Proper planning and operation are essential.
Pump cavitation is a destructive issue that can befall even the highest-quality pumping equipment. The symptoms range from excess noise and energy usage to serious damage to the pump itself. Thankfully, with the right planning and troubleshooting protocols in place, pump cavitation can be easily avoided.
What is pump cavitation?
Cavitation occurs when air bubbles are generated inside a pump because of the partial pressure drop of the flowing liquid, resulting in a cavity at the relevant part. Changes in pressure inside the pump turn the liquid into vapor and, as the pump’s impellers spin, back to liquid again. The air bubbles move, pressure is increased and the air bubbles instantaneously implode. The collapse of vapor bubbles erodes the impeller surface, and if strong cavitation occurs at the impeller inlet, pump performance decreases, which can lead to pumping failure.
Cavitation usually happens while using centrifugal pumps — these types of pumps depend on changing pressure inside the unit to create a vacuum, pushing the liquid into the unit as opposed to pulling it in. Submersible pumps can also experience cavitation, but the instance is less frequent.
This phenomenon is especially destructive to metal surfaces, which have little elasticity and will eventually become pitted by the high-pressure jets formed by the collapsing vapor bubbles. Acrylic pumps are more pliable than metal surfaces and are therefore more resilient against damage from cavitation, but steps should still be taken with these pumps to avoid cavitation at all costs.
Two types of cavitation are possible: suction and discharge.
In the case of suction cavitation, low-pressure or high-vacuum conditions “starve” the pump of incoming liquid, resulting in low flow. Bubbles form near the eye of the impeller, and as they move toward the discharge side of the pump, the bubbles compress into liquid and implode against the impeller’s edge.
Suction cavitation can be caused by several factors, including an obstructed strainer, excessively high suction lift or fluid that is overly heated to the point of vaporization. If the pump is running too fast, vortexing — or sucking air into the line — can occur. After excessive exposure to suction cavitation, an impeller begins to wear away and look a lot like Swiss cheese.
Discharge cavitation happens when a pump’s discharge pressure is inordinately high — In other words, the pump is running at less than 10 percent of its best efficiency point (BEP). High discharge pressure prevents fluid from easily flowing out, which leads to recirculation of fluid within the pump. The liquid gets stuck in a pattern of high-speed flow between the impeller and the housing, creating a vacuum effect that forms bubbles near the housing wall. The vapor bubbles collapse, causing impact damage that can wear away at the impeller until the shaft breaks.
The sound of pump cavitation in a centrifugal pump is unmistakable. Many industry professionals describe it as the sound of pumping rocks, marbles or gravel. The sound and action is pronounced and distinct, leading most end users to swiftly correct the issue.
In the case of a submersible pump — whether hydraulic or electric — pump cavitation instances are much harder to detect, but thankfully are also rare. If it is evident that performance has drifted too far to the right or left of the BEP curve, steps should be taken to increase pressure on the suction side of the pump to eliminate the vacuum. The end user must remove the pump from the application to check for cavitation damage. Taking a good, hard look at the impeller will show the telltale signs of wear immediately.
What can be done?
One of the simplest ways to prevent pump cavitation is to properly operate a pump best suited for the application. In the rental industry, for example, it is common for the end user to lack a working knowledge of pump technology. Instead of running a pump at the ideal rpm for the job at hand, some well-meaning rental customers push pumps too hard to move fluid at faster rates. If a pump works well at 1,800 rpm, the belief is that it will work even better at 2,300 rpm. This is not the case because forcing a pump’s performance too far to the right or left of its BEP will result in cavitation over time. If a pump is correctly sized and not starved, the pump will run at the intended speed while maintaining the BEP.
Altitude also has a major effect on pump cavitation. When pumps operate at higher altitudes, special attention must be given to make sure that cavitation does not occur since liquids boil at a much lower temperature. The boiling point of a liquid depends on the vapor pressure of that liquid matching the pressure of the gas above it. The lower the pressure of a gas above a liquid — as happens at higher altitudes — the lower the temperature at which the liquid will boil. This effect increases the likelihood of water turning to a gas inside a pump, potentially leading to damage from cavitation.
High-lift applications must be carefully handled as well. It is necessary that the height of the pump reference plane be set in the safety range relative to the suction water level. Net positive suction head (NPSH) hsv is a characteristic value that expresses a pump’s suction condition. It represents the total head provided by water at a certain temperature relative to the vapor pressure. The required NPSH hsv for pressure decreases at the pump impeller inlet to the minimum pressure, which should be lower than hsv.
Keeping an eye on fluid temperature will also keep cavitation at bay because the conditions for vaporization become more favorable as liquids heat up. Closely monitoring fluid levels will also help since neglecting a pump as it continues to create suction in muddy conditions will only hasten cavitation.
Pump cavitation is only as likely as the end user’s know-how allows it to be. With careful planning — and the knowledge of job site parameters that most industry professionals possess — a cavitation crisis can be easily avoided, ensuring functioning pumps and proper flow for the life cycle of the job.