Method for Sizing and Overflow on an Atmospheric Tank

[bif]

Is there an accepted/recommended method for sizing an overflow for an atmospheric tank where an independent vent line is also incorporated

The tank I am looking at contains hot water and has an independent vent line coming off the top. It also has an overflow set ~ 0.5 m from tan line coming out of the vertical vessel wall

I have seen various articles regarding self venting flows and Froude Nos. These appear to be applicable only when the vent is also the overflow?

If I size based on a self venting flow, the overflow line size seems excessive to me (nearly .08 m at a flow of 0.33m³/sec)

Is this correct, or can I reduce the size and accept that the flow doesn't have to be self venting as there is a separate vent line.

Is it accepatble to assume the overflow line is flooded to a set height and then work out the flow based on static head as the driving force

 

[zerosum]

The allowable static head on an atmospheric tank is usually set by the allowable uplifting force which in turn is set by the weight of the sidewall and top of the tank. This is typically a very low value, usually inches of water. An 80 mm vent or overflow does not seem large to me.

Undersizing the overflow on an atmospheric tank can and often has resulted in a rounded bottom on a formerly flat-bottomed tank, should there ever be an upset.

 

[bif]

Apologies, I've got my decimal place in the wrong place!

The overflow size should read 0.8 m rather than 0.08 m as I stated earlier

 

[katmar]

A flowrate of 0.33 m³/s is not a trivial quantity to get out of an overflow pipe. 800 mm diameter sounds about right to me. I would not rely on any head above the outlet pipe to increase the flowrate. Maybe you got the decimal place wrong in the flowrate as well?

Katmar Software
Engineering & Risk Analysis Software

http://katmarsoftware.com

 

[hahor]

bif,

you may want to have a look at "Applied Chemical Process Design" by F. Aerstin & G. Street. There you will find a diagram for sizing discharge lines from gravity decanters. With the given flowrate (5230 gpm) a 30 inch line seems to be sufficient.

Kind Regards,
hahor

 

[MJCronin]

bif,

Generally speaking, size an overflow for 2.5 - 3.5 ft/second velocity.

Base your calculation on the maximum filling rate.

A pipe of 26 or 28 inch NPS would be my choice..

-MJC

 

[JStephen]

Typically, an overflow weir box is used with the overflow on a water tank if the flow rate is substantial. The weir length will be set such that design flow rate can be achieved when the liquid is 6"/150mm above the lip of the weir. The depth of the box can be adjusted to give enough head to achieve the desired overflow rate. Size the pipe based on orifice flow, and then if necessary the entire pipe to make sure the flow rate can be achieved through it.

 

[shahyar]

1) Minimum diameter based on Q=CA(2gH)^0.5
Where:
Q= Flow in
H= Vertical distance from overflow centre up to upper edge tank

2) Check with: Fr>0.3 (self venting) if a PIPE is connected to the nozzle. However, it is not a VITAL criteria, not satisfying Fr>0.3 just causes some vibration during overflow. The first item is vital.

Hope this help