Prevent cavitation in your pump
You have just installed a pump and when you turn it on you immediately hear a crackling sound; the pump is cavitating. Technicians know it well, if you keep running with cavitation in pumps it will cost you money. Fortunately, cavitation in pumps can be easily avoided. Just a few small adjustments to the pump installation can make a big difference.
What causes cavitation in pumps?Cavitation in pumps occurs when bubbles form in a liquid as a result of rapid changes in pressure and then implode when the pressure increases further. We can use the example of a centrifugal pump to explain just what causes cavitation in pumps. Due to the high liquid velocity in the suction line, the pressure can fall below the vapour pressure, causing vapour bubbles to form. As soon as the liquid flow reaches the pump impeller, negative pressure changes into positive pressure, causing the vapour bubbles to implode.
Cavitation in pumps is therefore caused by the implosion of vapour bubbles. This is what causes the crackling sound and if left unchecked will also lead to significant damage to your pump.
Can cavitation in pumps be easily prevented?It all starts by making the right pump choice. If you still hear a crackling sound in an existing or temporary pump installation, then try the following:
- Install the pump closer to the water line
- Submerge the suction pipe deeper under water
- Increase the diameter of the suction hose
- Remove any obstacles* in the suction line
- Provide higher back pressure in the discharge line
- Reducing the pump speed
- Reducing the temperature of the liquid
- Reducing the fluid velocity
- Increasing the pre-pressure of the pump
* Obstacles can include: unnecessary pump fittings, kinks in the suction hose, incorrectly selected suction strainer, contamination. Anything that can disrupt the flow of liquid on the suction side.
And for the pump professionals…How do vapour bubbles form?
The explanation for this lies in the basic laws of physics. At a certain temperature and pressure, a liquid forms vapor bubbles. This is the boiling point. Water on your stove forms vapour bubbles at 212⁰F (100⁰C) and then boils. This takes place at the atmospheric pressure at sea level of 1 bar. However, if we reduce the pressure, water will boil at a lower temperature. For example, if we heat water on Mount Everest, the atmospheric pressure is only 0.31 bar and the water is already boiling at 156⁰C (69⁰C).
NPSHr – Net Positive Suction Head Required
Cavitation arises because the suction pressure is so low that vapour bubbles form in the liquid. You can avoid this by creating a certain absolute pre-pressure on the suction side of the pump. This minimum required pressure to ensure proper functioning and to avoid cavitation in pumps is called the Net Positive Suction Head or NPSHr, with the r standing for “required”. The NPSHr differs for each pump – even each duty point – and is shown in the manufacturers performance curves diagram of the particular pump.