API PUBL 937:1996 pdf download.Evaluation of Design Criteria for Storage Tanks With Frangible Roof Joints
2.0 Background Information
2.1 Description of Typical Tank Design
(Note: Drawings used in the following description come from more than one source, and so do not represent any particular tank. However, they do represent typical construction.) The storage tanks addressed in this research in- clude the typical tanks seen in oil refineries and chemical plants. The tanks are cylindrical with coni- cal roofs. The roof slope is quite small, limited by A P I 650 to a rise not greater than 2 inches in 12 inches. The tanks typically have a flat floor that rests on a sand foundation, with a hard concrete ringwall at the periphery of the bottom. The tanks are manufactured in the field using fillet or seam welded plates. Overlap- ping joints (with fillet welds) are typical on the roof and floor. Butt welds are used for the shell. Diameters of the tanks of interest range from 10 to over 200 feet, with heights of 10 to 60 feet. The material used in construction is steel, with yield strengths ranging from 30,000 to 60,000 psi. Figure 2.1 shows a photograph of a typical tank. The overall construction of a typical 25 feet diameter tank is shown in Figure 2.2, and includes all the features described above. The roof is constructed using overlapping flat plates, with fillet welds at the joints. The top angle is butt-welded to the top of the shell. The roof is then fillet-welded to the top angle around the entire periphery. The construction of larger tanks is similar.
The relatively flat roof is supported by interior rafters (Fig. 2.3). As can be seen, the rafters extend from a central column to the periphery of the tank. The rafters are welded to the column and bolted to the tank wall. The roof is not attached to the rafters, but rests on them. When walking on the roof, one can feel the deformation of the roof between the rafters. Also, there is typically some noticeable “waviness” in the roof. On larger tanks, there can be additional columns and intermediate supports for the rafters.
2.2 API 650 Design Rules for Frangible Roofs
The design rules given in AP I 650 cover vertical cylindrical aboveground, welded steel storage tanks for small internal pressures. Several sections of the code address the design of a frangible roof joint for supported cone roofs. Summary descriptions of these sections are given below, followed by a discussion of the apparent intent of the rules, and some difficulties with the present rules. The present description is based on wording in the Ninth Edition of API Stan- dard 650, July 1993 (API, 1993). Given below are descriptions of the sections of AP I 650 that impact the design of frangible roof joints. Section 3.1.5.9 (Item e): This section specifies mini- mum sizes for the top angles. For tanks with diam- eters of 35 feet and smaller, the minimum size is 2 by 2 by 3/16 inches; for tanks with diameters of 35 to 60 feet, 2 by 2 by ‘/4 inches; for larger tanks 3 by 3 by 3/8 inches.