Cavitation

What is Cavitation?

Cavitation is a commonly observed phenomenon in liquid systems which occurs any time local pressure reaches the liquid’s vapor pressure. At vapor pressure, the liquid boils, and forms vapor pockets within the liquid. These vapor pockets grow while the system pressure remains at the vapor pressure, and will collapse when the local pressure increases above the vapor pressure.

Cavitation is seen in many situations. An everyday example is boiling a pot of water. Raising the temperature of the water raises the vapor pressure of the water. Eventually, the water at the bottom of the pot is heated enough that the vapor pressure is equal to atmospheric pressure, and the water begins to boil. Vapor pockets are formed, and as they rise to cooler regions of water where the vapor pressure is lower, they collapse.

Another common example is cavitation at a pump impeller. Pressure in the liquid decreases as it approaches the impeller, and vapor pockets will form if the pressure at the impeller eye reaches the liquid’s vapor pressure. The vapor pockets move out away from the impeller eye, and collapse as the pressure rises above the vapor pressure.

A final example is cavitation downstream of a valve. Pressure downstream of this valve is initially above vapor pressure during normal operation. The valve is closed rapidly which generates a low-pressure wave and causes the downstream pressure to reach vapor pressure. A vapor void forms while the pressure is at vapor pressure. The low-pressure wave is reflected as a high-pressure wave, returns to the valve, and collapses the vapor pocket.

Transient Cavitation

The three examples of cavitation above all represent situations where the liquid pressure reaches vapor pressure and cavitation will occur. However, Impulse only attempts to model situations similar to the final example, where pressure waves are generated by transient events in a system forming vapor voids. This type of cavitation is called transient cavitation.

Transient cavitation can be particularly harmful to a system because the vapor void formed must eventually collapse when high pressures return. The liquid on each side of the vapor void will slam together causing large pressure surges, large transient forces, and potentially significant damage to a system. The pressure surge following a transient cavitation event can often be larger than the pressure rise predicted by the Joukowsky Equation for the system, meaning engineers should always evaluate these events carefully.

Any transient event can generate a pressure wave which results in transient cavitation. Some common events include:

Pump trips
Valve closures
Pump startups
Sudden changes in control valve position
Constructive wave interference

Cavitation Modeling in Impulse

AFT Impulse attempts to model transient cavitation in pipes using one of two cavitation models: the Discrete Vapor Cavity Model (DVCM) and Discrete Gas Cavity Model (DGCM). These models are described further on the Cavitation Models page. Both models are meant to capture limited, transient cavitation in pipes, and are not meant to capture cavitation inside a junction, or cavitation which exists for a sustained period of time. Impulse also does not allow simulations to proceed when cavitation is present in the steady-state solution.

A fundamental assumption behind Impulse’s solution methodology is that all pipes are 100% liquid-full. Cavitation inherently violates this assumption because vapor is introduced to the system. Users should always treat results with any amount of cavitation as uncertain results and carefully review all results.

AFT Impulse reports cavitation in a system with two parameters: Vapor Volume and Percent Vapor Volume. The Vapor Volume reports the actual volume predicted by Impulse’s cavitation models. The Percent Vapor Volume reports the vapor volume as a percent of the pipe section volume.

AFT Impulse also uses Percent Vapor Volume to trigger two different warning messages in the output. A Warning message is generated when the Percent Vapor Volume is greater than 10%, while a Critical Warning message is generated when the Percent Vapor Volume is greater than 100%. Each of these warnings is meant to convey increasing uncertainty in the results to the user.