Understanding And Preventing Steam Turbine Overspeeds.

Abstract

Destructive overspeeding of steam turbines due to poor component design, inadequate maintenance, and improper operating practices are all too common. The total cost of a turbine runaway can run into the millions of dollars and often include lives lost. Determining the root cause of the failure afterwards is confusing. Because so many variables contribute to an uncontrolled turbine, assessing all of them is difficult. Design ideas of the trip systems for the many turbine manufacturers are varied and span many years. Frequently, the causes of an overspeed disaster are not fully understood even after an exhaustive study is completed. Little hard data are available because things happen very fast. The accounts of eyewitnesses are highly unreliable, because so much happened that one individual could not see it all. No one wants to talk about the event due to legal entanglements. Most conclusions must be based on an event reconstruction done by looking at some broken parts, with limited information from data sources and witnesses. The unfortunate result is little movement among either users or manufacturers toward improving equipment to reduce the possibilities of an overspeed episode. Engineering design and application of the steam turbine and its appendage devices are important factors in the reliability of the machinery train. Lack of training for maintenance and operating personnel are also big factors in the possibility of a runaway. Nearly half the more than forty destructive overspeed failures encountered by the authors occurred during periodic testing activities. Considering that only a few hundred turbines are involved in the authors ' experience, overspeed failure represents a major problem.