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ANSI Flange Alignment Standard

ANSI Flange Alignment Standard

ANSI Flange Alignment Standard

We weld flanges onto water process cells, and I can not find a standard relating to the hole orientation.  This relates mostly to 4 bolt flanges.  Is there a standard orientation relative to the axis of the cell or chamber?

If so, can you tell me what the standard is and which of the possible alignments is correct?

O - Hole
- - axis of chamber/cell
X - axis of flange

Alignment A:

  O O           O O
  O O           O O

Alignment B:

   O             O
   O             O

We have been using B for quite some time and just recently a customer told us that we do not meet the ANSI standard.

Any insight would be greatly appreciated.


RE: ANSI Flange Alignment Standard

"A" is correct for the vast majority of cases and commonly called "two holed" or straddle centerlines.  Although I see pumps occasionally that are "B" which is commonly called "one holed" or on centerlines

RE: ANSI Flange Alignment Standard

So, that sounds like either there is no ANSI standard, more an industry standard (two holed) and occasionally people deviate (one holed) for some reason, or there is a standard (two holed) and occasionally people don't know about it or aren't subject to the standard.

Anybody know if there is an ANSI standard?

RE: ANSI Flange Alignment Standard

What you are calling ANSI is actually found in ASME B16.5.

RE: ANSI Flange Alignment Standard

Yes, thank you.  I have ASME B16.5 and have looked through it.  
After you mentioned it though, I've taken another look.  The only reference to bolt hole location though is (am I allowed to quote this here?):
6.5: Flange Bolt Holes

Bolt holes are in multiples of four.  Bolt holes shall be equally spaced and pairs of bolt holes shall straddle fitting centerlines.

To me, that just indicates that pairs of bolt holes need to be 180deg apart from each other.  It makes no reference to how they line up when welded to a chamber with a perpendicular axis.

RE: ANSI Flange Alignment Standard

The normal orientation for every pipefitter is "A". I don't know if it's an official code orientation, but it certainly is considered "normal". Unless there's some special reason, why do anything else? All you're doing with "B" is screwing up the guys in the shop/field.

RE: ANSI Flange Alignment Standard

JoelGuelph, in alignment B, no way can pairs of holes be said to straddle centerlines.



RE: ANSI Flange Alignment Standard

I disagree.  The standard refers to the flanges & fittings only, and does not mention weldments at all.  The way I see it, the standard refers to the central axis of the fitting (& flange) and therefore as long as pairs of holes are 180degs apart, they straddle the centerline of the fitting.

As for why we use B, well that's the way our jigs have been set-up for years.  I imagine whoever originally built the jig found it much easier to align the holes along the centerline of the chamber then to figure out the spacing/alignment for alignment 'A'.

I am just try to justify retooling our jigs for alignment 'A'.  Is it worth spending the time and money on if it isn't necessary?  Isn't the job of pipefitters to fit pipe?  Surely they deal with odd angles all the time, which would lead to bolt holes not being exactly where they want them.

So, nobody knows of a standard for pressure vessels?  Can anyone suggest a good way to prove there isn't a standard?

RE: ANSI Flange Alignment Standard

My interpretation/opinion would be that alignment "A" is correct.  The bolt hole pattern would be at a single radius or bolt circle so that uniform compression could/would be achieved on a presumably circular flange.  Alignment "B" would not meet the criteria of equal spacing.  If orientation of a mating part/assembly is of concern, you would likely need to highlight or make note of it for processing the assembly/weldment.

   O             O
-O X O---------O X O--  
   O             O


RE: ANSI Flange Alignment Standard

Hmm, that illustration did not come out right when posted but looked OK in draft.  Sorry

RE: ANSI Flange Alignment Standard

PSE, I'm not sure I understand what you are saying.  To be clear, both alignment A and B are the exact same flange, both with a single radius bolt circle.  

I am asking about the alignment of the fitting (a weldneck flange for example) when welded to a chamber.

RE: ANSI Flange Alignment Standard

fig A is standard for pressure vessels.  I do not know if it is a ""government or regulatory rule"" or industry standard.  It is JUST THE WAY IT IS & HAS BEEN FOR THE LAST 40 YEARS THAT I HAVE BEEN BUILDING VESSELS.

RE: ANSI Flange Alignment Standard

Thanks, vesselfab.  That is the general consensus I seem to be getting.

I'm also discovering it is much easier to prove that something meets a standard, then to prove there is no standard for something.  How do you prove that something doesn't exist?!?

Does anyone know much about ASME B31 series of codes?  They seem to relate to process piping.  Is there anything in there that might help me out?

RE: ANSI Flange Alignment Standard

Why not just change what you are doing to match the rest of the world.  Just because there is no ""official published standard"" does not negate the fact that if you are using "B" one holed orientation, you are causing problems for anyone that falls in behind your operation.  You can probably prove that you within your rights to go on doing what you are doing....but does it really do any good in the overall scheme of things?

RE: ANSI Flange Alignment Standard


I have discovered a note in the detailing section of a Pressure Vessel Handbook that says they should be "Alignment A".  Also, after talking to our welders, changing our jigs will not be a big deal as they already use a modular type system for different size flanges.

In conclusion, I did not find any codes or regulations describing the issue, just a note in a design handbook.  I have made my recommendation to management to make the change, in the interest of conforming to an "industry standard".

Thanks to all who have helped.

RE: ANSI Flange Alignment Standard

Hello all,

I am surprised that nobody has mentioned the ASME PCC-1 Document.

ASME PCC-1-2000,  Guidelines for Pressure Boundary Bolted Flange Joint Assembly.

Regards, John.

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