Advanced Pulsation Analysis For Reciprocating Compressors

Pulsation fluid models for reciprocating and screw compressor piping have become more sophisticated as the horsepower and speed ranges for the compressors have increased. This is due, in large part, to the need to more reliably predict system responses for control of high vibrations.

Using more advanced computational models, fluid system designers at Southwest Research Institute (SwRI) have been able to more accurately determine expected fluid pulsation levels and corresponding response frequencies and mode shapes, resulting in more efficient and reliable pulsation bottle designs with lower horsepower consumption.

This article highlights some examples of the use of advanced fluid system models at SwRI to provide integrated fluid mechanical design solutions for reciprocating compressor systems and screw compressor silencers.

3D Analysis
Pulsation models of reciprocating compressor systems commonly utilize a one-dimensional (1D) representation with acoustic length modifications to represent the three-dimensional (3D) system, given the simplicity and cost-effectiveness of this approach. One-dimensional transient fluid models are generally accurate for pulsating flow piping systems where the dominant physical length is in the flow direction. In the areas near the compressor cylinder, very close to the fluid force excitation from the piston, the 1D assumptions break down since many of the high-frequency energy components have not diminished.

For new high-speed reciprocating compressor cylinder designs, certain 1D representations of the gas passage may not be valid and could lead to incorrect predictions.

Uncontrolled responses associated with the cylinder gas passage system are primarily evident as higher frequency vibrations and high cycle fatigue failures at the compressor valves, cylinder body, and in the cylinder nozzles. Inaccurate designs will lead to use of cylinder nozzle orifice plates, poor valve life, and low compressor efficiency due to high dynamic pressure drop.

SwRI conducted an investigation comparing field data on installed high-speed units to predicted 1D and 3D acoustic responses near the compressor cylinder gas passageways. The combination of a 3D acoustic response model and a 1D fluid representation model was used to provide accurate predictions of all gas passage system responses in a cost effective manner. The testing discussed herein validated the new integrated approach and refined the SwRI transient fluid modeling methods.

Field-measured Pulsations
In order to characterize the responses in the system, pulsation data from the field site was recorded during a speed sweep of the reciprocating compressor unit which spanned the speeds from 825 -1,000 RPM. (This meant that the primary first order of excitation occurred at 13.75-16.6 Hz and the dominant energy at 2x spanned frequencies from 27.5-33.2 Hz). The four-cylinder compressor with the instrumentation installed is shown from its instrumented side in Figure 1. In this design, the two cylinders on each side of the machine fed into a three-chamber discharge pulsation filter bottle. Suction-side filtering was accomplished with a two-bottle system.