Agitator Bridge Supports API 650 Open Top Tank 1

[marcleblanc12]

In the case of having an open top API 650 tank with an agitator bridge. After calculating what the reaction loads will be where the beams sit at the top of the tank, we have to determine what is required to support those beams. (Either nothing or we need to add a vertical shell stiffener under the beam).

Does anyone have a method for determining whether or not vertical shell stiffeners are required to support an agitator bridge? Then if a stiffener is needed, how does one determine what size of a stiffener is required and how long should the stiffener be?

I believe I've determined a method by using a couple of different procedures in the "Pressure Vessel Design Manual" by Dennis Moss, but I'm not sure I trust it fully. Does anyone have a fool proof method to determine this?

 

[MJCronin]

On an open topped tank, the agitator bridge cannot be balanced on the sharp edge of the top of the tank shell. Welded Bridge supports must extend down the sides of the shell.

Alternately, some designers use a bridge with a "wide stance" and attach to an extra strong "top of the shell" reinforcing angle. Gussets welded to the shell are often used to stiffen the assembly

http://www.westpromachinery.com/product.php?id=16

https://www.machinio.com/listings/1...-8-10-straight-side-20400-06-in-hainesport-nj

Stress analysis of the tank bridge is an easy task for a competent structural steel designer

Bridges on closed topped tanks are much easier ...

MJCronin
Sr. Process Engineer

 

[JStephen]

One approach: Find the allowable compressive stress in the upper shell (API-620 for that). Figure the arc length of shell needed to support the load at that stress. Assume that the load spreads out 30 degrees either side of your vertical stiffener, and adjust stiffener length so adequate shell is engaged.

 

[marcleblanc12]

JStephen, I used AWWA D103 to determine the allowable compressive stress. It's a slightly different formula than API 620, but it basically gives the same result. The only thing I forgot was to consider the "Corroded Thickness" of the shell.

From that I determined my stiffener length required and then consider the stiffener as a column of that length to determine the size of the stiffener.

The question is, when determining what area of shell is required, can you include the area of the stiffener as part of that calculation? Therefore the bigger the stiffener, the less arc length of shell and therefore the shorter the stiffener. I have the "Pressure Vessel Design Manual" by Dennis Moss, and it appears to be a bit unclear as far as that goes. He uses 45 degrees rather than 30 degrees.

Thanks,

Marc

 

[JStephen]

I would say that the area of shell you're calculating is immediately below the stiffener, so the stiffener itself would not be included in that.
On the compressive stress- the only formula in API-650 itself is for seismic loading, so it includes the effects of internal pressure, which would not be considered for that case.