system should actively measure the compressor head and flow through the compressor system controls and determine the resulting operating point. The recycle valve should be opened in a specified time to a valve set point determined by the control system. This signal to the valve is based on the compressor operation, its proximity and its movement (rate) relevant to the surge-control line. Opening of the recycle valve in the surge-control system effectively avoids surge by providing more flow and reducing compressor head, to move the compressor away from its surge point. In Figure 1, the compressor rundown behavior is plotted over a head vs. flow map. The compressor flow begins to drop from 900 cfm to 500 cfm. Shortly before reaching the measured surge line, the recycle valve opens and the flow through the compressor increases, to effectively avoid reducing the flow further to the left of the surge line.
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