An intriguing phenomenon can occur when liquid passes through a flow restriction, such as a control valve. The valve’s restricted flow path causes the liquid velocity to accelerate. When velocity increases, it corresponds to a decrease in the local pressure (Bernoulli’s Principle for incompressible liquid flow). If the drop in pressure is great enough, it may fall below the liquid’s vapor pressure (the pressure at which a liquid becomes a gas), resulting in the formation of vapor bubbles.
Upon exiting the control valve, the liquid velocity decreases, pressure recovers, and the vapor bubbles implode. This phenomenon is called ‘cavitation’, and it can be quite destructive. When a vapor bubble implodes, the bubble’s center pulls in a jet of liquid at supersonic speed. If these implosions occur close to the valve or pipe wall, the powerful microjet will erode material from that surface. If left unchecked, cavitation can lead to significant noise, vibration, erosion, and mechanical failure.