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Riveted tank - welds

Wednesday, November 15, 2017   By: Carlos [1885] 0 Stars
Hi,
I am in a new role, and one of the cases I am receiving is a riveted AST whose rivets were recently welded all around exterior shell, the same the laps of shell plates.
The tank is around 90 years old, now I am asked to certify the tank repair was done in accordance with technical standards such as API653. I cant find such case in API.
Does anybody knows if welding the rivets and laps of the shell is allowed, or is a case of rejection for API standards?

Thanks
Carlos

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Wednesday, November 15, 2017   By: DavidR@Fast.net [39] 5 Stars
The repair you describe is quite common, see API 653 figure 9.2, etc.

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Friday, November 17, 2017   By: Tankee [235] 4 Stars
API 653 9.2.4.8 discusses the why of welding existing rivets and the lap seams. Maybe just near a new welded insert, door sheet, or bottom, or even the whole tank. It doesnt actually give you any limitations to how far or how much welding you may do, but the included NOTE gives you the main reason you go ahead and might weld everything.

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Friday, November 17, 2017   By: ITS [1678] 3 Stars
I just wanted to add that riveted aboveground storage tanks are getting harder to find, but many still have useful life left in them. Over the decades, I cannot think of one riveted tank that leaked.

As you know, these riveted tanks are sized about what we consider a process or production sized tank so they may have a questionable past service life, so their past service life seems important to me because if the content is corrosive and or the tank had experienced a hydrostatic bump and or a foundation or something else then quality assurance activities like exploratory UT straight beam would be informative and perhaps some shear wave UT to get under something.

The important thing I try to remember is that the 2 shell plates need to be tightly together, and also the rivet bearing surface and shell rivet hole needs to be tight too and without voids. If voids are found between the shells plates then there was a past leak that may have corroded the near by rivets. Not good! So, it is important that the rivet bearing surfaces be clean without voids, which prevents movement between the 2 plates being held together. I know you will not forget that corner joint rivets at the bottom shell and tank floor.

Tanks that were seal welded on the rivet heads as well as the shell seams should show no leaks either. Also, keep in mind the welds are not for strength but for leak prevention.

We tend to stay away for riveted tanks, but riveted tanks are repairable, but finding some to repair them is nearly impossible today.

So, I would start to find out the history of the tank and look previous evidence of leaks. Do some NDE at a level of effort that is acceptable to you and your client. Of course make sure you follow the code because there is a lot of experience in the code. Good luck.

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Friday, November 17, 2017   By: Dunedi [37] 4 Stars
Is there an effective rim gap seal system for riveted tanks?

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Friday, November 17, 2017   By: ITS [1678] 3 Stars
I think you know the answer, but riveted aboveground storage tanks where not built with emission losses in mind.

If we see a riveted tank then it is likely to be pre-1950 and more likely pre-1940, for riveting was used prior to what old people call modern arc or fusion welding techniques and people today just call welding. So, I dont believe seals were used then.

These are flat bottom small or medium tanks with a conical roof with structural supports for the bigger ones and these tanks cant hold much more than 3000 barrels.

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Friday, November 17, 2017   By: Tankee [235] 4 Stars
Dunedin, are you referring to an IFR seal system?

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Saturday, November 18, 2017   By: Dunedin [37] 4 Stars
I am looking at either EFRT or FRT with an IFR.

I accept that the riveted tank was built in a time when emission control was not a priority for tank owners but if these same tank owners are still using these same tanks then they must comply with local, national and international emission limits and must be fitted with effective rim gap seals - so what are they using?

In my experience the seals have fingers to negotiate the rivets but they are far from effective at controlling vapor emissions.

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Friday, November 17, 2017   By: DavidR@Fast.net [39] 5 Stars
Single or double wiper seals, and shoe seals with or without a secondary have been used with reasonable success. They present mechanical obstructions but as well, those tanks are sometimes badly our of round, etc. The EPA document AP-42 lists rim seal loss factors for riveted tanks, and of course they are higher than those for riveted tanks.

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Friday, November 17, 2017   By: ITS [1678] 3 Stars
Excellent post...

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Sunday, November 19, 2017   By: BABRTs973 [53] 5 Stars
The HMT Mobil design with two tiers of wipers and fingers is a very effective rivet tank seal. Be sure they supply the correct followers or they will hang up on the horizontals. Another thing to be aware of is the floating roof itself. Due to settlement and earlier modifications such as adding welded courses not is the tank out of round, if it has a Wiggons Pan with the alligator teeth braces on the topside, round is just a concept. I have seen 12 inch rim gaps with 6 inch in other places on the tanks. The old CBI Buffalo seal was a lot more forgiving than the things we have today. And the operators ran the things past the top angle as a matter of course. If you see a riveted EFR and it has a piece of channel sticking up 4 or 5 feet across from the platform, that was to allow the seal to hold together as they ran it up past the top. They would get these things within 8 inches of the top. And yes, they occasionally ran them over. That is what all the holes drilled below the top angle was for. So the roof did not float off the tank.

Some companies install and two tiered wiper with 4 inches between the wipers to go above and below the horizontal rivet row. I would go for 8 inches between the tiers if you are using a 7 inch wide wiper tip. When they age they tend to bend different ways around the tank,

The biggest issues I see with riveted tanks, other than leaking rivets, is laminations. You can see some scary stuff especially in the pre 1920s steel. And the leaking rivets is something operators worry about more than they should. They are a maintenance hassle more than anything, You just get good with using tin solder, punches to fill the holes and leaking laps with the solder and the Belzona rep on speed dial.

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