Design of ventilation of potable water ground storage tank 1

[WissamH]

Hi All,
I am designing a ground-level potable water storage tank that is 31 meters long, 21 meters wide, and 3 meters deep. At present, I am working on determining the appropriate number and sizing of ventilation standpipes to install on the tank roof. The goals are to properly ventilate the tank and prevent air entrapment during filling and aid air inflow during water emptying.

The tank will receive an inflow of 42 liters per second. I would appreciate any guidance or recommendations for calculating the:

Number of standpipes required
The diameter of each standpipe
Positioning/spacing of the standpipes
Having these design details for the ventilation standpipes would be very helpful as I continue working to complete the tank design.

Thank You!

 

[LittleInch]

I assume this is related to your other posts?

So a rectangular tank?

Design pressure?

API 2000 is used for above ground storage tanks.

As a ROT, you want at least the same size / square area of vents as the incoming pipe(s)
Or outgoing as this is a storage if this is a higher flowrate.

That's quite a big volume so at least two vents would be my guess.

Potable water will need insect grills? Any other filtration which can add a differential pressure?

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

 

[WissamH]

LittleInch, I am also thinking the same way. However, I have seen similar tanks with many vents coming out of the roof.

Thank you for your comment.

 

[LittleInch]

Probably goes back to your earlier question about how close to the roof can you fill it.

Variation of roof height over a space 31m x 21m could be considerable so you risk getting isolated gas pockets if you fill it too high.

got a drawing of this thing?
Or a picture?

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

 

[WissamH]

Dear LittleInch, I have a 50 cm of freeboard maintained above the water surface.

 

[bimr]

For a typical water tank with a shop fabricated vent, just size the vent the same size as the largest inlet or outlet. Use a #24 316 SS mesh screen, keep the open screen area similar to that size, and it should be good.

The tank is very large, consider 2 or 3 vents.

ASHRAE has duct flow data that covers screens, exhaust hoods, etc., and some of that can be applied.

 

[fel3]

At one company I worked for, the rule of thumb for tank ventilation that I was given by one of our mechanical engineers was to target about 1 foot per second gross air velocity (0.3 m/s). The true velocity through the screens would be higher, of course, due to the flow area given up to the screening material.

So, on this basis, using Q = 42 l/s and V = 0.3 m/s (±), you would need a total gross vent area of approximately 0.14 m^2. The following selections meet this criteria: two circular vents with a diameter of 300 mm, 3 circular vents with a diameter of 250 mm, and 4 circular vents with a diameter of 200 mm. The first two slightly overshoot the area required for a gross air volume of 0.3 m/s and the third one slightly undershoots it. You could certainly add more smaller vents and you could certainly use a different criteria if you want.

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"Is it the only lesson of history that mankind is unteachable?"
--Winston S. Churchill

 

[WissamH]

Dear bimr, thank you for your kind response.

Dear fel3, this is exactly the approach I have used before. The only difference is that I used 100 mm pipes, so I came up with a larger number of pipes. Your response has boosted my confidence in the validity of my approach. Thank you.