water-air slack flow

[ratash]

If your water pipeline is designed for slack flow with no air release or vacuum breaker, and you start with a fixed amonut of dissolved air in your water at pump suction, pump it through the pipeline (hilly)and then discharge it to the same pressure as source pressure (pump suction pressure, will the amount of air be the same at the discharge point(in = out = conservation of mass)?

Thanks.

 

[cvg]

short answer is no

 

[ratash]

Thanks, but why no?

 

[cvg]

dissolved air will collect in the high points in the pipeline. without air release valves, the air may eventually cause airlock in the pipeline and will reduce the capacity of the pipeline

 

[katmar]

As the air collects at the high points the pump delivery pressure will go higher and higher, but eventually you will reach an equilibrium point where the pressure no longer increases and the air exits at the same rate it enters.

Katmar Software
Engineering & Risk Analysis Software

http://katmarsoftware.com

 

[ratash]

Thanks.

 

[bimr]

One would think that the dissolved air prior to the pump would be changed to entrained air through the action of the pump.

Entrapped air in a pipeline can lead to the development of sudden shifts of water column at very high velocity, sometimes high enough to fracture the pipe. For this reason, when plastic piping systems are first filled with water for operation, the piping should be filled carefully and relatively slowly to minimize air entrapment. Air should be vented from the highpoints before the system is pressurized..

Automatic air an vacuum valves have been used to allow the quick automatic admission of air that might be needed to prevent collapse of a thin walled pipe during the fast drainage what would take place through a broken force main or during a water-column separation following a power failure.

It would be preferred, that the problem of possible collapse of pipes should be solved by the use of pipe having walls sufficiently strong to withstand the induced added crushing loads.

The Water Pollution Control Federation Manual of Practice No. FD-4 "Design of Wastewater and Stormwater Pumping Stations" states "A minimum velocity of 1.2 m/s (4 ft/sec) is required in the pipeline to shear the bubble and keep it moving downgrade."

 

[ratash]

Thank you for the great information. Is there an article on the net about start up, normal, and shutdown procedure of a water pipeline?

 

[SteveWag]

I have a different take on this. Dissolved air will not collect anywhere in the main. What collects is air that has become un-dissolved. Now, if the pressure in the main, everywhere along its length, is higher than the suction pressure, no air will come out and all the air will exit the end of the pipe. This is not true for entrained air, but the first poster was asking about dissolved air. I will be verifying my take tonight by carefully examining every single unopened bottle and then carefully lower the pressure and check for gasses becoming “un-dissolved”. The bottle contents will then be safely disposed of.
Steve

 

[ratash]

In slack flow the pressure reaches vapor pressure of the water, water will vaporize and as a result of reduction in pressure the dissolved air will come out(although it is very small). In fact, the air bubble come out first followed by water bubbles.

 

[cvg]

we can argue about whether it is dissolved or entrained or otherwise, however it is well documented that the air in the pipeline can form bubbles which may accumulate in high points. When this happens it reduces the effective flow area of the pipeline and can cause pressure to go up suddenly as discussed by katmar and bimr. sudden vacuum is another issue which can be just as serious causing large pressure surges. air and vacuum release valves are part of the solution to this problem. proper design should include a surge analysis to help with locating and sizing the air/vac valves and specifying the pipe strength. Soft start and vfd pump controllers, surge tanks, surge anticipator valves, slow closing check valves and other methods at the pump station can also be used to mitigate the pressure fluctuations

 

[ratash]

Thanks. Why sudden vacuum causes pressure surge. Are saying reverse flow occurs when we get column separation?

What kind of check valve down stream of the pump is used?

 

[cvg]

recommend anti-slam (slow closing) check valve

 

[ratash]

Thanks.