Question 7 of 8: Cavitation and Erosion

Question 7: Cavitation and Erosion (10 marks)

Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.

Part I — hydraulic cavitation. Cavitation is governed by the local static pressure relative to the liquid's vapour pressure. Wherever the flow accelerates — over the suction surface of a pump impeller vane, at a turbine runner exit, or in the low-pressure core of a draft tube — the static pressure falls, and if it drops to the vapour pressure at the local temperature the liquid boils and vapour bubbles nucleate. These bubbles are swept downstream into a region of higher pressure, where they collapse violently. The collapse is asymmetric near a solid wall: a high-speed micro-jet of liquid is fired at the surface, and the repeated hammering, at pressures of thousands of atmospheres, fatigues and pits the metal.

The damage therefore appears where bubbles collapse against a surface rather than where they form. In pumps the low-pressure suction sides of the impeller vanes near the eye are attacked; in reaction turbines the runner blade backs near the trailing edge and the draft-tube entry are typical sites. The result is a spongy, eroded surface, loss of efficiency and head, vibration and noise. Cavitation is controlled by keeping the net positive suction head above the required value — limiting suction lift, raising inlet pressure, or lowering the machine setting relative to tailwater — and by using cavitation-resistant materials such as stainless steels at the vulnerable surfaces.

Part II — moisture erosion in steam turbines. In the low-pressure stages of a condensing steam turbine the steam expands below the saturation line and becomes wet: fine water droplets condense out of the vapour. These droplets travel much more slowly than the vapour and lag the flow, so when they reach a moving blade row they strike the blades at a large relative velocity on the wrong side. The repeated impact of water droplets erodes the leading edges of the last-stage moving blades, particularly near the tips where the blade speed — and hence the impact velocity — is greatest.

Moisture erosion blunts and roughens the leading edges, reduces stage efficiency, and, if unchecked, threatens the mechanical integrity of the highly stressed long last-stage blades. It is limited by keeping the exhaust wetness within a few percent, by draining moisture through slots and grooves in the preceding fixed rows, by using reheat to keep the expansion drier, and by protecting the leading edges of the last-stage blades with hardened or Stellite-shielded strips.