Structural Design Weight --- Tank Capacity vs Actual Contents 1

[kgengr]

I have a client who needs structural support (gravity and seismic anchorage, calcs and sketches) for a Demister Tank located on a roof. The tank will fume scrub for gold & copper for an industrial building (high-tech). The tank capacity is 5174 gallons, but client insists it will only ever see 24 gallons (200 lbs) maximum contents + tank self-wt = 1450 lbs. I'm looking for a code reference that would show my client that we need to design for the tank capacity and cannot design for an arbitrary limit.
It would not be efficient to design for the capacity (~45 kips), and I'm hoping to steer him to a smaller tank that we can design for the capacity of. But does anyone know of a reference for something like this?

If they choose to stick with the enormous tank I think I will walk away from this little project for liability reasons (future building owners, facility managers, etc.). Thanks in advance!

 

[JStephen]

Do you have the manufacturer's information available? If they give a maximum design weight or design loads, it would be reasonable to go with them.

I'm not familiar with demister tanks. On tanks in general, you'd design for a liquid level to the overflow, not necessarily full to the top. So if the tank configuration there is such that it couldn't reasonably be filled full, it would make sense to design for actual operating weight. If you're just kind of on the honor system that it won't be filled, I'd opt for "full" design weight. If any foreseeable circumstances would result in the tank being filled (clogged pipes, faulty controls, whatever), I'd opt for "full" design weight. It sounds like a failure there would result in the tank falling through the roof, so it's a bit more critical than a free-standing tank would be.

Example: If you had an air compressor tank, it might accumulate minor amounts of water in it, but you wouldn't assume it was full of water for design purposes.

 

[RPMG]

The operating weight is to be used. If the client wants 24 gallons to be its operating weight, it should be written clearly on the tank itself. Otherwise, the tank says its a 5174 gallon tank, so 5174 gallons is the operating weight.

 

[kgengr]

I was considering that - like live load posting - but seems risky even still, with rooftop installation.
Yes, capacity is 5174 gal with 419 gal unfillable in dome. Not sure why the need for such a large tank.

 

[dhengr]

Kgengr:
Tell your client that the design weight is going to involve the 5174 gal. cap’y. weight. Or alternatively, you plan to cut several 12" dia. holes in the side of the tank at the 24 gal. cap’y. height.

 

[thaidavid40]

I asked myself if I personally would work under a tank that could, accidentally, through process and/or control mistakes hold 5174 gallons of fluid, but which was sitting on a structure only designed to hold 24 gallons. My personal answer was no. Anything that can possibly hold 5174 gallons will, at some unlikely point in the future, hold that volume. I recommend designing for the 5174 gallon volume, as I doubt that you will be on the spot in the future to prevent all occurrences of possible overload. If there were an un-valved, atmospheric, gravity drain to daylight at the 24-gallon limit elevation, I MIGHT consider that, depending on the other specifics.
Dave

Thaidavid

 

[SAIL3]

I would use 24gal as operating weight and 5174gal as emergency(upset) weight...

 

[IDS]

Clearly the building should be designed so that it does not sustain significant damage if the tank is loaded to its full capacity.

For combined seismic and tank loading I would suggest designing to normal seismic requirements with the tank at operating weight, and check that the structure would not collapse with combined seismic load and the tank at its maximum possible load.

Doug Jenkins
Interactive Design Services

http://newtonexcelbach.wordpress.com/

 

[allgoodnamestaken]

Would you bet your license and a lawsuit that the client never puts more than 24 gallons in there?

 

[kgengr]

Further conversations with my client have led to plans for drilled holes in the tank to prevent overfill. Need to brush up on my calculus to check the sizing as they are proposing (1) 3" diameter hole with filter screen. Additionally they will have a sign made indicating not for liquid. The tank itself will receive "air" discharge from a couple mech units, with water used for fume scrubbing, and tank free draining at the base. There is potential for drains to clog - so the extra drilled holes would be for overflow/overfill protection. The holes at 1 ft above base still allow for accumulation up to that point, so that weight is intended to be used for the design content weight (with added tank weight) for bracing and support.

 

[IDS]

The question remains, if they are so confident that this volume will never be filled that you don't need to design the building to carry the weight of the full volume, why provide that large volume in the first place?

Doug Jenkins
Interactive Design Services

http://newtonexcelbach.wordpress.com/

 

[kgengr]

It is for gas discharged from mech units out of an air plenum from a clean room. They wish to capture and fume scrub to reduce contaminants from exhaust.