Tank roof min thickness

[kalluna]

What tank roof thickness criteria is used during API 653? From the section in 653 on Suitablity for Service, it sounds as if roof plates corroded to an av thickness less than 0.09 inches in any 100 inch sq area means the plats should be reapiared or replaced. Is this true then whatever the original thickness of roof plates? We had a case where the API reports specified (as it turns out,wrongly!) a t min of 0.163, not 0.09 in. Of course this gave a too-short life remaining. But is 0.09 in thick roof always the determining criteria for repalcement or repair?

 

[IFRs]

I would apply the 0.090" minimum thickness to ordinary rafter supported cone roof plates that were 3/16" thick when new and did not have any unusual loading. For self supporting cone, dome or umbrella roofs I would other (thicker) minimums based on good engineering judgement

 

[kalluna]

Thanks-these were originally 3/16 inch (most dome and some were cone) roofs with no particular extra loads except maybe seismic(and I am not sure if they were designed for that)- they are 27 years old and roofs had corroded from 0.188 in thick to 0.170 inches in that time so Cr was 0.00067. Report does not say if there is are central supports nor are there any photos of tank inside.