Accelerated Fatigue Phenomena in High-Horsepower Turboexpander Wheels

Abstract

Abstract Radial expansion turbines or "turboex-panders" are widely used in the gas industry in both process and power recovery applications. The increasing cost of plant inputs such as power and feedstocks, coupled with abrupt swings in plant throughput as a function of markets and pipeline system linepack, places a premium on turbooexpander designs capable of operating efficiently and reliably over a broad range of operating conditions. High horsepower turboexpander wheels which operate over a wide speed band are subject to fatigue failures if the wheel's operating frequency window is too narrow. The fatigue failure mechanism is accelerated by resonance. Appropriate wheel designs, which will deliver extended operating life, may require modified wheel hub profiles and the use of higher strength alloys. Strict control of forging and heat treatments must be maintained to ensure metallurgy of a uniform high quality. This case study outlines the systematic methods adopted in identifying the cause of turboexpander wheel failures at the Kaff Creek Gas Plant, and the engineering design revisions required to eliminate them. Introduction The Karr Creek Gas Plant located in section 10, township 65, range 2 west of the 6th meridian in northwestern Alberta, Canada, first came on stream in 1981. This "shallow-cut" plant was initially designed as a dewpoint control facility utilizing a conventional refrigeration process. The products of the plant were natural gas and C3+ liquid. In 1990, a major addition was completed in the form of a new "deep-cut" plant employing the Ortloff Process. This addition boosted natural gas production capability from 45 to 105 MMscf/Day, improved C3+ liquid recovery from approximately 50% to 88% and allowed for the production of a stabilized C5+ liquid product. Integral to the new "deep-cut" facility is the rotary expansion turbine or "turboexpander". The turboexpander replaces the "JT valve" commonly found in cryogenic processes and allows the recovery of "free" horsepower for other plant requirements by converting the pressure energy of the gas stream to mechanical energy. At Karr the turboexpander powers post-boost compression into the sales compressors. Without boost compression, the plant capacity is restricted by as much as 35% depending on sales line pressures. Therefore, a dependable turboexpander is essential for the mechanical and economic efficiency of the plant. Shortly after startup of the "deep-cut" facility, it became apparent that the turboexpander system was not dependable. A catastrophic failure of the turboexpander wheel occurred without warning after 1300 hours of operation. Subsequent wheel failures have occurred after 4500, 2500, 750 and 9000 hours of operation (refer to Table 1 for a summary of wheel failures). P. 195^