Advanced Pulsation Analysis For Reciprocating Compressors
Through the analysis of the 3D modal responses, the field data pulsations were found to correspond with the second mode of the choke tube length response near 240 Hz and the nozzle quarter-wave mode near 320 Hz. Adjustments were needed to adjust the internal compressor cylinder volume, since the original OEM compressor design drawings were not available at the time of this study. This gas volume adjustment provided a more correct “calibrated” compressor gas volume, which brought the nozzle response down near 300 Hz to better match the field data frequencies.
The silencer redesign effort utilized a shift in the Helmholtz frequency upward slightly to open up the choke tubes and result in less pressure drop and more compressor through-put. The redesigned volume and its new Helmholtz response is 98.85 Hz. The new silencer design utilized an overall smaller volume to limit the 3D acoustic modes and thicker walls to the vessel for noise and vibration control.
Through the use of 3D acoustic modal analysis and field test data, the silencer redesign resulted in the following overall benefits:
1) A more compact silencer volume with less 3D acoustic modes in the primary excitation range and higher frequency length responses outside of the excitation range.
2) Avoidance of the choke tube length responses, compared with the previous design, which had shown severe pulsations due to a second mode of the choke tube length response.
3) A shift in the Helmholtz frequency upward (but still well below the necessary cut-off frequency), which provides less restrictive choke tubes and more throughput for the compressor.
4) Additional damping through the use of multi-hole orifice plates to attenuate shorter wavelength, high frequency pulsations which could still exist at the silencer outlet.
5) A smaller overall vessel size, which could be mechanically braced better and made more rigid with thicker vessel walls. This also resulted in a better mechanical solution for the discharge piping on the outlet, which was mounted on and supported by the silencer vessel.
Marybeth G. Nored is the group leader for the Fluid Machinery Systems Group at Southwest Research Institute (SwRI) which investigates machinery related fluid dynamics in piping systems, surge modeling and control, pulsating flows and detailed aero / thermal studies for turbomachinery design. She is experienced in reciprocating and screw compressor pulsation analysis, thermodynamic power cycles, gas property determination, machinery performance, and natural gas flow measurement. She can be reached at firstname.lastname@example.org.
Stephen M. James is engineer in the rotating machinery dynamics group at SwRI. He holds a master’s degree in mechanical engineering from Texas A&M (2010). His research interests are in the areas of rotating machinery rotordynamics, structural dynamics, acoustic analysis, and finite element analysis. He evaluates and analyzes problems in fluids machinery and associated plant systems. He has a broad knowledge of programming languages and development platforms, and supports the group's internal software applications..
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