Suction Pump Theory

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.

locum-motion

If the outside pressure is 1 bar, there is absolutely no amount of suction that will raise a column of water more than approx 10m.

The same as there is no amount of suction that will raise a column of mercury to more than about 76cm.

When firefighters are sucking water out of a river, lake or swimming pool, they have to site their pump so that it is within 10 vertical metres of the surface of the water. If they put it at 11m, their pump can run as long as it wants, it ain't getting any throughput of water.

You are quite right to not be able to get your head round 'negative 4 bar'; it cannot happen. But your calculation is about right. If it were possible, then 'negative 4 bar' is about what would be required to lift your water to 50m.

gctest50

Ravenerabnorm wrote: »

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.


Questions:
Any fundamental flaws in either of our thinking?


Any information is appreciated.
Cheers.

Might be an eductor on the bottom end of the pipe - so really the 'pump' is below water level and its power source ( the centrifugal pump you were looking at ) is elsewhere

Or maybe a submersible pump on the end of the pipe

Or or or etc

Have another nose around and ask

gctest50

.

Labarbapostiza

Ravenerabnorm wrote: »

Is it really possible to have negative absolute pressure?

There's a fundamental conceptual thing with vacuum cleaners, etc, that's difficult for people to get their head around.

When you're using a hoover. It may appear the hoover is sucking the air up. This is not happening. What is happening, is the air is pushing itself up. The pump creates a void, this changes the air pressure and air rushes in to balance the difference out. There's a limit to how much pressure you can get with this method, it can't go negative of the atmospheric pressure.

Some kind of high pressure system would need to push the water, not suck it.

What you trying to stop catching fire?....They're are other fire retardants worth exploring.