Variable speed operation has become increasingly common for reciprocating compressors, driven by the efficiency advantages of matching compression capacity to pipeline demand. What is less commonly understood is that variable speed operation significantly complicates pulsation analysis.
The Fixed Speed Baseline
For a fixed-speed compressor, the acoustic analysis evaluates pulsation at a single operating point. The excitation frequencies are known — they are multiples of the fixed running speed — and the bottle sizing is optimized for that one condition. If the analysis shows compliance at the design speed, the package is compliant.
The Variable Speed Problem
For a variable speed compressor, pulsation levels and the risk of acoustic resonance must be evaluated across the entire operating speed range. A bottle that provides adequate attenuation at 1,000 RPM may be inadequate at 700 RPM, where the excitation frequencies shift and the relationship between acoustic and mechanical modes changes.
More critically, a mechanical or acoustic resonance that falls outside the excitation frequency range at full speed may fall squarely within it at a reduced speed. This is the most common source of field vibration problems on variable speed packages — the analysis was performed only at rated speed and a resonance condition at part speed was never identified.
What a Proper Variable Speed Analysis Requires
A proper pulsation analysis for a variable speed compressor must sweep the acoustic simulation across the full operating speed range, typically in 50 or 100 RPM increments, and identify all resonance conditions at all speeds. Where resonances exist, the analysis must quantify the pulsation levels at those conditions and either confirm compliance or recommend corrective action.
Tech Transfer performs variable speed analyses as a standard sweep across the full RPM range. The additional computational effort is straightforward. The alternative — discovering a resonance condition in the field — is not.
