Paper 3: Wetness in Steam Turbines: A General Progress Survey

The subject of wetness in steam cycles divides naturally into three aspects, erosion, wetness loss, and supersaturation effects. The close relation once postulated between one or the other is no longer accepted. Erosion is no longer thought to be closely related to braking and this, after addition of slip loss and Coriolis loss, does not seem to account for the observed wetness loss which must therefore be at least partly attributed to supersaturation or to some other effect not yet isolated. Supersaturation theory does not offer any practical means of calculating its effect and therefore continues to be included in the classic type of wetness formula which is to be regarded as purely empirical. There is no evidence that drainage reduces the loss markedly though it may reduce erosion. Droplets created in suspension within the body of the steam are smaller than about 1 micron and make no contribution to erosion. Large droplets (about 0.5 mm diameter) shed by the stator vanes and protected from disintegration by lying in the wake of the vanes are now the accepted cause. The precise mechanism by which films of water collect on the vanes is not fully explained. Better understanding of erosion mechanism has resulted in several devices with expectations of significant amelioration. Among these the simplest is widening of the gap between the vanes and blades. The problem of making high-speed blades last a reasonable time does not now seem as formidable as some people thought a few years ago.