When a liquid flows through a control valve there are changes in the pressure and velocity, which may result to a process known as cavitation. The possibility of cavitation occurring must be considered when sizing control valves for liquid flow applications.
Cavitation is a two-phase process occurring in liquids under certain flow conditions. The cavitation within a control valve can be easily understood by treating the valve as an orifice plate in a pipe as shown in the schematic below. The changes in pressure and velocity conditions across a control valve which may lead to cavitation are shown in the schematic below:
As liquid flows through a control valve, a reduction in the area available for the flow leads to large increase in fluid velocity, which in turn causes a drop in the fluid pressure due to the principle of conservation of energy. This point of minimum pressure and maximum velocity along the flow stream is called the vena contracta The pressure is at its lowest level at the vena contracta point, which is located a little down stream of the physical valve orifice. When the pressure at the vena contracta is lower than the liquid vapour pressure, cavitation takes place.
Cavitation occurs in two stages. First, as the pressure of the liquid falls below the liquid vapour pressure, vapour bubbles form in the liquid. Second, recovery of pressure after the vena contracta raises the pressure above the liquid vapour pressure causing the collapse of the vapour bubbles. The collapsing of bubbles cause large pressure shocks in the order of 150,000psi! These shocks tend to cause yield or surface cracks on the material. Cavitation damaged surfaces are always rough and spongy. Cavitation produces noise, vibration and physical damage to the valve and/or down stream piping.
Avoiding Cavitation in Control Valves
Cavitation in control valves can in general be avoided by:
- Re-engineering components initiating high speed velocities and low static pressures
- Increasing the total or local static pressure in the system.
- Reducing the temperature of the fluid.
Ways to Manage Cavitation in Control Valves
In practice, it is not always possible to avoid cavitation but rather there are of controlling or managing cavitation. These include:
- Use special valve trim designs to control pressure drop and reduce velocities to ensure that the liquid pressure does not fall below the vapour pressure
- Control where bubbles collapse and keep it away from the vulnerable components (cavitation control trims)
- Ensure the plug and seat are made of material that can resist the damage(use stainless steel, stellite or tungsten carbide surfaced materials in the impact regions)
- Special valve trim methods using disk stack technology for severe duties in which the flowing energies are distributed through the valve trim into multiple levels of disks containing exit flow paths.