The devastating effects of cavitation
– By Martijn Blok, 13/03/19
Have you ever seen a pump impeller that looks like it’s been blown away by dynamite? Leaking pipes? Valves that have been eroded so much that they no longer function properly? All of this is caused by tiny vapour bubbles: behold the immense power of cavitation.
Most technicians have an idea what cavitation is (‘it has something to do with your circulation pump, right?’). While this is true, there is more to it. For one thing, it also happens outside pumps – in regulating valves, the bends of a pipe section or transition pieces.
Water vapour pressure
At sea level, atmospheric pressure is around one bar. In these conditions, water freezes at 0 °C and starts to boil at 100°C. You can see the relationship between pressure, temperature and state of matter (solid, liquid or vapour) in the phase diagram below.
At lower ambient pressure, water starts to boil at lower temperatures. The pressure of the vapour becomes higher than the surrounding pressure, causing vapour bubbles to appear in the liquid.
Vapour bubble appearing
Places with low pressure
There are several spots in a pump installation where the pressure is temporarily reduced: for example, the suction side of a circulation pump or the tips of the pump impeller. When the speed at which the water is moving increases, the pressure is reduced. This can happen in poorly mounted valves, pipe elements or transition pieces. Vapour occupies 50 times more space than water.
Places with low pressure and high speed occur only temporarily. Upstream the pressure will be normal (high) again. So in the low pressure part the newly created vapour bubble start to grow and grow until the pressure rises. When the surrounding pressure becomes higher than that of the vapour bubble, the bubble implodes.
Implosion of a vapour bubble
The surface of the bubble starts to break at its weakest spot. When it collapses, water flows into the volume that had previously been occupied by the vapour. This creates a micro jet which pierces the bubble and bursts through the system.
When this occurs in the direct vicinity of the sidewall of the tube, it’s like a little rocket impacting the wall. You can imagine the kind of damage thousands of these occurrences can cause to your system over time. And the impact is very noisy too: it is like running a handful of marbles through the pipe.
Martijn Blok | R&D Engineer
Martijn Blok has been working at Heinen & Hopman since 2013. During his study Integrated Product Development he learned to analyse and engineer the functional side of a new product. At Heinen & Hopman, Martijn is responsible for detail engineering using CFD-analysis amongst others to optimise our new products.