Water Storage Tank - Feedpipe

[Lexwater]

Hi,
I am designing a new 500,000 Gallon Water storage and this tank is going to have a isoloated feedpipe coming in near the top of the tank and a discharge pipe located at the bottom of the tank.

My question is how high up on the tank are you suppose to put your feed pipe? I have heard 2/3 height of the tank or 10 feet from the top, but neither knew for sure they were just rules of thumb they had heard over the years.

If anyone knows I would appreciate any info.

Thanks

 

[dicksewerrat]

to avoid a siphon condition, put the pipe over the top and leave 2 feet of space between highest water surface and bottom of pipe.

Richard A. Cornelius, P.E.

http://WWW.amlinereast.com

 

[JStephen]

It's really pretty arbitrary. We see lots of tanks with inlets near the top, but I couldn't tell you what that actually accomplishes.

Currently the state of Texas requires an air gap in certain situations- usually when the tank is taking water from a wholesaler- but otherwise, it's not the general practice.

 

[hydrae]

If the feed pipe is above the high water level there will be some level of airation (sp) which will reduce the level of some disolved gases in the water such as radon, hydrogen sulfied, or carbon dioxide.
It will also provide a method of reducing the amount of very old water
Hydrae

 

[semo]

I've seen and designed multiple tanks that have one common inlet/outlet pipe at the base of the riser. They operate with no problems.

I've also designed them with separate inlet/outlet pipes. This was done to improve the turnover in the tank as Hydrae mentioned. It was also done for tanks near the water supply to provide disinfection detention time. The height above the bottom of the bowl was determined by the detention needed.

I've never seen an inlet pipe located above the overflow however. In a sealed system, I don't really see what that will accomplish. I don't think there would be enough aeration to do any real treatment to the water.

 

[Lexwater]

Thanks for the help

 

[olmedo]

Feedpipes should be at the bottom of the pipe. This would serve also as the discharge pipe. This will allow you to reduce pumping cost as you will have lower pumping head than if you feed it near the overflow pipe.

 

[JStephen]

olmedo, your response makes sense to me, but I find that more often than not, tanks are not built this way. I assume pumping costs just aren't that critical in the grand scheme of things.

 

[Seels]

If the storage reservoir is intended to be used to help meet CT requirements, a common header/feeder pipe cannot be used (at least in my jurisdiction) as it causes the tank volume to not be in series (i.e. worse case senario is that water doesn't pass through the tank).

Additionally, water can stratify in a tank, a common header/feeder does not provide mixing to prevent stagnant water. I seem to remember another post on this topic.

 

[zabrab]

I have seen a single feed/withdrawal pipe design using a manifold with the "duckbill” style check valves to promote mixing and minimize short-circuiting. The check valves were at each end of the manifold and oriented such that filling occurred at one end and discharge was from the other end.

In the interests of full disclosure, my recollection is that I saw this design idea in a manufacturer’s literature promoting the use of the product, rather than me using or reviewing it in an actual design. However, if the concept appears to address the needs in your application, it may be worth the time to verify its practicability.

 

[Bjegovic]

as pointed above, it saves energy if you are pumping against actual water height in the tank, instead to the top of the tank.

other issue is if the pumps can stand varying head? if you don't have VFD, and pumps are pumping mostly against static head, it is not vise to put inlet on the bottom because you will exceed max flow so you will have to throtle when the tank is empty.

i think this is where the 2/3H rule comes from.

 

[sokos]

I think 0.5m space between the max. water level and the bottom of the pipe is sufficient.
I saw several tanks constructed in this way.

 

[JStephen]

Those duckbill valves are deceptively expensive- check actual pricing before you ever specify them for anything.

 

[bimr]

Municipal tanks generally have one common inlet/outlet pipe at the base of the riser. They operate with no problems.

CBI sells an internal eductor mixing device to assist tank turnover. The inlet flow to the tank through the eductor causes the tank contents to turn over.

 

[lochan017]

hi Lexwater

just calculate up to what height of the tank volume you would get 500000 gallons. simply place the feeder pipes up to that height. ussually there is free board if 1 to 1.5 feet left in the tank

regards

Rajeevalochana HN