The surge region of the centrifugal compressor may be viewed in terms of flow, head or speed. These three descriptions affect how the surge region responds to changes in gas composition. The surge line will change based on gas composition if the surge line is represented by variables that are calculated based on gas properties (enthalpy, density, or entropy). The different views of surge are more advantageous to particular operating environments.
Three views of the surge line are shown in below Figure.
Viewing the surge line in terms of speed versus flow rate provides an estimate of proximity to surge within a flow rate range or surge margin. As the compressor speed increases, the surge margin may be reduced. Flow rate changes based on density effects (or changes in the gas composition) will affect the surge margin and surge line in this view. The surge margin is useful during normal process operation when the flow must be reduced due to pipeline requirements.
The surge line viewed in terms of compressor head versus speed provides an estimate of the head rise to surge. The head rise to surge tells the operator how quickly surge will occur during shutdown operation. If the surge line is more flat, the head rise to surge will be smaller and surge can occur more quickly in an emergency shutdown event.
The third view of surge (shown at right in Figure) is the head versus flow representation. This view shows the compressor turndown or operational window. At a constant head, the compressor will operate within the operational region to the right of the surge line. The allowable operational flow range determines the turndown of the compressor. The surge limit model is best represented on a head versus flow map because this representation normalizes the surge line. The compressor head for the performance map is calculated using the measured temperature, pressure ratio and flow across the compressor, as well as the gas properties.
The reduced head versus reduced flow equation should be used to simplify the equations because the temperature, specific gravity and compressibility cancel out in both the head and flow equations. The reduced head is only a function of pressure ratio and flow.
Reducing the head and flow to these variables allows the surge limit model to be used at all speeds and gas compositional variations with minimal shifts in the surge line location on the performance map. The reduced head view is the best representation for the surge control system because the density effects cannot affect the uncertainty in the surge control line. This provides more precision in the surge prediction within the control system algorithm.
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