A Stage Efficiency Prediction Method and Related Performance Aspects of Retrofits on Disc/Diaphragm Steam Turbines

A general and well established ‘loss-debit’ method of stage-efficiency prediction, based on detailed physical blade-path modelling is described and reviewed in this paper. The continuous evolution of this process within the authors company with R&D support is also described. In particular, and through recent years, the method has been applied in Retrofit Projects, where ‘hardware is changed to improve turbine performance’. Initial retrofit applications have been on our Company’s own machines where we are the OEM (Original Equipment Manufacturer). Later this involved ‘upgrading’ part of non-OEM turbines, where dimensional and operational restraints have to be properly accommodated and the consequences on design have to be correctly accounted-for in performance prediction. The accuracy of the method is also examined with respect to application to individual component replacements, such as a specific turbine cylinder in a multi-cylinder steam turbine. It is here that ‘section’ efficiency may have more direct relevance than, say, overall heat rate and the method needs to be more discerning in this respect. Efficiency trends from possible design variants need to be accurately predicted in ‘optimisation studies’ that lead to the final ‘bespoke’ design giving the best techno-economic solution. Accurate trend prediction is, therefore, vital for the success of the method. Verification testing of the ‘performance improvement’ actually achieved in a Retrofit, by a ‘new’ component installed in an otherwise ‘old’ machine, has many special aspects and these are also reviewed in the paper.