MALFUNCTIONS OF A STEAM-TURBINE MECHANICAL CONTROL-SYSTEM

This paper is aimed at elucidating the cause of a series of malfunctio ns involving the bending or breaking of main steam turbine throttle va lve spindles which occurred at service times ranging from hundreds to several thousand hours in a number of 270 MW steam raising units. It w as clearly established, by two distinct approaches (one engineering, o ne micromechanistic) that the stresses which produced these malfunctio ns were bending in nature and were the result of out-of-alignment defl ections. In the case of the bent spindles the stresses were Very high and approached flow strength levels of around 8000 MPa while the broke n spindles were the results of fatigue initiation and subsequent growt h from a thread root (stress concentration) location on the spindle. U sing relevant fatigue crack propagation data for the valve spindle mat erial at 300 degrees C it was demonstrated that fatigue failures occur red at spindle deflections of between 0.9 and 1.6 mm. Finally, it was demonstrated that the fatigue breakage problem could be significantly reduced, especially at the lower end of the valve spindle deflection r ange, by a combination of re-profiling the thread root and shot peenin g. (C) 1998 Published by Elsevier Science Ltd. All rights reserved.