Butt welds of unequal thickness (sour service)
Butt welds of unequal thickness (sour service)
(OP)
Given:
Service: Produced Gas (sour service)
Mother pipe: NPS 20 with 11.9mm W.T.
Line pipe: NPS 20 with 9.5mm W.T.
Design to: CSA Z662-07
Background:
Due to stress concerns, we've had to order mother pipe (for our induction bends) that has a difference in wall thickness greater than 1.6mm from our line pipe (with outer diameters being equal).
According to CSA Z662-07 clause 16.6.3 (c) we should be using a machined transition piece that matches the bore of the adjacent pipe at each end for 25mm and then is tapered at an angle between 14 deg. and 30 deg.
Question:
If we choose to simply bore out the mother pipe for 25mm to match the wall thickness of our line pipe (9.5mm) and then taper down to the wall thickness of our mother pipe (11.9mm) have we satisfied the code? I prefer this approach since it saves purchasing a machined transition piece and saves on welding costs (approximately $3,000 each weld).
Any thoughts?
Thanks in advance
Service: Produced Gas (sour service)
Mother pipe: NPS 20 with 11.9mm W.T.
Line pipe: NPS 20 with 9.5mm W.T.
Design to: CSA Z662-07
Background:
Due to stress concerns, we've had to order mother pipe (for our induction bends) that has a difference in wall thickness greater than 1.6mm from our line pipe (with outer diameters being equal).
According to CSA Z662-07 clause 16.6.3 (c) we should be using a machined transition piece that matches the bore of the adjacent pipe at each end for 25mm and then is tapered at an angle between 14 deg. and 30 deg.
Question:
If we choose to simply bore out the mother pipe for 25mm to match the wall thickness of our line pipe (9.5mm) and then taper down to the wall thickness of our mother pipe (11.9mm) have we satisfied the code? I prefer this approach since it saves purchasing a machined transition piece and saves on welding costs (approximately $3,000 each weld).
Any thoughts?
Thanks in advance





RE: Butt welds of unequal thickness (sour service)
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RE: Butt welds of unequal thickness (sour service)
RE: Butt welds of unequal thickness (sour service)
My interpretation is that you can use any of the applicable details in CSA Z662-07 Figure 7-2 without a "transition piece" per se, thereby saving the extra weld on the thick end.
What you *will* need to check is that you will still meet the requirements for pressure design and Notch Toughness qualification on both sides of the weld in accordance with Clauses 4 and 5 of CSA Z662-07 and / or supplemental or superseding Provincial Regulations. This is especially true if you have a difference in material SMYS and Notch Toughness qualification on either side of the weld. Where an actual transition piece "fitting" (i.e., per CSA Z245.11) is used, it would be highly desirable for its SMYS to be equal to or greater than that for the thinner wall line pipe, and *my* interpretation is that if the line pipe Notch Toughness *requirement* is Category II or Category III, you will *have to* use a Category II fitting.
All this assumes that the line pipe itself has been correctly specified.
Regards,
SNORGY.
RE: Butt welds of unequal thickness (sour service)
All pipe is the same grade and CAT II tested to the same MDMT (as are fittings when selected).
Regarding CSA Z662-07 figure 7.2:
Only Clause 16.6.3 (a) and (b) reference CSA Z662-07 figure 7.2 directly and neither are applicable since delta w.t. > 1.6mm and O.D.'s are equal.
Clause 16.6.3 (c) requires a "machined transition piece" that meets the following provisions:
(i) A minimum 25mm length from each butt weld end shall match the bore of the adjacent item for each end
(ii) The dimensions for the transition portion shall conform to the applicable details in Figure 7.2
(iii) The thickness and strength of the transition piece shall be selected to meet the design provision for the pipeline.
I'm not 100% sure that a machined transition piece means a separate fitting (designed to Z245.11-09) but the wording does imply a separate transition fitting (i.e. "each butt weld end shall match the bore of the adjacent item for each end").
Anyway, the tangent lengths on these bends are long so it's straightforward to end-prep them with the boring/tapering details described previously. I believe all (ii) is saying is that the taper must be 14 to 30 degrees and that the mother pipe must be bored to a w.t. between 9.5 and 11.1mm (as per dimensions in figure 7.2 (a)).
RE: Butt welds of unequal thickness (sour service)
After all, if you were to do this and then cut that piece off of the bend, you would have a separate transition piece that met Z662 so leaving it attached is better since continuous parent metal is preferable to a weld joint.