Pressure inside pipe, high rise building 1

[Junkwisch]

Hi guys, I have a question I would like to ask about calculating water in high-rise building.

Assuming that there is a water tank on the roof of a 200m tall building. The height of water in this tank is 2metres and will always remain 2metres. For simplicity, assume that each floor has a height of 40m, thus 5 floor total in the building. The water from the tank flow vertically downward, with elbow in every floor (to supply water). The question is, "in the second floor" there is a value position exactly 1m away from the elbow, if the water coming out of the value is 3bars (flow rate is 0.00333 m^3/s) what is its Lequivalent of the valve?

I believe mechanical energy equation can easily solve this problem, but the problem I have is that "the pressure entering the value is way too high casuing Leq to be larger than the building itself".

I calculate the pressure entering the valve using this formula, P(valve entrance)=P(free surface in this case)+water density*g*height(which is 120m in this case). Thus I got the pressure entering the valve to be little less than 1.4Mpa. Which is way too high

I also tried using mechanical energy equation but still the pressure is too high (see attachment)

I'm sure that there must be something that I left out during my calculation for pressure at the valve entrance. Can anyone give me a head up of what I missed?

 

[LittleInch]

Coursework? Homework? whatever.

No there is nothing you've left out. Static pressure at the valve entrance will be about 12 bar for 120m head of water. It's called the effect of gravity. In flowing conditions a bit less due to friction, but probably not too much unless you have a really really small pipe

I'm puzzled though by what the question is "what is its Lequivalent of the valve?" If you mean equivalent length then yes this will be a very big number depending on the size of the pipe you are trying too make it equivalent to. This is not a normal thing to use for a throttling valve, only a valve which is wide open.

You have a valve which is throttling approx. 9 bar across it at a flow of 12m3/hr. Your question is??

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[Junkwisch]

Ah I see, thank you for your reply. Yes it is the equivalent length of the valve, the question is more like "What is the equivalent length of the pressure reduction valve". I really thought that I made a big mistake somewhere, thank for clearing it up. Cheers

 

[bimr]

In the equivalent length method of determining total pressure loss in a system, the pressure drop through a valve; fitting, miter elbow, or any pressure-reducing component is equated with the length of straight, round pipe that will have the same pressure loss under the same flow conditions.

For valves, you would have to consider that the valve is almost closed to obtain the pressure reduction that you have. Use of the nomograph would provide an estimate. The nomographs show equivalent lengths for closed, full open, and partially opened valves. However, the equivalent length estimate will be very rough because of the extreme pressure drop in your example. The equivalent length method is not really appropriate for your example.

http://www.daube.ch/share/nomo01.html

Refer to the discussion on water systems in the link. High rise buildings are generally zoned off to minimize large pressure differentials.

http://www.ctaspe.com/uploads/TS_400-1-1.pdf

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LittleInch (Petroleum) 28 Apr 15 16:32

In a closed loop

 

[racookpe1978]

Be very aware that your problem statement is for a 5 story building, 200 meters high.
Each "story" then is 40 meters.

I think you need to consider a 5 story building of 4 meters each story is more realistic.

 

[Junkwisch]

Hey everyone, thanks for your reply. Another question though, using similar example as the one above, 200m higher but each story is now 4metres (to keep it more realistic). What would happen to waterflow in pipe at the top of the building though? If the static pressure is already less than 300kPa (since the effect of gravity is smaller). Would water flow into the valve at those top floors at all? Or I would need a pump to increase the pressure.

 

[LittleInch]

If yon needed 3 barg exactly then yes you would need a pump if the inlet pressure us Leeds than 3 barg.
Most valves limit pressure to a maximum.If three is less pressure then it just opens.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.