The ASME Section VIII is the construction code for pressure vessel and it covers design, manufacturing, inspection, and testing in th...
This article deals with the different subsections and guidelines for the use and application of this code. If you want to know about the ASME Section VIII scope and boundaries, please read the Pressure Vessel Definition article.
You may know ASME Section VIII has three divisions. Division 1 covers pressure up to 3000 psi; Division 2 has an alternative rule and covers up to 10,000 psi and Division 3 can be used for pressure higher than 10,000 psi.
Law and Regulation at Location of Installation
Mandatory application of ASME pressure vessel code is determined by rule and regulation at the location of installation. An example, if you are living in the state of Minnesota, the application of the ASME Code for construction and stamping is mandatory in your location, but if you are living in the state of South Carolina, it is not mandatory.
ASME Boiler and Pressure Vessel Code
The next item in this hierarchy is the ASME Code itself; the ASME Code generally is divided into three groups as follows:
Group 1: Construction Codes
Some of them are: Section VIII for pressure vessel, Section I for Power Boiler, section III for Nuclear Power Plant and Section IV for heating Boiler
Group 2: Reference Codes
These are the codes, which are referenced from construction codes as explained in group 1.
The ASME Section IX for welding and Section V for Non-Destructive Testing both are in this Group.
An example, You are fabricating a pressure vessel as per ASME Code Section VIII, for welding requirement such as Welding Procedure Specification - WPS, Procedure Qualification Record - PQR Welder Performance Qualification - WPQ, etc. it refers you to ASME Section IX.
Group 3: In-Service Codes
These are the codes for in-service inspection after placing the equipment into service.
The ASME Section VI for the heating boiler and Section VII are from this group.
National Board Inspection Code (NBIC)
The NBIC is making certification for ASME Authorised Inspectors and is also certifying R stamp for Repair services for stamped pressure vessels.
ASME Code Section VIII Content
See below table; it shows ASME Code Section VIII Content:
| Foreword | |||
| Introduction | |||
| U-1 | Scope | ||
| U-2 | General | ||
| U-3 | Standards Referenced by this Division | ||
| U-4 | Units of Measurement | ||
| U-5 | Tolerances | ||
| Subsection A General Requirements | |||
| UG | General Requirements for All Methods of Construction and All Materials | ||
| Subsection B Requirements Pertaining to Methods of Fabrication of Pressure Vessels | |||
| UW | Requirements for Pressure Vessels Fabricated by Welding | ||
| UF | Requirements for Pressure Vessels Fabricated by Forging | ||
| UB | Requirements for Pressure Vessels Fabricated by Brazing | ||
| Subsection C Requirements Pertaining to Classes of Materials | |||
| UCS | Requirements for Pressure Vessels Constructed of Carbon and Low Alloy Steels | ||
| UNF | Requirements for Pressure Vessels Constructed of Nonferrous Materials | ||
| UHA | Requirements for Pressure Vessels Constructed of High Alloy Steel | ||
| UCI | Requirements for Pressure Vessels Constructed of Cast Iron | ||
| UCL | Requirements for Welded Pressure Vessels Constructed of Material with Corrosion Resistant Integral Cladding, Weld Metal Overlay Cladding, or Applied Linings | ||
| UCD | Requirements for Pressure Vessels Constructed of Cast Ductile Iron | ||
| UHT | Requirements for Pressure Vessels Constructed of Ferritic Steels with Tensile Properties Enhanced by Heat Treatment | ||
| ULW | Requirements for Pressure Vessels Fabricated by Layered Construction | ||
| ULT | Alternative Rules for Pressure Vessels Constructed of Materials Having Higher Allowable Stresses at Low Temperature | ||
| UHX | Rules for Shell-and-Tube Heat Exchangers< | ||
| UIG | Requirements for Pressure Vessels Constructed of Impregnated Graphite | ||
| Mandatory Appendices | |||
| Non-Mandatory Appendices | |||
ASME section VIII is divided into three Subsections, Mandatory Appendices, and Nonmandatory Appendices. Subsection A consists of Part UG, covering the general requirements applicable to all pressure vessels. Subsection B consists of Parts UW, UF, and UB dealing with specific requirements that are applicable to the various methods used in the fabrication of pressure vessels. Subsection C consists of Parts UCS, UNF, UHA, UCI, UCL, UCD, UHT, ULW, and ULT dealing with specific requirements applicable to the several classes of materials used in pressure vessel construction.
An example, if you want to manufacture a pressure vessel with SA 516 Gr.70 material (Carbon Steel), then you shall comply with the U-1 to U-5, UW for welded construction, UCS for material class and mandatory appendices.
ASME Code Section VIII, does not provide you fabrication tolerances, refer ASME Section VII, U-5 which states that “When dimensions, sizes, or other parameters are not specified with tolerances, the values of these parameters are considered nominal, and allowable tolerances or local variances may be considered acceptable when based on engineering judgment and standard practices as determined by the designer”
An example, you cannot find tolerances for nozzle orientation, projection, elevation, and others except for misalignment and weld reinforcement in the code, and you may refer the pressure vessel handbooks for such information. Please read the Pressure Vessel Dimension Inspection article, it may help you to get an idea in such tolerances.
If you read the ASME Forward statement it clearly says “The Code does not address all aspects of construction activities, and those aspects which are not specifically addressed should not be considered prohibited.” In continuing it says, “The Code is not a handbook and cannot replace education, experience, and the use of engineering judgments.”
An example, ASME Code Section VIII Division 1 in UG-28 mandates all loading to be considered in pressure vessel design, but the method for calculation of all of them has not been addressed. The formula for wind or earthquakes loads is not provided in the ASME Code Section VIII, and these items and other similar loading considerations need to be designed by using the local legislative requirements and the information provided in the pressure vessel handbooks.
Summary of Important Points in ASME Code Section VIII
ASME Code Section VIII edition is issued once every 3 years and addenda, once a year – both will be issued 1st July. Edition and addenda become effective on the 1st, January of the following year (i.e., after 6 months).
The thickness of cylindrical shell t = PR/(SE-0.6P) + C
Longitudinal weld is more critical because it is subjected to double the stress than Circumferential Weld.
Weld joint categories A, B, C, D – are based on joint locations in the vessel and stress levels encountered. Weld Types 1, 2, 3, … 8 describe the weld itself.
Depths of 2:1 Ellipsoidal and hemispherical heads are D/4 and D/2 respectively. (D= Head diameter.)
Weld Joint categories (Refer below figure UW-3)
Category A:
- All longitudinal welds in shell and nozzles.
- All welds in heads, Hemispherical head to shell weld joint
Category B:
- All circumferential welds in shell and nozzles
- Head to shell joint (other than Hemispherical)
Category C and D are flange welds and nozzle attachment welds respectively.
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 Double full fillet lap joint
Type 5 Single full fillet lap joints with plug welds
Type 6 Single full fillet lap joints without plug welds
Type 7 Corner joints, full penetration, partial penetration, and/or fillet welded
Type 8 angle joints (a joint between two members located in intersecting planes with an angle greater than 30 deg but less than 90 deg)
Full penetration welds (type 1): Joint efficiency (E) = 1.00, 0.85 or 0.70 which is chosen for the pressure vessel design, will determine the radiography, it shall be Full, Spot or Nil respectively.
Radiography marking on nameplates (typically for Type-1 welds)
RT 1: (E=1) All butt welds – full-length radiography
RT 2: (E=1.0) All Category A Butt welds Full length, Category B, spot radiography
RT 3: (E=0.85) Spot radiography of both Category A and B welds
RT 4: (E=0.7) Partial/No radiography
Where joint efficiency (E) 1 is required, all welds within the head requires full-length radiography since they are all Category A weld. If seamless heads, head to shell weld is fully radiographed (if weld category is A), and at least spot radiographed (if weld category is B).
Approximate Head thickness compared to Cylindrical shell as follows, 2:1 Ellipsoidal head is the same as shell, Hemispherical head half of shell, and Torispherical head is 77% higher than the shell.
MAWP is calculated for Working conditions (Hot & Corroded). Vessel MAWP is always taken at the top of the Vessel and is the lowest of all part MAWPs adjusted for static pressure.
Hydrotest is a Standard Pressure test on Completed Vessels.
Hydro test Pressure is = 1.3 x MAWP x LSR (lowest stress ratio)
Minimum Test temperature = MDMT + 30°F
All joints and connections shall be visually inspected at the Pressure of test pressure divided by 1.3
Maximum temperature = 120°F for the above visual inspection.
The pneumatic test is performed if hydro is not possible due to design or process reasons. Prior to the test, the mandatory non-destructive test to be completed as per UW-50.
Pneumatic test pressure = 1.1 x MAWP x lowest stress ratio, Pressure should be increased in steps (Total 6).
The pressure in the vessel shall be gradually increased to 1st step – 50% of test pressure and the remaining steps at every 10% of the test pressure.
All joints and connections shall be visually inspected at the Pressure of test pressure divided by 1.1 of the test pressure.
The pressure gauge range should be about twice the test pressure. However, in any case, it shall not be lower than 1.5 times and not higher than 4 times the test pressure.
Vessel MAWP represents the maximum safe pressure holding capacity of the vessel. Vessel MAWP is measured at the top-most point and is the lowest of vessel part MAWPs, adjusted for the static head.
Hydrostatic pressure shall be adjusted to account for any static head conditions depending on the difference in elevation as per the below calculation;
Calculation based on (water) specific gravity = 1, 1ft of height = 0.43psig. or 1 meter of height = 0.1 Bar
So, Gauge Pressure at the lowest point while hydrostatic test = Calculated Test Pressure + h x 0.1
(h = height of Pressure vessel in meter)
If part MAWP and elevations are known, Vessel MAWP can be calculated by the deducting static head from part MAWP.
Maximum allowable external working pressure is worked out on basis of Geometric factor A (which depends on L/Do and Do/t ratios) and factor B (depends on A,)
Using the value of B, Allowable External Pressure, Pa=4B/3(Do/t)
Values of A falling to the left of the applicable material/temperature line in the material chart:
Allowable External Pressure Pa = 2AE/(3(Do/t))
Nameplate shows the Code stamping, MAWP, design temperature, MDMT, and Extent of Radiography.
ASME materials (SA) shall be used for code stamped vessel fabrication instead of ASTM (A) materials.
Openings in vessels not subject to rapid fluctuations in pressure do not require reinforcement pad if the size of the finished opening is (Refer UG-36)
- Not exceeding 3 1⁄2 in. (89 mm) diameter if vessel thickness is ≤ 3/8 (10mm)
- Not exceeding 2 3⁄8 in. (60 mm) diameter” for all thicknesses of vessel
No two isolated unreinforced openings, in accordance with the above, shall have their centers closer to each other than the sum of their diameters.
The size of the safe reinforcement pad is OD = 2d and thickness = vessel thickness. (d = diameter of the finished opening)
Reinforcement limit along vessel wall = 2d
Reinforcement limit normal to vessel wall = smaller of 2.5t or 2.5tn (t = shell thickness and tn = nozzle thickness)
During reinforcement pad calculations, credit shall be taken for the area available in shell and nozzle.
Fillet weld throat dimension = 0.707 x leg of weld
Adequacy of weld sizes shall be checked as required by UW-16. The nozzles construction shall be one of the Code acceptable types.
The maximum permitted out of roundness in the shell (tolerance for ovality) for internal pressure (D max – D min) shall not exceed 1% of the nominal diameter of the pressure vessel. If there is an opening, then the tolerance can be increased by 2% x d (d = diameter of the opening) if the measurement is taken within a distance of "d" from the axis of the opening.
The mismatch tolerances and the maximum allowable weld reinforcement is stricter on longitudinal welds compared to circumferential welds (UW-35).
Principle of reinforcement:
The area removed = The area compensated.
The compensation area shall be within reinforcement limits.
Under tolerance plates should be avoided. Plates in pressure parts shall be fully identified. The maximum permitted under tolerance on plates is 0.01” (0.3 mm) or 6% of the design thickness, whichever is less.
All welding (including tack, seal, etc.) shall be done using qualified procedures and welders.
ASME Code Section VIII mandates full radiography for lethal service vessels and unfired boilers with design pressure more than 50psig. and all welding with thickness exceeding Table UCS-57 and related table for the material class.
PWHT is ASME Code Section VIII requirement if the thickness exceeds those given in tables UCS-56 (given in notes under the tables). These tables also give the minimum post-weld heat treatment temperature and minimum holding time (soaking period) based on P Numbers and thickness respectively.
While loading for PWHT in the furnace, the temperature shall not exceed 800°F (425°C), the rate of heating is 400-degree F/hr (222°C/h) divided by the maximum metal thickness, cooling rate 500°F/hr (280°C/h) divided by material thickness. Still, air cooling permitted below 800°F.
During the soaking period, the temperature difference between the hottest and coldest parts shall not exceed 150°F.
Multiple heats in a furnace PWHT minimum overlap shall be 5 ft. (1.5m)
ASME Code Section VIII impact test requirement, UCS 66 curve. If the MDMT thickness combination falls on or above the curve, impact testing is exempted. Additional exemptions are given as per UG-20(f) and UCS=68 (c).
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