ASME Pressure Vessel Joint Efficiencies

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ASME Pressure Vessel Joint Efficiencies The ASME Pressure Vessel Joint Efficiencies article provides you with information about pressure vessel joint efficiency requirements and their connection with radiography testing. You may know Pressure Vessel Joint Efficiencies are linked to the radiography testing grades and there is a concession for full radiography testing as per the UW-11(a) (5) (b) clause which it is a little bit confusing. This article provides you the ASME pressure vessel joint efficiencies requirements and guidelines for the above clause. Based on the ASME Code requirement, manufacturers have to mark the type of RT i.e. RT1, RT2, RT3 and RT4 in the pressure vessel name plate and state the same in Pressure Vessel Data Report.

We have seen many professionals, from inspectors to quality control engineers who are confused between RT1 and RT2, specifically when they see ASME Pressure Vessel Joint Efficiencies for both RT1 and RT2 is the same and equal to 1(E=1). They say both RT1 and RT2 are categorized in the “Full Radiography” part in UW-11 clause ... So why are some joints in RT2 radiographed in spots? We are making spot radiography, but it is categorized in full radiography!!!

So in this "ASME Pressure Vessel Joint Efficiencies" article we want to answer this question in very simple way, but before this, we need review joint categories and summarize them as below: Category A:

 All longitudinal welds in shell and nozzles  All welds in heads, Hemisph-head to shell weld joint Category B:

 All circumferential welds in shell and nozzles  Head to shell joint (other than Hemisph.) Category C and D are flange welds and nozzle attachment welds respectively Longitudinal welds (Category A) are more critical than Circumferential welds (Category B) because they are under double stress. This the reason why in different part of ASME code we have stringent rules in category A joint compared to category B joint. See the following Fig. for joint categories:

Now let's get back to the ASME Pressure Vessel Joint Efficiencies subject, to remove the above confusion about RT1 and RT2. We need to know: When and where is there a code requirement for full radiography? Item 1: All butt welds in vessels used to contain a lethal substance (UW11(a)).Lethal substances have specific definitions in ASME Code in UW-2 and it

is the responsibility of the end user to determine if they ordered a vessel that contains lethal substances. Item 2: All butt welds in vessels in which the nominal thickness exceeds specified values (UW-11(a). You can find these values in subsection C, in UCS57, UNF-57, etc. For example, this value for P-No.1 in UCS-57 is 1 ¼ inch. Item 3: All butt welds in an unfired steam boiler with design pressure > 50 psi (UW-11(a)). Item 4: All category A and D butt welds in vessel when “Full Radiography” optionally selected from table UW-12(column (a) in this table is selected); and categories B and C which intersect Category A shall meet the spot radiography requirement (UW-11(a) (5) (b)). The point is this: items 1, 2 and 3 are similar, but item 4 is completely different. In items 1, 2 and 3 it is mandated by code; to do full radiography in all butt welds in vessel so it means it is mandatory for designer to select column (a) in UW-12 table. But in item 4, there is no mandating rule. A manufacturer with its own decision has chosen to use column (a) in table UW-12 for full radiography. So here there is a concession or bonus to manufacturers for categories B and C.

What is concept behind this concession or bonus in pressure vessel RT test? If you review item 1, 2 and 3 one more time, you will see that the pressure vessel RT tests are related to the type of welds and services. You can see the pressure vessels in these items are critical from a safety point of view, one contains a lethal substance, the other one has a high thickness, which implicates high pressure, and the last one is an unfired steam boiler. But item 4 has no criticality like the other items have. But you should note all 4 items have been categorized in full radiography clause( U-11(a)), so to differentiate item 1, 2 and 3 from item 4, the RT symbols are used in Code (UG-116). RT 1: Items 1, 2 and 3, (E=1), All butt welds-full length radiography RT 2: Item 4 (E=1), Category A and D butt welds full length radiography and category B and C butt welds spot Radiography

RT 3: (E=0.85), Spot radiography butt welds RT 4: (E=0.7), Partial / No radiography You need to consider the hemispherical head joint to shell as category A, but ellipsoidal and torispherical head joint to shell as category B; Do you know why? Why ASME considered the stringent rule for pressure vessel RT test in hemispherical head joint? It is because this joint is more critical, because the thickness obtained from the formula for hemispherical head approximately would be half of the shell thickness; It means if the shell thickness is 1 inch, the hemispherical head thickness would be 0.5 inch. For more detail, you may review the Pressure Vessel Heads article. ASME Pressure Vessel Joint Efficiencies for welded Heads For Welded Heads, the joint efficiency of the vessel will be 1(E=1), if all welds within the head's full length are radiographed (since they are all Cat. A welds). See above figure. ASME Pressure Vessel Joint Efficiencies for Seamless Heads For seamless heads, the joint efficiency of the vessel will be 1(E=1) if the head to shell weld is fully radiographed for the hemispherical Head (Cat A); See the following Figure for RT types:

Spot radiographed for ellipsoidal and torispherical heads(Cat. B).

Weld Types: Here is some clarification about the different type of welds that have specific definitions in ASME Code SEC VIII DIV 1 and related to the pressure vessel RT test. The concept is to define the different types and then introduce some restriction for using them. For example, a Type 1 weld is defined as a full penetration weld, typically double welded and Type 2 is welds with backing strips. So when you go to service restriction for a vessel containing a lethal substance, you see there is a restriction there that says all category A joints shall be weld Type 1 and Category B and C shall be type 1 or type 2. You should take this point in to account, which is this: the same joint category with different weld types have different joint efficiencies. Summary of weld types: Type 1: Full penetration welds (Typically Double welded) Type 2: Welds with backing strip Type 3: Single welded partial penetration welds Type 4, 5 and 6: Various Lap welds (rarely used)

Pressure Vessel RT Test The Pressure Vessel RT Test article provides you with information about Radiography testing in the pressure vessel manufacturing process and related items in pressure vessel inspection. Do you know what your pressure vessel RT test requirements are? Is full radiography mandatory for your vessel? When is full radiography mandatory? What are the acceptance criteria? What are the RT symbols? So if you need this information, this article answers all of these questions. We recommend that you to review this article in conjunction with the ASME Pressure Vessel Joint Efficiencies article.

Before going into the RT test, we need to know about joint categories. These categories are based on ASME Code Section VIII:

Category A:

 All longitudinal welds in shell and nozzles.  All welds in heads, Hemisph-head to shell weld joint Category B:

 All circumferential welds in shell and nozzles  Head to shell joint (other than Hemisph.) Category C and D are flange welds and nozzle attachment welds respectively Longitudinal weld (Category A) is more critical because it is subjected to double the stress than Circ. Weld (Category B) and this the reason in different part of the ASME code, we have stringent rules in category A joints compared to category B joints. Pressure Vessel RT Test -When We Need to Do a Full Radiography Test? When one of following conditions is existing, you need to do the full radiography:

1. All butt welds in vessels used to contain a lethal substance 2. All butt welds in vessels in which the nominal thickness exceeds specified values 3. All butt welds in unfired steam boilers with design pressure > 50 psi

4. All category A and D butt welds in a vessel when “Full Radiography” is optionally selected As you see, the item numbers 1, 2 and 3 are really mandatory for a full RT test; But pressure vessel manufacturers can make an optional decision for full radiography in item number 4 Pressure Vessel RT Test - Why do pressure vessel manufacturers want to spent more money for full radiography in item # 4? Because the joint efficiency in the full radiography condition is 1, the higher joint efficiency in the pressure vessel wall thickness formula causes less wall thickness. The manufacturer might save lots of money with a lower thickness plate material. But code has given some bonus to manufacturers in item 4, because it is not mandated to do full radiography in all butt welds. Manufacturers can do spot radiography in B and C joints with the same joint efficiency of item 1. Item number 2 describes thickness limitations. Any pressure vessel material is designated to the specific P. Number by ASME section IX, so there are several tables in ASME Code Section VIII Div. 1 Subsection C, which determine this limitation. For example, SA 516 material is P Number 1, and needs to be fully radiographed if its thickness is greater than 1.25 of an inch. At the same time, SA 204 material is P Number 3 and needs to be fully radiographed if the thickness is greater than 0.75 of an inch. Pressure Vessel RT Test - Is Acceptance Criteria in Full Radiography or in Spot Radiography More Stringent? Acceptance criteria for welding defects in full radiography are stringent. These criteria are stated in UW-51 and UW-52 in ASME Code Section VIII Div. 1 It means there is a defect if interpreted based on the full radiography criteria in UW-51, and it might be rejected, but if it is interpreted by the spot radiography criteria in UW-52, it might be accepted. Pressure Vessel RT Test - What is the Important Spot Radiography Requirement?

 One spot shall be examined on each vessel for each 50 ft. increment

 For each increment of weld to be examined, a sufficient number of spot radiographs will be taken to examine the welding of each welder or welding operator  Each spot examination will be made as soon as practicable after the completion of the increment of weld to be examined  The location of the spot shall be chosen by the Inspector after the completion of the increment of welding to be examined Pressure Vessel RT Test - Radiographic Personnel Qualification: The radiographic personnel need to be certified by the pressure vessel manufacturer according to their written practice. Holding the ASNT Radiographic certificate is not enough In fact, SNT-TC-1A can be used as a guideline for manufacturers to establish their written practice for qualification and certification of their personnel. Radiographic Examination Procedure and method ASME Code Section VIII Div 1 mandates that all tests shall be done based on ASME Code Section V, article number 2.

Radiographic Testing Procedure This content provides you with a example Radiographic Testing Procedure. This is a general and sample RT procedure and you need to modify it to meet your project specifications. 1. SCOPE: 1.1-This Procedure describes the general requirements for radiography examination (RT) according to related approved weld map for the metallic welding and casting as may be required by the specification or under which component is being designed and manufactured.

1.2-This radiographic testing procedure provides the material, equipment, calibration, personnel qualification, examination process, evaluation, records and acceptance standards for XXX Project which will be fabricated in YYY. 2. SURFACE CONDITION According to T.222.2, the weld ripples or weld surface irregularities on the both the inside (where accessible) and out side shall be removed by any suitable process to such a degree that the resulting radiographic testing image due to any surface irregularities cannot mask or be confused with the image of any discontinuity. the finished surface of all butt welded joints mat be flush with the base material or may have reasonably uniform crowns, with reinforcement not to exceed that specified in the referencing code section. 3. RADIATION SOURCE 3.1- X-Radiation: The radiography testing techniques shall demonstrate that the required radiography sensitivity has been obtained. Maximum x-ray voltage is 300 KV. 3.2- Gamma radiation: The recommended minimum thickness for which Radio-active isotopes may be used as follow: Table – 3.2 Material

Iridium192

Cobalt 60

Steel

0.75 in

1.50 in

Copper or high nickel copper

0.65 in

1.30 in

Aluminum

2.50 in

--

The maximum thickness for the use of radioactive isotopes is primarily dictated by exposure time, therefore; upper limits are not shown. The minimum Recommended thickness limitation may be reduced when the radiography techniques are used to demonstrate that the required radiographic testing sensitivity have been obtained, by purchaser approval. 4. RADIGRAPHIC FILMS Any commercially available industrial radiography films may be used in accordance with SE 1815(ASTM) standard test method for film system in industrial radiography. Radiography film shall be fine grain high definition, high contrast film (Kodak type AA 400, FUJI 100 or AGFA D7).

5. SCREENS Any commercially available intensifying screen, except those of the fluorescent type, may be used. Intensifying screen for x-ray or Gama ray method divided in two categories: 1-front screen 2-back screen. Commonly lead screens use with 27 micron thickness. (Front screen) 6. PENETRAMETER (I.Q.I) Penetrameters shall be either the whole type or the wire type and shall be manufactured and identified in accordance with the requirements or alternatives allowed in SE 142 or SE 1025 (for whole type) and SE-747 (for wire type), and appending. ASME V 2007 ED & ASME Sec VIII Div I ED 2007. Penetrameters shall consist of those in table 233.1 for hole type and those in table 233.2 for wire type. (Wire type IQI shall be used for welds.)

7. SELECTION OF PENETRAMETER (I.Q.I) 7.1. Material. IQIs shall be selected from either the same alloy material group or grade as identified inSE-1025, or SE-747, as applicable, or from an alloy material Group or grade with less radiation absorption than the material being radiographed. 7.2 Size. The designated hole IQI or essential wire listed in Table T-276 provided an equivalent IQI sensitivity is maintained. See T-283.2.shall be as specified in Table T-276. A thinner or thicker hole-type IQI may be substituted for any section thickness (a) Welds With Reinforcements. The thickness on which the IQI is based is the nominal single-wall thickness plus the estimated weld reinforcement not to exceed the maximum permitted by the referencing Code Section. Backing rings or strips shall not be considered as part of the thickness in IQI selection. The actual measurement of the weld reinforcement is not required.

(b) Welds Without Reinforcements. The thickness on which the IQI is based is the nominal single-wall thickness. Backing rings or strips shall not be considered as part of the weld thickness in IQI selection. 7.3 Welds Joining Dissimilar Materials or Welds with Dissimilar Filler Metal. When the weld metal is of an alloy group or grade that has a radiation attenuation that differs from the base material, the IQI material selection shall be based on the weld metal and be in accordance with T-276.1. When the density limits of T-282.2 cannot be met with one IQI and the exceptional density area is at the interface of the weld metal and the base metal, the material selection for the additional IQIs shall be based on the base material and is in accordance with T-276.1

8. PLACEMENT OF RADIOGRAPHIC TESTING PENETRAMETER (I.Q.I) 8.1- Source side penetrameters: The penetrameters shall be placed on the source side of the part being examined, except for the condition described in chapter 8.2. 8.2- film side penetrameters: Sensitivity:

The sensitivity required using wire type IQI shall be 2%. Sensitivity:(Diameter of thinnest wire visible on radiograph / Part thickness at IQI location) x 100 Where inaccessibility prevents hand placing the penetrameter (s) on the source side, it shall be placed on the film side in contact with the part being examined. A lead letter “F” shall be placed adjacent to or on the penetrameter (s).

9. NUMBER OF PENETRAMETER (I.Q.I)

When one or more film holders are used for an exposure, ate least one penetrameter imager shall appear on each radiograph. If the requirements of T-282 are met by using more than one penetrameter, one shall be representative of the lightest area of interest and the other the darkest area of interest. The intervening densities, on the radiograph, shall be considered as having acceptable density. Number of I.Q.I shall be according to ASME SEC V.T.277.2.

10. RADIOGRAPHIC TESTING TECHNIQUE A single-wall exposure technique shall be used for radiography whenever practical. When it is not practical to use a single-wall radiographic testing technique, a double-wall technique shall be used. An adequate number of exposures shall be made to demonstrate that the required coverage has been obtained.

10.1 Single-Wall Technique. In the single-wall radiographic testing technique, the radiation passes through only one wall of the weld (material), which is viewed for acceptance on the radiograph.

10.2 Double-Wall Technique. When it is not practical to use a single-wall technique, one of the following double-wall techniques shall be used. (a) Single-Wall Viewing. For materials and for welds in components, a technique may be used in which the radiation passes through two walls and only the weld (material) on the film-side wall is viewed for acceptance on the radiograph. When complete coverage is required for circumferential welds (materials), a minimum of three exposures taken 120 deg to each other shall be made. (b) Double-Wall Viewing. For materials and for welds in components 31⁄2 in. (89 mm) or less in nominal outside diameter, a technique may be used in which the radiation passes through two walls and the weld (material) in both walls is viewed for acceptance on the same radiograph .For double-wall viewing, only a sourceside IQI shall be used. Care should be exercised to ensure that the required geometric unsharpness is not exceeded. If the geometric unsharpness requirement cannot be met, then single-wall viewing shall be used. (1) For welds, the radiation beam may be offset from the plane of the weld at an angle sufficient to separate the images of the source-side and film-side portions of the weld so that there is no overlap of the areas to be interpreted. When complete coverage is required, a minimum of two exposures taken 90 deg to each other shall be made for each joint. (2) As an alternative, the weld may be radio graphed with the radiation beam positioned so that the images of both walls are superimposed. When complete coverage is required, a minimum of three exposures taken at either 60 deg or 120 deg to each other shall be made for each joint.

11. SOURCE TO OBJECT AND OBJECT TO FILM DISTANCE (SOD & OFD) According to geometric unsharpness formula (Ug = f *OFD/FOD) for minimizing the Ug value, OFD value shall be minimizing therefore object to film distance shall be minimum. Source to object distance (SOD) shall be set according radiographic technique, object shape and strength of source. 12. RADIOGRAPHIC TESTING IDENTIFICATION SYSTEM The method shall be used to produce permanent identification to the radiographies traceable to the contract, components, welds or weld seams, or part numbers, as appropriate. This identification mark shall not obscure the area of interest. 13. RADIOGRAPHIC TESTING ACCEPTANCE STANDARD Refer to ASME Sec VIII, Div. I a) Butt welded joints surfaces shall be sufficiently free from coarse ripples ,grooves , overlaps and abrupt ridges and valleys to permit proper interpretation of radiographic and the required non-destructive examinations. If there is a question regarding the surface condition of the weld when interpreting a radiographic film, the film shall be compared to the actual weld surface for determination of acceptability. b) Indications shown on the radiographies of welds and characterized as imperfections are un-acceptable under the following condition: 1) Any indications characterized as a crack or zone of incomplete fusion or penetration. 2) Any other elongated indication at radiography, which has length greater than: (a) 1/4 in. (6mm) for t up to 3/4 in. (19mm) (b) 1/3 t for t from 3/4 in. (19mm) to 2 1/4 in. (57mm) (c) 3/4 t (19mm) for t over 2 1/4 in. (57mm) Where: t= thickness of weld excluding any allowable reinforcement.

3) Any group of aligned indications that have an aggregate length greater than t in a length of 12t , except when the distance between the successive imperfections exceed 6L where L is the length of the longest imperfection in the group. 4) Rounded indications in excess of that specified by the acceptance standards given in ASME sec. VIII, DIV I, appendix 4 fig. 4-2 to 4-8 Note: spot RT shall be done as per ASME Sec. VIII, Div. 1 UW-52; however the acceptance criteria shall be according to UW-51 (as specification). 14. DEFECT REMOVAL Repair area shall be located on the weld line after evaluation & interpretation of radiograph .defects shall be removed by suitable method such as grinding, chipping or gouging (if permitted).welding of the repair area shall meet the requirement of related WPS,PQR. 15. CERTIFICATION AND PERSONNEL QUALIFICATION IN RADIOGRAPHIC TESTING. Personnel performing radiography examination to this procedure shall be qualified and certified by XXX also shall meet the requirements of ASNT-SNTTC-1A-2001 EDITION at least level II and on ASNT-SNT-TC-IA for code section I and sec VII div 2. Film interpreter shall have level II as a minimum.

How to determine joint efficiencies for tank inspection Here at Apiexam.com, we continue with the explanation of concepts necessary for passing the API 653 exam. One of the topics you should study in order to pass is calculation of joint efficiencies for shell joints. What is joint efficiency? Joint efficiency is concept found in several API and ASME codes. It is a numerical value, which represents a percentage, expressed as the ratio of the strength of a riveted,

welded, or brazed joint to the strength of the base material. It is also a way to introduce safety factors in welding of shells for containment, and can be expressed as follows:

In other standards, values for Joint Efficiency in welds are assumed according to 2 traits 1.

Type of welded joint

2.

Extent of NDE required for the welded joint

Joint efficiency varies with weld type. Various weld types and joint efficiencies for them can be found in Table UW-12 “Maximum allowable joint efficiencies for arc and gas welded joints” of ASME VIII, Div 1, Sec B. That way, a butt welded joint will have a greater value of E than a fillet welded joint. Joint efficiency also varies with the extent of NDE required for the joint. A value of 1 for E will make one joint go full radiography. In tank design, however, joint efficiency depends on the edition of the standard and the type of joint. Please take a look at the following tables based on API 653

Joint efficiencies for weldedjoints

For riveted joints, tank efficiency depends on the type of joint and the number of rows of rivets.

Joint efficiencies for riveted joints

Of course, regarding welded aboveground storage tanks, we will speak only about butt welded joint and riveted joints, acknowledging that you can find other types of joints, but having in mind that these are the most used types of joints in tank design. According to

API 650 5.1.5, vertical and horizontal shell joints shall be butt joints with complete penetration and complete fusion. Since the seventh edition of API 650, all tanks are constructed with a default 1 joint efficiency, meaning that shells are the thinner they can be. This only was reached, obviously, with improvements in base material (which improves the yield strenght value), weld consumables and welding processes. What has to do joint efficiency with tank inspections? According to API 653 4.3.3.1, the value of E is used in two ocasions in tank inspection. 1. When determining the minimum acceptable thickness for an entire shell course, for any other portions of a shell course, according to the following equations*

2. When determining the maximum level of water to be used during hydrostatic test of a tank (for example, if you need to change tank service to a higher specific gravity liquid) , according to the following equations *

*Details about the meaning of the variables can be found in API 653 4.3.3 When you know the standard by which a tank was built, and solve any of the equations above, you can see if it is fit for continued service. Have in mind that joint efficiency evaluation makes sense only when the corrosion is in close proximity to the joints. In fact, the value of E is 1 for any spot 6 inches apart from the weld in welded joints or to the outermost rivet away from the centerline in a riveted joint. In the exam, one or two questions about joint efficiency will show up. That´s why you need this subject fully understood. EXAMPLE 1 Q: A riveted tank built in 1984 is being inspected. Calculate the joint efficiency for a butt joint with a total of 4 rows of rivets A: Joint Efficiency is 0.75 according to table 4-3 of API 653.

Quick hint about riveted tanks. As an inspector looking to increase inspection interval, you can propose the owner to increase the efficency of riveted joint by applying a full fillet lap welded joint additionally I hope this is a good explanatory article about joint efficiencies in tank inspection. Next article will be more questions on the Body of Knowledge for API 653, in continuation to the API 653 Questions series.