overflow nozzle location
overflow nozzle location
(OP)
A design proposal for an atmospheric storage tank shows the high liquid level all the way up to the top angle. This corresponds to the requested nominal capacity of the tank.
Does API 650 allow the overflow nozzle to be located on the roof? Or is there a minimum freeboard above the stated capacity - which means this supplier has really offered a smaller tank than we asked for?
Thanks in advance.
Does API 650 allow the overflow nozzle to be located on the roof? Or is there a minimum freeboard above the stated capacity - which means this supplier has really offered a smaller tank than we asked for?
Thanks in advance.





RE: overflow nozzle location
Steve Braune
Tank Industry Consultants
www.tankindustry.com
RE: overflow nozzle location
API Standards are often used in design of tanks for other liquids as well, many non-volatile.
Are there any other standards/ references commonly used in conjunction with API, for issues not addressed in API, then?
RE: overflow nozzle location
The normal arrangement for the overflow is via an internal 90 deg spool from the HLL suface down and out through a side nozzle below HLL.
RE: overflow nozzle location
But in case of other than petroleum products?
RE: overflow nozzle location
RE: overflow nozzle location
Typically, water tanks are built to AWWA or NFPA standards. These standards require an overflow, with weir box if required, at some point below the top of the shell. The capacity is supposed to be that below the overflow, although many people still use the API terminology and use a nominal capacity which is figured to the top of the shell.
Depending on the size and configuration of the tank, it might be possible to put an overflow nozzle in the roof, but this would be uncommon. If doing so, it would be helpful to come up with a lower configuration than a normal nozzle + ell arrangement.
In the design of API tanks, assuming the design liquid level is at the top of the shell is a conservative assumption that need not relate to the actual overflow depth.
RE: overflow nozzle location
JStephen: In the design of API tanks, assuming the design liquid level is at the top of the shell is a conservative assumption that need not relate to the actual overflow depth. - absolutely right.
As for sethoflagos: The design capacity by API 650 is limited to the lower of the top angle or bottom of the overflow. I was wondering if the argument should have really been the other way round - do we locate the overflow nozzle at a point such that it establishes the maximum liquid level required? Isn't there at least some rule of thumb for the minimum freeboard? Assuming non-petroleum products, of course.
RE: overflow nozzle location
RE: overflow nozzle location
For fixed roof tanks, if you set the overflow level below the top angle, the tank is going to require more (and possibly thicker) plate for the same working capacity so optimum economic design is to have both at the same level.
RE: overflow nozzle location
Thank you, sethoflagos. Now, there's a very useful design guideline.
RE: overflow nozzle location
I would stress that this is only a general guide - there are many dozens of tank specifications worldwide, and even more special process cases. So there are some exceptions to the rule.
RE: overflow nozzle location
How high above overflow will you let product depth go during overflow event? AWWA D100 limits it to 6 inches.
The limiting factor on most overflow systems is the ability to overcomne entrance losses. Therefore, overbox with generous weir length is often needed.
Steve Braune
Tank Industry Consultants
www.tankindustry.com
RE: overflow nozzle location
Knowing this volume flow you can calculate a weir length for a particular dh above weir lip (2" is an acceptable first estimate for a simple pipe overflow - the volume flow varies with dh^1.5). If, say this weir length computes to 36", the perimeter of a 12" pipe looks like a good candidate for the duty.
If the 'simple' solution starts giving an uncomfortably large overflow pipe, as you say a part or full circumferential weir will become the preferred option.