SEAMS IN A SPHERE
SEAMS IN FLAT HEAD
SEAMS IN FORMED HEADS SUCH AS TORISPHERICAL
FLAT SIDES OF VESSELS
HEMI HEAD TO TRANSISTION TRANSISTIONS HEMI HEAD TO NOZZLE NOZZLE LONGITUDINAL SEAM HEMI HEAD COMMUNICATIONS CHAMBER
UW -3 Welded Joint Category Case Study 2
The term "Category" as used here in defines the location of a joint in a vessel, but not the type of joint.
UW-3(a)(2) Category B. Circumferential welded joints within the main shell, communicating chambers, nozzles, or transitions in diameter including joints between the transition and a cylinder at either the large or small end; circumferential welded joints connecting formed heads other than hemispherical to main shell, to transitions in diameter, to nozzles or to communicating chambers.
FORM HEAD TO SHELL
MAIN SHELL
SMALL END OF TRANSISITION
FORM HEAD TO COMMUNICATING CHAMBER NOZZLE
LARGE END OF TRANSISTION
UW -3 Welded Joint Category Case Study 3
The term "Category" as used here in defines the location of a joint in a vessel, but not the type of joint.
UW-3 (a)(3) Category C. Welded joints connecting flanges. Van Stone laps, tubesheets, or flat heads to main shell, to formed heads, to transitions in diameter, to nozzles, or communicating chambers; any welded joint connecting one side plate to another side plate of a flat sided vessel.
NOZZLE NOZZLE VAN STONE LAP NOZZLW
CATEGORY C CATEGORY C BUTT WELD CATEGORY C
FLAT HEAD TO SHELL TUBE SHEET TO SHELL FORGED FLAT HEAD TO SHELL OR NOZZLE
C C CATEGORY C C C SIDE PLATES CAT. C BUTT WELD CAT. C
FILLET WELD CAT. C
UW -3 Welded Joint Category Case Study 4
The term "Category" as used here in defines the location of a joint in a vessel but not the type of joint.
UW-3 (a)(3)Category D. Welded joints connecting communicating chambers or nozzles, to main shell, to spheres, to transitions in diameter, to heads, or to flat sided vessels, and those joints connecting nozzles to communicating chambers (for nozzles at the small end of a transition in diameter, see Category B).
COMMUNICATING CHAMBER COMMUNICATING CHAMBER
CAT. D CAT. D
FILLET BUTT WELD
CAT. D CAT. D
FILLET NOZZLE BUTT WELD NOZZLE
NOZZLE TO SPHERE CATEGORY D CATEGORY D HEAD CATEGORY D
NOZZEL TO FLAT SIDE
MAIN SHELL MAIN SHELL
UW -3 Welded Joint Category Exercises
1. The category of a joint depends on:
a. What kind of weld was made: fillet or butt. b. The process used to make the weld.
c. Whether it is vertical or horizontal in the vessel d. None of the above.
2. A circumferential weld to attach a flange is what Category'?
a. D b. C c. E d. A
3. In the drawing below identify all of the of joints by Category.
D B B B A A A B C A A A B
UW-51 Radiographic and Radioscopic Examination of Welded Joints Overview
In UW-51 the requirements for radiographic examination are detailed. When performing radiography to Section VIII Div. 1 of the Code your are directed to Article 2 of Section V for the techniques to be used. The following are highlights of the requirements:
1. A complete set of radiographs shall be kept on file until the final acceptance of the inspector.
2. Personnel performing and evaluating radiographs shall be qualified using SNT-TC- 1A as a guideline for written practices used in their qualification.
3. That paragraph T-285 of Article 2 is a guide only and that final acceptance of radiographs is based on the ability to see the correct penetrameters image and the specified hole or wire size as applies.
4. How repairs of defects shall be made in accordance with UW-35 and the techniques for re-inspecting the weld after repair. The repair need not be radiographed if prior to the repair it has been demonstrated to the inspector's satisfaction that Ultrasonic Testing can disclose the defect. In which case ultrasonics can be used to examine the repair for acceptance.
5. That any indication on a radiographed characterized as a crack or zone of incomplete fusion or penetration is unacceptable.
6. That the limits of elongated indications are based on the materials thickness.
7. That unacceptable aligned indications are based on total length of a group and the material's thickness.
UW-51 contains the unacceptable indications for Full Radiography. Also definitions of nominal thicknesses for welded joints and weld repairs. Details of Spot Radiography are covered in UW-52.
UW-52 Spot Examination of Welded Joints Overview
Spot radiographs use the same techniques as those in UW-51, but of course are not for the full length of the weld. The basis for selecting Spot radiography is the desire to use a joint efficiency that will come from Column B of table UW-12. The small print note above the subparagraphs explains the Code's intent for the use of spot radiography. The following are highlights of the requirements for Spot Radiography.
1. One spot radiograph for every 50 ft of weld or fraction thereof for a joint efficiency from column b of Table UW- 12.
2. A sufficient number of spots shall be radiographed to examine each welder or welding operator in the 50 foot increment. In the case where welders weld on opposite sides of the same weld one shot will serve to examine both.
3. The inspector chooses the location of the spot radiography. If the inspector approves and cannot be present the fabricator can then choose the location of the spot radiography. Notice that there is no specific location; the welders should never be able to predict the inspector's choice of location.
4. The spot radiography used to pick a joint efficiency from column b of TableUW-12 will not satisfy the requirements of other paragraphs such as UW-11 (a)(5)(b); a spot radiograph required for the choosing of a joint efficiency from column A of Table 12.
5. Spot radiographs must follow the same rules as full radiographs for techniques. The minimum length of the spot examined must be 6 inches.
6. Indications described as cracks or zones of incomplete fusion or lack of penetration are unacceptable.
7. Slag inclusion or cavity evaluation is based on the thickness of the weld excluding any weld reinforcement (cap). The thickness is based on thinner member if two different thickness that have been joined by a butt weld. If a fillet is welded over a full penetration weld its throat must be included in the thickness (t). Indications in a line are described with acceptance standards.
8. Rounded indications are not a factor in the acceptability of welds not required to be fully radiographed.
UW-52 Spot Examination of Welded Joints Overview
9. When a spot radiograph is acceptable the entire weld increment represented is accepted. For example if a longitudinal weld has 65 feet of weld metal only the first 50 feet could be accepted by a single 6 inch spot radiograph. The remaining 15 feet is represented in the next declared 50 feet increment.
10. If the first spot radiograph reveals welding that does not comply then two additional spots in the same weld increment away from the first spot shall be radiographed (tracers). The choosing of the two spots follow the same rule as the first spot radiograph.
11. If the tracers pass then repair and radiography is allowed for the area that was rejected in the first spot radiograph.
12. If either of the tracers fail there are two options. Cut out the entire increment, re-weld then applies spot radiography again or apply full radiography and repair all defects found.
The spot radiography described above is not applied to any specific Category of weld. In a given 50 feet of weld increment there may be Category A, B, C, and D butt welds. The inspector will choose the exact location of the spot radiograph. In cases where spot radiography is a specific requirement of another paragraph of the Code the location for the spot radiograph is stated within that paragraph. The spot radiography of UW-52 cannot serve double duty; it will not satisfy the spot radiography requirements of any other paragraph. It allows the use of a joint efficiency from column B of Table UW-12 for all categories of butt joints in that 50 feet increment. If the 50 feet increment were to stop in the middle of a joint the efficiency of that joint could not come from column B until the next 50 feet increment was spot radiographed.
UW- 11 Radiographic and Ultrasonic Examination
The Code demands 100 % Quality Assurance for some butt welds (Category A butt welds in Lethal Service is one example). In other services, choices for level of Quality Assurance for butt welded joints can range from 100 % down to 60 %.
The Quality of a butt welded joint determines its Joint Efficiency in the Code. Joint Efficiency depends on the Type of butt joint and the amount of radiography applied. There are other Types of joints besides butt welded allowed in the Code. However they cannot produce Code acceptable radiographs. The term "Joint Efficiency" is a hold over from the days of riveted vessels. More will be said about this in the coverage of UW-12.
There are three levels of radiography per Code. Full, Spot and None. The Code demands Full RT in some cases and allows Full RT, Spot RT or None at all in others.
UW-11(a) Full Radiography specifies when Full Radiography must be performed. There are five instances sited.
1. Butt welds in the shell and heads of vessels used to contain a lethal substance.
2. When the least nominal thickness at a butt weld exceeds a limiting thickness, which is based on the type of material used in the vessels welded construction.
3. Butt welds in the shells and heads of unfired steam boilers having an operating pressure greater than 50 psi.
4. Butt welds in nozzles, communicating chambers, etc. in (1) or (3) above attached to vessels sections or heads that exceed certain limits on thickness or diameter.
5. Categories A & D butt joints. Where full radiography is not mandatory; but desired to obtain a joint efficiency from column A of Table UW- 12. Spot radiography must also be applied to Category B and C butt joints.
UW- 11 Radiographic and Ultrasonic Examination
UW-11(b) Spot Radiography. The next option, if full radiography is not mandatory under 1 through 5 above, is spot radiography. This spot radiography can be applied to Category A, B, C, or D butt joints and will allow a joint efficiency from Column B of Table UW- 12.
UW-11(c) No Radiography. If radiography is not mandatory under any Code requirements it may be omitted for butt welded joints. If this is the case the joint efficiency must come from Column C of Table UW- 12.
UW-11 contains the when and where for radiography and ultrasonic examinations. The effect of the degree of radiography is reflected in paragraph UW- 12 with a resulting Joint Efficiency "E". The "E" will be used in the thickness required or pressure allowed calculations for shells, heads etc.. The following pages contain graphical representations of the UW-11.
UW-11 Radiographic and Ultrasonic Examination
(a) Full Radiography. The following welded joints shall be examined for their full length in a manner prescribed in UW- 51:
UW-11 (a)(1) All butt welds in the shells and heads of vessels used to contain lethal substances [see UW-2(a)];
[UW-2(a) limits Category A butt welds to Type 1 and Category B to Type 1 or 2 of Table UW- 12].
HEMI HEAD ELLIPTICAL
LETHAL SERVICE MUST HAVE FULL RT
CATEGORY A CATEGORY B
UW-11 Radiographic and Ultrasonic Examination
(a) Full Radiography. The following welded joints shall be examined for their full length in a manner prescribed in UW-51:
UW - 11 (a)(2) All butt welds in which the least nominal thickness at the welded joint exceeds 1 1/2 in. or exceeds the lesser thickness prescribed in UCS-57. Category B and C butt welds in nozzles and communicating chambers that neither exceed NPS 10 nor 1 1/8 in. wall thickness do not require any radiographic examination;
P-1 Material Per UCS-57 >1-1/4" Full RT
10" Category 1-1/4" [Least Nominal
thickness)
C butt weld 1 NO RT required
1-1/8" thick 1-1/2" thick full RT
No RT required required
24” NPS 20 Category B
Category A 1-1/2" thick
and C butt Full RT
weld Full
RT 2” thick Full RT
RT will change based on the P No. of the material used in construction. See UCS-57, UNF-57 etc., for mandatory Full RT based on thickness.
UW-11 Radiographic and Ultrasonic Examination
(a) Full Radiography. The following welded joints shall be examined for their full length in a manner prescribed in UW-51:
UW- 11 (a)(3) All butt welds in the shells and heads of unfired steam boilers having a design pressure exceeding 50 psi, [see UW-2(c)];
[UW-2(c) limits Category A butt welds to Type 1 and Category B to Type 1 or 2 of Table UW- 12 ].
UNFIRED STEAM BOILER PRESSURE EXCEEDS 50 PSI
MUST HAVE FULL RT
HEMI HEAD ELLIPTICAL HEAD
Category A Category B Type 1 Only Type 1 or 2
UW-11 Radiographic and Ultrasonic Examination
(a) Full Radiography. The following welded joints shall be examined for their full length in a manner prescribed in UW-51.
UW- 11 (a)(4) All butt welds in nozzles and communicating chambers, etc., attached to vessel sections or heads that are required to be fully radiographed under (1) or (3) above; however, except as required by UHT-57(a), Categories B and C butt welds in nozzles and communicating that neither exceed NPS 10 nor 1-1/8 in, wall thickness do not require any radiographic examination;
>NPS 10 Category B Type 1 or 2
FULL RT REQUIRED
LETHAL SERVICE OR UNFIRED STEAM BOILER > 50 psi
HEMI HEAD ELLIPTICAL HEAD
MUST HAVE FULL RT
Category A Category B
Type 1 Only Type l or 2
NPS 10 1-1/8” thick
UW-11 Radiographic and Ultrasonic Examination
(a) Full Radiography. The following welded joints shall be examined for their full length in a manner prescribed in UW-51:
UW- 11 (a)(5) All Category A and D butt welds in vessel sections and heads where the design of the joint or part is based on joint efficiency by UW- 12 (a), in which case:
(a) Category A and B butt welds connecting the vessel sections or heads shall be of Type No. 1 or Type No. 2 of Table UW- 12;
(b) Category B or C butt welds [but not including those in nozzles or communicating chambers except as required in (2) above] which intersect the Category A butt welds in vessel 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, Spot radiographs required by this paragraph shall not be used to satisfy the spot radiography rules as applied to any other weld increment.
Cat. A Full RT Type 1 or 2
E= 1.0 or.90 For shell calcs.
Spot RT Type l or 2 Cat. D Full RT
Seamless Elliptical
Type 1 E = 1.0 head see UW-12 (d)
Type 2 E - .90
For hemi head and shell calculations only
Spot RT
Exercises
1. For a vessel in lethal service what butt joints must be radiographed in addition to all butt joints in the shell and heads? (\f by
2. A joint efficiency from Column A of Table UW-12 is desired for a Category A butt joint in a shell, what extent of radiography must be applied to this Category A butt joint? What additional requirement must be met?
3. If the least nominal thickness of a butt joint in a vessel exceeds a certain thickness based on the material used in its construction what amount of radiography must be applied?
4. Full radiography is required by UW-11 (a)(2) may it be assumed that all butt joints have been fully radiographed? Why or why not?
5. A vessel shell contains a Category A butt welded longitudinal joint and a Category D butt welded joint. Must both of these be fully radiographed to use a joint efficiency from Column A of Table UW-12?
ANSWERS TO UW-11 Exercises:
1. Category A, B & C that exceed diameter 10” NPS or 1-1/8” thickness in nozzles and chamber
2. Full RT and Spot RT
3. Full RT for all Butt joints that exceed the specified thickness except B category joints that do not exceed 10" NPS or 1-1/8” thickness.
4. No-Some thickness requirements may exceed the limit for the material used. It’s the thickness of the welded joint that determines the RT requirement.
Allowable Stresses and Efficiencies Overview
There is a relationship between efficiencies and stresses the Code; that when understood, will allow making calculations with more confidence. What is joint efficiency? What is stress?
STRESS
Stress as it relates to internal pressure on a vessel is a load in the vessel's material. Stress is measured in pounds per square inch. Our examples use a material that will fail at 60,000 pounds per square inch.
ONE SQUARE INCH OF MATERIAL
15,000 LBS
STRESS EQUALS 15,000 POUNDS PER SQUARE INCH
Ultimate Stress is the stress value at which a material breaks (fails)
ULTIMATE STRESS
ONE SQUARE INCH OF MATERIAL
Allowable Stresses and Efficiencies
The Code allows the working stress in a material to be only a fraction of its Ultimate Stress. The term used is Maximum Allowable Stress. The Maximum Allowable Stress is about 25% of the Ultimate Stress for a given material. In the first example above the material is loaded to only 25% of the second example which failed at 60,000 pounds per square inch. The limiting of stress in the Code gives a safety factor of about 4 to 1. This is under ideal conditions with no known flaws in the vessel's material. This of course would be seamless materials properly inspected or a welded material joined by a Code approved method and fully radiographed as required in the Code. Most vessels are constructed using welding and welding will introduce flaws into the vessel material. How many and how bad are the flaws? This is answered by the use of nondestructive examination, primarily visual and radiographic.
If a large enough flaw is present in the base material or the weld, failure can occur at a much lower value of stress.
ONE SQUARE INCH
CRACK OF MATERIAL
IN WELD
28,000 LBS.
FAILURE STRESS DUE TO FLAW 28,000 POUNDS PER SQUARE INCH
In the Code formulas the Stress Allowed must be multiplied by the joint efficiency 'E'. So SE always appear in the formulas. The reason for using E is to make an adjustment for how certain it is that the welded joint is equal to a seamless piece of material. In the case of full radiography the conclusion that the material is as strong as seamless is made and an Efficiency for a Type No. 1 joint can be 1.0. For a Type No. 2 .90 can be used. Spot Radiography allows lower joint efficiencies and No Radiography still lower.
Allowable Stresses and Efficiencies
The previous examples showed heavy weights causing a stress in tension in one square inch of bar material. In a pressure vessel the stress in tension is caused by the internal pressure over an area. There will be a given amount of pounds per square inch over an area that has the same total effect as the heavy weights and a resulting stress is set up in the vessel's material. This force wants to tear the vessel apart and must be resisted by the cross sectional area of the vessel's wall.
The Code limits the amount of stress that can be applied to a vessel's material and this will limit the pressure allowed or increase the thickness required. The stress in the material caused by the internal pressure is given special concern when there is a welded joint present in the vessel's wall. The expected strength of the material is known but how sure can we be if there is a potential flaw contained in a weld or its heat affected zone. Often the weld joint