tank sizing 1

[kunal2603]

We have a cylindrical tank with 40" id and 60" height. I have to come up with a over flow line at 5" from top of the tank( this gives sufficient head to the pump that pumps at 5gpm) Water flows at 90gpm in to the tank.

Using Q = A*V ( Q = 85gpm)
I have two unkowns here. Even if i use 3" pipe, how do i know if the velocity is 2.02 ft/sec( from cranes). Plz help me out to size the overflow line.

 

[JStephen]

Apply Bernouli's equation between the surface of the liquid and the orifice out of the tank, using zero velocity at the surface and zero pressure at both places; only unknown is then outlet velocity and you can then solve for area.

 

[katmar]

Sometimes the sizing of overflow connections requires more than just the velocity through the outlet nozzle. You could use the procedure suggested by JStephen and this would give you an outlet of about 3" ID for an allowable head of 4" above the centerline of the overflow nozzle. This would work well if the inflow is steady, and the overflow simply falls from the outlet nozzle through the air to an open drain.

The complications come in when you have varying inflow, and the overflow goes somewhere where you do not want entrained air. Another potential complication is the formation of a siphon.

Imagine that you have put a 3" outflow nozzle near the top of your tank with a pipe down the side of the tank discharging into an open drain. Now you start filling the tank. As the level reaches the bottom of the overflow nozzle water starts flowing out. But the rate of outflow is less than 85 gpm so the level in the tank continues to rise. As the level gets towards the top of the nozzle air will be entrained with the exiting water and you will have gurgling and maybe a bit of vibration in your pipe. This may or may not be a problem.

Once the level reaches the top of the nozzle no more air can be entrained. When the water level reaches approx 4" above the centerline of the outlet nozzle the outflow will match the inflow and you might think this is now a steady state - but it is not. What happens is that the pipe running down the side of the tank starts to fill with water and causes a siphon sucking the water down. At this stage the overflow rate jumps to around 300 gpm and the level in the tank drops rapidly until air is entrained once more and the siphon is broken. Now the level slowly increases and you get into a cycle of alternating high and low flowrates. Again, this may or may not be a problem.

The usual practice with overflow pipes is to size them so that the level in the tank never goes above the centerline of the outlet nozzle, and the piping system is designed to cope with (or prevent) air entrainment. Try to get a copy of the article "Designing piping for gravity flow" by P.D. Hills which was published in Chemical Engineering, Sept 5, 1983, pgs 111-114. This is the best reference I have seen for this type of piping.

Katmar Software
Engineering & Risk Analysis Software

http://katmarsoftware.com

 

[JoeTank]

Both commenters offer good info. I'd like to suggest that the pipe size is a secondary consideration. The first is to size a weir in order to get the flow into the pipe. The weir design is often overlooked and is the reason why many overflow sytems lead to tank overflows. Standard hydraulics handbooks provide equations for weir flows based upon a low head... which is almost always the case for tanks. The pipe size will typically govern how deep the weir box must be.

Joe Tank

 

[katmar]

A weir is useful if it is important to hold the tank level as constant as possible, but even with a weir all the considerations I mentioned before for the overflow nozzle and piping still apply.

Katmar Software
Engineering & Risk Analysis Software

http://katmarsoftware.com

 

[kunal2603]

thanks for the valuable posts. Exisitng tank has a 3" overflow line. It is 4" below from the open tank top. Water is still overflowing from the tank top inspite of the existing 3" overflow line. We encounter unsteady flow every 1 minute( line will be flushed with water to get rid of polymer in the line.Polymer crumbs are separated using screenshaker prior to this water tank. During normal inflow, tank works properly but when the line is being flushed at a 80gpm, thats when the tank level rises above the overflow line and overflows.

 

[katmar]

How long is the 3" overflow line, what fittings does it have and at what elevation does it discharge? My calculation showed that a 3" was sufficient if it was just a nozzle and discharged directly to atmosphere at the same level, but if there is any back pressure it could struggle.

Could the overflow line be partially blocked with polymer?

What is normal inflow, and what is the level in the tank under those conditions?

Katmar Software
Engineering & Risk Analysis Software

http://katmarsoftware.com

 

[kunal2603]

3" overflow line is 21' long. It goes 70" down from the overflow nozzle and another 15ft horizontal line that runs through the floor in to a waste pit opened to atmosphere. A total of 3 tees are in the line. No other valves or fittings.polymer crumbs wont pass thru the screen shaker.

Normal inflow is 60gpm but when th eline is flushed with water, flow rate goes up to 85-90gpm. Inflow is from the tank top. During normal flow, tank level is right up to the overflow nozzle.

I feel 4" space above the overflow line is not sifficient to hold the 90gpm flow rate( this flow lasts for less than a minute in a cycle of 3min)

 

[chicopee]

Rotor router your discharge line, pitch your horizontal run and sealup the open top. I dont think you'll have any problem afterward.

 

[katmar]

My calcs show that you would need only 8 or 9 inches of head to drive 60 gpm through this line, increasing to around 20 inches when the flow goes up to 90 gpm. If the vertical section (the 70") is flooded this should give you more than enough head. I would guess you have a blockage or a build up of polymer somewhere.

Katmar Software
Engineering & Risk Analysis Software

http://katmarsoftware.com

 

[kunal2603]

Katmar,

Can you plz provide me with the link for "Designing piping for gravity flow" by P.D. Hills.Google gave me two links but those are in chinese and another one in french( i suppose).

 

[katmar]

I don't think that you will find the Hills article anywhere on the web. The best would be to visit your local university library.

Katmar Software
Engineering & Risk Analysis Software

http://katmarsoftware.com