Pump Cavitation 101
Many of you may have experience with a rattling pump that sounds like it might be churning rocks! Pump cavitation - a predominant condition in centrifugal pumps - is the likeliest cause for unusually noisy and excessively vibrating pumps.
Cavitation affects a pump's operating flow and pressure, thereby having a significant impact on its performance and energy consumption. It can damage a pump's impeller, bearings and other components, and cause untimely mechanical failure of the entire pumping system.
Cavitation is when gas bubbles or cavities form and subsequently implode in the liquid in a pump's relatively low-pressure areas. These bubbles are formed when the liquid's pressure dips below the vapour pressure in the eye of the impeller. They take up space around the impeller. This causes the pump to spend more energy on dealing the bubbles, rather than on pumping liquid. The resulting noise and vibration are formed by the implosion of these bubbles.
Pump cavitation usually arises out of a difference between the Net Positive Suction Head Available (NSPHa) and the Net Positive Suction Head Required (NPSHr). NPSHa is a measure of the resulting drop in pressure as liquid travels from the suction nozzle to the impeller. NPSHr is the minimum net positive suction head required to avoid cavitation.
Pump cavitation is typically symptomatic of:
1. Pressure imbalances and fluctuated flow rates created by
blockages in the suction or discharge sides of the pumping system;
How can it be prevented?
With careful planning and monitoring, pump cavitation can be avoided. Here's how:
Choose the right design: The best way to avoid cavitation is to get your pumping design system right! While designing the pumping system and choosing the pump size, it is imperative that NPSH is taken into consideration. If there is insufficient NPSHa, this would be the best stage to catch it. Avoid sharp bends, curves, and obstructions in your designs that can increase frictional pressure loss and cause low pump suction pressure.
Re-evaluate design: If your pumping system is already in place, it would be wise to explore options to increase NPSHa by raising the liquid level in the suction head or using a booster pump. Frictional losses in the pipework can be reduced by using larger diameter pipes or less fittings. Precautions should also be taken against excess turbulence in the suction line. However, these measures are often impractical due to space and cost constraints.
Check pump efficiency: Ensure your pump is operating at its best efficiency point by installing pressure gauges and flow meters.
Monitor and Maintain: There's no compromising on this prevention technique. It's always better to check filters and strainers at regular intervals to ensure there are no clogs or leaks in the pumping system. Condition Monitoring Technology and automatic self-cleaning filters can also help protect against cavitation. Keep an eye on the liquid's temperature to avoid vaporization.
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