Check Valves

Check valves are required in the piping system at any point where backflow of gas after a shutdown has the ability to restart the compres sor, running it backwards or, for that matter, even in the normal direction. Reverse rotation is totally bad, as many components of the various compressor types are not designed for reverse rotation, and there is some possibility, generally remote, that the compressor could reach a destructive over speed. Forward rotation is bad primarily because the intent was to stop the compressor, and it is now operating out of control. This is a problem, particularly if the shutdown was caused by a compressor failure indication, and the need to stop was to prevent further damage. In this mode, it is unlikely that the compressor can attain an overspeed condition. An application with a high potential for backflow is the parallel operation of two or more compressors.

Another application with a risk of compressor uncontrolled restart is refrigeration service, where condensed liquid is normally present. On shutdown, the vapor from the boiling liquid has a high probability of producing a significant vapor flow, either for reverse or possibly forward rotation depending on the geometry of the piping and compressor nozzles.

Having presented all of the reasons for appropriately located check valves, what about the reliability of the check valve? There are some schools of thought that would argue the use of a check valve as detracting from reliability. Without an argument, check valves do fail if not properly engineered for the intended service. If the check valve is subjected to pulsating flow as is expected with many of the positive displacement compressors, it is certainly subject to fatigue failure. If a dynamic compressor is surged for extended periods, the stress on the check valve may become excessive and the valve may fail. The check valve should be of a rugged design and be properly selected for the application. It may need power assist or damping.

In some cases, power-operated block valves may be substituted for the classic chcck valve. However, these valves depend on an outside motive fluid and a signal from an external source to operate. It goes without saying that these ancillary items in themselves must be highly reliable. In a strict sense, overall reliability may be reduced because of the higher complexity; however, the entire system must be considered in making this type of judgment. In cases where forward flow can cause a problem, the power-operated block valve may be the only choice, because a common check won't work. The flow in this situation is in the normal direction.

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