flange orientation

[KimBellingrath]

Can someone tell me why most piping diamgrams show drilled holes in bolted flanges straddling the vertical centerline, so as to not have a hole at top dead center?
Thanks

 

[BigInch]

there is less stress per bolt under gravity loads.

http://virtualpipeline.spaces.msn.com

 

[khardy]

I think it's just convention.

 

[eliebl]

By straddling the centerlines there is less chance of having a bolt interfere with an elbow when a flange is close to the elbow.

EJL

 

[pennpiper]

KimBellingrath,
In addition to the wise and appropriate answers you have already gotten I will give you two others.

1 - You will find that with the bolts on the vertical centerline there would be a problem with getting a wrench on the nut on some valves.
2 - Many valves, both cast and forged have a reinforcement "Web" right at the "12 O'Clock" position connecting the back of the flange and the rest of the valve body so a bolt hole is out of the question.

 

[Shmulik]

thread378-150578

 

[11echo]

I too would like to add one more answer to the already "wise" answers you have already received. When welders’ assembly flanged piping systems they are required to get the system on a horizontal or perpendicular orientation. This requires the welders to be able to weld components exact level, to achieve this the welders use a technique called "two holing it"! Basically they put two pins in the top flange holes and then put a level on these pins. When the bubble in the level is centered, so is the flange in relationship to the assembly! This is a common assemble procedure here in the oil field.

 

[CountOlaf]

While we're on the subject, on a recent job I was on, a vendor referred to the bolt-hole orientation of a flanged device as "one-hole" or "two-hole". It took me a while to find this description elsewhere, but apparently the "one hole" orientation is the 12:00 scenario described above and the "two hole" is the straddle scenario.

My question to the responders of this thread is, "Is this common terminology?" It wasn't for me, but I don't normally get involved in such details.

 

[deanc]

http://www.pipefitter.com/

Explore

 

[Artisi]

Maybe cause it looks good on the drawing (-:

 

[BigInch]

Hummm! costs $20 just to look at that pipefitter site.

http://virtualpipeline.spaces.msn.com

 

[TBP]

Flanged valves & fittings are made to be installed on two-holed flanges. One-holing flanges means that any flanged components will be left tipped to one side on an odd angle. For instance, no flanged swing check valve could be installed in a horizontal line such that it sat properly.

Two-holing makes it easy to fabricate piping using common tools. A fitter with a tape measure, torpedo level, a 4 foot level, and a couple of framing squares will be much more productive than the same guy who's wrestling with the goofy angles left by one-holing.

Unless there's some special reason - and they CAN occur from time to time - why would anybody design a system like this? It's like handing the carpenter the framing plans for a house, with all of the studs on 15" centres.

 

[MTPipeliner]

I'll second the comment about the valves being tilted if they are installed by one-holing. We have one in an existing system that was accidently done that way and it bugs me every time I see it.