UNDERSTANDING PUMP CAVITATION: Causes and Prevention. 

Pumps are central to most industrial processes and the second most common machine in the world (after the electric motor). Because they are so common, pumps are often overlooked as a potential source of improved productivity or a cause of excess costs if not operated properly.

Pump cavitation, a phenomenon that all pumps experience, can cause many issues including excess noise, vibration, and energy usage, not to mention serious damage to the pump itself.

CAUSES OF PUMP CAVITATION

Pump cavitation is a physical phenomenon that occurs when the pressure of the liquid incoming becomes lower than the vapor pressure of the liquid. A reduction in pressure is typically caused by increased speed of the fluid. When fluid pressure on the trailing side of the impeller blade (opposite the pump intake) falls below the vaporization point of the fluid, vapor bubbles begin to form. 

As the bubbles/cavities travel to the discharge side of the pump, moving to a high-pressure area, the cavities implode. The imploding or collapsing of these bubbles trigger intense shockwaves inside the pump, causing damage to the impeller, vibration, and excess noise.

When a pump is subjected to shocks or rapid fluctuations in pressure, its moving parts can be damaged. This problem worsens over time and can eventually lead to failure. The degree of compression for a vapor bubble is contingent upon the speed and shape of the impeller blades. Liquids of higher temperatures have a greater vapor pressure, creating an increased risk for these occurrences. 

HOW TO KNOW IF THE PUMP IS CAVITATING 

The good news is that there are technologies and techniques you can use to detect, monitor, and prevent pump cavitation. Cavitation damage can shorten the life of the pump’s impeller, mechanical seals, bearings, and possibly other components—but it’s easy to avoid that scenario. In fact, it will increase your pump’s MTBF (mean time between failure), which means fewer maintenance costs and downtime.

If you suspect your pump is suffering from cavitation, one way to confirm this would be to listen for the cracking sound. The sound tends to be audible when the pump is running at full power. The level of vibration experienced by the pump can also be observed since it will increase when cavitation occurs.

Pump cavitation is often accompanied by temperature and/or power spikes that are detectable at the outboard motor. Pumping dry like this can cause the motor to overheat, reducing its lifespan and risking damage to the impeller and the pump itself.

Alternatively, the outboard motor of a pump that is cavitating would experience a low power event. 

Once an event is identified, it is important to identify the cause of the pressure drop and correct it. In most cases, the problem can be solved by simplifying the suction pipework, removing as many bends and valves as possible:

  • Move the pump closer to the fluid source.
  • Clean pipework and remove any blockage.
  • Increase the diameter of the suction pipework so it reduces the inlet velocity.

Routine maintenance can be performed to avoid a future occurrence of cavitating. Suggested maintenance includes regular review and cleaning of the filters and strainers leading into the pipework.

HOW DOES MONITORING AVOID CAVITATION?

Most industries use large, complex pumps. Failure of these critical pumps could result in downtime costs that exceed $200,000 per day in operations cost alone, not counting the cost of equipment replacement due to damage. Users are thus looking for reliable solutions that would allow them to assess and prevent pump cavitation.

When vapor bubbles burst, shockwaves will travel through the water. This in turn raises the temperature of the water. The high temperature of the water can be damaging to an outboard motor. High temperatures will cause damage to the pump and impeller in the outboard motor. Motor management relays (MMRs) monitor and protect against overheating by tripping, or even shutting down, the power source if high temperatures are detected. Thus, MMRs offer a high level of protection for your entire power system while maximizing uptime.

To prevent cavitation, users should carry out an effective maintenance program on the pump. A regular maintenance schedule from RPM will increase the pump’s efficiency and lengthen its life.

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