7 - Steam Turbine Key Safety and Reliability Issues

Abstract

Steam turbines extract the energy from the process fluid steam to produce useful work (power) to drive the driven machinery. Steam turbines, like driven machinery, are used for critical (unspared) and spared applications. Spared applications usually use single stage turbines and have used them mainly due to the unreliability of the plant electrical system. In recent years, the use of single stage turbines has been decreasing since plant electrical system reliability is greatly improved. The reliability of single stage turbines has been reduced by the following issues: Not having a position indicator on the inlet throttle valve, throttle valve friction binding, improper position of hand valves, steam seal leakages, and lack of bearing oil condition monitoring. Critical (unspared) applications use multistage, multivalve custom-designed steam turbines to optimize their efficiency based on the plant steam system requirements. Reliability issues with these turbines have been the risks of vane passing frequencies causing damage to turbine diaphragms and buckets. Understanding the Campbell and Goodman diagrams is necessary to minimize the possibility of vane passing frequency excitations. Like pumps and compressors, poor sealing steam designs resulting in water in the lube oil is a major cause of unreliability in steam turbine-driven trains. Oil leaks on high-pressure steam turbines have caused fires. Use TMR electronic governors and fully electronic overspeed trip systems for all critical (unspared) steam turbines to prevent catastrophic failures and negate the need for uncoupled overspeed tests. Also, the use of electronic governors on smaller steam turbines will avoid the necessity of uncoupled and dangerous overspeed trip tests.