Hello Syed Waqar Hasan.

The bubble formation could be attributed to many factors. Mostly, if the tank is open to the atmosphere, there would be dissolved air in the water over time. When the thermodynamics conditions permits (pressure & temperature), this air will turn into gas phase again and form the bubble. Moreover, the water itself might evaporate under certain circumstances and form vapor bubbles. But this would require heating, which I assume does not occur in your case.

Further, the bubbling process requires roughness in the surface of the pipe to form the bubble nuclei (this could be in the scale of micrometers).

Check also the NPSH of the pump. If there is no enough water height in the tank, the suction of the pump may form vortices in the water surface inside the tank and this is a cause of the air introduction into the pipe.

Syed,

I add to Ali's reply - the bubbles may caused by cavitation in the pump.

If the local pressure at any point in the pump's volume falls below the vapour pressure of air dissolved in water, then that gas will exsolve.

If you were to try this with degassed water, it would be interesting to see if the bubbles do not appear.

Ali Jraisheh , Emilio Casado , James Garry

Thank you very much for your kind responses. I highly appreciate it.

Ali Jraisheh I also thought about the dissolved air in water. But the amount of the air possible to be dissolved in water (per volume) is not too high (~4%). But if you see the sizes of the air bubbles in the picture i attached, the bubbles are considerably huge. Can these huge bubbles really be from the dissolved air in the water.

Emilio Casado I should check NPSH and let u know the updates.

James Garry shouldnt the cavitation bubbles be bursted at the diffuser section of pump? Or maybe the air bubbles are so big that they survive the diffuser part of the pump and appears in the pipe.

I will update you all about future developments. Maybe together we form a collaboration.

Syed,

On reflection, bubbles created by cavitation generally recollapse when the pressure returns to something like ambient conditions. To have a train of bubbles sustained in the flow suggests that the liquid was super saturated with air (unlikely).

You know? I'd be tempted to look for a leak.

Can you gently pressurize the pump?

Cap the inlet/outlet, add a syringe to the inlet and squeeze? It could be something as simple as a leaky o-ring around the shaft of the impeller - no leak when stationary, but when given a little flow (hence the syringe) it might leak under pressure?

When running, a subtle leak in a gasket might draw in air before it allows water out... lower viscosity, etc.etc.

Hi Syed Waqar Hasan .

As James Garry suggested, a dissolved air cannot cause a sustainable train of bubbles as suggested in the photo. The bubbles seem to be consistent in your experiment and most probably there is a consistent (although small) source of air that is introduced in the flow. Try what James has suggested and close every possible leak.

Ali, and you know the best bit? We get to speculate and stay dry while Syed is wrestling with getting water-tight fittings for random pipes.