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I am setting up an experimental station for which I am using a small gear pump to pump water. I have noticed that on the discharge port of the pump, a short row of bubbles comes out at high flows and low pressures.

For example, at 650 ml/min and 2 bar of system pressure I might observe a relatively long row of bubbles coming out from the port. By increasing the system pressure up to 7 bar, the row shortens and eventually disappears. The same happens if I decrease the flow. I have checked the feed, and it does not contain any bubbles.

This has led me to believe I am having some degree of cavitation in my pump, is this correct?

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    $\begingroup$ can you draw a sketch indicating where the flows and pressures you mention are, and give some details on the suction pipe of the pump? $\endgroup$ – mart May 11 '16 at 8:14
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    $\begingroup$ Possibly cavitation. $\endgroup$ – Carl Witthoft May 11 '16 at 13:59
  • $\begingroup$ Also, what's the temeperature of your water? $\endgroup$ – mart May 12 '16 at 6:44
  • $\begingroup$ Room temperature, between 20 and 25 celsius $\endgroup$ – Keine May 12 '16 at 6:58
  • $\begingroup$ I will get a cpiture and a sketch when I return to the lab $\endgroup$ – Keine May 12 '16 at 6:58
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As well as 'proper' cavitation which is a fairly high energy phenomena and certainly a concern in terms of wear on the working surfaces of the pump it is possible that pressure changes are causing dissolved gasses in the water (most likely air) to come out of solution.

The pressures and flow-rates here aren't huge but having said that gear pumps can produce very high local pressures. It would be interesting to know if you get the same bubbles when the pump is connected to a short tube to an open outlet.

The unknown is if this is happening on the surfaces of the pump with enough energy to be a problem or if it is happening downstream of the pump itself.

This could also be indicative of a small leak somewhere, note that a leak which admits small bubbles of air may not be very noticeable in terms of the amount of fluid which is leaking.

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I have noticed that on the discharge port of the pump, a short row of bubbles comes out at high flows and low pressures.

This does indeed sound like cavitation to me.

Cavitation occurs usually in areas with a rapid change in pressure (from high to low, usually). As you say, when you increase the system pressure, the bubbles disappear.

Think of it this way - you either have too fast or too low pressure of a flow. The fluid can't go through the outlet side fast enough, so bubbles form as the fluid tries to keep up. By decreasing the flow or increasing the pressure, the fluid can keep up with itself on its way out.

You should try to balance your system such that cavitation is minimized. It's not good for the pump parts.

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Here the small bubbles are formed due to low pressure occurred at the discharge port of the pump. Generally cavitation is formed due to the sudden fluctuations in the fluid i.e. sudden decrease and increase or vice versa in the pressure of the fluid. When the pressure of the fluid is above vapor pressure there will not be any cavitation effect but as it decreases below vapor pressure it started forming cavitation bubbles. When the fluid pressure starts decreasing below vapor (saturation) pressure and at a particular pressure below vapour pressure i.e. at negative pressure (negative pressure is also known as tensile strength of the fluid), the fluid starts forming small cavitation bubbles (cavitation nuclei) which are in the size of microns or lesser. As the pressure decreases more, bubble size starts to increase and it increases up to certain limit. The maximum bubble size depend on pressure fluctuations and fluid properties. Now, when the pressure increases above vapor pressure then formed bubbles starts collapsing. The growth and the collapse of the bubble produce the sound which is also known as the cavitation noise. At the time of collapse of the bubble, the pressure generated is very high which will erode the material of the pump very fastly and it will also affect performance and the life of the pump. Small impurities present in the fluid such as in condensed gases, solid particles, etc. also increases cavitation effect. Cavitation bubbles are also formed due to the high flow rate. Cavitation can be avoided by maintaining pressure in the system above vapor pressure or above atmospheric pressure. High flow rate and velocity of the fluid is also reason of the formation small cavitation bubbles. Use standards for maintaining the flow rate and velocity of the fluid.

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Air leak in the suction. No cavitation ; cavitation voids ( almost vacuum ) do not leave the immediate area of the pump.

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  • $\begingroup$ Yup, shaft seal is sucking air. $\endgroup$ – Phil Sweet Feb 15 at 23:30

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