Roof Frangibility

[xema77]

Tank Information
Diameter = 60m
Internal Design Pressure = 1.62kPa
Roof = Supported Cone
Roof Slope = 9º

The client is requesting that the tank roof be frangible. The tank meets the frangibility requirements per 5.10.2.6, with maybe the exception of the compression ring type.
The Appendix F.5.1 calculation results in a large compression area required and it is difficult to get the required compression area using Fig F-2 details a-e.
1) Is it the intention of figure F-2 that standard structural angles be used?
2) Can a large fabricated angle (eg. 300 x 300 x 25) be used to meet the frangibility requirements and gain the area required, keeping within the wc and Le limits?

Thanks

 

[JStephen]

I would assume that it is the intent that standard structural angles be used. Are you checking the area requirement in 5.10.2.6-5 as well?

Frangibility implies a low failure pressure, and pressurized construction per Appendix F imvolves raising the design and failure pressure, so you will reach a point where the tank simply can't be considered frangible per API rules due to the pressure design.

 

[vesselguy]

JStephen,
Your last sentence is a good point. It might be true that for certain configuration of tank sizes that frangibility might be limited by the amount of compression resistance area provided on the roof joint offered by the standard structural angle shapes.

I always thought that the idea of a frangible joint is applicable to the entire pressure range (0- 2.5 psi) permitted by this standard as long as the required top ring stiffness is met. Hence, I would think that we should be able to "leap" to detail h or detail k to provide the needed area.