large api620 tanks 1

[vesselguy]

The largest API620 tanks I have designed in the past were around 30ft and had self supporting cone or dome roofs. For API650 tanks, I have designed them to 120ft with supporting roof. No problem. However, I now have to consider the possiblility of changing several 100ft diameter API650 tanks (with Appendix F)into full blown API-620 design because process engineers want to increase the pressure!

I don't have experience with this large diameter API620 design. Can the more experienced tank engineers confirm for me if I can still have a supported roof (ie. internal rafters & columns) and still be an API-620 design? I think it is possible as long as I meet the top shell course's compression area requirements and the roof and shell thickness is sufficient to resist the internal pressure.

Would the fabrication cost of an API-620 design be much more than an API-650 design? Thanks.

 

[JoeTank]

LNG tanks have self supported dome roofs. Most of the larger LNG tanks exceed 200ft in diameter. They are too big to be designed by plate stability rules, so they are typically supported by a system of radial raftera, an inner comprerssion ring, outer tension ring and several intermediate rings. While the design is a challenge that is manageable, the erection will be much more difficult than most tank fabricators can handle. Select your contractor very carefully. Large dome roofs are often air-raised into position, but that may prove to be difficult on a retro-fit basis.

Joe Tank

 

[vesselguy]

Joe Tank,
I was hoping you would share your input. Thank you.

Your input give me some confidence that my thinking is correct in that a supported roof can be used in API620. But as such, why would the building of the dome roof be challenging, and may even require air-lifted into position? I would think they can weld the roof plates together while the plates rests right on the roof rafters. The plate will not be welded to the rafters flanges of course.

Thanks again, Joe.

 

[JStephen]

I think the roof he is referring to is still a columnless roof, although it has radial stiffeners and circumferential tension/compession rings. Part of the challenge is that the roof can be very high. See the roof on this tank, for example- 260' diameter x 80' shell (not an API-620 tank, but for illustration):

http://www2.sherwin-williams.com/im/wastewater/largestwatertank.asp

 

[JoeTank]

JStephen hit the nail right on the head. Erecting a large dome roof on a completed shell is possible, but requires lots of stick on the crane. Air-raising requires an incredibly round tank with few flat spots. Much more care is employed during the shell erection of a tank that is have an air-raised roof. So, I'd check the existing tank very carefully before contemplating an air raise. If the dome gets jammed up during the air-raise, it often ends up looking like a big taco shell.

Joe Tank