API 650 Drawoff Sumps

[TankDude]

Can anyone tell me or hypothesize as to what is the controlling factor for the location of drawoff sumps relative to the tank shell per API 650? I have a client who wants to use a 6" draw line with only a 24" diameter sump, and after unsuccessfully trying to convince them to use a larger sump I specified that the sump be located 8'6" away from the tank shell in accordance with table 3-16 for a 6" line. Any thoughts? I think that it may be to avoid the sump being pulled up by the suction of the draw line which would potentially induce a bending stress on the bottom to shell connection. I think that this combination of draw line and sump size would increase the probability of this happening. Am I even remotely on the right track? Thanks in advance.

 

[JStephen]

I think if suction was the issue, you could just as easily argue that it ought to be closer to the shell to hold it down and to keep the nozzle from rotating.

You do develop some hoop stress in the outer part of the floor plate, and I assume you want to hold some minimum clearance on that account. And on a tank with seismic (or wind) uplift that depends on the product weight to help hold it down, you're assume that a certain part of the floor can lift to develop that force. You'd want to maintain weld clearances. Those are the only reasons I can think of for limiting how close the sump is to the shell.

 

[LeonEarle]

As I see it, there are a few issues to consider. First, you want to locate the sump flange outside the critical zone so that when doing repairs, you won't violate API 653. Second,weld spacing. You want to be certain that locating the sump within the bottom layout does not result in a three plate lap closer than the 1' minimum required. Third is actual installation of the sump relative to the type of foundation. If you have an earthen foundation, the further away from the tank shell, the better (within reason). As much as API requires that you compact the soil around the sump, many sumps are installed after the bottom is laid. As a result, you never get proper compaction immediately around the sump. This is a really good reason to install the sump during foundation construction. If you have a concrete ring-wall, you would want the wall of the sump any closer than 18" or so to ensure you have enough room to achieve proper compaction around the sump. So, [ Critical Zone = 3" + typical flange = 4" + 1/2 sump diameter = 19" to center ][ Three Plate lap = 12" from shell + 4" flange + 12" (1/2 sump) = 28" to center ][ For Compaction (this is preferred minimum) = 6" (1/2 ringwall) + 18" to wall of sump from ringwall + 12" (1/2 sump) = 36" to center. As regards suction, most sumps are provided for water draw-off rather than product out so the discharge rate should be pretty low. In your example, you have a 6" line. This is pretty large for water draw-off unless you are looking at a pretty big tank. The bigger concern, especially with earthen foundations, is having a disturbed area that could propigate under the shell resulting in a lack of uniform support to the shell thus introducing added stress into the shell and corner weld.