Gas Compression Magazine

October 2016

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ABOUT THE AUTHORS Sarah Simons, research scientist; Klaus Brun, program di- rector; Augusto Hernandez-Garcia, group leader; and Jeffrey Bennett, research engineer, are all with Southwest Research Institute (SwRI), San Antonio, Texas, USA. C ompressor manufacturers require that surge control sys- tems must keep a compressor out of surge, even during a short transient surge during a fast emergency shut down (ESD). With increasing compression ratios and ever more-demanding operating conditions, the use of hot gas bypass valves to avoid compressor surge during ESD events has become common, as shown in Figure 1. But most operators dislike a second hot gas bypass valve in their surge control system because of the added complexity and maintenance headaches they create. These valves can also be expensive, and the extra piping is problematic when space is at a premium near the compressor. In most cases these valves are installed as a result of an over-abundance of caution and could be eliminated if the rest of the surge control system piping, check valve placement, valve selection, and controller design are done properly. Hot gas bypass valve avoidance is effectively the same as transient ESD surge avoidance, since the only time the hot gas bypass valve is utilized is to avoid transient surge during a fast shutdown. So, a system design that avoids a hot gas bypass valve requires a surge con- trol system that avoids transient surge during quick shutdowns. This can be achieved with a carefully de- signed and optimized primary surge control system, which, in turn, requires a detailed engineering analy- sis of the system with a high degree of accuracy. This cannot be achieved with a basic analysis using simplified rules of thumb, non-dimensional number models, or basic lumped volume models. Typical surge controller designs are based on a 15% design surge margin and a 10% surge control line using a combination of compressor flow and head. If a compressor surge dynamic analysis is not capable of being accurate within a fraction of that percentage, it is not useful in determining whether the compressor will go into surge during an ESD or, for that mat- ter, what type of valve or whether a hot gas bypass valve will be required. Neither the non-dimensional number nor the low fidel- ity lumped volume approaches can be sufficiently accurate for this since their inherent uncertainty is significantly higher than 15%. Specifically, the accuracy of these simplified approaches in deter- mining at what head or what speed the compressor crosses the surge line during a fast shutdown is at best 20 to 30% in flow and 15 to 20% in head. Therefore, these methods are not useful when trying to determine whether a system is susceptible to dynamic surge or whether a single- or two-valve system is required. gascompressionmagazine.com | OCTOBER 2016 12 SURGE COnTROl Are SecondAry Surge control VAlVeS outdAted with AdVAnceS in trAnSient AnAlySiS? By SARAH SImonS, KlAUS BRUn, AUgUSTo HERnAnDEz-gARcIA, AnD JEFFREy BEnnETT Figure 2: Transient Simulation Predictions vs. Test Data Before And After Calibration Figure 1: Typical Hot Gas Bypass Configuration

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