Weal top roof on API 650 roof

[ctopher63]

We have an API 650 AST, 36'dia, 30' tall. There is a flame arrester and pressure vacuum vent valve (PVVV) on the roof that weighs in the neighborhood of 800#, as well as steam, nitrogen, and process inlet piping. When the flame arrester and PVVV were initially installed there was a noticeable movement of the roof so a support was designed (by a licensed structural engineer) to come up from the shell of the tank with box tubing and carry 2 pieces of channel iron across the top of the tank at the elevation of the PVVV, then two pieces of angle would be drilled and bolted below the flange set to pick up the load of the PVVV, the flame arrester would be below. The vertical supports do not interfere with the roof shell connection at all; it is held outside this area.

My question is what does this do to the function of the weak top roof from and interference / hold down standpoint? The plant operators are concerned that if there is a failure the function may be impaired by the support structure actually holding the roof down. What assumptions are used as far as process piping on the top of the tank as far as holding the roof down? There is very little structural steel in the area to support the PVVV and piping with spring hangers.

Thanks in advance.

Chris

 

[JoeTank]

Unless the tank is heavily anchored, a 36 ft diameter was never frangible to begin with. API-650 gives a set of criteria for frangibility that may be useful to you. Usually a tank needs to be well above 50 ft in diameter to be frangibile. In addition, any tank with a self-supported roof will have a shell to roof joint that is too strong for frangible behaviour.

Steve Braune
Tank Industry Consultants

http://www.tankindustry.com