Floating Roof Tank Overflow Protection
Floating Roof Tank Overflow Protection
(OP)
I have been assigned to size an overflow nozzle for both internal and external floating roof tanks. My question is that if I know the maximum fill rate of the tank, the 95% fill height of the tank, the sum of the fill nozzle sizes to the tank, what else is needed to calculate the size of the overflow nozzle(s) needed for the tank. I also have been told that typically there should be two nozzles (or holes) spaced 90 degrees from one another. My initial thought was that the overflow nozzle (or hole) should have a crosssectional area at least one size larger than the sum of the fill nozzles to the floating roof tank. I have as yet been able to find any calculation or standard to support this. Also, would I need to know the maximum roof height as well so as not to damage the floating roof seals?





RE: Floating Roof Tank Overflow Protection
RE: Floating Roof Tank Overflow Protection
As far as I can see it does not give a recommendation or method of calculating the size of an overflow nozzle in the event that the detector/switch would malfunction or give a false reading.
Any further help is appreciated
RE: Floating Roof Tank Overflow Protection
RE: Floating Roof Tank Overflow Protection
RE: Floating Roof Tank Overflow Protection
From the practical point, any nozzle on a side of a tank would interfere with external floating roof seals. As the liquid reaches the nozzle elevation, the seal would be breached and some vapor would be released to the atmosphere resulting in an Air Permit Violation. For an internal floating roof (not Vapor Recovery), tank overfill has identical results. Vapor Recovery tanks (with or without an internal floating pan) would not be affected by this type of an incident.
Second unknown is the ability to recover the overflow volume. The inflow rate to a tank would need to be recovered in a second tank with adequate capacity. Considering that the overflowing liquid will trap some air, the second tank must be able to decant and purge that air.
Tank overfill prevention is achieved by a combination of properly designed tank gauging hardware and customized operating procedures. Consider redundant tank gauging, high level switches, level interlocks, inventory management software, and operator training.
RE: Floating Roof Tank Overflow Protection
RE: Floating Roof Tank Overflow Protection
If the nozzle or slot is completely under the fluid surface, you can apply Bernouli's equation between the fluid surface and the opening of the overflow. Assume zero velocity at the fluid surface and zero pressure at the opening and you can solve for velocity and then flow rate as a function of head.
If the nozzle or slot is only partially covered by the fluid, you get into a weir-flow situation. Refer to various fluid references for information on weir flow.
Use of the overflow may require that seals rise high enough to clear the opening.
RE: Floating Roof Tank Overflow Protection
And yes it will probably start as weir flow and then progress to full orifice flow if the level rises enough.
I don't like rules of thumb for such critical issues. I assume that this is a rather large and expensive tank with a liquid one does not wish to escape to the environment. It is worth the effort to do it correctly.
Sorry if this appears to be a lecture, that is what happens when an ME reads a ChE forum.
A Happy and Safe New Year to all.
Paul
www.ostand.com
RE: Floating Roof Tank Overflow Protection
RE: Floating Roof Tank Overflow Protection
The overflow nozzle (piped to about 12” above ground) is a common solution to prevent an “smaller” atmospheric tank from being overfilled. This design uses a small vent at top of the tank to equalize the pressure and a separate overflow line. The overflow line may have a liquid seal to retain tank vapors. In this case, there is no floating roof seal to interfere with the nozzle operation. As an example, this type of tank could be used to store NaOH or Acid. Please note that tank overfill causes a hazzard and should have secondary containment.
Expected roof damage in an overfill situation is directly related to roof design. External Floating Roof can have severe damage if the gauging platform limits travel. If not mechanically limited, the roof will float high and spill the contents over the side. Internal Floating Roof will have severe damage when it comes in contact with the fixed roof. Atmospheric or Vapor Recovery Tanks (no floating pan) can be damaged when the rising liquid pushes against the fixed roof. This normally causes the fixed roof to separate at its connection to the tank wall.
All tank overfills should be avoided.
RE: Floating Roof Tank Overflow Protection
RE: Floating Roof Tank Overflow Protection