Considering the large number of high-rise buildings in New York City, booster pumps play a very important role in ensuring a continuous water supply. In the absence of a booster system, most buildings in NYC only get a reliable water supply for the first five floors. For this same reason, keeping booster pumps under optimal operating conditions is very important, and one of the main causes of impeller failure is an hydraulic phenomenon called cavitation. This article will provide an overview of cavitation and how it can be prevented.
What is Cavitation?
Everyone knows that water can be boiled with heat, turning it into vapor. However, low pressure can also vaporize water, and this can happen inside a pump if water is not supplied with enough pressure at the intake. When the pressure of a fluid drops below a critical value called the vapor pressure, small bubbles form in the flow, and these bubbles collapse violently once pressure increases again - the phenomenon is called cavitation, because the bubbles are cavities in the fluid.
You may be wondering how a pump reduces fluid pressure, when its actual purpose is to increase it. The answer can be explained with Bernoulli’s principle, which states that a fluid loses pressure as it speeds up or as it rises to a higher elevation. Water speeds up at the pump suction, and its pressure drops momentarily before being increased.
One bubble forming and collapsing does not cause major issues, but consider that thousands are continuously forming and imploding when a pump has severe cavitation issues. The combined shockwaves of all these bubbles gradually erode the pump impeller. When removed, the impeller blades will seem to have corroded, even though cavitation does not involve any chemical processes.
Other than impeller erosion, cavitation has many negative consequences in water booster pumps and other similar systems:
- Vibration: The ongoing formation and collapse of bubbles not only wears down the impeller. The resulting shockwaves also shake the impeller, inducing vibrations in the entire shaft, with the potential to damage other system components. Seals and bearings are especially vulnerable to vibration.
- Noise: Cavitation is very noisy due to the imploding bubbles. For a person close to the affected pump, it may sound like there are small rocks or marbles are being pumped along with water.
- Decreased performance: Cavitation represents wasted energy, and this can be reflected as a reduction in flow or discharge pressure. A sudden drop in pump performance without an evident reason may indicate cavitation.
The technical specifications for pump manufacturers typically include a value called the Net Positive Suction Head (NPSH) required, which can be defined as simple terms as the minimum water head required at the pump suction for normal operation. If the actual head is above the NPSH required, no cavitation occurs.
In theory, cavitation can be prevented by increasing the suction pressure or by reducing the speed of water as it flows through the pump impeller. In practice, there are many ways to accomplish this effect.
- Reduce pump speed: Cavitation is less likely at lower RPM values, so a booster pump can be slowed down with a variable frequency drive (VFD), as long as the system continues to meet the pressure and flow requirements in the NYC plumbing code.
- Install the pump at a lower level: Static water pressure is higher at the lower levels of a building, so installing it at the lowest elevation possible reduces the chance of cavitation.
- Reduce temperature: The critical pressure at which cavitation occurs increases as fluid temperature increases. If water must be pumped and heated, make sure the pump is installed upstream from the water heater.
- Selecting the right pump: Many cavitation issues can be attributed to poor pump selection, and the issue disappears when a pump that matches the application is used.
The best solution for cavitation is not allowing it to occur in the first place, and this can be accomplished by working with qualified design professionals from the start of a project. Modifying actual projects is far more expensive and time consuming than editing construction plans and specifications. A high-level professional design will not only prevent cavitation, but also optimal equipment capacity, energy efficiency and NYC code compliance.