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API 650 two-inch chime requirement

API 650 two-inch chime requirement

API 650 two-inch chime requirement

(OP)
Why does API 650 require a two-inch chime? Is there a structural basis?

RE: API 650 two-inch chime requirement

The chime provides a compression member to resist the 1 inch of water vacuum rating (minimum) required by the standard. This vacuum must be resisted even with the tank empty.

RE: API 650 two-inch chime requirement

(OP)
In the case of an open top tank, I would be comfortable allowing the chime to be reduced (forced in by incorrectly placed anchor bolts) to one-inch instead of trimming shell plate to maintain the two-inch chime. Is there anything else I should consider?

RE: API 650 two-inch chime requirement

I'm not aware of a structural basis for it.

If it was for vacuum loading, then larger tanks would require a larger chime, which is not the case. Appendix A or J may allow a smaller chime.

I don't think API-650 addresses the situation with clearance between anchor bolts and the chime.

RE: API 650 two-inch chime requirement

(OP)
In the case where a reinforcing plate takes up a portion of the chime, the standard (API 650) recommends at least 1/2 of the bottom plate to project past the toe of the fillet weld at the reinforcing plate-bottom plate joint. His seems like a distortion limiting, or weld quality issue.?.?

RE: API 650 two-inch chime requirement

JStephen,

Appendix (Annex) J tanks do have a smaller chime requirement. See J.3.2.3 and note that the bottom may be flat flanged (no chime) or a minimum of 1" beyond the toe of the shell-to-bottom weld. Appendix (Annex) A tanks can be quite large. For instance, it is possible to construct a 100 ft diameter x 32 ft high tank to Appendix (Annex) A so I do not see how the chime requirement should be relaxed for that portion of the standard.

RE: API 650 two-inch chime requirement

In the AWWA water tank standard, the Section 14 design (ie, high-strength design comparable to API-650) requires the 2" chime, while the base design (more similar to the App. A design) requires 1" beyond the toe of the fillet. Meanwhile, API-12D for tanks up to 55' diameter doesn't require any specific chime. So it's hard to imagine that this is some critical number.

If I remember right, there is an issue in the shell analysis theory as to whether the bottom plate is assumed to have a plastic moment both inside and outside, or inside only, and perhaps the additional length was to help justify the assumptions there. If that's the case, then notching around a bolt should have minimal effect on it.

RE: API 650 two-inch chime requirement

It's a "traditional" requirement. A small ammount of chime [chine] is required for welding the OD of the shell, and a further small ammount is required for the 1-Foot calculation method to be valid, i.e. using the floor and floor extension [chine] as a reinforcing member for the bottom of the shell. And it is a very good idea to have some more chine sticking out for 'sacrificial anode' to rust away first, before the corrosion gets to the weld.

When I add these up in my mind I get an inch or more; i presume that is how the 2-inch requirement arrived. When a tank is built in the field using 'classical' methods, the floor sketch plates are cut from straight plates. At that point, any reasonable floor extension - chine - can be established 1", 1.5", 2", 2.5" etc. For a new tank, I personally am perfectly happy with a nominal 2-inch chine, that never goes below 1.5" from the outer face of the shell.

RE: API 650 two-inch chime requirement

In the EEMUA 159 document there is a specific requirement for a tank to retain at least 9.5mm of chime outside the toe of the weld.

RE: API 650 two-inch chime requirement

OK what'a a chime (chine?)? Is this a non-USA term? I looked in api 650 and didn't see it. I googled the word. It took me to some web sites but I didn't see any definition. I looked in the dictionary and it said that it was a boat term where the bottom meets the side. This could be the shell meeting the bottom but where does the 2" come in? Is it just the bottom plate overlap?

RE: API 650 two-inch chime requirement

Bobfromoh,

Chime is a term to describe the required projection of the bottom plates beyond the shell as described in 5.4.2 of API 650.

RE: API 650 two-inch chime requirement

"The rim of a cask"
"the edge or rim of a cask or drum"
"the projecting rim at the end of a cask"

Specifically, the bit of bottom plate that extends outside the shell on a tank.

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