API 650 Flat Roof Design

[minorchord2000]

I am a structural engineer and I am asking for assistance from the mechanical engineering community relative to the API 650 code. We are tasked with designing a flat roof (steel plate) for an acid reservoir with an internal diameter of 14 feet. When reading section 3.10 of the code, it specifies cone or dome type roofs but does not specifically mention a flat roof. However, section 3.10.2.7 intimates that flat roofs may be permitted if the plates are stiffened by sections welded to the supporting rafters or girders. The central area of the roof includes 4' x 2' rectangular opening.

Any design suggestions would be appreciated.

 

[IFRs]

API 650 is mostly concerned with the integrity of the tank. It tends to offer a cookbook type of design to keep ordinary detailers out of trouble. If you need to deviate from what is in 650 for your tank roof, see section 5.10.2.8 which covers alternate roof designs.

My take on your question is that with approval of the purchaser of the tank, as long as your design is as safe and functional as other designs in the code, you should be OK.

 

[JStephen]

I was thinking somewhere in there, it specified a minimum 3/4:12 roof slope, so if the tank needed to comply with API-650, it wouldn't be flat.
A flat roof wouldn't necessarily be a problem. One reason for the slope is so that rainwater or condensation will drain off the roof, which reduces corrosion problems.
Normally, structure in a tank is radial due to the cone shape, but for a flat roof, could just be parallel beams spanning across, inside or outside the tank.
Watch the terminology. I have seen roofs referred to as "flat" when they were still 3/4:12. Meaning "flat enough to walk on" as compared to a dome roof.