What is Pump Cavitation and Its Common Causes?August 13, 2018
There's a drive-hampering phenomenon that we've mentioned several times in past articles. Referred to by engineers as "Pump Cavitation," the issue attenuates flow performance. Again, that fact has been covered several times, but what is cavitation? Pump repairers must understand this fluid effect, must know all about its causes and the conditions that produce the flow-hampering issue. For starters, is cavitation really just a verbose way of talking about bubbles?
What is Pump Cavitation?
At heart, the effect does look like a fizz of bubbles. If the pump drive mechanism was transparent, an interested observer would see the fluid cavities forming around the drive mechanism. In a conventional spinning centrifugal pump, the turbulence caused by the effect would be obvious. The disturbance in the flow would be seen to be collapsing and generating new wavefronts. That chaotic energy, left unchecked, would eventually damage the pump impeller.
Cavitation in Helical Pumps
The condition isn't limited to spinning impellers. No, the abnormal flow condition also occurs inside helical equipment. A progressive cavity pump, functioning as designed, suffers a significant flow drop. Bubbles are propagating and collapsing. The energy released by the dynamic event is creating small shockwaves. This is bad enough when water is the discharge medium, but the problem grows progressively worse when other fluid types are in attendance. Discrete food pockets, driven as a mix-free discharge, begin to combine. Sewage streams no longer move their semi-solid masses through the long pump cylinders. As for caustic chemicals, the potentially sensitive compounds could react destructively if they're exposed to a cavitating mechanism.
Finding the Common Causes
There are two primary types of pump cavitation, two forms that can be detected then repaired. First of all, Suction Cavitation occurs when the equipment is experiencing a low-pressure event or near vacuum conditions. The pump is working as normal here, but it's not receiving intake. Experiencing destructive fluid conditions, the aforementioned shockwaves, the helical pump generates noise and vibration. Alternatively, a gaseous or mildly vaporous effect is occurring inside the equipment. Whether this is due to heat or chemical excitation, the local static pressure inside the housing will invariably drop below the vapour pressure of the fluid. This effect is sometimes known as "air binding."
In both cavitation incidents, bubbles seemingly appear out of nowhere. They're caused by intake suction/discharge imbalances, by vapour and gaseous fluid pressure differentials, and by vacuums, perhaps ones generated by a pipe blockage. Regardless of the cause, the equipment must be taken out of service until the shockwaves are eliminated. To do otherwise would be to court disaster.
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