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

Sketch of long seam welded from one side

A long seam welded from one side with no backing strip – always Type 1

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

Sketch of circ seam

A circ seam on a small vessel welded from one side only with no backing strip – is it Type (1) or Type (3) ?

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.
2) 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.