Cavitation Erosion

Cavitation occurs when a fluid's operational pressure drops below it's vapor pressure causing gas pockets and bubbles to form and collapse. This can occur in what can be a rather explosive and dramatic fashion. In fact, this can actually produce steam at the suction of a pump in a matter of minutes. When a process fluid is supposed to be water in the 20-35°C range, this is entirely unacceptable. Additionally, this condition can form an airlock, which prevents any incoming fluid from offering cooling effects, further exacerbating the problem. The locations where this is most likely to occur, such as:

•  At the suction of a pump, especially if operating near the net positive suction head required (NPSHR)

•  At the discharge of a valve or regulator, especially when operating in a near-closed position

•  At other geometry-affected flow areas such as pipe elbows and expansions

•  Also, by processes incurring sudden expansion, which can lead to dramatic pressure drops

This form of corrosion will eat out the volutes and impellers of centrifugal pumps with ultrapure water as the fluid. It will eat valve seats. It will contribute to other forms of erosion corrosion, such as found in elbows and tees. Cavitation should be designed out by reducing hydrodynamic pressure gradients and designing to avoid pressure drops below the vapor pressure of the liquid and air ingress. The use of resilient coatings and cathodic protection can also be considered as supplementary control methods.

Cavitation corrosion of a deaerator