16 May 2021
Merriam-Webster defines cavitation as “the formation of partial vacuums in a liquid by a swiftly moving solid body (such as a propeller) or by high-intensity sound waves. Also, the pitting and wearing away of solid surfaces (as of metal or concrete) as a result of the collapse of these vacuums in surrounding liquid".
Cavitation is a very common problem with pumps, centrifugal pumps in particular, as this condition is directly related to the pump’s impeller function. As you may already know, centrifugal pumps rely on an impeller to move the fluids from intake to output.
An impeller is a rotor that produces a sucking force and increases the pressure and creates flow in a pump. Conversely, a propeller is a bladed fan that creates thrust by pushing against a liquid. They both move fluid, but for a different purpose. However, they both face the same challenge and can suffer the same damage.
Cavitation in centrifugal pumps is usually due to insufficient NPSH (Net Positive Suction Head) energy on the intake side of the pump. NPSH is the energy required to push the liquid into the pump. Insufficient NPSH can be the result of a number of conditions: running the pump to fast, using a suction hose that is too small in diameter or placing the pump too far or too high from the fluid source.
Put your centrifugal pump in any of these situations and here is what happens. Centrifugal pumps operate on the principle of creating low pressure on one side of the impeller and high pressure on the other. Meanwhile, science tells us that, when a liquid is being moved at any speed, with the subsequent pressure change there is, the potential for the fluid to drop below its vapor pressure.
If the pressure of the fluid drops below the vapor pressure, it causes vapor bubbles to form because the fluid is boiling at a non-usual temperature. You can get water boiling at 21 ºC (70 ºF).
As the boiling fluid leaves the output side of the impeller, the higher pressure on that side causes these bubbles to burst. This bursting is almost like a small explosion, one that will reduce the machine's pumping capacity and can damage its internal pumps, the impeller in particular. The collapsing vapor bubbles then cause excessive vibration, which in turn causes rotating parts, such as the impeller, to contact non-rotating parts, such as the wear plates or wear rings.
An excessive vibration can also lead to the premature failure of mechanical seals and bearings. In these instances, the energy that is released when the vapor bubbles burst can cause pieces of metal to break off and collide with other moving parts. The damage typically occurs to the impeller and can range from pitting to cracking to chipping. The damaged impeller then leads to even more extensive and expensive damage to the pump.
When you are facing a cavitation problem, more often than not, the tip-off will come in the form of noise, sounding like pumping rocks where there are no, vibration, reduced performance, or a combination of all three. At the earliest sign of one of the above, shut down your pump immediately.
Reduce the speed of the engine so it creates less flow output. When reducing the speed, we force less volume to get into the pump and that reduces the velocity to the acceptable level where we effectively stop the formation of cavitation. Often enough, this accepted reduced flow level ends up to be more volume pumped since we actively avoided cavitation.
Re-check all of your pump’s specs and compare them with your application needs. Is the pump at too high a distance above the fluid source? Are your pipe diameters correct? Are you running the pump too fast? In short, make sure that you are not asking your pump to do more than it can.
What’s the best advice we can give, all of the above considered? Seek professional help when choosing a pump. That way, you can be certain that the equipment you’re using aligns with the needs of your application.
About Andreas Neufeld
Our Pumps Product Marketing Manager, Andreas, started his pumps career in technical service, working through applications with customers and now manages the pumps portfolio for the North American region.