Ask Greg McMillan - What role do you see dynamic simulation playing in the future of best understanding of compressor surge control?

Greg's Response:

The surge controller is a suction low controller with a remote set point that is parallel to the surge curve. Not only is surge prevented for all possible positions of the operating point on the compressor map, but energy is saved by less unnecessary recycle or vent flow and higher operating efficiency. Since the long axis of the efficiency ellipses are parallel to the surge curve set point, the operating point moves along the long axis for load changes.

The remote set point is the output of a ratio station whose bias is set to give the desired offset (margin) from the surge curve, and whose input is the pressure rise developed by the compressor. The margin size depends on the speed of the measurements and control valve and the speed of the fastest disturbance that should not trip the open-loop backup system.

An open loop backup detailed in next post helps prevent surge and is critical since even a fast feedback controller is unable to get a compressor out of severe surge because of the huge jumps in flow. If the instruments are fast, the feedback controller is tuned properly, and overcorrection is a problem, it is the downstream disturbance that determines the margin. A characterizer eliminates the need for the use of square root extractors to generate a set point curve parallel to the compressor surge curve. High compression ratios and multiple stages cause break points in the surge curve requiring a signal characterizer used in place of the ratio station to match the curvature.

A poorly designed surge control system may require the feed¬back set point to be 20 percent or more from surge curve, whereas well-designed feedback plus open loop backup surge control system may have its feedback set point less than 10 percent and the backup set point less than 5 percent from the surge curve.

The surge control system principles basically are the same for surge vent valves and surge recycle valves. At least two valves in parallel are used to provide redundancy, particularly because surge valves might not open after sustained operation in closed position, where stiction from seal or seat friction is greatest.

For the latest details on advances in compressor modeling and control see the Control Global article Compressor surge control: Deeper understanding, simulation can eliminate instabilities

For much more knowledge, you can download for FREE the Momentum Press book Centrifugal and Axial Compressor Control.