Baghouse on roof of App. F tank

[KLee777]

Not really a "code" question, I guess....but, I've got a self-supported domed-roof tank where the existing baghouse on the roof is being replaced with a much heavier one. The new one was installed before the engineering was completed. Now they're seeing that the baghouse sways when someone climbs on it, or a stiff wind comes along.

So, I was asked to check the roof for this new weight. This is an Appendix F tank, design internal pressure = 2 in-wc and design external pressure = 3 in-wc. I used App. V to check external pressures.

The weight of the equipment was added into the roof dead load. Now, how do I account for the wind load (V=120 mph in this area) on the baghouse itself? It's 7 feet in diameter and 12 feet tall. I thought perhaps the approach would be to calculate the wind pressure on the vertical projection of the cylinder, and add this to the roof live load.

I should note that this tank is not an API tank, but we were asked to evaluate it as such. Hard to do!

Am I headed down the wrong path? Any advice would be much appreciated.

Oh and, if it turns out the roof is structurally sound, any ideas how to brace this thing up there?

Thanks,
KLee777

 

[JStephen]

My suggestion: Calculate the moment about the base of the baghouse due to wind, convert that to an equivalent vertical load of 2M/R, with R being the baghouse radius, then use the API-620 dome roof equations to check stresses in the roof right at the edge of the baghouse.

This will give stress vs allowable stress, but won't give deflection.

If bracing is required, I'd consider putting radial stiffeners out from the baghouse running down the roof- not necessarily to support the full weight of the baghouse, but to reduce deflection. You could use guys, but pressurizing the roof would put some (unknown) amount of tension in them.

 

[JoeTank]

I believe that your methodolgy of adding the baghouse weight into the roof design weight will not adequately address the issue of local stresses and overall stablilty of the the dome roof plate. None of the API codes address this condition adequately, so the evaluation will be by non-code procedures. I suspect that the movements observed under nominal loads are signs that deflections calculated by normal methods (based on small deflection theory) will be invalid due to the large defelction issues. I suggest you contact a tank consultant for assistance.

The wind moment on the baghouse can be converteted to a maximum unit load at the extreme axis of the circular baghouse as N = 4M/(PiD^2).
M = wind overturning moment
N = Vertical unit load
D = Baghouse diameter.

Joe Tank