Cavitation is defined as the phenomenon of forming and imploding vapor bubbles in a region where the pressure of the liquid falls below its vapor pressure. Cavitation and the resultant damage can occur in any fluid-handling equipment, especially in pumps. Technological advances in industrial protective coatings and composite repair materials have made it possible to repair pumps operating in a cavitating environment rather than simply replacing them after damage occurs. Cavitation-resistant (CR) elastomers have the ability to retain adhesion under long-term immersion, dissipate energy created under high-intensity cavitation, and provide outstanding resistance to corrosion and other forms of erosion.
Cavitation is a serious problem for pumps. In simple terms, the main utility of a pump is to move a fluid from one location to another under sometimes very extreme conditions. The impeller vane is subject to pressure gradients, which cause bubbles to form and implode and strike the surface underneath. The resulting damage to the pump’s internal working parts can cause loss of pump performance and even pump failure.
The phase diagram of water in Figure 6 is a practical aid to understanding the theory behind cavitation. This diagram illustrates the three physical states of water at different values of temperature and pressure. Water is most commonly boiled by heating it at a constant pressure, as we do when boiling a pot of water on a stovetop (white arrow). As temperature increases at constant pressure, water remains in a liquid phase until it reaches the normal boiling point (100C at 1 atm).
What is less intuitive is that water can also be boiled by dropping the pressure at a constant temperature (red arrow in Figure 6). This is exactly what occurs just behind the leading edge of a pump impeller vane. As water (or any other fluid) enters the pump, it is deflected by the vane. Above the leading edge of the vane, the fluid is compressed, creating a high local pressure area. Directly after the leading edge, there’s a small area of decreased pressure. If this decrease in fluid pressure moves below the vaporization curve at constant temperature, the fluid will begin to boil, and vapor bubbles will form in the fluid. Behind this low-pressure area there is another high-pressure region. As the vapor bubbles entrained in the fluid move into this high-pressure region, they condense and collapse violently against the surface of the impeller vane. This rapid production of vapor bubbles, followed by their violent collapse, is described as cavitation (Figure 7).
6. Two ways to boil water. The curves on the graph represent equilibrium states. The curve bordering the liquid and gas phases is referred to as a vaporization curve. At normal conditions of pressure and temperature, a fluid is at 1 atm (14.7 psi) and 25C (77F). The white arrow illustrates a typical heating process that occurs at atmospheric pressure. The red arrow illustrates that saturation temperature (hence, boiling) of a liquid can also occur by reducing the liquid’s pressure. Source: Belzona Inc.
7. How to damage a pump. A cavitating fluid can cause extensive damage to a pump impeller even during normal operation. The imploding pressure caused by cavitation has been recorded as high as 145,000,000 psi, which exceeds the elastic limit of any exotic alloy. These vapor bubbles are responsible for the mechanical damage found on pump impellers placed in any type of service that causes cavitation. Courtesy: Belzona Inc.
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