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The compressor map for a speed controlled compressor identifies the possible operating points for the machine. The possible conditions are limited by maximum and minimum operating speed, maximum available power, choke flow, and stability (surge) limit (Figure).
This article is extracted from "Dynamic simulation and testing to assess rundown speed of a compressor" by Rainer Kurz, Solar Turbines Incorporated Rienk Zwerver, Gasunie Sarah Simons, Southwest Research Institute, Adrian M. Alvarado, Southwest Research Institute and Klaus Brun, Southwest Research Institute, at the 2018 Asia Turbomachinery & Pump Symposium.
Typical performance map for a variable speed
Surge is the flow reversal within the compressor. It is often accompanied by high fluctuating load on the compressor bearings and should be avoided to protect the compressor. A surge avoidance system (“anti-surge-control”) consists of a recycle loop with a fast acting valve (“anti-surge valve”). When the control system detects the compressor approaching its surge limit , the anti surge valve is opened.
The control system uses the measured suction and discharge pressure and temperature, together with the inlet flow into the compressor as input to calculate the relative distance (“surge margin” or “turndown”) of the present operating point to the predicted or measured surge line of the compressor “(Kurz and White, 2004). The control system is set up to open the recycle valve at a preset turndown (typically 10%), thus keeping the compressor from crossing the surge line. This is the normal control function of a compressor control system. A well designed system acts without upsetting the process, and can allow the compressor station flow to be reduced down to zero station flow.
Following the description in White et al (2006): “There are five essentials for successful surge avoidance:
1) A Precise Surge Limit Model: It must predict the surge limit over the applicable range of gas conditions and characteristics.
2) An Appropriate Control Algorithm: It must ensure surge avoidance without unnecessarily upsetting the process
3) The Right Instrumentation: Instruments must be selected to meet the requirements for speed, range, and accuracy.
4) Recycle Valve Correctly Selected for the Compressor: Valves must fit the compressor. They must be capable of large and rapid, as well a small and slow, changes in capacity.
5) Recycle Valve Correctly Selected for the System Volumes: The valve must be fast enough and large enough to ensure the surge limit is not reached during a shutdown. The piping system is the dominant factor in the overall system response. It must be analyzed and understood. Large volumes will preclude the implementation of a single valve surge avoidance system.”