A Review of Cavitation-Erosion Resistant Weld Surfacing Alloys for Hydroturbines
An improved weld surfacing alloy has been developed and tested to resist cavitation-erosion in hydroturbines. Typical wear characteristics experienced in laboratory testing has been correlated to actual service conditions. A metallurgical evaluation shows that a high strain, work hardening austenitic stainless steel produces superior resistance to cavitational erosion. Several industrial alloys were evaluated using the vibratory and high velocity cavitation test, to produce a new alloy development in weld surfacing. Field testing shows an improvement in cavitation-erosion resistance of up to 800% relative to 308 stainless steel.
Keywords: Welding, Surfacing, Cavitation, Erosion, Wear, Turbines,
Cavitation occurs to various degrees in all types of fluid handling equipment including propellers, pumps, piping systems and large turbines. Equipment used for the movement of fluids is an important part of most manufacturing industries.
Hydro-electricity, coal-fired power utilities, marine, chemical, pulp & paper and petrochemical industries are such examples. With reference to the generation of electrical domestic power, most of the electrical energy requirements are satisfied using traditional coal-fired steam turbines. However this process has energy extraction efficiencies as low as 30% and is a major contributor to pollution. Electrical energy extraction up to 90% is regularly achieved using hydraulic turbines (hydro-turbines). Hydro-stations form a very important part of the overall electricity system for many different countries. In New Zealand, approximately 80% of operational power stations are hydro stations .
For efficient operation of a hydro-turbine, it must have specific shape and contour. Cavitation-erosion leaves behind cavities or pits which affect these important contours, creating obstacles to smooth flow of water through the turbine. This leads to a loss of operating efficiency of the turbine. Considering the cost of electrical energy, even a relatively small change in the operating efficiency can be very expensive. Cavitation causes surface penetration damage of up to 10 mm per year to critical components such as impellors, turbine blades, and casings . The end result is a reduction in energy extraction capacity that can lead to losses in terms of downtime, productivity, efficiency and money. Today, hydro-turbines are more powerful and more compact, two factors that can lead to greater risk of cavitation.