Cavitation

Cavitation
The cavitation phenomenon

Cavitation refers to the formation of fine bubble particles in a liquid. cavitation is derived from the word cavity and means cavitation, and cavitation means to create a cavity. the cavitation phenomenon is limited to liquids.

 

 

Given the current economic situation and the future ahead in the industry, we must try to maintain and improve quality and consider also making projects more economical. therefore, we intend to tell you about two destructive factors in all hydraulic tools and equipment, namely “cavitation” and “water hammer”, which both lead to crushing and wear of equipment during this article and the next article.

The cavitation phenomenon can be both beneficial and harmful, and at the end we will mention its useful applications. it goes without saying that the focus of this article and most of the existing articles is on the disadvantages of cavitation in the industry and hydraulic equipment and solutions to prevent it.

The cavity causes wear on hydraulic equipment components and on equipment such as pumps, reducing the lifespan of the impeller and other pump components.

In general, the cavitation phenomenon has 2 stages:

  1. Bubble formation in the liquid
  2. Destruction of the surface due to the destruction and bursting of bubbles

To better understand these two steps, it is enough to get acquainted with the thermodynamic laws related to cavitation. Therefore, we will examine these rules. as you know, temperature and pressure are two factors that affect the evaporation of liquids. As the temperature increases and the pressure decreases, the liquid gets closer to saturation (on the verge of evaporation). well, it is natural for you to understand the effect of the temperature factor on the evaporation process, but we should not ignore the pressure factor.

The effect of pressure on the evaporation process is such that by reducing the pressure, in fact, the pressure is removed from the liquid surface and approaches the saturation state faster and consequently evaporates more easily. and if the pressure increases and the temperature decreases, then it is normal for the liquid to move away from the saturation state and not evaporate.

Now we can explain the first stage of cavitation, that means the formation of bubbles due to the rotation of the pump impeller.

When we move an object rapidly in a liquid (such as a pump impeller), changes in particle pressure occur when it strikes water particles. For example, look at the figure below:

The arrow shown indicates the direction of rotation.

As a result of the rotation of the impeller, the area shown in blue becomes vacuum, in other words, its pressure decreases and the liquid becomes saturated, forming the bubbles mentioned above. now we want to talk about what happens in the second stage:

The bubbles formed in the first stage move by the rotation of the impeller and continue their movement towards the area indicated in red. as mentioned above. when the pressure increases, the liquid moves away from saturation, which causes the bubbles to disappear. (this is exactly what happens in the red area.) after the bubbles burst and disappear, the fluids around the bubbles that do not exist now try to maintain fluid coherence, so they rush to the cavity. the result is a high-pressure beam of liquid called microjets.

The following figure shows the events described above

Due to the impact of microjets on the surface of the part, very small damage occurs on the surface of the object, which is indicated by a red arrow. as time goes on and the number of times increases, the piece is completely destroyed and loses its original shape.

 

 
Cases that cause cavitation:
  • Excessive pressure drop at the pump inlet that causes the liquid to evaporate and bubble to form.
  • Increasing the temperature of the liquid inside the pump
  • High suction height of the pump in case the pump inlet tank is lower than the pump.
  • High pump speed which leads to increased flow and increase of fluid velocity at the pump inlet and finally decrease the pressure at the pump inlet.

 

Cavitation prevention:

It is not difficult to detect the occurrence of cavitation. because, as mentioned, cavitation creates a wave in the fluid that causes a lot of mechanical vibrations that are accompanied by a lot of noise that can be heard.

The important point to prevent such damage to the system is the timely repair, service and maintenance of equipment and pumps.

But there are also points and actions that should be considered in the system design, installation and commissioning phase to prevent cavitation or even delay it as much as possible:

  • Reducing fluid velocity at pump inlet
  • Avoid overuse of valves, elbows and other fittings that cause pressure drop in the pump inlet.
  • Increasing tank pressure (pressurized tank)
  • Increasing the height of the tank
  • Increasing the liquid level in the tank
  • Preventing turbulence in the fluid flow and help the fluid move at a constant speed.
  • Reducing fluid temperature, the lower the temperature, the more the fluid moves away from the saturation state.
  • Using cooling supplements and injecting them into the pump inlet.
  • Proper insulation of pipes
  • Pump operation in design conditions

and etc.

 

 

It should be noted that a small number of metals can resist cavitation degradation. Steel and chromium alloys have better strength than other metals such as bronze, brass, cast iron and ordinary steel, etc.

 
Useful applications of cavitation phenomenon:

Well, as we said at the beginning of the article, cavitation, although mostly discussed because of its disadvantages and negative points, but it also has positive points and benefits in various industries such as chemical industry, food industry, defense industry and submarine equipment, medical equipment, etc. for example, in medical equipment, this phenomenon is used to destroy kidney stones, open kidney ducts and cardiovascular ducts.

 

 

In the chemical industry, cavitation is used to better mix and homogenize materials. for example, in the milk processing stage, cavitation is used to homogenize milk and its fat. As you are aware, milk fat is normally placed on the surface of the milk, which uses cavitation to solve this problem.

 

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