Page 250 - Failure Analysis Case Studies II

Failure Analysis Case Studies II
D.R.H. Jones (Editor)
0 2001 Elsevier Science Ltd. All rights reserved 235

MALFUNCTIONS OF A STEAM TURBINE MECHANICAL
CONTROL SYSTEM

J. H. BULLOCH* and A. G. CALLAGY
Power Generation, ESB., Lower Fitzwilliam Street, Dublin 2, Ireland
(Received 13 October 1997)

Abstract--This paper is aimed at elucidating the cause of a series of malfunctions involving the bending or
breaking of main steam turbine throttle valve spindles which occurred at service times ranging from hundreds
to several thousand hours in a number of 270 MW steam raising units. It was clearly established, by two distinct
approaches (one engineering, one micromechanistic) that the stresses which produced these malfunctions were
bending in nature and were the result of out-of-alignment deflections. In the case of the bent spindles the
stresses were very high and approached flow strength levels of around 8000 MPa while the broken spindles
were the results of fatigue initiation and subsequent growth from a thread root (stress concentration) location
on the spindle. Using relevant fatigue crack propagation data for the valve spindle material at 300°C it was
demonstrated that fatigue failures occurred 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 range, by a combination of re-profiling the thread root and shot peening. 0 1998
Published by Elsevier Science Ltd. All rights reserved.
Keywords: Fatigue, fatigue crack growth, fatigue markings, plastic deformation, power-plant failures

1. INTRODUCTION

During the past decade or so certain ESB stations that operate 270 MW units have encountered
operational problems involving main steam turbine throttle valve spindles. Basically the problems
were identified as:
(i) valve sticking as a direct result of spindle undergoing permanent plastic deformation or bending;
or
(ii) spindle fracturing during operation near the top of the valve spindle in the threaded section.
The present study considered four separate spindle failures which occurred over a three year period
and involved:
(i) a detailed failure analysis or micromechanic assessment; and
(ii) a basic engineering stress analysis approach.

2 MECHANICAL CONSIDERATIONS

A detailed view uf the valve spindle arrangement is shown in Fig. 1 and in this instance the valve
was in the closed position. All failures occurred at position A at the root of a spindle thread while
the spindle bending problem was observed at position B. At these positions the working temperature
was assessed at about 300°C. Also, the forces acting on the valve spindle are illustrated in Fig. 1
between piston C and the actuating lever where the vertical force is the result of steam pressure and
the angular force was caused by the angle of the actuating or lifting lever. The resultant force is a
sideways bending force on the valve spindle and it was calculated that the relationship between the
bending stress uB, and value of spindle deflection 1, at position A could be expressed as follows:
~g = 342(%), (1)

a Author to whom correspondence should be addressed.
Reprinted from Engineering Failure Analysis 5 (3), 235-240 (1998)