5.8.3    Protection and erosion shield materials

 

Methods have been investigated for collecting water from the stationary blade passage, including removal by suction through slots near the outer edge of hollow fixed blades or by bled-steam belts immediately before the last stages.

These methods have the disadvantage that they automatically reduce performance. The adoption of higher heat drops in the last stage, together with higher mass flow loading and higher pressure at inlet to the moving blades, can lead to smaller droplets whose velocities more rapidly approach that of the entraining steam. This can result in reduced impact velocity and erosion effect.

It has been the practice for many years to protect the leading edge and part of the convex surface of the last-stage moving blades by surface hardening or by the attachment (by silver soldering, or more recently by electron beam welding) of a shield made from a hard material such as tungsten, chromium tool steel or Stellite 6B.

Attachment is always followed by the most stringent non-destructive examination of the joints. Erosion of the original blade material around the shield can lead to undercutting of the shield material and eventual shield detachment. Coverbands, blade upstands and tip ties can also suffer from droplet erosion. It is therefore considered prudent to inspect the blades during overhauls. Even if the erosion shield does not become detached, the shields will erode in time and will probably need to be replaced at least once during the life of the machine.

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