N Parida

Failure analysis of coal pulverizer mill shaft

Failure of a ball and race type coal pulverizer mill shaft has been analysed. It was found that the shaft, made of EN 25 steel, failed by fatigue. The fatigue cracks originated from the keyway area, unusually from the top edge. The presence of elongated manganese sulphide inclusions due to improper heat treatment had reduced the ductility and CVN toughness of the material, and thus made the material more prone to failure.

Probability, confidence, and sample size in fatigue testing

The sample size necessary for a statistically significant test has been determined, and a comparison of actual and predicted lives has been made for a desired probability of survival with given confidence level, statistical error, and coefficient of variation diffeient stress levels. This approach has been used for cases where to follow a log-normal distribution. On the basis of this approxiation, it is possible to estimate the lower tolerance limit using a minimum number of samples. It has also been concluded that, whater the sample size, the estimate always results in some error, Ro, which mainly depends on confidence level, probability of survival, and stress level, and that, beyond a specified sample size, fatigue life nation is independent of sample size.

Failure analysis of high temperature studs

Studs in the interceptor valve of a 110 MW unit failed after a service life of 148,700 h. The studs were operated under a steam pressure of 35 kg/cm2 and a temperature of 535°C. The studs were fractured at one end of the threaded end. Various techniques were employed to analyse the failure of the studs. It has been concluded that the failure of the studs was due to reverse temper embrittlement. The failure was delayed due to the presence of Mo and V. To reduce the tendency to this kind of failure, the following steps were recommended: (a) reduce the phosphorus content in the steel to a low level or (b) reduce the grain size to about 10 μm.