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engr_mustehsan
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Join Date: Jul 2013
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Two Hole Rule For Flange

08/07/2013 2:28 AM

All Flange Bolt Holes Straddle the Centerlines, Why ?
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Pathfinder Tags: flange flange fastening two hole rule
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PWSlack
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#1

Re: two Hole rule for Flange

08/07/2013 3:10 AM

It makes it easier to align the drill and put the hole in the right place. No-one needs wonky bolt holes.

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jesw55
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#2

Re: two Hole rule for Flange

08/07/2013 3:24 AM

What possible advantage would there be in putting the holes "off centre"?

Why ask such a question?

John

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Tornado
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#3

Re: two Hole rule for Flange

08/07/2013 3:37 AM

See your previous thread on this same subject.

So valve stems and other stuff can be horizontal or vertical, rather than goofy 22.5° angles.

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jesw55
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#4
In reply to #3

Re: two Hole rule for Flange

08/07/2013 3:54 AM

Hi Tornado,

By "off centre" I really meant anything but concentric about the centre.

Best regards,

John

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Tornado
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#5
In reply to #4

Re: two Hole rule for Flange

08/07/2013 4:55 AM

Please note I was responding to the OP.

(There is some confusion in the thread between center lines and quadrant lines, however.)

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jesw55
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#6
In reply to #5

Re: two Hole rule for Flange

08/07/2013 6:49 AM

Guilty conscience on my part?

Should have looked at the response number DOH!

John

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KeepItSimpleStupid
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#7

Re: Two Hole Rule For Flange

08/07/2013 10:58 PM

I nominate this thread for the next "Capton this". It ought to be entertaining.
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edressler
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#8

Re: Two Hole Rule For Flange

08/08/2013 2:10 PM

Here's a copy/paste from Yahoo answers, of all places. Looks right to me.

"Bolt holes on flanges straddle the center lines (one on each side of the center line) for a very simple reason, To assure proper alignment of flanges from piece to piece there needs to be a simple method of consistently orienting the bolt holes.

By using a simple spirit level it is possible to set the hole orientation. A bolt can be placed in each of two holes and a level is place on them. The flange is rotated until the bubble in the level is centered. This places the two bolt holes equidistant from the center line.

A simple but low tech way of getting the job done regardless of the location."
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old salt
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#9
In reply to #8

Re: Two Hole Rule For Flange

08/13/2013 6:22 PM

Small note, usually not significant---

If the bolts are not centered in the center of the flange hole it is possible to get a progressive rotation of the flange holes as more lengths are added

For example- on a 4 bolt flange with 1/8" clearance for the bolts, if one bolt is at the top of the hole and the adjacent one is at the bottom, the flange bolts will not be inline. Do this with both ends of the pipe and it possibly doubles the error. Do this with enough sections of pipe bolted together and you can get some weird angles produced, such as the 22-1/2o mentioned in another post.

In order to prevent this, make sure the bolts are located in the same place in the hole, I.e. top-top, bottom-bottom. Also there are devices called "pipe flange aligner pins" to take this error out. They have conical ends that center it in the flange hole. Do this for each hole and put a level across them. Turn flange till the bubble is level. McMaster # 2745A242

Good Luck, Old Salt

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Randy Conner
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#10

Re: Two Hole Rule For Flange

08/19/2013 9:20 AM

I know bolted and flanged joints have been used in the piping field for at least 350 years. I suspect bolt hole location has been pretty much "straddle the vertical centerline" for most of that time, and for most of those applications (in addition to the very good reasons mentioned by others, the bolts are also arguably at least a little easier to reach with wrenches on larger pipes, when having to reach under or over the pipes).
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Users who posted comments:

edressler (1); jesw55 (3); KeepItSimpleStupid (1); old salt (1); PWSlack (1); Randy Conner (1); Tornado (2)

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