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Repad in the form of a pipe flange

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chicopee

Mechanical
Feb 15, 2003
6,199
Owner has received permission to cut out an overflow hole at the 19'elevation of a 73' dia. X 30' high tank so as to keep the capacity around 750,000 gallons. The current plan is to cut out 8" hole so that a 1/4" thick repad assembled with short piece of pipe and elbow can be bolted to the tank. The question now is can a 1" thick pipe flange(13 1/5" OD) be used instead of the 1/4" X 17 1/5" OD repad? I, for one thought, correctly or wrongly, that the repad thickness could not exceed the 1/4" thick shell plate. I also stated to form the repad to the tank OD which I don't believe that you could do to the flange. Owner also wants only four bolts but I believe that 8 bolts would be better for such large OD.
 
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Duwe6 thanks for your prompt reply. I've been looking at the shell/head mount on pg 60 and from the drawing, it appears to be mounted by welding on the outside of the tank. Is this correct? if it is so, unfortunately, welding is out of the question and therefore bolting was suggested which could viewed to similar situations with riveted repads. At most, I don't expect the total weight to exceed 30 to 50 lbs as there will be no outlet pipe longer than a nipple from the elbow outlet. Any input is appreciated.
 
Welding would be the simplest and preferred solution in most cases.

If you don't need a riser pipe and it's just an overflow, you could cut the hole or slot, weld a hood over it similar to the overflow slots on petroleum tanks.

Generally, on a tank of that size, I would expect the shell to be flexible enough that you could bolt a flange to it. That flexibility might also make it hard to get a seal with a standard bolt pattern (or fewer bolts). Might take a thicker and/or softer gasket. Due to the flexibility, you'd want more bolts smaller and closer together. On this, page note the "nozzle flange" about halfway down, intended for bolting to bolted tanks:
Note that a standard flange connection is intended to keep product inside the pipe and isn't necessarily watertight through bolt holes across the flange. I think they sell bolts with a rubber ring under the head to help with that.
 
Oh, also- note that stresses at the overflow level of a tank are very low, so it doesn't much matter what you do as far as tank stresses are concerned. If the tank needs to comply with API-650 or API-653, you need to refer to them for details of the nozzle. Also, you'll need to do coating touch-up at some point, or you'll have corrosion problems down the road.
 
Not that it really matters but unless I'm sadly mistaken, 19' high gets you about 595,000 gallons. You would need about 24' to get 750,000 gallons. In any case, what happens above the liquid level is of little concern, so bolt or weld to your heart's content. Since there is no hoop stress above the liquid level that we care about, there is no reinforcing needed.

On the other hand, if you are using an internal riser pipe whose opening is at the 24 foot mark, 5 feet above the shell opening then there is hoop stress and reinforcing may be needed which you can check with simple calculation and leaking in this bolted connection will be a concern which you will have to address.

I would be careful to make sure this overflow could pass the maximum liquid inflow rate with the available head pressure to help.
 
Yes, the tank volume is actually 600,000 gallons. That's what happens when the information is passed down thru a few people.
 
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