Cavitation and Water Hammering in Valves

Cavitation in Valves

When a liquid flows through a partially closed valve, the static pressure drops in the region of rising velocity and in the wake of the closure member and may reach the liquid's vapour pressure. The liquid begins to vaporise and produce vapor-filled cavities in the low-pressure zone, which build around tiny gas bubbles and contaminants transported by the liquid.

When the liquid reaches a high static pressure region again, the vapour bubbles collapse or implode. This is known as cavitation.

The collapsing vapour bubble's opposing liquid particles impinging on each other causes locally high but short-lived pressures. If the implosions occur at or near the valve body or pipe wall borders, the pressure intensities can match the tensile strength of these sections. Rapid stress reversals on the surface and pressure shocks in the pores of the boundary surface eventually result in local fatigue failures that cause the boundary surface to roughen until huge cavities emerge.

A valve's cavitation performance is typical for a specific valve type, and it is typically specified by a cavitation index, which shows the degree of cavitation or the valve's proclivity to cavitate. 

The cavitation index in valves measures the likelihood of cavitation in a fluid passing through a valve. Typically, the cavitation index is determined using the pressure drop across the valve as well as fluid parameters such as density and viscosity. The index is a dimensionless measure that indicates the possibility of cavitation happening in the valve. A higher cavitation index suggests a higher risk of cavitation.

Cavitation can be avoided by allowing the pressure drop to occur in phases. By increasing the ambient pressure, the injection of compressed air immediately downstream of the valve reduces the production of vapour bubbles. The entrained air will interfere with the readout of any downstream sensor on the debit side.

Water hammering caused by valve operation

The variation in kinetic energy of the flowing fluid column creates a transitory change in the static pressure in the pipe when a valve is opened or closed to vary the flow rate. This brief shift in static pressure in a liquid is sometimes accompanied with pipe shaking and a pounding sound, hence the name waterhammer.

The transient pressure shift does not occur instantly along the entire pipeline, but rather gradually from the point where the flow change is triggered. If, for example, a valve at the very end of a pipeline is closed instantly, only the liquid elements at the valve are affected. The kinetic energy contained in the liquid elements compresses and stretches the adjacent pipe walls. The rest of the liquid column continues to flow at its previous velocity until it reaches the liquid column at rest.

The velocity of sound in the liquid within the pipe is equal to the speed at which the compression zone spreads towards the inlet end of the pipeline. When the compression zone reaches the inlet pipe end, the entire liquid is at rest but at a pressure greater than the normal static pressure.

The unbalanced pressure now causes a flow in the opposite direction, relieving the static pressure rise and pipe wall expansion.

When this pressure drop reaches the valve, the entire liquid column returns to normal static pressure, but continues to discharge towards the inlet pipe end, creating a subnormal pressure wave that begins at the valve. When this pressure wave has completed its circuit, the normal pressure and flow direction are restored. The cycle now begins again and continues until the kinetic energy of the liquid column is wasted through friction and other losses.

To avoid the production of excessively high surge pressures when opening or closing valves, stop valves should be operated gently and with a uniform rate of change of the flow velocity. Check valves, on the other hand, are operated by the flowing fluid, and the speed with which they close is determined by the valve design and the deceleration characteristic of the retarding fluid column.

If the surge pressure is caused by a pump stopping, the surge pressure must be calculated using the pump characteristic and the rate of change of the pump speed after the power supply has been turned off.

The system tolerates a slow-closing check valve if the distance between the check valve and the site of pressure wave reflection is long and the elevation and pressure at this point are low. If the distance between the check valve and the point of reflection is small, and the pressure at this point is strong, the flow reverses nearly instantly, and the check valve must close extremely quickly. Such near-instantaneous reverse flow occurs, for example, in multipump installations when one of the pumps fails unexpectedly.

There are various methods for calculating fluid pressure and velocity as a function of time and location along a pipe. Graphical and algebraic methods can be employed for simple scenarios. However, the widespread availability of digital computers has made the use of numerical methods more convenient, allowing solutions to be obtained with any desired accuracy. 

In some circumstances, reducing the impacts of waterhammer by altering the valve characteristic may be difficult or impractical. The pipe system's characteristics should then be changed into consideration. One of the most typical methods is to install one or more surge protection devices at strategic positions throughout the piping system. A standpipe holding gas in direct contact with the liquid or separated from the liquid by a flexible wall or a pressure relief valve is one example of such a device.

Waterhammer effects can also be modified by purposely modifying the fluid's acoustic characteristics. This can be accomplished, for example, by directly injecting bubbles of a non-dissolvable gas into the fluid stream.

This has the effect of lowering the fluid's effective density and bulk modulus. A similar effect can be obtained by enclosing the gas in a flexible walled tube, or hose, that runs the length of the pipe.