Evaluation of surge pressure dynamics in a closed fluid circuit for centrifugal compressors applications

Elisabetta Belardini, Libero Tapinassi
GE Oil&Gas

Alberto Deponti

Centrifugal compressors operability at low flow is normally limited by the onset of surge that occurs when operating near maximum achievable pressure rise at constant speed. Surge is characterized by large amplitude and periodic pressure oscillations in which the compressor can also experience a series of flow reversal and recovery. The accurate estimation of surge frequency and inception is a key task for high-pressure centrifugal compressors since its occurrence can cause the onset of dangerous vibrations that can compromise the operability of the machine. Therefore operating close to surge limit must be prevented.
For the modeling of compressor surge the most widespread approach is represented by the low order state-space model developed by Moore and Greitzer, which is basically capable to capture the fundamental nonlinear features of rotating stall and surge. According to this model the whole compression system (including compressor, pipes of different diameters, bending, coolers…) can be basically represented as a compressible plenum in which flow is at rest connected by a control valve to an incompressible pipe with flowing fluid. This approach, despite the extreme simplicity, showed good capabilities in predicting the existence and development of both rotating stall and surge. Important applications have been documented (TNO, MIT).
The assumptions behind the Moore Greitzer model of the system are strong: large regions of the system are cumulated into a single big volume in which fluid is at rest. Besides, amongst the different components of the network which may have an impact on surge dynamics, only the control valve is accounted for. Real systems are for sure more complicated. For instance, even if with a velocity significantly lower compared with the compressor speed, fluid is moving all over the portions of the system so that many components can be hardly approached as plenums. Dynamic pressure fluctuations during surge cycles may be transmitted or smoothed by the various elements according to the geometrical characteristics (area or diameter changes, presence of junctions, coolers, friction). Such details cannot be accounted for in the basic Greitzer approximation. Moreover, fluid properties can vary in the different regions of the system as a consequence of changes in both fluid velocity and temperature. Flowmaster® is a 1D software which can solve fluid systems (pipes, vessels, compressors, valves...) linking various components from a library of standard elements accounting for the fundamental features of the geometry. It also allows the modeling of both flow properties (mainly compressibility) for different technical gases and liquids.
Pressure data during surge occurrence have been acquired during testing activity in a closed test loop equipped with a single stage compressor. Several dynamic pressure probes were installed in the testing section to understand the basic features of the surge dynamics. The closed loop has been firstly modeled according to the plenum-pipe assumption suggested by Greitzer. Then a more detailed circuit was built using Flowmaster® standard components and fluid libraries. The results of both approaches in terms of amplitude and frequency of pressure fluctuations are compared and discussed against the measures.