Welding Spacing in Pipe Spool 1

[FranciscoG]

I need to make a pipe spool with an 8" Sch 40 A-106 GR. B and two welding neck flanges A-105 Class 300 and I need to know the minimum spacing for the welds of the flanges, so as to make the spool as small as possible.

 

[MikeHalloran]

Is this some kind of academic problem, having to do with allowable spacing between adjacent welds?

Or do you just need a really short spool?

I've made 'em as short as 11mm overall length. No welding involved, just a piece of plate, faced, bored, and turned with an o.d. to fit just inside the bolt pattern, and a radial hole for a pressure tap. Like a thick gasket, made of metal.



Mike Halloran
NOT speaking for
DeAngelo Marine Exhaust Inc.
Ft. Lauderdale, FL, USA

 

[pipesnpumps]

Surely this a trick question? assuming both flanges are bored for sch.40 weld them together

 

[quark]

Say 150mm. With SO flanges it can be as small as 50mm (a bit difficult while tightening bolts)

 

[FranciscoG]

Thank you for your replies.

Really I am concerned with the allowable spacing between adjacent welds, because the spools have to be a given length, the flanges have to be WN and the welds of the flanges are resulting 78 mm apart.

 

[mohtogh]

In some specs, I read 200 mm is min. allowable distance between welds.

 

[unclesyd]

I see no problem with a 78 mm pup piece in your case. We use smaller sizes all time on steam and vaporized Therminol.

 

[JStephen]

I don't know if there is any minimum weld spacing. But I would like to point out that you CAN buy flanges with extended necks, made for nozzles on tanks and vessels. With one of these and a welding neck or slip-on flange, you would only have one weld.

 

[gmorin]

I've seen make-up spools that were pretty short. 2" / 50 mm pipe between weld necks. We try to use no less than 1 diameter of pipe in a pup piece.

I'm no welding expert, but I think the trick used was to make the first weld to a longer piece of pipe, then cut the pipe to length, then weld the second flange on. That way you get a "normal" heat pattern in both welds as there's more material to carry the heat away.

I'm sure there's a metallurgist out there cringing because there would be two heat affected zones very close to each other if not overlapping, but if we're talking 150# flanges, you're probably in business.

Good luck

Greg

 

[MickMc]

Twice the wall thickness of the pipe or 50mm between the welds, whichever is the greatest. in your case it will be 50mm between the welds to avoid Heat Affected Zone (HAZ)

 

[FranciscoG]

Thank you all.
I see that there is no clear definition on this subject.
I have been looking at ANSI B31.3 and there is no reference about this.

Someone mentioned that in ASME SEC VIII DIV 1, UW-14 you were allowed weld-on-weld connections. Can anyone check this for me as I don't have the code?

MickMc,
From where did you obtain the information of twice the wall thickness or 50mm minimum?

 

[tickle]

One thing that has not been discussed is to ensure that there is enough space between the flanges to allow you install and remove the bolting.

It may not seem like a problem with the 8" connections that you state, but if the spool is to go between two valves then you may have difficulty getting the bolting in and out, the stud bolts required are 111mm long.

If the spool is to be removed regularly, you may want to include jacking bolts.

Another consideration may be to make the spool piece long enough so that you can replace it with a valve in the future.

Further, the smallest separation that could be obtained would be with a spectacle blind (cannot find my sheet with the dimensions).

 

[MickMc]

The information I gave you is common as a rule of thumb to the UK onshore & Offshore industries but may differ slightly between Operators. I have added an excerpt from a major international operator from their Welding & Inspection Standard.

4.6.3.5 Spacing

Longitudinal seams on adjacent pipes shall be separated by at least 45 degrees of rotation or 150mm, whichever is the less. Circumferential welds shall be separated by at least 4 times the nominal pipe wall thickness (toe to toe). Attachments shall be at least 2 times the nominal pipe thickness or 40mm (toe to toe) from any weld, whichever is the smaller.

 

[DSB123]

MickMc,
The 40mm you quote agrees with BS2633 (Class 1 Welding)We tend to opt for 50mm between toes of the welds on our site.

 

[FranciscoG]

Thank you all. I think I will stick to what ASME VIII par. UW-14 b) says that provided the weld meets the radiographic requirements in Par, UW-51(m) you can make an openning in or adjacent to welds.

May be also testing the hardness of the welds, could give another assurance. A fitting manufacturer recommended a Brinell Hardness not more than 215 for the circumferential welds.

 

[jay165]

Why not insert a bleed ring and avoid the problem altogether?