How To Reduce Cavitation in Centrifugal Pumps
Saoirse Stephan | Aug 3, 2023
What is ‘Cavitation’?
Cavitation occurs when fluctuations in the pressure of a liquid causes it to drop below then rise above its vapor pressure, resulting in the formation and collapse of gaseous vapor bubbles. This results in a microjet of pressure blasts from the bubble with enough pressure to damage steel. Why does this happen? To understand it, you need to understand the strange relationship between pressure and temperature.
Pressure and temperature are intrinsically linked. As pressure decreases, the heat needed to cause a substance to change state (from solid to liquid, or liquid to gas) decreases. As pressure increases, the heat needed to maintain a state increases. This seems complicated, but it’s actually quite simple: as pressure is decreased, matter needs less energy for all of its atoms to spread apart. As pressure increases, you need more. This is why you can boil water at only 68°c at the top of Mt. Everest, and yet it remains fluid at 400°c in the hydrothermal vents at the bottom of the ocean.
While not that extreme, these pressure differentials affect your pump as well. As a centrifugal pump draws in fluid through its inlet, the rotating blades of the impeller push the fluid towards the casing. This increases the velocity (or speed) of the fluid as it moves away from the center of the impeller to its outer edges, before expelling it at high pressure through the outlet. This creates vortices of pressure within the pump head, moving the fluid from high to low and vice versa.
Pumps are designed not to cavitate, but in the wrong conditions pressure imbalances can cause the fluid’s pressure to drop low enough that, given sufficient temperature, it will boil. As a result, vapor bubbles begin to form, moving from the low-pressure area towards an area of high pressure. As this pressure increases, the bubbles undergo rapid state change back to fluid. This causes the bubbles to form toruses and then collapse in on themselves. This collapse shoots tiny microjets which, due to the physical properties of the collapsing bubble, are strong enough to blast apart steel. The repeated occurrence of this results in erosion, vibration, excess noise, and potential pump failure.
Combine this with hot fluids coming from a kettle or HLT, and in the wrong conditions even water can damage your pump.
Ways a Pump Can Cavitate
There are two common types of cavitation that affect pumps: suction cavitation and discharge cavitation.
Suction Cavitation
Suction cavitation is caused by low intake flow due to low pressure or high vacuum conditions. This can be a product of inadequate flow, a poor coupling, a small hose, or issues in the pipeline. The low pressure at the inlet of the pump drops the pressure at the center of the impeller, causing bubbles to form. As these gaseous bubbles move towards the high pressure zones, they implode.
To prevent suction cavitation from occurring, it is crucial to ensure that the pressure at the inlet remains above the vapor pressure of the fluid being pumped. This can be achieved by maintaining an adequate NPSHa (Net Positive Suction Head available). In order to do so you must avoid excessive suction lift and by ensuring the fluid temperature is within acceptable limits.
Discharge Cavitation
Discharge cavitation occurs when the pump’s discharge pressure is too high, possibly because the discharge flow is restricted. This results in the majority of the pumped fluid recirculating back into the pump. There’s typically two ways in which this can occur. First being the internal recirculation of the fluid is forced through the clearance between the impeller and the pump housing at a high velocity creating a low pressure region. This mixture of high and low pressure creates the formation of gaseous bubbles. Second is the liquid recirculating inside the volute of the pump in which it rapidly overheats, forming gaseous bubbles. Regardless of the type of cavitation, it will have similar consequences. The formation and collapse of gaseous bubbles, causing premature wear on the impeller and pump housing. In extreme cases, discharge cavitation can cause the impeller shaft to break.
How to Recognize Cavitation?
Cavitation can shorten the life of the pump’s impeller, mechanical seals, bearings, and possibly other pump components. Early detection helps prolong the life of pump components, reduces maintenance costs, and minimizes pump downtime.
Several Methods to Identify and Detect Cavitation:
Sound and vibration
Cavitation in centrifugal pumps often produces distinct sounds and vibrations. Listen for a popping, crackling, or rumbling noise resembling bubbles or rocks passing through the system. This sound is a key indicator that cavitation is occurring within your pump.
Visual indicators
Take the time to inspect the internal surfaces of your pump for clear signs of metal loss, pitting, or erosion to the impeller. Regular visual inspections can help detect cavitation at an early stage and prevent further deterioration.
Flow and pressure
Cavitation often leads to fluctuations in flow rate and pressure, indicating irregularities in the pump’s operation. It is recommended to continuously monitor any sudden changes from the normal operating conditions.
How to Reduce Cavitation?
To reduce cavitation in centrifugal pumps, several measures can be taken depending on the type of cavitation occurring:
Suction (Classic) Cavitation:
Hose compatibility
To minimize friction along the hose and prevent flow restrictions caused by kinks or bends, it is advised to maintain suction side hoses that are both wide and short.
Suction capabilities
It is important to avoid attempting suction through equipment such as filters, heat exchangers, or other components, as this can impede inflow and diminish low pressure.
Hard angles
To ensure optimal flow, it is recommended to avoid the presence of sharp angles immediately before the inlet of your pump, as they have the potential to restrict the flow.
NPSHa is greater than the NPSHr
Ensure that the Net Positive Suction Head Available (NPSHa) is greater than the pump's Net Positive Suction Head Required (NPSHr). This ensures sufficient pressure at the pump inlet to prevent cavitation.
Discharge Cavitation:
Clear pathway
Inspect your hoses and fittings to ensure that the passage through which the liquid travels to and from your pump is not becoming constricted or choked.
Check for blockage
Regularly inspect your filters and strainers to detect any clogs, as blockages occurring on either side of the pump can disrupt the pressure balance.
Suction and Discharge Cavitation:
Spacing
When setting up your system, it is important to consider the distance between your tank or kettle and the pump to minimize friction loss due to piping.
Proper pump selection
When selecting a pump, make sure to choose one that is capable of handling the viscosity of the fluid being pumped. It is also crucial to verify that the pump is configured correctly to meet the necessary requirements.
Altitude
The altitude at which the pump operates affects cavitation. Liquids boil at lower temperatures in higher altitudes, increasing the likelihood of cavitation. Special attention should be given to prevent cavitation in such cases.
Fluid Temperature
Keeping track of the fluid temperature helps prevent cavitation. Lower temperatures are generally more favorable in reducing the risk of cavitation.
High lift applications
For applications with high lift requirements, ensure that the pump's reference plane height is within the proper and safe range compared to the suction water level.
Conclusion
It is essential to understand and address cavitation when it is occurring to maintain pump efficiency, and prolong the life of the pump’s components. By ensuring proper suction and discharge conditions, minimizing flow restrictions, conducting regular inspections, and selecting appropriate equipment, operators can effectively reduce the risk of cavitation and its damaging effects on pump performance.