Gravity Fed System for a High Rise tower

[kayfactor]

Guys,
Your input would be very much appreciated. The following problem is building services related, best person to answer - hydraulic engineer, fire engineer, fluid mechanics expert

Situation: Water storage tank at level Z, with effective head at outlet 2m (20kPa). Outlet is re-entrant tube Ø150 mm Gravity feed stack/riser (i.e serving levels below the tank)
I.e Q=Cd*Ao*Sqrt(2gH)
Cd= 0.72
This gives me a flow of 80 L/s.
i.e V1=4.52 m/s (too high) (v1= flow at inlet to pipe from tank)
Now at level Z-X (so further down the high rise tower)
I change my riser from 150mm to 100mm (as demand becomes less and less)
Using continuity equation A1V1=A2V2, my V2 is too high

Problem: V1 and V2 too high. Code only allows 3m/s Max
How do i reduce the velocity ? (solving this means i meet code requirement, and reduce my pressure loss in kPa/100m length, i.e have enough pressure for my most remote fixtures, at the levels closest to the tank)
I have already tried to change outlet size of the water supply pipe at the tank from 150mm to 100mm, which only reduces the velocity negligibly (4.45m/s). Head is not a problem as I will specify a PRV station for excess of 552kPa anyway

All your inputs would be very much appreciated. I am pretty sure I have missed some crucial information in my text above, please ask if something is missing

Kayfactor
Hydraulic Engineer

 

[vt2012]

Your calculation of flow is giving you more of the maximum flow rate through your 150 mm pipe with only gravity flow. That will not necessarily be the actual demand from the building. I believe that number would be much lower!

With a quick check of Crane - you could hope to get about 55 L/s of water through your pipe keeping the velocity under 3 m/s. If your actual demand is lower than 55 L/s I think you will be okay. Increasing pipe size will reduce velocity and also reduce pressure drop (but cost more up front in piping purchases). If you truly have 80 L/s of demand, you will need 200 mm pipe diameter to keep it under 3 m/s (double check me on that!)

 

[LittleInch]

As vt says, you're using the wrong formula. You're assuming your pipe can carry away all the water flowing out of a hole. I don't think it can.

Also which code says 3 m. I's not a bad number to use, but I don't know of any decent code which absolutely limits flow velocity.

This seems an odd way around to size something like this, i.e. what can I flow versus what is the demand.

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

 

[zdas04]

As the previous posters have said, this is odd. It looks like you are calculating an AOF (absolute open flow) which is quite meaningless unless you are concerned about the flow during a pipe rupture.

Every one of the fixtures in the building will have a flow restrictor on it, so if you assume some number of open faucets (10%? 20%?) then look at the cumulative opening in the restrictors and you'll likely find that the amount that can be pushed through them gets your velocity down where you need it. On this kind of application, it is normal to always use the biggest pipe you can afford. Fifty openings sized for 0.5 L/min each will give you a cumulative flow of 25 L/min, which provides reasonable velocities.

David Simpson, PE
MuleShoe Engineering

In questions of science, the authority of a thousand is not worth the humble reasoning of a single individual. Galileo Galilei, Italian Physicist

 

[bimr]

Your entire concept is invalid.

For adequate pressure in a building, you will need at least 13m of water pressure.

Building service piping is less than 25mm for residential services. A little larger pipe size for commercial use.

High rise towers do no use gravity fed water systems. The water systems consist of pumped pressure zones installed every 5-6 floors.

https://www.aspe.org/sites/default/...h-RisePlumbingDesignForHigh-RiseBuildings.pdf

 

[kayfactor]

Guys thanks for your responses.
I know what my demand, pipe Ø and velocity need to be. That was not my question, my question is what will be the actual velocity in a 150mm pipe for a tank with a given head of 2m, and that I got as 4.52m/s. And that is not full flow, full flow would be when Cd=1!
AS3500.1, Carrier Design Manual Chapter 3 - Both recommend maximum 3m/s
where as IPC, UPC recommend 1.5-2.4 m/s.

My question is how to reduce velocity. I cant change tank size.

bimr - I know how gravity fed systems work. Ofcourse I am not serving the level below the initial head, it will serve once i have sufficient pressure at most remote fixture after accounting for frictional and fitting losses - you went off topic.

 

[kayfactor]

vt2012 - you are spot on. so how do i reduce the velocity down ? in that 150mm pipe which is giving 80 l/s at 4.52m/s to 35 l/s at 2m/s

 

[itdepends]

Actual velocity = demand divided by the cross sectional area of your 150mm pipe.

Whether your system can deliver you demand flow is then a function of the available head and pressure drop (which is dependent on the total network).

If you're going to assume an open outlet then the flow will increase until you overcome the frictional losses of your pipe- and the velocity will probably go above 3m/s unless you use a very small pipe. The only way to stop that would be to install a restriction (e.g. orifice) in the 150mm pipe- but why you would want to is beyond me.

Recommended maximum pipe velocities are normally only for economic reasons (excessive pressure drop or erosion). In the case of copper pipe it is possible to get flow induced corrosion if the velocity is too high as it scrubs off the oxide layer- but I couldn't quote the velocity from memory.

As a chem eng/metallurgist the first part of any answer I give starts with "It Depends"

 

[kayfactor]

itdepends
you are right, but i think i am confused
if i have a tank with a 150mm outlet with the given head and cd value, then the flow is the flow, its irrelevant what the demand is ?

 

[LittleInch]

You're definitely confused - if the demand is less than the inflow, which is what we're all saying, then where does the water go??

If your pipe size is fixed, then the only way to reduce velocity is to reduce flow.

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

 

[kayfactor]

Hi all,
your comments helped.
i have now reduced the pipe size to meet my demand of 22.2 l/s
with velocity at 1.81 m/s
currently/now running calculations to see the affects of this on my frictional pressure loss