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Cavitation Erosion in Reducer Valve

A flanged reducer with a severely worn inner surface was received for failure analysis. The reducer was on the downstream side of an 8-inch V-Ball pressure reducing control valve.

Procedure:

The valve was cross-sectioned, visually inspected, dimensionally measured and photographed.

Discussion:

The inner diameter of the KTM V-ball pressure reducing control valve was 0.865” larger than the inner diameter of the mating reducer. This diametral mismatch allowed the water coming into the reducer from the control valve to impact the edge of the reducer flange creating turbulence in the water stream, which led to cavitation erosion.

Cavitation erosion is caused by turbulence that allows the formation and rapid collapse of air bubbles. The force of an air bubble collapse causes the air vapor to impinge and erode the inner pipe surface. The wear patterns and the severity of metal loss on one side of the reducer indicate the water stream exiting the control valve was primarily oriented toward one side of the reducer pipe. The orientation of the water stream coming from the control valve accelerated the rate of the cavitation erosion on the reducer.

Conclusion:

The causes of failure were diametral mismatch and control valve orientation. Diametral mismatch was present in the abrupt decrease in inner diameter between the control valve and the reducer pipe. The mismatched joint created a 0.865 inch “shelf” that was impacted by the water stream from the control valve. The turbulence created by the water stream impacting this “shelf” resulted in cavitation erosion of the reducer pipe. The control valve located upstream was adjusted in such a way that the vector created by the water stream targeted a localized area of the reducer pipe, accelerating the rate of erosion and causing severe metal loss.

one side

Figure 1 – The extensive erosion on one side of the reducer pipe indicates that the water stream from the V-Ball pressure reducing control valve was directed primarily towards one side (left side as shown in photograph) of the reducer pipe.