Performance Loss of Modern Steam-Turbine Plant Due to Surface Roughness

Abstract

The serious consequences of blade-path deterioration that may result from boiler carry-over has led to active research into the performance aspects by the Air Flow Laboratory of the English Electric Company at Rugby. This paper reviews some of this work and illustrates the conclusions. Data are required to determine nozzle and blade efficiencies with various types of surface finish, from polished specimens which can be regarded as hydraulically smooth, to the type of deposit build-up and/or surface pitting which can be experienced after periods of operation at site. There is a great variety and randomness embraced by the term ‘surface roughness’. In a practical machine the difficulty is not only to obtain an accurate record of the roughened surface, but to assess its effect on the frictional loss. However, very many data have been obtained on the resistance to flow in pipes and along flat plates with varying degress of sand-grain roughness, and the simple theoretical treatment for turbine nozzles and blades explained in the paper makes use of these data. In this way, it has been possible to derive curves showing efficiency correction factors for both Reynolds number and roughness. The enormous variation in Reynolds number through a modern steam turbine makes this a highly important factor. Experimental work using cascades of typical turbine profiles has also been carried out in the laboratory to try to substantiate the theoretical analysis, various grades of emery paper being used as a yardstick. A fair measure of agreement was obtained, and other workers' results are also available for comparison. The cascade tests of efficiency were also extended to reveal the particular areas on the nozzle convex and concave faces which were more susceptible to performance loss owing to localized roughening. Finally both a 200-MW and a 500-MW steam turbine have been worked through to arrive at the worsening in heat rate that might be expected for specific sand-grain roughness values applied to all nozzle and blade surfaces. This sort of information assists the turbine manufacturer in deciding the extent of refinement necessary in his manufacturing processes to obtain satisfactory finishes. On the other hand, these ex-works finishes may well deteriorate after only short periods of site running according to evidence from many sources. Evidently increasing collaboration on these matters between manufacturers, the electricity boards, and station operators is necessary.