Paper 16: Effect of Thermal Cycling on Pre-Stressed Steam Pipelines

Investigations into the effect of temperature and pressure cycling on a pre-loaded model main steam line of 0·5% Cr-0·5% Mo-0·25% V steel have shown that a relatively large increase in length occurred over a few cycles at 100 per cent cold pull, and a decrease in length occurred at 50 per cent cold pull. Using high temperature platinum-tungsten strain gauges, it has been found that large changes in strain occur round an expansion loop in the model pipeline. It is suggested that large changes in the design stress distribution in a main steam line occur during the commissioning stages of a generating unit, making creep behaviour of the line during base loading difficult to predict. It is further suggested that failure in pipelines when a unit is working under a twoshift system is due to low cycle fatigue, as the strain changes occurring do not appear to approach an equilibrium minimum under cycling conditions.

It would appear that strain changes, and hence the effect of low cycle fatigue, can be minimized by choosing a level of cold pull between 50 and 100 per cent, the actual level depending upon the physical characteristics of the pipe material, the more important ones being thermal expansion and the change of yield stress with temperature.

Using the plastic strain range measured in the test a prediction can be made regarding the time to failure.