Compression Systems are designed and operated in a manner to eliminate or minimize the potential for surge, which is a dynamic instability that is very detrimental to the integrity of the unit. Compressor surge can occur when compressors are subjected to rapid transients such as one following emergency shutdown (ESD) or power failure. To prevent this from occurring, compressor stations are designed with recycle systems and special types of recycle valves, which are required to open upon ESD. These recycle valves must have specific characteristics to cope with such fast transients and protect the compressor for undergoing surge. This paper describes three methods for determining the effectiveness of any specific recycle system and recycle valve characteristics. These three methods are: i) the concept of the Inertia number, ii) a simplified method based on system impedance, and iii) full dynamic simulation of the compression system. These three methods are applied to a high pressure ratio (up to 3.5) natural gas compressor station involving very high volume of piping and equipment contained within the recycle loop. There is a heat exchanger to utilize the heat of compression to heat the condensate separated downstream in the discharge scrubber, two large aerial coolers to cool the high temperature discharge gas from the compressor, and the discharge scrubber itself. This equipment and associated large volume capacitance contained therein presented a significant challenge and demand on the surge suppression system in all aspects of operation; particularly during ESD. Results are presented from all three methods which show the consistency and complementary nature of these methods.

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