What he said.
On small flanged elbows, you sometimes cant use bolts if holes are on centre because there isnt enough clearance to fit them. Even stud bolts can be a pain as you first need to push the stud in from the face side on one flange.
From my perspective the most important reason is that poprietary such as valves & instrumentation is drilled for staggered bolting. Few things look worse than valve spindles and the like installed "on the piss" because they are bolted to on-centre flange drillings.
Cheers
Steve
>From: Al <alwynk@shaw.ca>
>Reply-To: PipingDesign@yahoogroups.com
>To: PipingDesign@yahoogroups.com
>Subject: [PipingDesign] thoughts- Flange Bolt Array Positioning
>Date: Sat, 20 May 2006 17:35:36 -0600
>
>i hear this a lot of times , the best answer from discussing with grey
>hairs
>was largely CONSTRUCTION
>you can get a true centreline TDC better and easier when marking and
>straddling
>a line down the face of the flange body circle. much more time consuming
>and
>never
>as good trying to find and mark centre of line.
>
>I think going Along a line doing this each time gets near perfect matchup
>with no worries.
>eg wont get axial misalignment loading and the same would apply at pump and
>hardware series
>bolt up, thats what i think
>
>I entirely agree chris comments ""Lower stress argument is correct, but I
>still can't imagine that it would make a lot of difference."", but avoids
>risk of introducing
>"twisted stresses" ...theres a good term.
>
>
>** Altecheng@shaw.ca
>
>' 001(780)465-9762
>6 Fax/Msg(780)465-9762
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>-----Original Message-----
>From: PipingDesign@yahoogroups.com
>[mailto:PipingDesign@yahoogroups.com]On Behalf Of Christopher Wright
>Sent: Saturday, May 20, 2006 1:32 PM
>To: PipingDesign@yahoogroups.com
>Subject: Re: [PipingDesign] Flange Bolt Array Positioning
>
>
>
>On May 20, 2006, at 4:02 AM, Paul Bowers wrote:
>
> > The question is based on flanged connections and is about the holes for
> > the bolts. Why are holes on a flange never positioned on the vertical
> > centerline?
>The story I was told is that such positioning forces the topmost bolt
>to carry more stress when the attached pipe is subject to bending about
>a horizontal axis. This is true but I can't imagine it's significant
>for flanges with more than 4 bolts. With 4 bolts the difference in
>bending stress is about 30%; with 8 bolts the difference is less than
>10%
>
>Since the number of bolts on standard flanges is always a multiple of 4
>straddling the vertical centerline also makes the bolt pattern straddle
>the horizontal centerline, so bolt stresses from horizontal loading
>would also be lower. Maybe the lower stress argument is correct, but I
>still can't imagine that it would make a lot of difference.
>
>I think I overheard someone say that it was for accessibility from
>underneath, but again, for more than six bolts it isn't going to make
>much difference.
>
>
>Christopher Wright P.E. |"They couldn't hit an elephant at
>chrisw@skypoint.com | this distance" (last words of Gen.
>.......................................| John Sedgwick, Spotsylvania
>1864)
>http://www.skypoint.com/~chrisw/
>
>
>
>
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