API 650 5.10.4.7 (c) Interpretation Question

[treemkr]

I have a question re: interpretation of API 650 5.10.4.7 - Roof support column base plate requirements, specifically as pertains to vertical movement.

Code indicates that the design shall "allow the columns to move vertically relative to the tank bottom without restraint in the event of tank overpressure or bottom settlement". In fact, Figure 5.26 shows no welding between colmun base plates and the base plate guides.

I am working on a large storage tank for UAN, which is highly corrosive to untreated carbon steel, especially when trapped in a crevice. The plant wants the internal column base plates seal welded to avoid a potential corrosion issues. The tank manufacturer is citing this provision in the code and indicates that if they do this they cannot certify the vessel per API 650.

The roof rafters that the internal columns are supporting are not attached to the colunn top pales, so I fail so see how the columns would pull away from the wear plates in the event of over pressure. There is not issue of settlement as the tank is set on a poured concrete pile cap.

Can someone shed some light on the failure mechanism that this provision is tryng to prevent, or if the code assumes that the roof rafters are attached to the column top plates?

Any insight would be appreciated.

 

[fegenbush]

My interpretation is that the standard is attempting to prevent an overpressure causing stress between the floor and the column base plate which could lead to tearing of the floor plates (or floor plate welds). This is a common theme in API 650, preventing the spill of the tank product.

A solution to this may be to use austenitic stainless steel for the column base plate, base plate guides, and bearing plate. Other solutions involving timing of coating application and shop fabrication of components are also possible.

 

[JStephen]

If the tank is on a concrete slab foundation, you shouldn't need much in the way of a base plate to support the load- just use a stainless steel base plate, leave it loose per API-650 with a wear plate.