Cavitation is the formation and subsequent collapse of a vapor inside a liquid. Cavitation occurs in centrifugal pumps and can cause severe damage and disrupt operations. The damage results from bubbles collapsing and discharging energy. The severity of the damage depends on the liquid being pumped as well as the temperature. For example, water cavitation causes more damage than oil cavitation, and cold-water cavitation causes more damage than hot-water cavitation. There are two types of cavitation: Classic Cavitation and Recirculation Cavitation.
Illustration of vapor bubble formation and collapse as liquid moves through the pump impeller.
Classic Cavitation
Classic Cavitation happens when the liquid at the impeller inlet (suction eye) is so low that some of the liquid vaporizes before it enters the pump, forming vapor bubbles. As the vapor bubbles move into the impeller passageways, and eventually the volute, pressure increases and the bubbles collapse. The cause of Classic Cavitation is low suction pressure given the suction temperature and flowrate through the pump.
- If a tank is the source of liquid for the pump, sometimes a low tank level can cause low suction pressure, and therefore cause cavitation.
- Operating conditions should be analyzed to discover other causes for low suction pressure. These may include partially shut valves, clogged filters or strainers, or other operational problems upstream of the pump.
- Keep in mind that suction temperature dictates the vapor pressure of a fluid. As the temperature rises, the vapor bubbles form more easily at a given pressure, and higher pressures may be required at the suction eye to prevent cavitation.
- As the flowrate through a pump increases, the pressure required to prevent cavitation at the suction eye also increases. This pressure is called the Net Positive Suction Head Available (NPSHA). The NSPHA must be higher than the manufacturer’s Net Positive Suction Head Required (NPSH3, or formerly known as NPSHR). Since NPSH3 is dynamic and changes with flowrate, it can generally be found plotted alongside the pump hydraulic curve for Total Developed Head (TDH).
An example of hydraulic curves. The top line shows the TDH. The NPSH3 is the bottom line. The “NPSH3 plus minimum margin” illustrates the desired minimum NPSHA, considering that the pump would already be cavitating when NPSHA crosses NPSH3.
Classic Cavitation Example
Refer to the hydraulic illustration above when visualizing this example. Let’s say a pump is operating at BEP, generating 50 gpm of flow, and the pressure at the suction eye is slightly higher than the pressure required to prevent cavitation. This results in a point on the NPSHA curve directly below the BEP caret illustrated above. At this point, the NPSHA is greater than the NPSH3 and cavitation is not happening. Then, the pump flow is increased to 75 gpm, resulting in a flowrate near the end of the hydraulic curve (on the right). Even if the pressure at the suction gauge remains constant, the NPSHA drops due to dynamic pressures inside the pump, causing the NPSHA to fall below the NPSH3 and resulting in cavitation. To prevent this, either raise the suction pressure of the pump, lower the temperature at the suction, or decrease the flowrate.
Recirculation Cavitation
Recirculation Cavitation happens because the flowrate through the pump is not ideal for pump operation. Theflow is too high or too low. Raising system pressures will not resolve recirculation cavitation. The only solution is to operate the pump inside of the Allowable Operating Region (AOR). For best pump reliability, the pump should be operated within the Preferred Operating Region (POR) which optimizes energy efficiency, minimizes vibration, and minimizes damage to the pump due to pump hydraulic conditions.
Recirculation Cavitation can be further broken into Suction Recirculation Cavitation and Discharge Recirculation Cavitation, but the damage they cause and solutions to resolve them are similar. In either case, flow reversals at the impeller suction or discharge create vortices inside the impeller passageways, creating local low-pressure spots in the fluid which cause cavitation.
The AOR is defined by the pump manufacturer and should be provided by the OEM on any purchase. The AOR is defined by an upper flow limit and a lower flow limit. The upper flow limit is usually the end of the pump curve (the right side of the curve, the highest flowrate drawn on the curve). However, in some instances this can be extended beyond, or limited to, values less than the end of the curve. The lower limit of the AOR is also called the Minimum Continuous Stable Flow (MCSF). Operation below the MCSF is the most frequent cause of recirculation cavitation and can cause short time to failure if it is severely violated.
The POR also has an upper flow limit and lower flow limit but is not always included by the OEM of the pump when it is purchased. Usually, the OEM can share that information upon request, or you can try to calculate it yourself. For most centrifugal pumps operating at a reasonable pumping speed, flow, and head, the lower POR limit is 70% of the BEP flowrate. The upper POR limit is 120% of the BEP flowrate. For pump designs with higher shaft speeds, or high flows with a relatively low TDH, the region may be restricted to 80%-120% or 85%-115% of the BEP flowrate. Under the most restrictive pump designs, with very high shaft speeds, or with a combination of very high flowrates with very little TDH, the region may be restricted to 90%-110% of the BEP flowrate.
Think this is too much? Feel free to ask an Augury Reliability Service Manager for assistance and we can help you determine the proper flowrates to ensure good pump reliability.
P. S.
Cavitation is different than entrained air or gas bubbles naturally present in a fluid. For example, if a tank level is drawn very low without vortex prevention, then vortices can form in the tank and draw air or gas into the fluid. The fluid can carry those gases into the pump causing flow and pressure fluctuations. These can damage the pump or bearings. However, this type of problem is not cavitation.
Cavitation sounds like pumping gravel. The gravel noise is actually the “pop” of the cavitation bubbles as they implode.
Not all cavitation is bad, and some pumps are designed to operate with cavitation. However, these are uncommon and unless you know your pumps are designed to do this, cavitation should be avoided.
