Sizing Tank Overflow Line for Gravity Flow

[Pavan Kumar]

Hi All,

We have Tank that is designed to overflow to a small tank that drains to a sewer. The smaller tank is vented to atmosphere. The system sketch is attached with this post. The overflow line is 6" Sch 10. I want to verify if the line is sized correctly. I read through paper by PD Hills " Designing Piping Systems for Gravity Flow". In the flow rate of the overflow is same as inflow rate which is 35 gpm. I calculated the line size as 4" by maintaining JL* to be less than 0.3 for side outlet piping from vessels in PD Hills paper. Also we want to install a 4" ON/OFF valve on the over flow at the bottom end as the overflow line size is required is only 4". he in flow valve will be closed when the 4" ON/OFF valve is closed. I can install a 6" valve too but a 4" would be cheaper and less heavier for installation and maintenance. The intention of installing the ON/OFF valve is have a control the drainage of the waste water until its PH is brought to spec.

1. I want inputs from you all if you the 4" line size for self venting flow is correct or not.

2. I also want to know if by installing the 4" ON/OFF valve am I changing anything. I understand that when the valve opens after a while initially there will be full pipe flow followed by half-pipe flow.

Thanks and Regards,

Pavan Kumar

 

[Pavan Kumar]

Attaching the line sizing calc here.

 

[bimr]

Gravity pipe fluid flow is calculated with the Mannings equation. Here is link to a calculator:

Mannings Calculator

The gravity pipe slope should be approximately 0.03 ft/ft to minimize settling of solids. That means you only need a 3-Inch horizontal pipe for 35 gpm. Size the pipe so that it will flow half full Use the same pipe size for the vertical segment.

Delete the tank. Use an overflow with a tee on the side of the tank. Vent through the tee.

Note that the side outlet will not maintain a fixed water level in the tank. If you want a fixed water level in the tank, you will need an overflow weir in the tank.

 

[vt2012]

[ol 1]
[li]4" line is correct. I ran a quick calc and req'd ID is 3.9 inches for vertical self-venting flow.[/li]
[li]Can you put the on/off valve in the horizontal section either at the top or bottom? I am concerned that the reduction in pipe size plus the valve in the vertical will disturb the annular nature of the flow. Even with a 6" valve (and no reducers) I would prefer it in the horizontal.[/li]
[/ol]

 

[Pavan Kumar]

Hi vt2012,

Can you please suggest me the source that you used to size the line for slef venting flow. I am referring to the paper " Designing Piping Systems for Gravity Flow" by PD Hills. In page 4 the author says that the line should be sized such that dimensionless superficial volumetric flux J[sub]L[/sub][sup]*[/sup] < 0.3

I am attaching the PD Hills paper with this message.

Also I have been suggested to use Manning's Equation, which I have not heard earlier.

Thanks and Regards,
Pavan Kumar

 

[pierreick]

Hi ,
Consider the document on gravity flow prepared by Art Montemayor a few year ago.
Pierre

 

[MJCronin]

I believe that this question has been answered a couple of times in the "Storage Tank" forum

https://www.eng-tips.com/viewthread.cfm?qid=325512

MJCronin
Sr. Process Engineer

 

[Pavan Kumar]

Hi pierreick,

My calculation of 4" line size for the overflow line is based on the formula given in the paper by PD Hills. Art Montemayor's spreadsheet is same as PD Hills paper.

Thanks and Regards,
Pavan Kumar

 

[Latexman]

Did anyone else get an alarm on Art's Gravity Flow.xls?

Good Luck,
Latexman

 

[Pavan Kumar]

I did not get it, as downloaded Art's spreadsheet from a CHEE resources forum not from pierreick's attachment above.

Thanks and Regards,
Pavan Kumar

 

[bimr]

Looked at this again and now see what you are talking about.

Your example (tank overflow) is different than the scenarios posted in the paper by PD Hills, unless you are dealing with a foaming wastewater.

The horizontal pipe outlet of the 6-Inch from the tank will never be full in any scenario with a fluid flow of 35 gpm. The horizontal pipe outlet will have an approximately fluid level of 4-Inches at 35 gpm. That means no air will be trapped in the horizontal or vertical pipe and the trapped air will always vent back into the larger tank.

Not clear why want to install a valve.

It is customary to size shutoff valves with the same line size as the pipe. Note that when valves are installed in vertical pipes like you have shown, the solids contained in wastewater may settle out and foul the valve.

The valve should be sized for full pipe flow. The head loss at the valve will be the fluid level in the vertical pipe.

 

[Pavan Kumar]

Hi bimr,

Per my calculation a 4" overflow line will be sufficient(based on the criteria that J[sub]L[/sub][sup]*[/sup] <0.3 per PD Hills paper) for unflooded self venting pipe flow. In our case the tank overflow line is 6". It is ok till here. We want to install a ON/OFF valve on this overflow as we want to overflow the waste water only when the P[sup]H[/sup] is on-spec. The tank shown in my sketch is part of the Neutralization system in our plant. As per my calcs since 4" overflow line size is sufficient I can use a 4" ON/OFF valve without any problem. I agree locating it on the vertical line will cause settled solids to clog the valve. The horizontal portion is not that long to fit the 4" ON/Off valve on it.

I would prefer a 4" ON/OFF valve over a 6" valve as the cost and weight of the valve are both less.

4" ON/OFF valve cost / weight - $ 2750 / 160 lbs
6" ON/OFF valve cost / weight - $ 5125 / 280 lbs


Thanks and Regards,
Pavan Kumar

 

[bimr]

Very well then, but would also suggest you use a full port valve.