I'm not clear what the configuration of your drains are.  Where do these drains go to?  Is the vertical elevation from the top of the double floating roof tank to whatever drain point fixed or does it vary?

Hi Td2k

The emergency drains are just the pipe that comunicates from top of roof to bottom of floating roof, say something as 800 to 900 milimeters. They are located away from the center and protrude 76 milimeters in relation to top surface.  When water reaches them, it flows directly to the botttom of the tank through the "stored liquid". The difficulty is to correctly consider the effect of the stored liquid in the flow of the drain.
 Thank you for your help

DKF

If I understand you correctly, you have an open drain from the top of the floating roof, through the floating roof, out the bottom of the floating roof and into the fluid.

The tank liquid, with no draining, should be somewhere in the drain line (800 mm to 900 mm plus 75 mm 'standpipe' or 'drain line') representing how far the floating roof is 'sitting' in the liquid.  You can either measure that or calculate that as you'll need it.  It may not be significant but I'd want to check it.  If the roof doesn't 'sink' far in the liquid (think of a canoe) the effect is minor.  If the roof does sink down in the liquid significantly (relative to its thickness, like a loaded ship), I'd want to consider the effect.

Doing the drain calculations, I'd determine the height of the liquid in the standpipe above its outlet.  This height is essentially the pressure at the outlet of the drain leg (where the water draining down the standpipe flows into the tank).  Now, when you are draining at maximum rate, the drain leg will be full of water going from atmospheric pressure at top, through the drain leg to the tank at the calculated outlet pressure (from above).  The driving force is the pressure created by the drain leg full of water.  Other than correcting for the 'pressure' where the drain leg ends, I don't see any problems caused by the product in the tank.

What you will get is the maximum drain capacity.  If you have less than this, than the water will only fill part of the cross section area (ie. run down the walls) or it won't completely fill the standpipe (which means less driving force and therefore less flow).

The only thing I might caution is that if you are overflowing water from the roof into the drain leg, how much water must accummulate on the roof in order to fully flood the drain leg and thus get your maximum capacity?  I'm not sure of the answer to this.  You might have to do some checking with the numbers to see how much water the roof can carry and how does this compare to the diameter of the roof drains.  If the drains don't completely fill, then you have a much more difficult calculation and one I'm not familar with.  

Have you talked to the roof vendor and ask them what approach they use or standards they reference.

This isn't very easy to explain.  I can send you a sketch if you want to give me a fax number.

The more I think about this, the more I think you won't flood the drain pipe though the calc is worthwhile to do to show it isn't the limiting factor.

Try this,

1. Determine the maximum water load to the roof based on rain fall, firefighting, etc.
2. Determine the amount of water to be handled by each drain (above number divided by the # of drains).
3. Compare this number to the maximum calculated drain capacity.  I suspect you find the maximum capacity is far greater than what the drain needs to handle which means the drain isn't flooded.  Instead, the water will flow over the top of the drain pipe and drain down the sides of the pipe into the liquid.
4. Determine how much of a 'crest' of water you need to get the necessary flow rate into the drain.  In doing some quick searches for weirs (which is sort of what the top of the drain pipe is like), one reference said that for a 1/2" high liquid 'crest', you get a flow rate of 13 gpm per foot of weir lenght.  For a 6" drain, you'd have a weir length of about 1.5 feet.  I'd suggest some more looking around though to decide on the number.

Other factors would be whether you assume all the drains are working or if some are plugged.  At the end of they day, you want to be able to confirm that the drains will be able to pass xx amount of water with no more than xx inches of water on the roof and show that the roof can support this amount of water.

TD2K
The tank we are talking  ( double deck floating roof)has a normal drain system ( 4 " pipe )with estimated capacity to release 44 cubic meters of water per hour. In addition there are two 6" emergency drains that show by experience that the overflow happens from time to time ( mainly on summer = heavy rain periods).
The floating roof is sitting 100 milimeters in the fluid, and roof can hold around 500 cubic meters of water on top of it without sinking.
The situation I am lookig at is when we have stong downpour say 400 cubic meters per hour, normal drain handles something as 40 to 50 cubic meters per hour, the emergency drains are reached in a few minutes and are completely overflooded. In this situation, say you have a head of 200 milimeters water over the emergency drains what would be a good estimate for drain flow. Roof surface area is around 5200 square meters.
Thank you for your kind help
DKF