Suction Pump Theory Help

Ravenerabnorm

Hi
A colleague and I were discussing a project recently which included a fire suppression system designed with a large length of pipe with one end in a reservoir and the other attached to a centrifugal pump so as to suck the water into the system. The system has a 50m static head rise from the mean water level outside the pipe to the pump. We had slightly different opinions on how this works.

We simplified the problem in terms of calculations by assuming the below:
Pressure required to lift a column of water of height 10m = 1 bar.
A simplified approach using P=ro*g*h taking specific density as 1 gravity as 10m/s2 and h 10m.

Having recently built a vacuum former, I was of the mind that it is impossible to get less than 0 absolute pressure. Seeing as atmospheric pressure is 0 bar gauge pressure and 1 bar absolute pressure then the smallest gauge pressure achievable is -1 bar (0 bar absolute pressure).

My colleague then opined that if the pipe was installed with air inside, the water levels inside and outside the pipe are the same. If the air was then removed from the pipe the level would rise inside the pipe by 10m. Due to the vacuum left by the air the water will fill the vacuum as it is driven by atmospheric pressure. Now to overcome the remaining 40m there must be pressure differential of 4 bar between the water level and the pump level. This pressure difference must also be negative as to cause the water to rise towards this point. Seeing as the water level is now at 0 absolute pressure and the pressure difference between this point and the pump must be negative 4 bar, then it appears that the required pressure at the suction side of the pump is 4 bar in <b>negative absolute pressure</b>.

I agree with him totally, that to lift the column of water then a pressure differential is required but I can't wrap my head around the negative absolute pressure.

Questions:
Any fundamental flaws in either of our thinking?
Can anyone provide some explanation to how this might work?
Is it really possible to have negative absolute pressure?
If it is then how does that work?

Any information is appreciated.
Cheers.

godtabh

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