API 510 Chapter 10

API 510 Chapter 10 – ASME VIII Welding and NDE

10.1 Introduction

This chapter is to familiarize you with the general welding approach contained in sections UW of ASME VIII. API 510 is for in-service inspection of vessels and therefore most welding carried out will be repair welds, rather than welds carried out on new systems. API 510 also makes it a mandatory requirement to comply with the welding rules contained in ASME VIII.

Take a quick look at the scope of sections UW-1 through to UW-65 of the code (which are all of the UW sections). Not all the numbers run consecutively; some are missing. Note the following:

  • Section UW is not just about welding; there are design and NDE-related subjects in there as well.
  • Coverage of welding processes really only starts properly at section UW-27. Before that, the content is more about welded joints themselves, rather than the processes used to weld them.

10.2 Sections UW-1 to UW-5: about joint design

You can think of these sections as an introduction to section UW (don’t ask where UW-4 has gone). Fundamentally, they are about joint design rather than welding techniques but you need them for background information. The best way to understand these is not to read them in the order presented in the code. Start with UW-3 (and its UW-3 figure) and then move to UW-2, which explains the restrictions placed on these joint categories by four special categories of vessel service.

10.2.1 UW-3: welded joint categories

The background to these joint categories is that the ASME design codes (unlike some other vessel codes) are built around the idea of a joint efficiency factor denoted by the symbol E. The factor E appears in the internal pressure equations and depends on:

  • The method of welding
  • The amount of NDE carried out on the weld

Various categories of joints are identified, which (as we will see later) are given different joint efficiencies.

ASME VIII pressure vessel welded joints are given a letter designation A, B, C or D depending on their location in the vessel. The designations are described in section UW-3 and illustrated on the page afterwards in figure UW-3. Note how this figure contains all the practical weld joint types that are found in standard types of pressure vessels. The most critical welds are those classified as category A, as these are the ones that require the most NDE. The content of ASME VIII figure UW-3 is shown in Fig 10.1. Note these points about it shown in the annotations:

Cat A includes all longitudinal welds and critical circumferential welds such as hemispherical head to shell welds.

Cat B includes most circumferential welded joints including formed heads (other than hemispherical) to main shells welds.

Cat C includes welded joints connecting:

  • Flanges to nozzles or shell components.
  • One side plate to another in a flat-sided vessel.

Cat D includes welded joints connecting nozzles to shells, heads or flat-sided vessels.

Note one specific point identified in figure UW-3: a Cat B angled butt weld connecting a transition in diameter (i.e. tapered section) to a cylinder is included as a special requirement provided the angle (see figure UW-3) does not

Figure 10.1 ASME VIII weld categories (courtesy ASME)
Figure 10.1 ASME VIII weld categories (courtesy ASME)

exceed 30°. All the requirements of a butt-welded joint are applied to this angle joint.

This figure UW-3 is of limited use on its own. Its purpose is mainly to link with section UW-12: joint efficiencies. This section, with its accompanying table UW-12, allows you to determine the joint efficiency to use for a weld, as long as you know the category of weld (A–D), the weld joint arrangement (single or double groove, etc.) and the extent of NDE that has been carried out. We will see how to use this soon.

10.2.2 UW-2: service restrictions

Stepping back one section, UW-2 gives guidance on which types of pressure vessels/parts have restrictions on what type of weld should be used for each joint category. The four types of vessels referenced are:

(a) Vessels for lethal service (containing a lethal substance)

(b) Low-temperature vessels that require impact testing

(c) Unfired steam boilers

(d) Direct-fired vessels

These are referenced a, b, c, d in the code. Unfortunately, the page containing these is formatted using an impenetrable hierarchy of subheadings, making the explanations some what difficult to follow.

Most of the ‘meat’ in this section resides in the subsection covering the first category of vessel identified: (a) vessels for lethal service. The other three categories are more or less based on this first one, with some selected changes. Look at the requirement for lethal service and highlight the following key points:

  • Butt-welded joints must be fully radiographed.
  • Carbon or low alloy steel vessels need PWHT.
  • Cat A joints need to be type 1(double vee or equivalent) welds (the types are given in table UW-12).
  • Cat B joints can be either type 1 or type 2 (single vee with backing strip)
  • Cat D must be full penetration welds.

There is a lot in this section about Cat C ‘lap joint stub end’ welds. These are an alternative to forged weld-neck flanges and not particularly common (even though there is a large section on them) except (presumably) on ASME VIII vessels for lethal fluid service.

Note how the (b) category, covering low-temperature vessels, has very similar requirements to the lethal service category, one slight difference being that Cat C welds have to be full penetration. The other two types of vessels, (c) unfired steam boilers and (d) direct-fired pressure vessels, are, again, similar, but with a few changes.

Remember the key point again: 

Most of the ‘meat’ in section UW-2 resides in subsection (a) vessels for lethal service. The other three categories are based on this first one, with some selected changes.

10.3 UW-12: joint efficiencies

UW-12 (text and table) are a core part of the welding requirements of ASME VIII. It is set out as shown in Fig. 10.2. Table UW-12 covers all types of gas and arc welding processes and spreads over two pages. Look at this table in your code and the notes 1–7 at the bottom of the second page of the table. These are often used as the subject matter for open-book examination questions in which table UW-12 is involved.

Now look at the body of table UW-12. It contains the following information:

  • The weld type number – these range from types 1 to 8
  • The joint description for each weld type
  • Any limitations associated with a weld type
  • The joint category (Cat A to D as already seen in UW-3)
  • The degree of RT carried out, subdivided into three levels as follows:
  • Column (a) – full radiography
Figure 10.2 The format of ASME VIII table UW-12
Figure 10.2 The format of ASME VIII table UW-12
  • Column (b) – ‘spot’ radiography
  • Column (c) – no radiography

These RT categories have their origin in section UG-116

Figure 10.2 shows where this is heading. Now look in more detail at ASME VIII table UW-12 and note the following key points:

Look how joint efficiency E depends only on the type of joint and the extent of RT examination carried out on it. The three columns a, b and c cover the situation for conditions of full, ‘spot’ and no RT.

The text is divided into six subsections (a to f) describing a variety of individual cases. You can think of this as elaborating on the content of table UW-12. Look through sections d, e and f in the code and note the points indicated below:

Item (d): seamless (forged) sections of vessels This is a special technical case, but a common area for examination questions. Note what the code says:

(d) Seamless vessel sections are considered equivalent to welded vessel sections of the same geometry in which all Cat A welds are type 1. This applies to both head and shell sections.

Although (by definition) seamless shells or heads have no seams, there is still the need to decide a joint efficiency to use in the pressure design calculations. It is best not to think too deeply about this or you might think it doesn’t quite make sense. ASME VIII obviously thinks it does. The E value to use is set out as follows:

  • E = 1.0 when ‘spot’ RT has been done.
  • E = 0.85 when ‘spot’ RT has not been done or when Cat

A/B welds joining seamless sections together are of type 3, 4, 5 or 6.

Item (e): welded pipe or tubing Following the same impeccable logic as for vessels, welded pipe or tubing is also treated in the same manner as seamless, but with the allowable tensile stress taken from the appropriate ‘welded product’ values in the material stress tables. The requirements of UW-12 (d) are applied as before.

10.4 UW-11: RT and UT examinations

Note the following principles of ASME VIII (UW-11).

10.4.1 RT levels

The three levels of RT are:

  • Full RT (100 % of weld length of code identified welds) .
  • Spot’ RT (a sample of weld length – minimum 6 inches)
  • No RT (radiography not required at all)

The simple principle is that critical welds (those with a high risk of failure due to high stresses) will generally require full radiography to determine whether defects are present that could lead to failure. Welds that are less critical or less likely to fail if they contain a defect may not require full RT but will still require ‘spot’ RT. Joints that are not under internal pressure/high loads are less likely to fail and do not require any RT at all.

10.4.2 Minimum specified RT/UT requirements

This is the most important part of UW-11 (see Fig. 10.3). It gives the six situations where full RT is mandatory under UW-11 (a):

1. All butt welds in the shell and heads of vessels containing lethal substances.

2. All butt welds in vessels over 1 1/2 in (38 mm) thick, or exceeding the thicknesses prescribed in table UCS-57 (have a quick look forward to this table). Note the exemption from this: Cat B/C butt welds in nozzles and communicating chambers ≤ NPS 10 or ≤ 1 1/8 in (29 mm) wall thickness do not require RT. This is a GENERAL

Figure 10.3 Important principles of ASME VIII (UW-11)
Figure 10.3 Important principles of ASME VIII (UW-11)

EXEMPTION FOR SMALL NOZZLES. THESE REQUIRE NO RT AT ALL (so effectively escape ‘under the radar’ from the RT requirements of the various categories RT1, RT2, etc.).

3. All butt welds in the shell/heads of unfired steam boilers exceeding 50 psi (345 kPa).

4. This one covers nozzles. Full RT is required for all butt welds in nozzles, etc., attached to vessel sections or heads that require full RT under (1) or (3) above.

UW-11 (b): ‘spot’ RT

This says that you may use ‘spot’ RT (and use a lower joint efficiency E ) instead of full RT on type 1 or 2 welds.

UW-11 (c): ‘no’ RT As a principle, no RT is required when the vessel or vessel part is designed for external pressure only, or when the joint design complies with UW-12 (c). Sections (d) to (f) cover a few additional UT requirements when specialist welding techniques are used (electrogas, electron beam, etc.).

Note this important point, hidden away at the end in (g):

(g) For RT and UT of butt welds, nominal thickness is the thickness of the thinner of the two parts to be joined. This nominal thickness may be needed to determine if RT or UT is required.

10.5 UW-9: design of welded joints

Finally, we will look at UW-9: design of welded joints. This is more a design issue than a welding one, and there is less to this section than first appears. The main content relates to two areas:

  • Taper transitions between welded sections of unequal thickness
  • Stagger’ of longitudinal welds in vessels

Don’t expect to have to consult detailed figures of weld joints in this section. There is only one figure UW-9, showing the requirement for tapers. Look first at this figure and notice the main points in its descriptive text UW-9 (c):

(c) Tapered transitions requires that tapered transitions must have a taper of at least 3:1 between sections if the sections differ by the smaller of:

  • More than 1/4 of the thickness of the thinner section or .
  • 1/8 in (3.2 mm).

Now move to UW-9 (d). This requires that longitudinal joints between courses must be staggered by at least five times the thickness of the thicker plate unless 4 inches (100 mm) of the joints on either side of the circumferential joint is radiographed (probably unlikely). Note the requirement hidden in the body of the text referencing UW-42. It means that if the taper is formed by weld build-up the additional metal must be examined by PT/MT.

The two points above appear very frequently as API 510 exam questions. Now try these familiarization questions.

10.6 ASME VIII section UW-11 familiarization questions (set 1)


Q1. ASME VIII section UW-11 (a)
Which of the following may not require full RT?



Q2. ASME VIII section UW-11 (a)
‘Full RT’ under ASME VIII means a vessel must have:



Q3. ASME VIII section UW-11(a)
A vessel is manufactured from P4 Group 2 material. It has a shell thickness of 18 mm and is used to contain lethal substances. What RT is required for a shell-to-shell circumferential butt weld?



Q4. ASME VIII section UW-11 (a)
A vessel is manufactured from P1 Group 2 material. It has a shell thickness of 3/4 in (19 mm) and does not contain a lethal substance. What RT is required for a shell longitudinal butt weld?



Q5. ASME VIII section UW-11 (a)(6) and UW-11 (d)
A 2 in (50 mm) thick P1 Group 1 material is welded using the Electrogas welding process to give a full-penetration groove weld in a vessel in non-lethal service. What are the NDE requirements?


10.7 Welding requirements of ASME VIII section UW-16

(a) UW-16 minimum requirements for attachment welds at openings

UW-16 deals with the configuration and size of vessel nozzles and attachments welded into vessels. It gives the location and minimum size of attachment welds and must be used in conjunction with the strength calculations required in UW 15. Note that weld strength calculations are not included in the API 510 syllabus. Note also that the terms nozzles, necks, fittings, pads, etc., mean almost the same thing. This is a fairly complex section about a fairly simple subject, and can be a bit tricky. Have a look at figure UW-16 spread over a few pages of ASME VIII. It produces a few open-book exam questions occasionally, but doesn’t seem to be mainstream content.

(b) Symbols

This paragraph defines the symbols used in UW-16 and in figures UW-16.1 and UW-16.2. You will need to recognize these symbols in order to understand figure UW-16.1, the one with the most important content for exam purposes. The main ones are:

  • t = nominal thickness of vessel shell or head .
  • tn = nominal thickness of nozzle wall

te = thickness of reinforcing plate .

tw = dimension of attachment welds (fillet, single-bevel or single-J), measured as shown in figure UW-16.1 .

tmin = the smaller of 3/4 in (19 mm) or the thickness of the thinner of the parts joined by a fillet, single-bevel or single J weld .

tc = not less than the smaller of 1/4 in (6 mm) or 0.7 tmin.

t1 or t2 = not less than the smaller of 1/4 in (6 mm) or 0.7tmin.

Don’t be put off by these definitions, which look a little complicated. If you have difficulty differentiating between tc, t1, t2 and tw, review them in conjunction with figure UW-16.1 itself.

Paragraphs (c) and (d) cover the two main options for connecting nozzles to shells using Cat D welds.

(c) Necks attached by a full penetration weld Paragraph (c) basically tells us that:

  • A set-on nozzle will have full penetration through the nozzle wall.
  • A set-in nozzle will have penetration through the vessel wall.

Examples of each are then given in sketches UW-16 (a) to UW-16 (e).

To ensure complete weld penetration, backing strips or similar must be used when welding from one side without any method of inspecting the internal root surface.

A nozzle requires a hole to be cut in the shell producing a weakened area that may require strengthening. This strengthening can be added in the following ways:

1. By integral reinforcement (also known as self-reinforcement). This consists of using a thicker shell and/or nozzle, forged inserts or weld build-up, which is integral to the shell or nozzle. Figure UW-16.1 sketches (a), (b), (c), (d), (e), (f-1), (f-2), (f-3), (f-4), (g), (x-1), (y-1) and (z-1) show examples.


2. By adding separate reinforcement pads. (These are also termed compensation pads). They are welded to the outside and/or inside surface of the shell wall to increase the thickness in the weakened area. Figure UW-16.1 sketches (a-1), (a-2) and (a-3) give examples of compensated nozzles.

Note the different ways of welding the reinforcement pads to the shell:

At the outer edge of the pad by a fillet weld, and either:

  • where it meets a set-on nozzle, by a full penetration butt weld plus a fillet weld with minimum throat dimension tw 5= 0.7tmin or .
  • where it meets a set-in nozzle, by a fillet weld with minimum throat dimension tw = 0.7tmin(figure UW16.1 sketch (h)).

At the outer and inner edge of the pad by a fillet weld if it does not meet the nozzle. The fillet weld will have a minimum throat dimension of 1/2 tmin. See figure UW-16.1 sketch (a-2) for an example of a fillet welded attachment.

Now try these familiarization questions.

10.8 ASME VIII section UW-16 familiarization questions (set 2)


Q1. ASME VIII section UW-16 (c) and sketches (a), (b).Necks attached by a full penetration weld
A nozzle is fitted abutting (i.e. set-on) the vessel wall. What is an acceptable method of attaching it?



Q2. ASME VIII section UW-16 (c)
By what means can reinforcement be added to an opening in a pressure vessel?



Q3. ASME VIII section UW-16 (c) figure 16.1(a)
A vessel is manufactured from P1 Grade 2 material. It has a shell thickness of 18 mm and is used to contain lethal substances. A set-on (abutting) nozzle of 12 mm thickness is attached using a category D full penetration weld with a reinforcing fillet.

What is the minimum required throat thickness of the reinforcing fillet weld?



Q4. ASME VIII section UW-16 (c)(2)(c)
A vessel has a shell thickness of 3/4 in (19 mm). A set-on (abutting) nozzle of 1/2 in (13 mm) thickness is attached using a category D full penetration weld. A reinforcing plate of 1/4 in (6 mm) is required.

What welds will be required to attach the reinforcing plate to the nozzle?



Q5. ASME VIII section UW-16 (d)(1)
A nozzle of NPS 10 (DN 250) is inserted through a vessel wall and protrudes into the vessel by an amount equal to the nozzle thickness. The nozzle thickness is one-half of the shell thickness.
Which of the following weld combinations are acceptable to attach the nozzle?


10.9 RT requirements of ASME VIII sections UW-51 and UW-52

Remember API 510? We saw some very general requirements for the NDE of repair welds, using the same principles to those for welds carried out on new systems. This made it a mandatory requirement to comply with the welding rules contained in ASME VIII. We have also seen that ASME VIII contains various requirements for RT, spread around several sections of the code. These included the RT ‘marking’ categories of UW-11 (RT1, RT2, etc.) and the joint efficiencies that result from the choice of RT scope, set out in UW-12. This worked on the general principle of ASME VIII of being able to choose the RT category to follow (within limits), as long as you are happy to live with the joint efficiency that results.

We will now look at some further RT requirements of ASME VIII as set out in sections UW-51 and UW-52. As with all parts of the ASME code, you will find the inevitable cross-references to other code sections, but they are not as extensive here as in some other parts of the code. Have a look at Fig. 10.4; this shows a summary of the referenced sections relating to RT.

Before progressing further with UW-51/UW-52 bear in mind the existence of table UCS-57: radiographic examination (see Fig. 10.5). This table is very important as it gives the nominal wall thickness above which it is mandatory to fully RT butt-welded joints. The content of UW-51 and UW-52 must therefore be seen against the background of these mandatory requirements.

Figure 10.4 A summary of ASME VIII RT
Figure 10.4 A summary of ASME VIII RT

10.9.1 UW-51: ‘full’ RT examination of welded joints

UW-51 and UW-52 are complementary sections. UW-51 deals with ‘full RT’ situations and UW-52 deals with those applicable to ‘spot’ RT.

Starting with UW-51 (a), this specifies that radiographed joints have to be examined in accordance with article 2 of section V. These are well defined and covered in the ASME V chapter of this book. There are a few differences that take

Figure 10.5 RT requirements of UCS-57. Courtesy of ASME
Figure 10.5 RT requirements of UCS-57. Courtesy of ASME

precedence over section V but they are mainly procedural (i.e. documentation/record-related). The main thrust of these is as follows:

  • The manufacturer must retain a complete set of radiographs and records for each vessel until the Inspector has signed the Manufacturer’s Data Report
  • The manufacturer must certify that only qualified and certified radiographers and radiographic interpreters are used
  • Radiographs will only be acceptable if the specified IQI hole or wire is visible

UW-51 (b) specifies conditions under which imperfections (‘indications’) are not acceptable and actions are to be taken. Note the following ‘principle’ point about the use of UT instead of RT:

Unacceptable imperfections must be repaired and reradiographed. The Manufacturer can specify UT instead of RT, providing the original defect has been confirmed by UT to the satisfaction of the Authorized Inspector prior to making the repair. For material > 1 in (25 mm) the User must also agree to its use. This UT examination must be noted under remarks on the Manufacturer’s Data Report Form.

Note: Historically, the ASME code has been built on the premise of using RT as the main volumetric NDE method, but in recent years has started to accept UT as a viable alternative. In reality, however, RT still forms the basis of the ASME code’s approach to integrity and it will probably take many years for this to change.

Defect acceptance criteria

ASME VIII, unlike some codes, does provide information on weld defect acceptance criteria. Note how these are slightly different for the ‘full’ and ‘spot’ RT scenarios. For ‘full’ RT, the following imperfections are unacceptable:

  • Cracks, or incomplete fusion or penetration
  • Elongated indications with lengths greater than the following
  • 1/4 in (6 mm) for t up to 3/4 in (19 mm)
  • 1/3t for t from 3 4 in (19 mm to 21 4 in (57 mm)
  • 3/4 in (19 mm) for t over 2 1/4 in (57 mm


t = the thickness of the weld excluding reinforcement

For a butt weld joining two members having different thicknesses at the weld, t is the thinner of these two thicknesses. If a full penetration weld includes a fillet weld, the thickness of the throat of the fillet must also be included in t.

This section also concentrates on acceptance criteria for aligned indications. The following conditions are cause for rejection:

A group of aligned indications with an aggregate length greater than t in a length of 12t unless the distance between the successive imperfections exceeds 6L, where L is the length of the longest imperfection in the group.

Rounded indications in excess of those given in ASME VIII appendix 4. Paragraph (c) gives examination requirements for real-time radioscopic examination. This is not a mainstream NDE technique in the pressure vessel industry so is unlikely to feature in the examination. Ignore it.

10.9.2 UW-52: ‘spot’ RT of welded joints

UW-52 begins with a note explaining the benefits and shortcomings of spot radiography. It basically points out that spot RT is useful for monitoring weld quality but can miss areas with weld defects present. If a weld must not have any defects in it then 100 % RT must be carried out.

Figure 10.6 shows the minimum extent of spot RT as specified by UW-52 (b). Look at these examples on its interpretation (it is fairly straightforward once you’ve got the idea):

  • A single vessel with 55 ft of weld will have two spots examined, one spot for the 50 ft and one spot for the remaining 5 ft.
  • Two identical vessels have 20 ft of weld each. This gives a total of 40 ft and therefore only one spot needs to be taken on one of the vessels.
  • Two identical vessels have 40 ft of weld each. This gives a total of 80 ft and therefore two spots need to be taken (one for the 50 ft and one for the remaining 30 ft). In this case one spot would be taken on each vessel.
  • Three identical vessels have 15 ft of weld each. This gives a total of 45 ft and therefore only one spot needs to be taken on one of the vessels.
  • There are also some more general points on choosing the number and location of the spots.

UW-52 (c) gives acceptance criteria for spot RT. Note how they differ slightly from those in UW-52 for ‘full’ RT. The main points are as follows:

Figure 10.6 ASME VIII (UW-52) spot RT
Figure 10.6 ASME VIII (UW-52) spot RT
  • Cracks or zones of incomplete fusion or penetration are unacceptable.
  • Slag inclusions or cavities with length > 2/3t are unacceptable (the value of t is given in UW-52 (c)(2), which also gives limits on multiple inclusions or cavities <2/3t).
  • Rounded indications need not be considered. (This is an important point . . . these are only relevant when a weld needs full RT.)

Re-test of rejected welds UW-52 (d) deals with re-tests when spot radiographs have failed their acceptance criteria. This uses the simple 2 for 1 principle, similar to that used in other codes (see Fig .10.7). Note the following main points in UW-52 (d):

1. When a spot radiograph is acceptable then the entire weld increment represented by it is acceptable.

2. When a spot radiograph shows a defect that is not acceptable then two additional spots must be examined in the same weld increment at locations chosen by the inspector:

If the two additional spots examined are acceptable the entire weld increment is acceptable provided the defect disclosed by the first radiograph is removed and the area repaired by welding. The weld-repaired area must then be radiographed again. .

If either of the two additional spots examined are unacceptable then the entire increment of weld represented must be rejected and either:

  • Replace the entire weld or
  • Full RT the weld and correct any defects found.

Repair welding must be performed using a qualified procedure and in a manner acceptable to the Inspector. The re-welded joint, or the weld-repaired areas, must then be spot RT examined at one location in accordance with the foregoing requirements of UW-52.

The 2 to 1 Replacement rule

Figure 10.7 RT re-tests
Figure 10.7 RT re-tests


Q1. ASME VIII section UW-51 (a)(4)
In ASME VIII Division 1 the final acceptance of the quality of a radiograph will be based on?



Q2. ASME VIII section UW-51 (a)(1)
When can a manufacturer destroy the radiographs relating to a vessel in lethal service?



Q3. ASME VIII section UW-51 (b)(4)
Full RT has been carried out on a vessel marked as RT-1. A 1 in (25 mm) thick butt weld shows rounded indications. The required action will be to:



Q4. ASME VIII section UW-52 (c)
What is the minimum length of a spot radiograph?



Q5. ASME VIII section UW-52 (d)(2)
A spot radiograph fails the acceptance criteria.
What should the API inspector request?


Click Here To Read Next API 510 Exam Chapter 11- ASME VIII and API 510 Heat Treatment


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