B31.3 Weld Transitions

[FlowNBE]

I'm designing a vessel to meet B31.3-2004 requirements. I have a piece of rolled plate welded to a piece of round bar; both 304 SS. The plate is .3125 thick and the wall of the round bar after boring will be about 1.74" thick. B31.1 references B16.25 (Fig. 1 page 2) for butt weld transitions which shows a 2T long (no minimum designation) transition zone on the inside and outside. So, my transition zone on the outside should be about .625" long with the 30 and 45 and 90 degree angles. Seems short.

Looking at B31.1-2004, Fig. 127.4.2 page 72 shows a minimum transition zone of 2T. This seems to be opposite of B16.25-2003

Is it better to have a longer transition zone? Is there a conflict between these two standards?

 

[metengr]

FlowNBE;
Before I can help you, you are mixing ASME Power Piping code references. Are you designing to B31.3 or B31.1?

 

[FlowNBE]

Please excuse! The code of construction is B31.3. My confusion is that B16.25, referenced in B31.3, 328.4.2 (5), shows a (max?) 2T transition zone, while B31.1 and B&PV code show much more.

 

[jte]

But why design a vessel to a piping code?

jt

 

[FlowNBE]

Per customer specification. It's going into their piping system.

 

[metengr]

FlowNBE;
I agree with jte. Are you sure that this vessel should be designed under B31 Piping Code? What is the vessel or component?

 

[MJCronin]

Flowdude...

I believe that there is a misunderstanding in concepts and codes.

ASME B31.1 and B31.3 apply specifically to piping systems. The forword to each code is quite specific about applicability. Generally speaking, piping systems use piping components and are designed for the transport of liquids and gasses operating at pressures above 15 psig.

Pressure vessels are different. They are not typically designed using piping components (i.e. rolled plate is OK); must be certified by an authorized inspector and must bear a code stamp. The most commonly used code is ASME Section VIII, Division I.

Sooooooo.... despite what you were told by your client, you seem to be using the wrong code for design and construction.

Where (in what country)will this "vessel" be installed ?

-MJC

 

[FlowNBE]

This is a piping component going into a process piping system. B31.3 doesn't exclude rolling plate into cylindrical shapes and welding with longitudinal joints. Various grades of bar and plate are listed in Table A-1 for use in piping systems. These can be welded to each other and to other piping components.

 

[metengr]

FlowNBE;
Ok, I reviewed B31.3, Section 328.4.2 End Preparation....
essentially, subparagraph (5) states that girth or miter welds used to join components of unequal thickness (where one component thickness exceeds the other by 1.5) end preparation shall be in accordance with B16.25.

This seems very straight forward to me (not a Piping Engineer). I have a copy of ASME B16.25, ButtWelding Ends, so I reviewed the applicable section for transitions, and again in Section 3, Welding Bevel Design, the information is clear to me. Keep in mind that Figure 1 in Section 3 is only for illustrative purposes and the final contour is at the option of the manufacturer unless specified by the Purchaser.

I still don't know why you were concerned with B31.1 if you are designing to B31.3. The B31 Piping Code bodies like to be unique with their domains .

 

[prex]

Coming back to your original question, FlowNBE, can't see where is the conflict. Both B31.1 and B31.3 reference B16.25, and, by the way, 2t_m is a minimum, not a maximum.
A longer transition will of course always be preferred to a shorter one, codes only specify minimum distance.


prex

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[FlowNBE]

Lacking experience, I make no assumptions. B16.25 shows a 2T transition. To me, this means 2T max length.

B31.1, figure 127.4.2, page 72, shows a minimum 2T (labeled minimum)transition zone, meaning it can be longer.

In my mind these contradict each other. If a longer transition is better than I should consider the B16.25 transition a minimum length.

I also wonder if the transition only applies in the heat affected zone. I could change the profile of the thicker piece, chamfering out to the full OD outside the weld area?

 

[prex]

Of course the best is what you propose, meaning that you would bevel the thicker plate creating a nose with the same thickness as the shell and with, say, 2t length, then going to the major thickness with 45 or even 30 degrees slopes.
However all this is not required by code: it's up to you to decide whether you want to do better than the prescribed minimum.
The 2t_min in B16.25 is also to be considered a minimum: please read note (5) to the figure.

prex

http://www.xcalcs.com

: Online tools for structural design

http://www.megamag.it

: Magnetic brakes for fun rides

http://www.levitans.com

: Air bearing pads

 

[FlowNBE]

prex,
Thanks for the help. I'll make sure my transitions are longer than 2T.

 

[davefitz]

Its better to have the longer transition zone. Also, regarding fatigue failure , the EU PED codes classify the thickness transition weld zone to be a highly stressed zone unless the slope of the thickness gradient is less than 1:3 ( 18 degree max).