Comparison of Erosion Resistance of Standard Steam Turbine Blade and Shield Materials on Four Test Rigs

To obtain a better understanding of the physical processes involved in erosion and permit a wider range of turbine erosion shield materials to be investigated, an attempt has been made to correlate the erosion behavior of three standard materials (FV 566 blade steel, high-speed tool steel, and wrought Stellite 6B) on four different test rigs over an impact velocity range from 1000 to 2000 ft/s. The four rigs were the Central Electricity Generating Board (CEGB) high-speed erosion machine (HSEM), and the Napier rig at Marchwod, and the rigs at English Electric and C. A. Parsons. All four rigs list the tertiary erosion rates (assessed in the tertiary zone where rates approach a steady-state value) of the three materials in the same order of merit for any given speed : the FV 566 highest, followed by the high-speed tool steel, and then the Stellite 6B. Physical differences between the four rigs, principally in the condition of impacting water (that is, size of droplets, etc.), make cross comparisons of erosion resistance difficult. If the relative erosion rate (expressed as a fraction of the maximum erosion rate) is plotted against the relative water quantity (expressed as a ratio of the quantity of water required to acheive maximum erosion), all four sets of test results show remarkably similar characteristics, with a tertiary rate averaging a quarter of the maximum. A simple multiplying factor is not however sufficient to predict tertiary erosion rates in one machine from those in another, principally because the frequency and duration of the impact stress is coupled to the size of the droplets. Tertiary erosion rates taken from the C. A. Parsons (CAP) and English electric (EEC) rigs follow an exponential relationship of the form (dM/dW)3 = K2eav where M is the erosion weight loss, W is the mass of impacting water, V is the impact velocity, and a and K2 are constants. The value of a lies between 0.0026 and 0.004 for the three materials tested, and an average value of 0.003 would suggest that, under similar conditions, the erosion shields of a 160-in.-tip-diameter turbine are likely to have a terminal erosion rate 2.6 times that of the shields in a 136-in.-tip-diameter turbine. Conditions, however, are unlikely to be the same, as the machine with the larger tip diameter could be expected to reduce the size of the impacting droplets which could more than offset the rise in impact velocity. A supplementary program conducted on the HSEM has demonstrated that droplet size plays an important part in the erosion process, conditioning not only the duration of the incubation period but also the maximum and tertiary erosion rates. The power relationship M = W (V-Vc)n (where Vc is the critical velocity below which no erosion would be expected to take place and n a constant) successfully correlates the HSEM results for 640-μm Sauter mean diameter (SMD) droplets over the complete test range, but additional factors such as the condition of the surface will have to be considered before the correlation can be extended to cover smaller droplets.