Cavitation is caused when a liquid experiences rapid changes in pressure, leading to small vapour-filled cavities forming in areas where the pressure drops below the vapour pressure for the liquid. These bubbles or voids then collapse suddenly when the pressure increases and the vapour returns to a liquid state. The very abrupt movement of the surrounding liquid, as the cavities collapse in this way, generates a highly-localised shock wave.
Bernoulli’s principle is based on the fundamental principle of conservation of energy. A fluid flow carries energy in the form of kinetic energy, static pressure and potential energy. Therefore, when one of these forms of energy changes, the other forms must also change to maintain a constant flow of energy. This assumes that there is no significant dissipation of energy through effects such as turbulence and dissipation through heat. Bernoulli’s principle is, therefore, an approximation, but one which explains many common effects well. Typically, as a fluid flow accelerates, causing its kinetic energy to increase, its pressure drops.
The formation and collapse of vapour cavities is closely related to changes in the velocity of a liquid. When a liquid flows around a sharp corner or around an impeller blade, it accelerates rapidly and, therefore, its pressure also drops rapidly. When the pressure in the liquid drops below the vapour pressure it changes state, effectively boiling, to produce a bubble of vapour. As the local flow slows towards the average speed of the overall flow, the pressure increases again, causing the vapour to return to a liquid state.
Cavitation occurs more easily when the cavitation number, Ca, is high. Ca is given by local pressure minus vapour pressure, divided by dynamic pressure. This shows that when the local pressure is high and the vapour pressure is low, cavitation is more likely. Therefore, high-pressure pipes are much more susceptible to cavitation.
Cavitation can be divided into two types. Inertial, or transient cavitation, is typical of the commonly-understood form of cavitation described so far, in which cavities form and then fully collapse, causing a shock wave. Non-inertial cavitation involves cavities which don’t fully collapse but instead oscillate in size or shape. This can be caused by acoustic waves propagating through a liquid and may be utilised in ultrasonic cleaning baths.
WEAR
When cavities collapse close to the surface of a component, the resulting shock wave generates large stresses in the component. These implosions typically occur repeatedly, producing cyclic stress and therefore causing surface fatigue. The sudden collapse of bubbles also results in small hot spots with very high temperatures, further increasing the surface wear on components. For these reasons, cavitation is generally to be avoided in pumped fluid systems, and can drastically reduce the life of components when it does occur. There are, however, processes which deliberately exploit cavitation. Examples including removing debris from surfaces as part of a mechanical cleaning process, emulsifying or homogenising liquids.
Damage only occurs when cavities collapse close to the surfaces of components. It is, therefore, specifically this form of cavitation that must be most avoided. In addition to the direct surface fatigue wear caused by vapour cavities collapsing close to surfaces, cavitation also causes very significant noise, vibration and loss of efficiency. ‘Supercavitation’ involves a large and sustained cavitation bubble forming around a component or even an entire vessel.
Cavitation causes an erosion of metal surfaces, with affected surfaces appearing pitted. The roughness of these surfaces can then lead to further wear and poor performance. The increased surface area accelerates corrosion, and surface imperfections act as stress concentrations. The rough surface causes turbulence and edges also provide nucleation sites for cavitation bubbles, exacerbating the problem still further.
Sudden changes in pressure, and therefore cavitation, often occurs in impellers, valves and tight bends where a liquid suddenly changes direction.
Within pumps, suction cavitation occurs were the inlet pressure drops below the vapour pressure and vapour cavities are carried through the pump and then collapse on the discharge side of the pump. Suction cavitation is often caused by blockages leading up to the pump, perhaps in pipework or due to clogged filters. A poorly-specified pump or badly-designed pipework may also lead to a more persistent problem with suction cavitation. Discharge cavitation is caused by a blockage or excessive pressure in the outlet of a pump. This prevents the fluid exiting the pump, and instead it recirculates, reaching very high velocities where it passes between the impeller blades and the pump housing. Wear therefore occurs at the impeller tips and interior faces of the pump housing.
In valves, there is often a peak velocity at the region of greatest flow restriction. This can lead to very low pressure and the formation of cavitation bubbles.
DETECTING CAVITATION
Cavitation often causes significant noise, and this can often be the first sign that it is becoming an issue. Cavitation in a pump typically sounds like gravel or marbles circulating through the pump. It can also sometimes sound like popping bubbles or a cracking noise. This will be clearly different to the constant hum of a correctly-operating pump.
Reduced performance, either in terms of lower flow-rate or higher energy consumption, is another sign that could indicate cavitation. The use of vibration sensors for condition monitoring can also be an effective way to detect cavitation as soon as it occurs.
Where cavitation has already caused damage, inspection of components can confirm cavitation is to blame, and help to identify the underlying problem. Cavitation causes surfaces to become pitted with chunks of varying sizes missing. In the worst case, the surface will start to look like a sponge.
PREVENTION
Proper pump selection is critical to avoiding cavitation. If a pump is operating close to the limit of its performance, there is a risk that under non-ideal conditions cavitation will be encountered. Pumps are designed and tested with fully-developed flows at their inlets. This means that they will only operate within their specified performance envelope if there is a straight section of pipe leading to their inlet. Bends or elbows close to the inlet will lead to turbulent flow entering the pump and increase the likelihood of cavitation, even when performance parameters suggest the pump is within its performance envelope.
Regular pump maintenance is also important to ensure that pumps continue to operate within their specified performance envelope. This should include checking, cleaning or replacing filters and strainers, monitoring pressure sensors, and checking for cracked or collapsed piping or hoses.
Often cavitation in a pump is caused by a flow restriction, with an upstream restriction causing suction cavitation and a downstream restriction causing discharge cavitation. In these cases, cavitation can be prevented by avoiding or removing such restrictions. This can be achieved by moving a pump closer to the fluid source, simplifying and shortening pipework, using larger diameter pipes, removing bends and valves, and cleaning pipes and filters. If cavitation suddenly starts in a pump that was previously operating without any issues, searching for blockages should be the first step.
Reduced surface tension can reduce cavitation by limiting the size of cavitation bubbles. In some cases is may be possible to use a different lubricant or hydraulic fluid to make use of this effect.
Tougher, more elastic materials are generally less susceptible to cavitation wear since they are better able to dissipate the shock loading caused by the collapse of cavitation bubbles. For example, components made from stainless steel will be better able to resist cavitation than those made from cast iron.
In conclusion, cavitation can cause a great deal of damage within pumps and pipework. However, correct system design and component specification can generally avoid this problem. The key is ensuring that pumps and valves operate well within their specified envelope, and that they do not encounter flow restrictions or disturbances. Listening to the noise and vibrations from pipes and pumps can also provide an early warning of any cavitation related problems.
BOX: Using cavitation for good
A German state-supported R&D project aims to apply cavitation to process water treatment sludge. The new process is being tested and improved as part of a pilot system at the Wupperverband’s Hückeswagen sewage treatment plant in northwest Germany.
Sewage sludge is a multi-substance mixture, which is mainly composed of water, organic substances, nitrogen and phosphorus compounds. In Germany alone, the volume produced in industrial and municipal sewage treatment plants amounted to 1.8m tonnes in 2015. So far, the sewage sludge has mainly been used in agriculture or incinerated. However, a 2017 ordinance requires that phosphorus must be recovered from the sludge in the medium term.
The joint project aims to make the valuable substances contained in the sewage sludge, in particular phosphorus, accessible through treatment. The process produces is a cellulose-rich fibre, a nutrient-rich gel phase and an easily-fermentable liquid phase.
Researchers say that, on one hand, the separated liquid phase, with its high proportion of dissolved organic matter, can be digested much more easily than untreated raw sludge for the production of biogas. On the other hand, the phosphorus and nitrogen compounds contained can be separated out, for example by magnesium ammonium phosphate precipitation (MAP), and recycled.
The focus of the process is an ultrasonic cavitation unit (at right). A special ultrasonic transducer causes the constant pressure change in the sewage sludge to create tiny bubbles that expand and implode, causing various physical and chemical effects that tear apart the structures of the sewage sludge. Both the extremely high power input of the new ultrasonic technology and the process parameters to be set such as temperature and pH value play a decisive role in the process, say the researchers.
German firm Aquattro, technical institute UMSICHT and the Wuppervand sewage treatment plant have cooperated on the three-year, €530,000 project due to complete in August 2021.