Consequence and Analysis of a Tank Being Overfilled

Unless the roof vent(s) are grossly undersize, there will be no significant upward force on the roof, or the roof-to-shell joint. Check all the possible "Fill" scenarios, and ensure that the vent(s) are bigger than the pump(s) fill capacity. If the tank is cross-connected to any other tanks, that take into account also. Simple.

If the venting is incorrect, fix it ASAP. You have a disaster waiting to happen. If that roof is actually frangible, it will be a small disaster. If the roof-to-shell joint holds, it could be VERY ugly.

You can check the compression effects in the top angle section using the methods from API-620. You could also get some uplift of the shell from the foundation.

For a lot of smaller tanks, shell thickness is based on minimum thickness anyway, and the extra 5' wouldn't matter. Check the stresses and see how it compares to the hydrotest stress or the allowable stress for existing shells from API-653.

We are planning to calculate the maximum allowable internal pressure using API 650 App F, then multiplying that pressure times the internal area of the dome to come up with what we believe the maximum total load on the roof to shell joint is.

Then we plan to calculate the normal forces on the dome due to the hydrostatic load from partial fill of the dome space up to the overflow, and then calculate the resulting overall load on the dome. If that load is less than the maximum load due to the maximum internal pressure, we think the roof would not fail.

This might even be conservative, as the liquid level above the joint results in an outward hydrostatic load on the joint, which should be ‘helping’ the compression ring resist the joint collapse.

Hi... I have some pictures of a 35000 barrels water tank that was overfilled. The roof-shell joint has now a visible plastic deformation. I´m posting one picture here. If you want, I could post more.
The tank was designed to receive 6 L/s with a 4" inlet nozzle. The overfill nozzle is 4". During a non autorized operation of the water source system, the tank received 60 L/s during night shift for 4 hours. The overfilling protection system was not conected, because the tank is under construction. I'm writing the technical report of what happened. Now, reading your posts, I have another ideas on how to explain it. thanks!.. if you have other comments which were the causes, please, let me know. I would appreciate them.

vargaslu,
When you say "the overfilling protection system was not connected", does that mean that the 4" overfill nozzle was blocked?

David Simpson, PE
MuleShoe Engineering

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