Cavitation: Why Your Pump May Act Like a Shrimp
The snapping shrimp, aka pistol shrimp, can literally shoot its prey. Using a plunger on its enormous claw, it can shoot water at 70mph/115kpm. This water velocity creates a vacuum pressure pocket that takes gas out of solution, and expands it rapidly. The resulting explosion of gas bubble generates light, sound (210 dB), and shockwaves that can kill or stun its prey.
Some species have been found to use their ‘pistol’ to blast out a cozy little home for themselves in a rock. While these shrimp do not cause cavitation in your pumps, the same forces have caused premature failure in many fish farm pumps. Hopefully this article will save more pumps from the same fate as the pistol shrimp’s prey.
In the case of a pump sucking water from a tank, well, or water body, these same laws of physics can cause rapid pump failure. The theoretical limit of a suction lift is 33.8 vertical feet… if the water surface is at sea level. This is due to the pressure exerted by 33.8 feet of water being the equivalent to atmospheric pressure acting on the water surface. If further suction was applied, the water column would begin “boiling” at the surface (evaporation).
Although 33.8 feet is the theoretical limit, there are other forces at play. Namely, the force of friction in the suction pipes. As soon as the flow begins, the friction force will decrease this limit below 33.8 feet. Then there is the pump design itself. In simple terms, a pump functions by thrusting water away from the eye of its impeller. This action creates a low pressure area that can allow localised vaporization to occur right on the impeller fin. This is known as cavitation.
Ship propellers also deal with this and suffer from premature wear, loss of thrust, and excessive noise and vibration as a result. The same symptoms occur in pumps. An impeller suffering from cavitation may suffer the following fates:
Wear prematurely, even to the point of complete disintegration of the fins
·Create shaft deflection from the cavitation action that will cause premature failure of the bearings and mechanical seal.
·Operate noisily (may sound like it’s pumping rocks)
If you have a centrifugal pump that is operating in its recommended range (verify this with pressure gauges) yet it suffers premature failure, the first culprit to look for is cavitation.
How to find cavitation?
Cavitation occurs when NPSHr is greater than NPSHa. Pump manufacturers publish NPSHr curves with their pumps. NPSHr (Net Positive Suction Head Required) is a pump-specific parameter. Each pump has its own value. This value changes with flow rate, pump rotational speed, and impeller diameter.
To find NPSHr, you need to know those three values and get a copy of the manufacturer’s pump curve specific to your model.
The next step is to determine NPSHa (Net Positive Suction Head Available). NPSHa is a system-specific parameter. This is the amount of vacuum that your pump has to produce on your system at your required flow rate. It varies with vertical lift, flow rate, suction pipe fouling or smoothness, and will increase if a strainer on the suction side becomes clogged.
To calculate NPSHa use the following steps in theory:
Start with the atmospheric pressure at the water surface.
Subtract the actual vertical distance from the water surface to the centerline of the pump suction
Subtract the friction loss created by any suction pipes.
Subtract the pressure drop across any strainers on the suction side of the pump (ensure each value is converted to the same pressure unit before adding/subtracting, feet or meters of water column).
A simpler method to determine NPSHa is to install a vacuum gauge near the pump suction. This reading represents the NPSH being experienced by the pump at a given time. Take a black marker and mark the pump’s NPSHr on the face of the gauge. If the needle rises past the mark, cavitation will occur.
If NPSHa is greater than the NPSHr for your pump, there will not be a cavitation issue. However, if NPSHa is less than the NPSHr of your pump, cavitation is certainly at play. Your pump will fail prematurely and will not pump to its full capacity.
If you have a pump that is cavitating already, there may be some options for you to explore before buying a new pump. The two options are to lower the NPSHr or increase the NPSHa.
Looking at the pump curve will tell you if NPSHr can be easily lowered. In some cases, it is as simple as restricting the amount of flow through the pump. It takes more effort to determine it, but slowing the speed of the pump down (using a drive or different sized pulleys perhaps) can lower the NPSHr.
It is possible for a pump that was not cavitating previously to wear naturally to a point where cavitation begins to develop. If this is the case, doing the math will determine that the pump’s NPSHr is lower than the NPSHa. If cavitation is still present, a new impeller or wear plate or other component may be needed.
NPSHa can be increased by:
Installing the pump closer to the water surface (as low as possible).
Using a larger diameter suction pipe or reducing water flow rate (to decrease friction loss)
Removing any bio-fouling from the inside of the suction pipe
Cleaning strainers on the suction line
Using larger strainers (or multiple units in parallel)
Some Special Cases
Submersible pumps do not have a suction line, but cavitation can still be an expensive issue if ignored. Most submersible pumps have an NPSHr greater than 33.8 feet. In practice, this means they must be below the water surface. ……makes sense, but take care to find out how many feet of water there must be above the pump to prevent cavitation. Upwards of 10 feet is required on some high flow well pumps.
If a suction lift greater than 25 vertical feet is required, you may have to consider other equipment that can be used to increase NPSHa. This is common in deep well jet pumps, for example, that use a two line system to prevent water from turning to vapour on the suction side of the pump.
Philip Nickerson, B.Eng.