Designing Oil-Free Screw Compressor Systems for Acoustic Resonance During Shop Testing, Field Startup, and Process Operation

Abstract

An OFS (Oil Free Screw) compressor experienced problems meeting the API 619 3rd Edition vibration level acceptance criteria during shop testing. Extensive testing identified the primary mechanism that increased the rotor vibration levels was high levels of dynamic pressure pulsations due to acoustic resonance. This presentation will identify the fundamentals of acoustic resonance and how these frequencies can be affected by the factors which define an acoustic system. An inherent characteristic of OFS compressor discharge flow is a pulsed flow defined by pocket or lobe passing frequency occurring when the discharge gas from the rotor lobe pocket passes into the discharge porting. Oil free screw compressors can experience high levels of vibration due to acoustic resonance coupling with high dynamic pressure pulsations generated by these positive displacement machines. When a screw compressor is not volumetrically matched and the gas MW, density or discharge pressure is high, the discharge dynamic pressure pulsations can be high.