2.3.7   Wetness loss

 

This is incurred by moisture entrained in the low pressure steam towards the exit plarie of the LP turbine. The loss is a combination of two effects; the first is the reduction in efficiency due to absorption of energy by the water droplets and the second is the erosion of the final moving blades leading-edges.

Erosion occurs because the water droplets travel far slower than steam, consequently their velocity relative to the blades is significantly different in direction as well as speed. The result is the impact of condensation droplets on the blades, in particular at the tip where the blade rotational velocity is highest.

In the past, the leading edge has been protected either by an edge-hardening process or by the brazing-on of stellite, which is a very hard material. More recently, a better understanding of the mechanism of droplet erosion has produced means of alleviating much of the problem. Higher last-stage heat drops are employed, along with higher mass flow loading. The result is a higher pressure at the inlet to the moving blades leading to the reduction in the size of droplets.

 

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