Page contents
What is cavitation?
Cavitation is the formation of vapour bubbles in a liquid whose local pressure falls below its saturation vapour pressure: the liquid boils without being heated. Carried into a zone of higher pressure, the bubbles collapse violently, causing a gravel-like noise, vibrations, a drop in flow and head, and rapid erosion of pump impellers, propellers and foils. In a pump it appears at the suction when the available NPSH of the installation is lower than the required NPSH; on a propeller or an airfoil, in the low-pressure zones of the suction side.
Cavitation condition and available NPSH
- Pvapour: saturation vapour pressure of the liquid at its temperature, in Pa (water at 20 °C: 2,339 Pa)
- Patm: absolute pressure at the surface of the suction tank (atmospheric pressure, 101,325 Pa at sea level, plus the tank pressure if it is closed)
- Hsuction: height of the pump above the suction surface, in m (negative for a flooded pump); ΔHsuction: head losses of the suction pipe, in m
- NPSH: net positive suction head, in m; condition: NPSHavailable ≥ NPSHrequired of the pump + 0.5 m
- σ (sigma): cavitation number of an airfoil or a propeller, dimensionless; cavitation as soon as the local pressure coefficient reaches −σ; V: flow velocity, in m/s; ρ: density, in kg/m³; g = 9.81 m/s²
Suction lift of water according to its temperature
| Water temperature | Saturation vapour pressure | Maximum theoretical suction lift | Practical suction lift |
|---|---|---|---|
| 10 °C | 1,228 Pa (12 mbar) | 10.2 m | 6.2 m |
| 20 °C | 2,339 Pa (23 mbar) | 10.1 m | 6.1 m |
| 40 °C | 7,384 Pa (74 mbar) | 9.7 m | 5.7 m |
| 60 °C | 19,940 Pa (199 mbar) | 8.4 m | 4.4 m |
| 80 °C | 47,390 Pa (474 mbar) | 5.7 m | 1.7 m |
| 100 °C | 101,325 Pa (1,013 mbar) | 0 m | impossible: flooded suction required |
Detailed explanations and calculation in Mecaflux
Cavitation phenomenon boiling, generates cavitation on the propeller pumps: explanation
It is possible to evaporate a liquid, without changing its temperature, by lowering the ambient pressure below the saturated vapor pressure.
For sucking a liquid in a duct is created a depression, If the pressure drop of the liquid falls below its saturation vapor pressure,the liquid begins to boil. (Steam production) This phenomenon is called cavitation.This vacuum vaporizing water is achieved when a column of water is sucked, around 10 meters in height, Water vaporizes and releases steam which fills the depression, it is thus impossible to pump more than 8-10 meters deep, by aspirating. It is therefore necessary to pump the fluid outlet and place the pump in the bottom of the well ...
These pressure drops below the saturation vapor pressure can be very localized, for example on the upper surfaces of the profiles, or in the narrowed areas of the pumping device: Bernoulli tells us that the acceleration of the fluid creates a pressure drop. These decreases internal pressure pumps are described by the NPSH of the pump..
Cavitation is the formation of vapor bubbles due to pressure drop:
In forming these bubbles increase the volume of fluid in the low pressure area, which has the effect of increasing pressure in some places where the gas bubble condenses violently imploding. Shocks created by the bursting bubbles destroy the walls of the organs in contact with the fluid. A pump which Cavite, wears down quickly.
Cavitation should be avoided,and to the design of a propeller, this must be verified. Much information on the cavitation characteristics of propellers is obtained from model tests. They are made of cavitation tunnels where the ambient pressure can be reduced so as to cause the cavitation under conditions of controlled observation. Cavitation on propellers can be predicted according to the depression calculated on the profiles (see modeling cavitation propellers and foils)
Another phenomenon is to take into account by pumping or by hydrodynamic: the ventilation
For more information: cavitation on propellers and hydrofoils with Heliciel.
With mecaflux standard it is possible to control, if the pumping system may cavitate::
Detection of cavitation according to network and pump parameters:
When you enter the network settings, and fluid, the results s' update, and thus highlight a possible cavitation of the network or the pump. Window cavitation becomes red, and indicate whether it's your pump or duct which Cavite
In this part of the record results, see inter alia, that the pump does not Cavite and that it consumes 1845 kW for a efficiency of 0.9,. But there is a cavitation conduct because the accumulated pressure losses on the suction side and the suction height makes the pressure drop below the saturation vapor pressure in the pipe!
Frequently asked questions
Why can't water be lifted by suction from more than 10 metres?
Because it is the atmospheric pressure that pushes the water up the suction pipe: it is worth 10.3 m of water column at sea level, minus the vapour pressure. As soon as the suction vacuum approaches that value, the water vaporises and the pump loses its prime. In practice, with head losses and the required NPSH, a surface pump rarely lifts cold water from more than 6 to 7 m; beyond that, a submersible pump or a pump placed lower down is needed.
What are the signs and the damage of cavitation?
A gravel or crackling noise in the pump, vibrations, a sudden drop in flow and head, then in time a honeycomb erosion of the impeller vanes, propeller blades or pump casing, and the destruction of seals and bearings. A cavitating pump wears out in weeks instead of years.
How do I avoid pump cavitation?
Increase the available NPSH or choose a pump with a lower required NPSH: place the pump lower, flooded below the tank; shorten and enlarge the suction pipe, remove unnecessary elbows and filters; limit the liquid temperature; pressurise the suction tank; reduce the rotation speed or the flow. Keep a margin of at least 0.5 m between available and required NPSH. Mecaflux Standard flags cavitation of the pump or of the pipe as soon as the parameters are entered.
How do I check that my pump will not cavitate?
In the pumps and turbines tab of Mecaflux Standard, you enter the fluid and its temperature, the suction lift and suction head losses and the pump's required NPSH: the cavitation window turns red if the pump or the pipe cavitates, and the online NPSH calculator gives a first opinion. For an installation that cavitates or a project to be secured, Mecaflux carries out the study of your project (suction side, corrections to make) and delivers the calculation report, or offers training on the software applied to your own installation.