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API 650 Tank with a Rolled Bottom Ring

API 650 Tank with a Rolled Bottom Ring

API 650 Tank with a Rolled Bottom Ring

(OP)
I have a 304 Stainless API650 Tank,  Tank is designed for 2.5 psig.  Capacity is about 10,000 gallons.  Diameter is about 12 ft. The tank sits on a sloped concrete pad.  (The bottom of the tank is sloped to drain out.
the Hold down ring has began to curl up, in some places it is 1" off the concrete, despite the concrete anchors currently in place.   I am wondering what should I do with it?   It is currently not leaking.    I am wondering if I should drill anchor bolts into the flange between all of the existing bolts and try to pull it back down.   Or if I sould leave it the way it is, maybe just caulk around the outer ring to keep snow and ice from getting in there.    Any thoughts?

Regards
Stonecold

RE: API 650 Tank with a Rolled Bottom Ring

If by "hold-down ring" you mean the bottom extension outside the tank shell, that is not normally intended to be a hold-down anything.  Normally there would be anchor chairs to restrain the tank from upwards movement.  If this is the configuration, I would remove all anchors through the bottom plate and anchor the tank in other ways.  The weld between the shell and the bottom is one of the most important welds on the tank, I would not stress it with anchor bolts.  My opinion only...

RE: API 650 Tank with a Rolled Bottom Ring

(OP)
The ring, I suppose it is a 4" rolled angle.  Had anchor bolt hole in it, as fabricated, so I am pretty sure that was the intention of the ring.   It purposely sticks out beyond the insulation.   However I must admit 99% of my experience is with ASME tanks, just the nature of our business.

RE: API 650 Tank with a Rolled Bottom Ring

Pictures... StoneCold ??

   

RE: API 650 Tank with a Rolled Bottom Ring

(OP)
Ok.
I have finally had a chance to put some pictures together to show you guys the problem with this tank.
I put them all in a word document, I hope you can open it.
Let me know what you think might be happening.
One more thing the specific gravity of the fluid is only 0.7 so I don't think that is a contributer to the problem.   The tank did not have this when it was originally installed.

Regards
StoneCold

RE: API 650 Tank with a Rolled Bottom Ring

From looking at the photos and your description, it appears the tank is being anchored by holding down the floor or something attached to the floor. Shame on the tank fabricator! He doesn't understand the API 650 requirement of anchoring the shell. Better not use this fabricator again!
Solution - design anchorage that is attached to the shell in accordance with API 650.

RE: API 650 Tank with a Rolled Bottom Ring

Can't read the name plate to see what code it was built to (maybe none?).  In any case, if the 2psig is enough to overcome the tank weight then it is trying to lift the tank off the slab and bulge the bottom downwards.  If this is the case, the anchors are not doing their job.  FTank's suggestion to retrofit properly designed anchors is then the proper solution.

RE: API 650 Tank with a Rolled Bottom Ring

I agree with the comments above....

I suspect that the 2 psig nitrogen is the cause of the bottom ring problem.

Perhaps the nitrogen pad was added to the tank some time after the tank was originally put into place ?

This type of sloped-bottom tank deserves proper anchorage.

Pictures can help a lot !!!

   

RE: API 650 Tank with a Rolled Bottom Ring

(OP)
The real question now is what do do about it?
About the only way to force it down would be to put anchors in the concrete around the rim and then make a hold down clamps from steel plate to try to squeeze the tank back down to the concrete.

I don't know how good that would be for the tank.   I wonder if it would be better to leave it and just caulk around the base to keep water out.  I certainly don't want to crack the bottom of the tank and end up with a leak.

Any thoughts on a repair?

Regards
StoneCold  

RE: API 650 Tank with a Rolled Bottom Ring

You don't really care about the bottom.  Leave it alone.  You need to anchor the shell.  Calculate the anchorage required.  Expose the shell.  Weld anchor chairs to the shell.  Epoxy anchors into the concrete.  Go home and sleep well.

RE: API 650 Tank with a Rolled Bottom Ring

By any chance did the tank get slighty over pressured.  Over
pressure will try to round the floor of the tank which would
cause the edges to lift,  You can see the anchor bolt chair ass mentioned above.  All our SS tanks have the T-Joint on
the floor plate which extends 3-4" from the wall, no angle at
the bottom.   One of the names for the floor plate that is outside the bottom ring is d compression ring even though it is part of the floor.

http://my.engineering.com/action/file/download?file_guid=427305

This tank is on grillege and has many bolts on th cover plate insted of a few big ones.

RE: API 650 Tank with a Rolled Bottom Ring

We have had small [10 to 15-ft dia.] tank bottoms due to overpressure in the past.  If tank passes an API-653 Internal & External Inspx, use non-shrink grout to fill the gap and bring the tank back to intimate contact with the slab.  If your tank has minimal wind exposure, just cut your incorrect anchors and do not reanchor the tank.  If the tank needs anchorage for wind loading or pressure, intstall proper anchors to the shell.

Off the top of my head, a 12-ft tank needs substantial anchorage for 2.5 psig internal pressure

RE: API 650 Tank with a Rolled Bottom Ring

(OP)
I think you guys are right. At about 12 ft in diameter and the bottom of the shell acting like a diapham because there are no stiffeners then the anchor bolts have to hold down 40,715 lbs when the tank is empty to keep from pulling up the ring.  There are currently 12 anchor bolts in the ring.  That is 3392 lbs each.  So if the embedment is 2" they are good for 2400lbs if it is in 3.375" then 4100 lbs.   So the anchors were either not drilled in deep enough or the concrete quality was not good.
Either way it is kind of difficult to fix now.

Regards

StoneCold

RE: API 650 Tank with a Rolled Bottom Ring

After the picture
I dare to say that the tank has beee overpressurize (gas o overfilled ) due to a low diameter averflow ,or inerte gas pressurized
The tank try to get the form of a barrel and the bottom goes up in order to get the spherical form.
so loose all bolt.
Youcan check it if you take the isolation of, an review the operating conditions
Make new chemical anchor in betwwen the old bolts
I hope you can repair it
Luis

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