Oil level measurement in oil-injected screw compressor packages used in the petroleum, petrochemical, refrigeration and fuel gas markets

摘要

Screw compressors used in the petroleum, petrochemical, refrigeration and fuel gas tend to feature lubrication, shaft-sealing and oil-control systems designed to the general requirements of API-614. These systems require oil level measurement for process control, alarm and trip functions. This paper focuses on the level measurement within the package's primary separator. Oil injected screw compressor packages inject oil not only into the drive train bearings but also into the compressors working chamber. During the compression process the gas becomes entrained within the oil and the primary separator vessel is used to separate the two mediums. The level of the oil within the separator requires to be monitored as too high a level would increase the amount of oil carry over into the process and too low a level could eventually cause a process trip. The process and internal design of the separator vessel does not lend itself to easy measurement such that the oil to be measured can be at a low or high temperature, the dielectric constant varies with the amount and type of gas entrained within the oil and the viscosity of the oil to be measured varies with its temperature. Add to this the oil and gas mixture discharged from the compressor is being deliberately blasted against a diffuser to separate the two mediums makes for difficult measurement. This paper discusses each of the different parameters in turn with a view to aligning them with measurement technologies before concluding with the most suitable technology for this application.