Modification of Oil Brake on Cryogenic Expander for Improved Efficiency Under Varying Load Conditions

Abstract

Investigation into the operation of a 200kW brake oil expander in a 1970’s vintage ethane cracker revealed that maximum gas throughput was significantly below design and that the gas throughput varied significantly in normal operation. Expander refrigeration duty was thus below design, and substantial production losses accrued due to the inability to achieve the low cryogenic refrigeration temperatures at the original design point and as plant throughput varied. The oil brake controls were redesigned based on a detailed understanding of the operation of both expansion turbine and the centrifugal pump brake. The OEM supplied fixed orifice was replaced with an engineered hydraulic selfregulating control block. Only proven industrial hydraulics components were used to maximize reliability and minimize cost. The control block was bench tested separately from the expander after manufacture to allow the control block to be accurately pre-set before installation in the field. Commissioning was trouble free. The result is a robust and reliable oil brake control that has increased turbine efficiency to be better than 85% of the original BEP at all times. Analysis of the impact of the resultant increased refrigeration duty indicates that this project will have a payback time of less than 1 year.