Shop Fabricated Tank Size

[xema77]

What is the reason for capping the tank diameter at 6m (20ft) in API 650 Appendix J? If the transportation envelope is such that it permits a larger diameter tank to be shop fabricated is this acceptable?

 

[JohnGP]

xema77,

Shop fabricating subjects a tank to additional loadings that a site fabricated tank doesn't experience. I don't have direct knowledge regarding the 6m diameter limitation imposed by Appendix J in API 650, but imagine that some thought was put into how big you could easily fabricate a tank in a "shop", using the nominal minimum plate thicknesses, then lift and transport it to its intended location.

Certainly it is feasible to shop fabricate tanks larger than 6m diameter. We have on my present project, mainly due to the high cost of site fabrication in this instance. The largest was over 13m diameter, and weighed a bit over 130 MT because of some internal bits. Most were around the 11-12m diameter weighing around 50MT with top platforms. While Appendix J is not applicable for tanks over 6m, its requirements were taken into consideration for our tanks - such as butt welding of floors, and lifting lugs design. Additional stiffening and lugs were also provided for transport restraint (both road and sea), and floors restrained during lifts.

Regards,
John

 

[FTank]

In Canada, specifically Alberta, we can ship items as wide as 24 ft. on special low-bed trucks. When writing a tank specification using API 650 Appendix F as the reference, we re-word par. J.1.1 (modification) so as to allow up to 7.163 m (23.5 ft.) Dia. tanks. The reason we max. out at 23.5 ft. is to allow for additional thicknesses like insulation or if unavoidable, nozzle projections so that the overall width of the shipping envelope does not exceed 24 ft. The tank height is dependant on the trucking company's imagination, but very seldom exceeds 32 ft. Of course most of these tanks require anchor chairs for wind loading & internal design pressures from Appendix F (very common in our industry).
As far as xema77's concern regarding additional loadings, he is quite correct if the tank shell is only 3/16" nom. thickness, there is a good chance that the tank shell will "egg shell" during transportation, causing amongst other things, a detremental affect to the interior coatings. This is why we also modify par. J.3.3 so that the nominal thickness of the shell plates is not less than 1/4". We also provide "shipping shoes" so that the tank shell or the insulation does not make direct contact with the truck deck.
Just as a footnote, we specify 1/4" as the min. nominal thickness on field erected tanks also as 3/16" tends to distort during welding, resulting in an unacceptable appearance to our clients.

 

[JohnGP]

Yes, we had a similar modification to shell thickness such that it was not less than 6mm. Shipping width at the fabrication shop was an issue. The fabricator was near a port in an industrial area, but some of the street hardware, such as light poles and signs, had to be temporarily removed during tank transfers. There was a port at the project site, with direct road access, so not an issue there.

 

[JStephen]

I'm sure part of the thinking was just standard practice as to what is field erected vs what is shop fabricated.