What is Cavitation?
Cavitation is an important term for rotating equipment engineers. The generation of vapour bubbles or cavities in a fluid flow due to low pressure and its sudden collapse causing damages in the related parts like impeller, pump housing, etc are termed as Cavitation. It is one of the major problems for Centrifugal pumps. There are various criteria that can describe the occurrence of cavitation, extent and impact of it.
What Causes Cavitation?
When the suction pressure drops below a certain value, the performance of centrifugal pump deteriorates. This suction pressure that is often decided with respect to vapour pressure at suction temperature is called Net Positive Suction Head that is popular by its acronym NPSH. If this NPSH drops, impeller inlet pressure may fall below the vapour pressure which causes vapour bubbles or voids to generate. If the flowing liquid is then subjected to pressures above the vapour pressure, these voids can implode causing damage, which is called cavitation.
The most well-known causes of cavitation are:
- Not meeting the required NPSH
- The pump is installed at too high of a distance above the fluid source
- Suction pipe diameter is too small
- Pipe blockage on the suction side
- Clogged filters
- Length of the Suction pipe is more.
- Poor piping design
- Flowing liquid is having a very low vapour pressure
- The pump is running too far right on the pump design curve
- The Pump Speed is more
- Due to high-vacuum or low-pressure environment, the flow in the pump is not proper.
Two types of cavitation are more prevalent in rotating equipment engineering; Vapour Cavitation and Gas Cavitation.
When the static pressure in a flowing fluid falls below the vapour pressure, Vapour cavitation develops. At the same time, the presence of nuclei or microscopically small vapour bubbles is required for the cavitation to form. The static pressure decreases if the local velocity is increased or the inlet conditions change.
Now, the generated vapour bubbles implode suddenly at a very high velocity, when the static pressure rises above the vapour pressure in the flow direction. This sudden implosion may lead to material erosion, a rise in noise levels, rough running of the pump and a drop in pump efficiency and head. Normally, as the implosion begins, vapour bubbles will dent inwards and later a water microjet is formed that is directed at the wall and strikes with a high velocity. This sequence of all events, along with with the fissured microstructure, very fine pores, indentations and cracks in the wall surface is the main reason behind the material’s destruction, which is accelerated in the presence of mechanical stress.
When the bubbles are generated due to release of dissolved gases from solution in conjunction with diffusion, Gas Cavitation occurs. When the fluid’s pressure drops below the saturated vapour pressure, Dissolved Gases come out from the solution. This mechanism is dependent on the concentration of the dissolved gases. In terms of material damage, Gas cavitation is not as destructive as vapour cavitation, because, with rising pressure, the gas diffuses into the liquid again which means that this process is much slower than the collapse of vapour bubbles.
The formation of Vapour and Gas may overlap, as well. Vapour bubbles which are generated when the fluid’s pressure reaches or drops below vapour pressure may also contain gas which can release from the solution via diffusion while the fluid approaching the suction of the pump. So both of these two cavitations together, intensify the extent of the impact on the fluid flow.
Symptoms/ Effects of Cavitation
As a result of cavitation, One or a combination of the following symptoms or effects can be generated:
- Unexpected Vibrations
- Decreased or Reduced Flow or Pressure
- Erosion of the Impeller
- Seal and Bearing Failure
- Erratic Power Consumption
How to prevent cavitation?
An obvious way to guarantee that there will be no cavitation is to restrict the void or bubble generation. This can be done in various ways like
- Lowering the pump speed.
- Raise the liquid level to increase the NPSH so that lower pressure scenario does not occur.
- Lowering the operating temperature.
- Reduction of Pump motor RPM.
- Using a booster pump to feed the principal pump.
- Increasing the impeller diameter.
- Selecting the impeller inlet geometry that ensures no vapour formation.
- Using two lower capacity pumps in parallel.
- Installation of an impeller inducer
- Strictly following the pump’s manufacturer performance guidelines.
- Increasing pump suction line size.
Few more resources for you..
MAJOR FACTORS AFFECTING THE PUMP PERFORMANCE”: A short article
NPSH for Pumps: Explanation and Effect
Water Hammer Basics in Pumps for beginners
Pumps & Pumping Systems: A basic presentation
A brief presentation on “CENTRIFUGAL PUMP WITH SPEED CONTROL”
Stress Analysis of Pump Piping (Centrifugal) System using Caesar II
Considerable points while Commissioning and starting-up a Process Pump
Considerable points while installing centrifugal pumps at site to reduce vibration