Compressors surge controls

It is not a bad idea for the process engineer to familiarize himself with compressor surge controls. The interaction of the compressor surge controls with downstream process control valves can become a problem area later, and this study phase is not too early to put such items on a checklist. An LNG plant example comes to mind where such an operating problem existed. 

Figure 12-61D. Centrifugal compressor surge control schematic diagram shows instrumentation required when primary flow-measuring device is located in centrifugal compressor discharge line. Symbols T = temperature P = pressure A = differential across compressor outlet to inlet. See Reference 89 for a detailed discussion. (Used by permission White, M. H. Chemical Engineering, p. 54, Dec. 25,1972. McGraw-Hill, Inc. All rights reserved.)...

In most cases, closed-loop control can be improved by moving the surge control line to a more conservative position in response to disturbances. When the flow measurement is sufficiently stable (good signal-to-noise ratio), the controller can calculate the time derivative of the compressor map. 

In all static surge-detection devices, the actual plienonemon of flow reversal (surge) is not directly monitored. What is monitored are other conditions related to surge. Control limits are set from past experience and a study of compressor characteristics. 

A compressor surges at certain conditions of low flow, and the compressor map, a plot of head versus flow, has a surge line defining the limits. Surge controls help the machine avoid surge by increasing flow. For an air compressor, a simple spill to the atmosphere is sufficient. For a hydrocarbon compressor, recirculation from discharge to suction is used. 

Boyce, Meherwan P., et al. Practical Aspects of Centrifugal Compressor Surge, and Surge Control, Proceedings of the 12th Turbomachinery S ni-posium, Purdue University, West Lafayette, IN, 1983, pp. 147-173. 

As with many consumer goods, compressors cannot be purchased without accessories. Some of the accessories are essential to the basic operation of the compressor, such as lubrication systems and couplings. Other accessories, such as the anti-surge control equipment, are optional in order to enhance operation but are not essential. The accessories may be purchased from the equipment vendor or they may be purchased from another party—a decision open to the purchaser. Generally, the accessories essential to the operation are purchased from the equipment vendor as part of the original purchase. The lubrication system and couplings are classic examples of this option. Intercoolers, while essential, are not normally purchased from the equipment vendor if the application is a process compressor. 

Brown, Royce N., An Experimental Investigation of a Pneumatic Closcrt Loop Anti-Surge Control for Centrifugal and Axial Flow Compressors Master s Thesis, University of Wisconsin, Madison, Wisconsin, 1966,... 

A recycle valve is needed for surge control as well as for the conditions listed above for reciprocating compressors. At constant speed the head-capacity relationship will vary in accordance with the performance curve. For a constant compressor speed ... 

The compressor can operate at any point on the performance curve. For the maximum value of suction pressure, the pressure rise across the machine at the surge control point must be less than the system pressure rating. If not, a relief valve should be installed,... 

Friedli, F, H. Lenz, and S. Ross, Controlling Compressor Surge, Instruments and Control Systems, p. 36, Feb. (1973). 

A) Compressor surge is prevented by using a low flow controller that opens the valve in the spill-back line from compressor discharge to compressor suction. 

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