Technical BulletinDocument Number: TB-000021Category: FabricationSubject: RT Designators for Section VIII, Division 1Abstract: A discussion on applying the RT Designator rules of UG-116(e)Body: To apply the RT designation rules of UG-116(e) correctly, it is necessary to understand a

few definitions. Section VIII-1 uses joint Category labels to concisely describe the locations of weld joints in a pressure vessel.

CategoriesPer UW-3, a joint category describes the location of a weld joint in a vessel. Four categories (A, B, C, D) are used to describe the most common joints in a pressure vessel. (Note: Not every weld joint is assigned a category; for example, welds attaching jacket closure bars are not assigned a category.)

α

D

A BA

C

A C

B

Illustration of Weld Joint Categories

Category A: Longitudinal welded joints within main shells, heads, cones, flat plates, nozzles, and the attachment weld of a hemispherical head to a shell.

Category B: Circumferential welded joints within the main shell, cone, nozzles, and the attachment joint between formed heads (ellipsoidal and torispherical) and shell.

Category C: Welded joints connecting flanges, tubesheets, flat heads to main shell, to formed heads, to transitions in diameter, to nozzles, or any welded joint connecting one side plate to another side plate of a flat-sided vessel.

Category D: Welded joints connecting nozzles to main shells, spheres, formed heads, flat heads, flat-sided vessels.

Butt Joint - UW-3(b). A butt joint is any joint in which the angle a is 30° or less. Joints with angles greater than this are not subject to radiography under the rules of Section VIII, Division 1.

RT-1 - Fully Radiographed VesselPer UG-116(e)(1), a vessel is marked "RT-1" when all pressure retaining butt welds, other than Category B and C butt welds associated with nozzles and communicating chambers that neither exceed NPS10 nor 1 1/8 in. wall thickness, have been radiographically

examined for their full length per UW-51.

RT-1 Examples

NPS 4

NPS 20

FULL RT

Fig. (1)

NPS 4

FULL RT

Flat Head

Corner Joint- No RT

Fig. (2)

The radiography and joint efficiency rules only apply to butt joints in which a < 30°; thus the corner joint attaching the flat head to the shell in Fig. (2) is not required to be examined by RT. The value of 30° is arbitrary; it represents an angle beyond which it becomes increasingly difficult to interpret radiographs of the joints. Section VIII-1 compensates for the lack of NDE of corner joints by building in a joint efficiency factor when sizing the welds.

RT-2 VesselsUnlike an ‘RT-1’ vessel in which all butt welds are required to be fully radiographed, radiographic examination in an ‘RT-2" vessel focuses on those joints most critical to the design of the vessel. An ‘RT-2’ vessel provides the best balance in terms of risk and design economy. Let’s see why that is.

UG-116(e)(2) defines "RT-2"as: when a complete vessel satisfies the requirements of UW-11(a)(5) and when the spot radiography requirements of UW-11(a)(5)(b) have been applied.

UW-2 requires full radiography for lethal service [UW-2(a)] and unfired steam boilers [UW-2(c)]. Absent these mandated examinations, current Section VIII-1 rules permit a Manufacturer to examine welds on a joint-by-joint basis. This feature is uniquely applied for ‘RT-2’ vessels.

Referring to UW-11(a)(5), this paragraph requires that all Category A and D butt welds in vessel sections and heads must be fully radiographed when designed with a joint efficiency from column (a) of Table UW-12 [column (a) is associated with full radiography]. Two additional conditions must be satisfied:

1.UW-11(a)(5)(a) – all Category A and B welds connecting the vessel sections or heads shall be Type No. (1) or (2) of Table UW-12.

2.UW-11(a)(5)(b) – all Category B or C butt welds which intersect the Category A butt welds in vessels sections or heads or connect seamless vessel sections or heads, shall as a minimum, meet the requirements for spot radiography in accordance with UW-52.

Simplifying the above rules, in an ‘RT-2’ vessel, only the Category A welds in shells and heads are fully radiographed and the intersecting Category B or C welds are spot radiographed. But the vessel sections and heads are designed using the joint efficiency associated with full radiography. Let’s look at a couple of examples:

NPS 4RT-1

Full RT

SPOT RT

NPS 4RT-2

seamless shell &heads

In this first example involving a seamless shell with two seamless heads, the ‘RT-2’ vessel only requires spot RT on the two Category B seams. However, if one welder welds these two seams and they are equal to or less than 50’, then only one spot RT will be required. [see UW-52(b)(1)]. The two vessels are identical in thickness since in both cases a joint efficiency equal to 1.0 will be used in the shell hoop stress calculation and the head calculations. The shell axial stress calculations do not control the design even though the joint efficiency E is equal to 0.7 for the RT-2 vessel. [assume Type 1 joints for the girth seams].

Both vessels calculate to identical thicknesswhen designed for internal pressure only.

NPS 20

FULL RT

RT-1

NPS 20

FULL RT

SPOT RT

RT-2

The same is true in this second example, since the long seams have been subjected to the same level of radiography. In a complex vessel, the savings due to reduced radiography based on ‘RT-2’ vs. ‘RT-1’ can be more dramatic. As in the first example, the number of spot RT’s can be reduced depending on the number of weld increments [see UW-52(b)(1)] in the vessel.

Some additional facts about UW -11(a)(5)(b) –

1.To stamp a vessel ‘RT-2’, the rules stated above must be applied to the entire vessel.

2.The spot x-ray taken to satisfy UW-11(a)(5)(b) is not at all related to spot RT’s required by UW-11(b).

3.The spot x-ray taken per UW-11(a)(5)(b) is sometimes referred to as the "quality shot". Looking back at the first vessel fabricated from seamless components, the stresses acting on the Category B welds do not control the design for an internal pressure only load case. The spot RT taken of these seams is a statistical "quality check" of the ability of the welder to deposit sound weld metal.

‘RT-3’ VesselsAn ‘RT-3’ vessel is one in which the entire vessel satisfies the spot radiography requirements of UW-52. The only exception is for any Category B & C butt welds in small nozzles and communicating chambers that neither exceed NPS10 nor 1 1/8" in thickness. Furthermore, spot radiographs to satisfy the requirements of UW-11(a)(5)(b) cannot be applied to any other increment.

In essence, spot radiography is a statistical check of a welder’s ability to deposit sound

weld metal. Per UW-52 (b)(1), only 1% (6 in. per 50 ft. of weld) of a welders work need beradiographed. The location of the spot radiograph is to be chosen by the AI, however when agreed to in advance, the fabricator may select the location of the radiograph. Each 50 ft. of weld placed by a welder is defined as a weld increment. A weld increment can be made up of different joint types [Type 1 or 2], different weld processes [SAW, GTAW, SMAW,…] and extend across multiple vessels.

Ex. (1) All welds made by same welder and totalless than 50 ft. - 1 Spot RT Required

NPS 4

RT-3 Examples

Design Efficiencies [All Type 1 Joints]:Shell: Longitudinal Seam - E = 0.85

Circumferential Seams - E = 0.85Heads: Seamless, however UW-11(a)(5)(b)

not satisfied; E = 0.85

NPS 20

SPOT RT

Ex. (2) Assume 3 weld increments requiring 3 Spot RT's

Spot RT

The two sketches show a couple of typical examples of Spot RT’d vessels meeting a RT-3 marking requirement. One item to note is that the seamless heads are designed with a joint efficiency of 0.85, since the additional requirements of UW-12(d) and UW-11(a)(5)(b) have not been met. If an additional spot radiograph is taken to satisfy UW-11(a)(5)(b) and hence design the heads with an E=1.0, then this would be a mixture of RT-2 and RT-3 rules, and the correct marking would be RT-4. More on that later. To summarize, spot radiography provides a reasonable alternative to either full or no radiography conditions. Although there is a reduction in the design efficiency from that used for full RT, often the savings in examination cost offset the increase in material cost. It is interesting to note, that other foreign pressure vessel standards also contain spot examination options, but unlike ASME, they typically require from 10-20% of the weld to be examined

‘RT-4’ Vessels.The purpose of the ‘RT’ marking system is to indicate that radiographic examination of the vessel welds was performed, and to what extent. The simplest way to describe ‘RT-4’ is to state that as a minimum, one or more welds was RT’d, but the extent of RT did not

satisfy an ‘RT-1, 2, or 3’ condition. Put another way, ‘RT-4’ means some radiographicexamination took place, but one cannot describe the amount or location with a simple numbering system.

One common application of RT-4 is the vessel shown in Ex. (2) above. As depicted this is an RT-3 vessel. However, if an additional spot RT is taken to satisfy UW-12 (d) and UW-11(a)(5)(b), thus allowing the seamless heads to be designed with an E=1.0, then it becomes RT-4. Why? Because there is a mixture of the RT-2 and RT-3 rules. This situation is further clarified in the following code interpretation:

Interpretation: VIII-1-89-210RSubject: Section VIII, Division 1 (1989 Edition), UG-116(e) and UW-12 (d)Date Issued: June 17, 1991File: BC90-418*

Question: Which of the marking requirements under UG-116 (e) apply for a vessel that consists of seamless ellipsoidal or torispherical heads when the vessel joints are spot radiographed and after the circumferential seams attaching the heads are separately spot radiographed per UW-11(a)(5)(b)?

Reply: RT-4.

The vertical tower shown below would be another example of an RT-4 vessel, where the lower shell course of the tower is fully radiographed, while the remainder of the vessel is spot RT’d per UW-11(b) and UW-11 (a)(5)(b).

Spot RT

Full RT

Spot RT

RT-4 Example

f

SUMMARYASME Section VIII, Division 1 provides great flexibility to the vessel manufacturer in the way of balancing radiographic examination of welds against design and fabrication requirements. Once vessel fabrication is completed, the RT marking system of UG-116 (e) provides a shorthand system for identifying that radiographic examination was performed, and to what extent.

References: Section VIII, Division 1Links:

Author: John SwezyDate: 11/18/2003 10:14:16 AMLast Modified: Alex Garbolevsky on 09/28/2007 08:21:00 AM

Approved: Sandy Babka

Approval Date: 09/28/2007 08:20:49 AM

Expires: