An Analytical Approach to the Estimation of the Performance of Steam Turbine Cycles Off-Design

In estimating the performance of a real steam turbine cycle off-design it is essential to ensure that the flows as determined by heat balance conditions are compatible with the flow-passing capabilities of the stage groups of the machine. Once this condition of compatibility is satisfied the stage group inlet and outlet pressures are uniquely established and these, in conjunction with the (assumed known) locus of the off-design stateline, allow the off-design performance in terms of output and heat rate to be determined.

In the case of a condensing-only cycle operating under straightforward part load condition it is a relatively simple matter to arrive at the stage group pressures consistent with both the heat balance and the flow-passing characteristics. Under certain more extreme off-design conditions, and also in the case of passout machines where the work output and the passout quality may vary independently over a rather wide range, the problem of ensuring compatibility is, however, less easily solved. This is an aspect of steam turbine cycle design which does not appear to have been systematically discussed in the open literature.

The method presented in this paper is based on an analytical approach which permits solution by a rapidly convergent iterative technique. The stage group pressures arrived at in this fashion, and hence the estimate of thermal performance based upon them, correspond to a degree of accuracy quite sufficient for all purposes to those of the equivalent real cycles.