Development of a Predictive Method for Quantifying Vortex-Shedding Pulsation Amplitudes in Compressor Piping Systems

Abstract

One of the design criteria for centrifugal compressor piping systems is the prevention of piping and valve failures from high pulsation amplitudes and vibration caused by vortex-shedding induced (VSI) pulsations of gas flow past closed piping branches (stubs). Most vortex-shedding analyses are performed on the basis of frequency avoidance. If a coincidence is predicted between the acoustic natural frequency of a piping segment and the flow induced vortex shedding frequency, piping changes must be made to avoid the coincidence, since there is currently no known method for accurately predicting the severity of the resulting pulsation amplitudes and, therefore, the possibility of piping failures. Often, these piping changes are expensive and time consuming and likely unnecessary for smaller bore piping (10-inch or less), assuming the flow-acoustic coincidence does not coincide with a mechanical resonance. The majority of research performed to date on vortex-shedding excitation of piping stubs focuses on predicting the frequencies of excitation using an experimentally determined Strouhal number or range of numbers for a given geometry or piping feature. However, to properly design a piping system using a combination of support structures and piping changes, reliable prediction of the pulsation amplitude is essential for determining the shaking forces acting on the piping system and the resulting vibration and stress amplitudes. Due to the expense of experimental testing and availability of equipment, it is difficult to obtain accurate amplitude measurements of VSI pulsations of fluids at Reynolds numbers typically associated with natural gas transmission. This paper describes a series of experimental tests performed to record vortex-shedding induced pulsation amplitudes in three test facilities. Three different process fluids were utilized varying the operating conditions to obtain gas flows with Reynold’s numbers between 1e5 and 2e7. Steady state flow, temperature and pressure data was recorded with ASME PTC-10 compliant instrumentation. Transient pressure data was taken with dynamic pressure transducers installed at each stub end as well as in the main piping near the tee. The geometry configurations, pulsation amplitudes, and calculated Strouhal numbers associated with each test configuration are presented. Comparison was made between the test results and several previously published formulas. From the test results, a predictive method was developed to best represent the limit of the pulsation amplitudes.