Seal Oil Piping Vibration On An Oil-Free Screw Compressor In An FPSO Application.

Abstract

A dry screw compressor skid was fabricated for offshore service that included oil buffered mechanical seals on the compressor. During shop testing, excessive seal oil piping vibration was observed that resulted in pipe component failures. The oil system was designed to include a common lube and seal oil system with a pump design pressure of 590 psi. There were six oil control valves on the system, including a lube oil pressure control valve, one flow control valve on the inlet end of the compressor, another flow control valve on the discharge end, two differential pressure controllers to maintain differential pressure on the seals for each end of the compressor, as well as a back pressure control valve for the oil system. Vibration and pressure pulsation measurements indicated that the response was at the pocket pass frequency of 206 Hz. The source was determined to be at the compressor. Arrangement of the control valves and control sensing lines were likely contributors to the severity of the response. Modification to the seal oil system included addition of several volumes at strategic locations to change the acoustical characteristics of the oil system to reduce the shaking forces in the pipe system. These changes along with pipe support modifications and changes to the control valve sensing lines were successful at reducing the pipe pulsations and vibration response to acceptable levels.