Emergency shutdowns in compressor stations cause fast transients in the operating conditions. The paper and presentation will address the physics of compressor surge, as well as the physics that have to be modeled to describe the system behavior during these fast transients. Sample calculations are presented. Surge avoidance generally falls into two classes: Avoidance of surge during normal (slow) process changes and surge avoidance during fast transients, which, far example occur during emergency shutdowns. In this paper, data from a compressor that surged during an emergency shutdown are presented. The data are analyzed to determine the effects of surge and the rate of deceleration. The issue of the rate of deceleration, in particular for different drivers, is discussed. A model to simulate shutdown events is developed and possible simplifications are evaluated. The compression system is analyzed, thus verifying the model and the simplifications. The impact of piping geometry, valve sizing, and instrumentation on the results are also covered.
A centrifugal compressor, operating within a compressor station (Figure 1), will exhibit a stability limit that prevents it from operating at conditions that require a flow lower than the flow at said stability limit (Figure 2). The stability limit is usually referred to as surge limit, because once the compressor operating point crosses the surge limit, the flow through the compressor will reverse. This can cause damages to the compressor. It is important to note that surge is a dynamic system behavior, that follows from the interaction of a centrifugal compressor (expressed by its head-flow characteristic) with the pipes, valves, coolers etc. of the compressor station upstream and downstream of the compressor. The possible operating points of a centrifugal gas compressor are limited by maximum and minimum operating speed, maximum available power, choke flow, and stability (surge) limit (Figure 2). Surge, which is the flow reversal within the compressor, accompanied by high fluctuating load on the compressor bearings, has to be avoided to protect the compressor. The usual method for surge avoidance ("antisurge-control") consists of a recycle loop that can be activated by a fast acting valve ("anti-surge valve") when the control system detects that the compressor approaches its surge limit If the surge margin reaches a preset value (often 10%), the anti-surge valve starts to open, thereby reducing the pressure ratio of the compressor and increasing the flow through the compressor. The situation is complicated by the fact that the surge valve also has to be capable of precisely controlling low. Additionally, some manufacturers place limits on how far into choke (or overload) they allow their compressors to operate. Properly designed centrifugal compressor systems can provide an extremely large operating range, that is further enhanced by the appropriate use of recycle systems.
