Pump cavitation is a serious issue that has a destructive impact on many aspects of a pump, but most likely will destroy the impeller, which leads to poor pumping operation and significantly reduces the lifespan of the pump. Cavitation causes serious damage and erosion to impellers, causing even new impellers to look as if they have been used for several years. Pump cavitation often causes impeller damage beyond repair, but luckily, with the right pump system design, pump cavitation can be avoided.
What Causes Pump Cavitation?
Cavitation can occur even when using high-quality pumping equipment and occurs when the fluid being pumped turns to a vapor at low pressure, which causes air bubbles to form. Cavitation occurs when the pressure and temperature of the liquid at the impeller suction point are equal to the vapor pressure. Vapor pressure varies with pressure and temperature and is different for all liquids. Cavitation occurs when the pressure inside the pump changes and turns liquid into vapor which can happen when there is insufficient pressure at the suction end of the pump.
These changes in pressure cause the impeller to turn liquid into vapor when pumping and back into liquid again as the impeller continues to spin. This moves the air bubbles, which increases pressure and causes the bubbles to implode instantly. This implosion of vapor bubbles is just as damaging as an explosion and causes continual damage and erosion to the surface of the impeller. Strong cavitation at the inlet of the impeller causes a decrease in pump performance and can lead to pumping failure.
What is Net Positive Suction Head Available (NPSHa)?
Net Positive Suction Head Available (NPSHa) is a calculation made from the suction side of the pump configuration and is basically the difference between the pressure on the suction side and the vapor pressure of the pumped fluid at that particular point. Cavitation occurs when there is insufficient NPSHa, causing bubbles to implode as the liquid passes from the suction side of the impeller to the delivery side.
These shockwaves hit the impeller hard and create vibration, which is most often the first sign of pump cavitation. Vibration causes serious problems and mechanical damage to many pump components, including the seals, bearings, and shaft. Pump cavitation in a centrifugal pump sounds like the system is pumping rocks and often leads to swift action to stop the damage.
Types of Pump Cavitation
There are two types of pump cavitation that can occur, suction and discharge.
Suction cavitation occurs under low pressure, or high vacuum conditions, which basically starve the pump of incoming liquid and cause low flow. This causes bubbles right at the eye of the impeller, and as they move outwards to discharge, they compress into liquid and implode on the edge of the impeller. This can be caused by many factors, such as overly heated fluid to the point of vaporization, too high of a suction lift, or an obstructed strainer. Pumps that run too fast will vortex and suck air into the line. Continual exposure to suction cavitation can put holes in the impeller as it wears away.
Discharge cavitation occurs when the pressure at discharge is exceptionally high, which causes the pump to run far from its best efficiency point (BEP). When high pressure at discharge prevents the fluid from flowing out easily, it recirculates within the pump and gets stuck in a high-speed flow pattern between the housing and the impeller, causing a vacuum effect to create bubbles near the housing wall. As the vapor bubbles implode, they damage the impeller and can also break the shaft.
Cavitation and Pumps Types
Pump cavitation usually occurs in centrifugal pumps, which rely on pressure changing inside the pump to create a vacuum to push liquid into the unit instead of pulling it in.
Cavitation can also occur in submersible pumps, although it is not as common. When cavitation occurs in submersible pumps, whether they are electric or hydraulic, it is more difficult to detect than cavitation in centrifugal pumps. Thankfully, this occurrence is rare, although if a change in performance is noted, either too far to the left or right of the BEP, the operator should take steps to increase pressure on the pump suction side and eliminate the vacuum.
Operators should remove the pump immediately to check for cavitation damage when noticing a change in performance or warning signs of cavitation. Examining the impeller would quickly reveal the wear and damage caused by cavitation.
Cavitation is particularly destructive to metal surfaces as high pressure will eventually cause pitting to form near the implosion of vapor bubbles. Acrylic pumps are more resilient against cavitation damage as they are more pliable than metal surfaces, although the right steps should always be taken to prevent cavitation.
What Is Vapor Pressure?
Vapor pressure is the pressure at which liquid molecules turn into vapor and varies with temperature for all liquids. For example, when boiling water to 100°C, atmospheric pressure bubbles start to form on the pan bottom, and steam rises, indicating the temperature and vapor pressure to cause boiling has been reached. Vapor pressure and temperature are linked, and water can even boil when subjected to a partial vacuum without exposure to any heat at all.
How to Avoid Pump Cavitation
Understand and Consider Net Positive Suction Head
Luckily, for experienced pumping engineers, it is fairly simple to avoid pump cavitation and should be done during the design phase. Pump engineers must make sure that the Net Positive Suction Head Available (NPSHa) is greater than the pump’s Net Positive Suction Head Required (NPSHr), which is determined by the pump manufacturer based on extensive testing under strictly controlled conditions. Cavitation can be avoided during the design stage, assuming that no changes to the liquid properties or suction conditions occur during operation. It is key to understand and consider Net Positive Suction Head throughout the entire design phase. Net Positive Suction Head, NPSH, is the amount of energy available at the pump suction to exert pressure on the fluid.
Net Positive Suction Head can be further explained by evaluating each term:
- Net – pressure that remains after all deductions have been made for friction loss, inlet and outlet losses, and velocity head loss. Velocity head loss represents the fluid’s kinetic energy and is measured as the height in feet that the fluid would rise if all kinetic energy were converted into potential energy. Head loss is simply the potential energy that is converted into kinetic energy due to frictional resistance such as pipes, fittings, valves, inlet and outlet losses.
- Positive – + always positive
- Suction Head – the pressure at the inlet flange of the pump
Pump design engineers know that the chosen pumps must supply the NPSHr as determined by cavitation tests carried out by pump manufacturers.
Choose Pumps Best Suited for the Application
Using the right pump suited to the application is one of the easiest ways to prevent cavitation. Pump cavitation commonly occurs in the rental industry when users lack the necessary understanding of pumping technology.
Understand BEP and the Ideal rpm for the Job
When operators lack a working knowledge of pumping technology, they may not understand the impact that pump speed has on cavitation. Some may consider that if the pump works well at 1,600 rpm, it might work even better at 2,200 rpm. This is not the case, as forcing a pump to perform too far to the left or right of its BEP will cause cavitation over time. Make sure pumps are sized correctly, are not starved to run at the intended speed, and maintain BEP.
Experienced pump designers know that the altitude at which a pump is running has a significant impact on pump cavitation. Liquids boil at a much lower temperature in higher altitudes, and special attention must be given to prevent pump cavitation. The liquid boiling point depends on that liquid’s vapor pressure equaling the pressure of the vapor above it. When the gas above a liquid is at a lower pressure, which happens at higher altitudes, the liquid will boil at lower temperatures. This drastically increases the likelihood that water will turn to a gas inside the pump, leading to cavitation and damage.
Watch Fluid Temperatures
Monitoring fluid temperature will help to prevent cavitation, which is more favorable as the temperature of liquids increases.
Evaluate High Lift Applications
Pump designers must give careful consideration to high lift applications to ensure that the height of the pump reference plane is in the proper and safe range compared to the suction water level.
Pump Design and Selection to Avoid Cavitation
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