Foam Dam Story -Floating roof tanks

Over design is not always SAFE! 

In one of the projects after completing the fabrication and erection of a floating roof storage tank, the Contractor failed to pass the foam test 3 consecutive times. The whole team was anxious as the production is delayed and they could not be able to find out how to rectify the issue.

They asked me to help them overcome the issue. I have checked the following possible causes:

1- Foam solution - concentration and composition 

2- Selection of foam maker

3- Design of the complete foam system

There were no issues with all the above-mentioned details. Then I contemplated for half an hour, and suddenly I realized there is one detail I haven't checked yet! and it was Foam Dam Drain slots. 

To allow drainage of rainwater from rim seal area to the centre of the tank drain system, slotted holes are allocated beneath foam dam. The optimised size of the drain hole is very crucial, if the drain hole is too big, it is beneficial to drain rainwater, but it is not suitable to keep the foam in the rim seal area during the foam test or actual fire case.

When I have calculated the drain hole area, it was 4 times more than the required area as per NFPA 11! consequently, the over-designed drain holes, make the foam escape the rim seal area, resulting in failure of the foam test.