Updated: 2016-12-20th MCT.
The articles below will hopefully provide some clarification on code weld requirements listed in ASME VIII-I. This initially simple topic often leads to confusion when it comes to efficiencies in calculations.
- Weld Efficiency for VIII-1 Vessels – Section UW-11(a)(5)(b)
- Use of Type (3) welds in ASME Pressure Vessel Design
Weld Efficiency for VIII-1 Vessels – Section UW-11(a)(5)(b)
Comment: Only ASME can make interpretations on the ASME VIII-1 Code
I have long struggled with the weld efficiencies presented in section UW of the ASME VIII-1 code. I have had more trouble with it than many other sections of the book combined. The ideas in this section are simple, but the ASME code written around it is anything but. Where ASME has not made the code readable, we must live with confused and diverging interpretations.
The problem area is sections UW-11 and UW-12 and any section of the code that references UW-11(a)(5)(b) – and there are many. What weld efficiencies to use when seams with different efficiencies intersect? I do not believe that circ weld efficiencies can affect longitudinal efficiencies however; these rules as presented in UW-12 and UW-11(a)(5)(b) exist and must be dealt with. I do believe that I will have to read the infinitely confusing sentence UW-11(a)(5)(b) many more times in my career.
Only ASME can provide interpretations as to what this code means, and someone can ask them, but – how about doing something simple instead? Samples of what the code committee wants can be found in appendix L. In specific samples L-1.5.1 through L-1.6.3 show the correct weld efficiency to use with differing radiography, and they also show the effect of circ efficiency on long seams. The meaning of UW-11(a)(5)(b) can be inferred.
This article comments on the 6 sample vessels found in Appendix L. A simple spreadsheet is introduced that calculates the same weld efficiencies (see below). The spreadsheet can be downloaded below. Each Appendix L sample is calculated at the end of this article.
Pressure Vessel Engineering Ltd. assumes no responsibility for this sheets use, and reminds you that only ASME can provide code interpretations. We would however be very happy if everyone could use the same interpretation to this difficult code section.
Update: Oct 2014
ASME has withdrawn the sample calculation Appendix L from ASME VIII-1 and released PTB-4-2013 ASME Section VIII – Division 1 Example Problem Manual as a replacement. The old Appendix L and the new PTB-4 disagree on when UW-11(a)(5)(b) has been met and the resulting weld efficiency. We are leaving the Weld Efficiency Notes posted as we and others have used the logic to design many vessels, but we are not longer using it going forward. The appropriate comments have been added to the document indicating what has changed. The weld efficiency calculator has been removed from this site.
Use of Type (3) welds in ASME Pressure Vessel Design
File: PVE-6022, Last Updated: March 23, 2012, By: LRB
Conclusion: Table UW-12 provides 8 types of welds with appropriate efficiencies to use when differing levels of radiography is applied. Confusion exists when trying to determine if a single welded circ weld in a small vessel is type 1 or type 3. Our answer based on experience and code interpretations is that it is a type (1) weld as long as the back side can be inspected; otherwise it becomes a type (3).
Definition of Type (1):
Joint Description: Butt joints as attained by double-welding or by other means which will obtain the same quality of deposited weld metal on the inside and outside weld surfaces to agree with the requirements of UW-35. Welds using metal backing strips which remain in place are excluded.
Efficiency: is 1, 0.85 or 0.7 depending upon the degree of radiography applied
Definition of Type (3):
Joint Description: Single-welded butt joint with- out use of backing strip
Restrictions: Circumferential butt joints only, not over 5/8″ thick and not over 24 inch outside diameter
Efficiency: is 0.60 only for no radiography
The confusion always comes from determining what is the equivalent of double welding? Can a joint be welded from one side only and still be considered to be the equivalent of double welding? ASME has a few of interpretations that directly address this issue:
Interpretation: VIII-1-83-220 Subject: Section VIII, Division 1; UW-12 Date Issued: February 22, 1984 File: BC83-557 Question (1): For vessels of small diameter, not accessible for welding from the inside, as well as for vessels of large diameter where welding from the inside is possible, it is proposed to weld both the longitudinal and circumferential seams with single side full penetration welds. TIG and SMAW or TIC; and SAW processes with argon backing for the root run will be used. May these be considered to be Type 1 joints in Table UW-12 of Section VIII, Division 1? Reply (1): Yes. Question (2): Will the degree of examination affect the determination of the type of joints? Reply (2): No. Interpretation: VIII-1-83-291 Subject: Section VIII, Division 1; UW-12 Date Issued: June 29, 1984 File: BC84-191 Question: For vessels of small diameter, not accessible for welding from inside, as well as for vessels of large diameters where welding from inside is possible, it is proposed to weld both longitudinal and circumferential seams with single side full penetration welds. GTAW, GMAW, SMAW, and SAW processes with fiberglass tape backing for the root run will be used. May these be considered to be Type No. (1) joints as described in Table UW-12 of Section VIII, Division 1? Reply: Yes. Interpretation: VIII-1-83-267 Subject: Section VIII, Division 1; Table UW-12, Joint Types Date Issued: May 31, 1984 File: BC84-090 Question: A circumferential joint of greater than 24 in. 0. D. is made with a single-welded full penetration butt weld. Is this a Type No. (1) joint as given in Table UW-12 of Section VIII, Division 1? Reply: Yes, provided the requirements in UW-35 and UW-37(d) are met.
So it is permissible to consider single sided welds as type 1. Is it also permissible to consider them as Type (3)?
Interpretation: VIII-1-92-138 Subject: Section VIII, Division 1 (1992 Edition, 1992 Addenda); Table UW-12 Date Issued: March 17, 1993 File: BC93-110 Questions: Under the limitation requirements for Type No. (3) joints given in Table UW-12 of Section VIII, Division 1, may the following be single welded and still be in compliance using only circumferential butt joints:
1) Weld a vessel that has a 3/8 in. thick wall and is 20 in. in diameter?
2) Weld a vessel that has a 3/8 in. thick wall and is 30 in. in diameter?
Replies: 1) No.
This interpretation prohibits the use of Type (3) joints regardless of the vessel size (over or under 24 inch in diameter). If Type (3) is not allowed, all that is left is Type (1). Personally I would have expected (1) above, the 20 inch diameter vessel to be allowed as a Type (3)…
Our Experience indicates that the use of Type (1) joints is acceptable as long as the back side of the weld can be visually inspected after welding. We have been asked to use Type (3) occasionally – primarily when the back sides of welds cannot be inspected. About once every few years we will be asked to change from a Type (1) to Type (3) weld for other reasons – which we will make as requested. These requests are not surprising as this section of the code book is a mess and very hard to understand or reach consensus on. The change from Type (3) to Type (1) usually does not affect the design of a vessel as the long seam efficiency normally governs the design thickness.