Pressure Vessel Dimension Inspection The Pressure Vessel Dimension Inspection article provides you with information about the dimensional requirements in pressure vessel construction and pressure vessel inspection. You may know some fabrication tolerances have not been addressed in ASME Code Section VIII. So you need to refer to other sources for inspection. This article provides you the most important dimensional inspection requirements. The dimensional check of a pressure vessel consists of the following items:
Mill Undertolerance of Plates and Pipes
Tolerances for Formed Heads
Out of Roundness of shell
Nozzles and attachments Orientation
Nozzles and attachments Projection
Nozzles and attachments elevation
Nozzles and attachments levelness
Weld mismatch
Weld reinforcement
Mill Undertolerance of Plates and Pipes Pressure Vessel Dimension Inspection - Plate:
Your pressure vessel plates with 0.01 inch or 6% undertolerance, whichever is smaller, may be used for full design pressure, instead of at the given design thickness specified. However, if the material specification allows greater undertolerance, then the ordered thickness for the material should be sufficiently greater. For example, if you have SA 516 Gr.70 plate with a 0.625 inch nominal thickness, and the actual thickness is 0.615, it is acceptable, and you may use this plate without any specific design consideration. Global TPI Services We Provide Pre-Shipment Inspection, Vendor Inspection and Commodity Inspection Services. Click Here for More Information
But if the same plate has an actual thickness t hickness of 0.595 inches, you need to consider mill undertolarence in your design calculation. This plate thickness is acceptable based on the material specification (see table in SA 20 for thickness tolerances). We cannot reject this plate because it is in the permissible tolerance of plate specification, but based on ASME Code SEC VIII Div 1, you need to consider this 0.03 inches in your design calculation: e.g. Thickness = Min Thickness + Corrosion Allowance + Mill Undertolrance Pressure Vessel Dimension Inspection - Pipe:
Contrary to the plate material, your pipe and tube mill undertolrances need to be taken into account for design consideration. Pipe and tube undertolrances are almost -12.5 % of the nominal wall thickness, but sometimes there are differences for different materials, so for any specific material, it shall be referred to the suggested pipe or tube material specification to obtain the exact amount of mill undertolrances. For example, if there is a pressure vessel with nominal shell thickness of 0.5 inch, it needs a 6” SA 6” SA 106 Gr.B nozzle to be attached attach ed to the shell plate. Assume that external loading is not a design controlling factor (is not governor), so as our minimum, nozzle thickness will be the same as the shell thickness. So with a consideration of -12.5%, the calculation will be: 0.5 / 0.875 = 0.571 inches. So when we refer to ASME B36.10 and select our pipe schedule it will be SA 106 Gr.B Schedule 160.
Pressure Vessel Dimension Inspection - Tolerances for Formed Head:
You may know that there is a specific rule for dimensional checking of formed heads, based the fact that the UG-81 inner surface shall not deviate outside of the specified shape more than 11⁄4% of D and inside the shape more than 5⁄8%. You can use sweep boards, which are made by cutting a thin steel sheet or piece of wood for checking your crown and knuckle radius. For your skirt, the difference between the maximum and minimum inside diameter, should not be more than 1%, and you can use a tape measure or laser measure for dimensional control of the skirt. So to fully understand, let's check the following head together:
Head ID = 3364 mm Head Type: Torispherical Limit for Outside of specified Shape = 3364 x 1 ¼% = 42.05 mm Limit for Inside of specified Shape = 3364 x 5/8 % = 21.025 mm So we check the depth, and the drawing is 656.3 mm and the actual is 672 mm, so it is outside of the shape. We need to check if it is in the range of tolerance, so the
maximum outside of the shape will be 42.05 + 656.3 = 698.35 so our actual value is 672 mm. 672<698.35, so it is OK. We cut a sweep board equal to the head drawing crown radius (3400 mm) and other one for the knuckle radius equal to 204 mm, so we try to fit the sweep boards in the heads for the above example of the sweep boards. The edge distance to the head surface should not be deviate from the above limited values. Skirt max and min ID should not be more than 1% of the ID, which is 33.64 mm. So, in the above example skirt is also is OK. Our nominal thickness is 20 mm, so up to 19.746 is acceptable then for the above example. The thickness dimension is also OK. So the above head dimensions are ok Pressure Vessel Dimension Inspection - Out of Roundness of Shell:
UG-80 deals with out of roundness of shell. The maximum permitted ovality tolerance (D max – max – D D min) shall not exceed 1% 1 % of the nominal diameter of the vessel. If you have an opening, then the tolerance can be increased by 2% x d (d = diameter of opening), if the measurement is taken within a distance of „d‟ from the axis of the opening. The out of roundness generally is measured in two directions with a normal measuring tape or laser measure, so one direction would be the D max and other one would be the D min. The following picture shows an out of roundness measurement: We Provide TPI Inspection Services in Most Industrial Areas Around the Globe Globe.. Click Here for More Information
Pressure Vessel Dimension Inspection - Nozzles and Attachments Orientation:
ASME Code SEC VIII Div 1 has not specified the orientation tolerances for nozzles and attachments, so you need refer to the pressure vessel handbook. It is almost +/- 1 degree. The orientation check can be done by a simple calculation by using a measuring tape in the actual measurement work on the vessel. For example, we want to check N3 orientation in following drawing.
N3 is located at 120 degrees in the drawing. The manufacturer has located this point in the vessel with a marker, and we want to check the location, and if it is OK, then we can allow the manufacturer to cut. So we run this simple calculation: Vessel OD = 97.875” Shell outside Circumference length = π x OD = 3.14 x 97.875 = 307.3275” 307.3275 distributed to 360 degrees so each degree represents 307.3275/ 360 =0.8536” N3 is located at the 120 degree position, so if we use a measuring tape and keep 0 at the zero reference point in the vessel and pull the tape, the nozzle center should be located at 120 x 0.8536 = 102.4425 inches. This 102.4425 is a perfect case so our tolerance is ±1 degrees. So if the tape shows in following range, it will be OK. 102.4425 – 0.8536 =101.5889” 102.4425 + 0.8536 =103.2961” Pressure Vessel Dimension Inspection - Nozzles and Attachments Projection:
Nozzle and attachment projection is the length from the nozzle or the attachment face to the vessel shell centerline. Projection dimensions are addressed in general assembly drawings. The projection tolerances are not addressed in ASME Code SEC VIII Div 1, but if you refer to the pressure vessel handbook, almost ± 0.25 of an inch is permissible. In following figure, we need to check the BD nozzle projection. It should be 660 mm in distance from shell centerline to the flange face.
In practical measurement, you may use a measuring tape to measure the distance between the shell outside circumference to the nozzle face. Then the measured value is summed with the shell thickness and the inside radius. So for the above example, the following range is acceptable:
660 – 6.35 = 653.65 660 + 6.35 =666.35 Pressure Vessel Dimension Inspection - Nozzles and Attachments Elevation:
Your Nozzles and attachments elevation is the length between the nozzles or the attachments centerline and the bottom or top head tangent line. The elevation dimension is addressed directly in the general assembly drawing. The elevation tolerances are not addressed in ASME Code SEC VIII Div 1, but if you refer to the pressure vessel handbook, almost ±0.25 inch is permissible. In following figure, we need to check Nozzle EB elevation. As you see in the drawing, the distance between the nozzle centerline to the bottom head tangent line is 700 mm.
In actual measurement, the measuring tape or laser measure is used to measure from the nozzle neck center line to the tangent line. In this example, the following range is acceptable: 700 – 6.35 = 693.65 700 + 6.35 = 706.35
Pressure Vessel Dimension Inspection - Nozzles and Attachments Levelness:
The nozzles and attachments levelness tolerances are not addressed in ASME Code SEC VIII Div 1, but in the pressure vessel handbook, a ½ ˚ deflection is permissible. For levelness checking, a level gage is used. If the bubble is in the middle of the designated lines, the nozzle is level. But if the bubble intersects the designated limit lines, you need to run a simple calculation and see if your deflection is within the tolerance limit.
You need to move one end of the level gage up to the bubble to be placed in middle. Then measure the distance between the level gage end and the flange face. The measured value is the X in the figure. Then obtain α, if α is less than 0.5˚, the deflection falls within the tolerances. And it is OK otherwise it will require cutting out and re-welding. Pressure Vessel Dimension Inspection - Weld Mismatch:
ASME Code SEC VIII Div. 1 specified tolerances for weld mismatch in UW-33. It is important to know that the limit for weld mismatch is stringent for a category A weld (Longitudinal joint and circumferential shell to hemispherical head). The concept behind this is that the longitudinal joint bears double the amount of stress, and inspectors should precisely check these joints. For example, you have a pressure vessel with a nominal thickness of 1 inch. You do a visual and measure the mismatch by the welding gauge. Assume you found a 0.143 inch mismatch in one longitudinal and one circumferential joint. So you look to the UW-33 table:
The permissible weld mismatch in the longitudinal joint for your case is 0.125 of an inch, and for the circumferential joint, it is 0.1875 of an inch. Your weld mismatches for both longitudinal and circumferential joints are 0.143, so your circumferential joint is OK, but your longitudinal joint should be repaired. Pressure Vessel Dimension Inspection - Weld Reinforcement:
The same concept for weld mismatch exists for weld reinforcement tolerances. The longitudinal joint weld reinforcement limit is more stringent than that for circumferential joints. This is because longitudinal joint bears double stress, and it is required that the stress concentration is minimized.
In the same example, assume that there are 0.150 of an inch welds reinforcement for both category A and B welds. Do you want to review your weld acceptance? Category A allowances = 0.093 inch Category B allowances = 0.1875 So our circumferential joint is OK, but the longitudinal is not OK and should be repaired by removing the excess weld reinforcement.
Spherical Pressure Vessel The Spherical Pressure Vessel article provides you with information about this vessels characteristics and related points for the pressure vessel inspection. What is construction code for spherical pressure vessel? What is In-Service code for spherical vessel? What are the applications? What are the advantages? This article either provides you the answers of your questions or refers you to the specific sources. What is the Difference Between the Design Formula of a Cylindrical Shell and a Spherical Shell?
In the same design condition with the same design pressure, design temperature and material, the thickness you obtain from the internal pressure formula for spherical pressure vessel will be half that of the cylindrical pressure vessel. The design formula for the cylindrical shell is t = PR/ (SE-0.6P) and for the spherical shell is t = PR/ (2SE-0.2P) When the “t” is represent Thickness, “R” the Inside Radius,” S “the Allowable Stress, “P” the Design Pressure and “E” the Joint Efficiency; For example, if your design pressure is 250 psi, inside radius 20 inch. , allowable stress 20,000 psi and joint efficiency 1. Your thickness for cylindrical shell will be 0.24 inch. or 6.10 mm, and for the spherical shell, it will be 0.125 inch. or 3.175 mm. What is the Application and Advantage of a Spherical Pressure Vessel?
When you need to make storage for a great amount of pressurized liquid or gas, meaning you need a big volume, then you need a big pressure vessel, so a spherical shell pressure vessel would be more economical. As you see above, the thickness is half that for a similar design condition. Also placing a long cylindrical pressure vessel might not be suitable in regards to the regarding plant layout. What is the Disadvantage of a Spherical Pressure Vessel?
Spherical shell pressure vessels are more expensive than cylindrical pressure vessels to fabricate, and this higher price is only justifiable for large vessels.
What is Difference in Fabrication Method Between Cylindrical and Spherical Shell Pressure Vessel?
Cylindrical shell pressure vessels generally are built in the shop and then transferred to the plant field except for long process towers, which might be built in two pieces and completed in the field by one circumferential weld. This is only because of the facilitating shipping process. But spherical shell pressure vessels are assembled in the field. The plates generally are formed in the rolling shop and then carefully transferred to the field for assembly. What is the Difference in the Post Weld Heat Treatment (PWHT) Process?
If PWHT is a code or process requirement, the PWHT for cylindrical shell pressure vessels generally are done in the furnace, and if the vessel is too long, it is done in two heating process with a 5 ft. overlap. But PWHT for spherical shell pressure vessels are done by one or more high velocity burners that are fired into the vessel using the top or bottom man ways (or both) as burner entry and exhaust positions. This is done because using a PWHT furnace is not possible. What Kinds of Materials are Stored in a Spherical Pressure Vessel?
Most of the LNG (Liquid Natural Gas) and LPG (Liquid Petroleum Gas) tanks are Spherical Vessels. Butane, Propane, Ammonia, oxygen, hydrogen and nitrogen also are stored in spherical vessels. What are the In-Service Inspection Requirements for Spherical Shell Pressure Vessels?
In-service inspection requirements are similar to those for the cylindrical pressure vessel and are based on the requirement of the API STD 510 Standard. Some important points about in-service inspection are: The inspection plan needs to be developed for the vessels, and generally, internal inspection is a mandatory requirement. You need to do internal inspection in intervals not exceeding one half of the remaining life, or 10 years, whichever is less. You can calculate the remaining life with the following simple formula:
RL = (Tc-Tmin)/CR ; When Tc is Actual Thickness, and Tmin is Minimum Required Thickness, it is can be obtained by the deduction of nominal thickness from corrosion allowance, and CR is corrosion rate. Example: you have a spherical shell storage vessel with an actual thickness of 1.121 in., and the minimum required thickness is 1.0 and the corrosion rate is 0.005 in./year, so the reaming life will be: (1.121-1.0)/0.005 = 24.2 years; So the internal inspection interval will be: Half-life = 24.2/2 = 12.1, so 10<12.1, then the internal inspection interval will be 10 years. But in most plants, evacuation of storage vessels is very difficult and costly, so integrity engineers try to find a new technique to replace internal inspection. API STD 510 allows the On-Stream inspection to be replaced with the internal inspection with some conditions. This is the definition of On-Stream Inspection per the API 510 Standard: “An inspection performed from the outside of a pressure vessel while it is on-stream using NDE procedures to establish the suitability of the pressure boundary for continued operation.” But only you may replace this technique if all of following conditions are met with your vessel: The general corrosion rate is less than 0.005 inch. per year, remaining life is greater than 10 years, there is no corrosive character in the contents, no questionable condition is observed in the External inspection and the vessel is not subject to SCC cracking. What is the Best Technique to Use as On-Stream Inspection?
The best technique, which is widely used, is acoustic emission testing. In this technique, the spherical vessel is completely filled by content fluid to maximize stresses. Meanwhile highly-sensitive proprietary acoustic sensors are attached to the vessel wall to detect the fracture of corrosion products during the monitoring period.
The suspect areas are identified and located. If the suspect shows a high risk issue, then the vessel would be evacuated for internal inspection and necessary repairs. What are the Repair Methods for a Spherical Shell Pressure Vessel?
There are three sources for repair of a spherical pressure vessel; API STD 510 has provided some repair requirements and instructions. Based the API 510, only repair organizations are allowed to do repairs, and the definition and condition for being a repair organization has been provided in the API 510. The other source is the ASME-PCC-2, which provides you detailed instructions and requirements. The other source is the NBIC Part 3 Code book, which similarly provides requirements and instructions. Please note if your spherical pressure vessel is a “U” or “U2” stamped pressure vessel, only repair organizations that are ”R” stamp holders from NBIC are allowed to do repair on your vessel.
Pressure Vessel Certification The Pressure Vessel Certification article provides you with information about the ASME stamp holder process for manufacturers and third party inspection companies. Global TPI Services We Provide Pre-Shipment Inspection, Vendor Inspection and Commodity Inspection Services. Click Here for More Information
Do you what the pressure vessel certification process is? Do you know how pressure vessel manufacturers can be ASME stamp holders? How Third Party inspection companies can be certified by ASME and be Authorized Inspection Agencies? What is a “U” stamped pressure vessel? This article describes all the required information regarding pressure vessel certification and regulation. Pressure Vessel Manufacturer Certification:
You may know pressure vessel manufacturer certification is the same as authorization for an ASME Stamp. The pressure vessel manufacturers can implement an ASME Quality Control System and then apply for the ASME Stamp. This means that if a manufacturer is accredited by the ASME organization for pressure vessels per ASME Code Section VIII Div. 1, they can stamp the letter “U” in the pressure vessel nameplate. See the following Fig. for Pressure Vessel Certification (The Stamp Item)
This is the process for ASME stamp accreditation: 1. Obtain Application Forms from ASME
2. Sign a Service Agreement with an Authorized Inspection Agency (AIA) Authorized inspection agencies are third party inspection companies, which have accredited by an ASME organization and their inspectors are certified by the National Board Inspection Code (NBIC). These inspectors are named authorized inspectors and hold commission cards issued by NBIC organization. Summary: AIAs are accredited by ASME, but these inspectors are certified by NBIC. 3. Submit Application Forms to ASME and transfer Fees 4. Purchase ASME Code Books 5. Describe a QC-System according to the ASME code quality control manual and have the procedures prepared by the manufacturer. 6. Prepare a Demonstration Item A representative demo pressure vessel needs to be constructed and all drawings, calculations, part lists, purchase orders, material test reports, fabrications, inspections, tests and reporting shall be based on ASME code section VIII requirements. 7. Qualify Procedures and Personnel Quality control procedures and personnel also need to be approved by the manufacturer. 8. Pre-Joint Review by the Supervisor of AIA It takes almost 4 months to fulfill the above requirements. Then an audit needs to be conducted by a supervisor authorized inspector, which is designated by an authorized inspection agency, which is under contract with the manufacture. We Provide TPI Inspection Services in Most Industrial Areas Around the Globe. Click Here for More Information
This auditor would report non-conformities found in the audit process, and then the manufacturer would have some time to correct them. 9. Joint Review (Audit) with ASME Designee, Inspector and Supervisor Finally, the audit would be conducted by an ASME designated person, supervisor authorized Inspector and authorized inspector. That is the reason this audit is named a joint review.
10. Issuance of Certificate and Stamp by ASME If the result of Audit was satisfactory, the certificate would be issued by ASME and then manufacture would be authorized to stamp name plate with “U” Stamp. What are the Different Pressure Vessel Stamps?
Manufacturing of Pressure Vessels (Shop and /or Field): U Alternative Rules Section VIII, Division 2(Shop and /or Field):U2 Manufacturing of High Pressure Vessels (Shop and /or Field): U3 What is the Quality Control System that Needs to be Implemented in Pressure Vessel Certification?
For U the stamp, the quality control system needs to be based on ASME section VIII Div. 1 Appendix 10. What is the Benefit for a Pressure Vessel Purchaser to Order a Stamped Pressure Vessel?
When a purchaser orders stamped pressure vessel from a stamp holder manufacturer, it is not necessary to hire a third party inspector. Because this pressure vessel would automatically be inspected by an AI(Authorized Inspector) and its data report would be signed by him. Another benefit can be the purchaser's assurance of the quality of the pressure vessel, because of the manufacturers responsibility to the ASME organization. At the same time, ordering a stamped pressure vessel would be more expensive than a non-stamped pressure vessel.
ASME Code Section 8 The ASME Code Section 8 is the construction code for pressure vessel and covers design, manufacturing and pressure vessel inspection and testing in the manufacturing shop. This Code section addresses the mandatory requirements, specific prohibitions, and non-mandatory guidance for Pressure Vessel Materials, design, fabrication, examination, inspection, testing, certification, and pressure relief. In this article you will learn about the different subsections and guidelines for the use and application of this code. For ASME Code Section 8 scope and boundaries, review the Pressure Vessel Definition article.
You may know ASME Code Section 8 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. Hierarchy of Standard 1. Law and Regulation at Location of Installation
Mandatory application of ASME pressure vessel code is determined by rule and regulation at location of installation. For example, if you are living in the state of Minnesota, the application of 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. Global TPI Services We Provide Pre-Shipment Inspection, Vendor Inspection and Commodity Inspection Services. Click Here for More Information
2. ASME Boiler and Pressure Vessel Code
The next item in this hierarchy is ASME Code itself; the ASME Code generally is divided into three groups as following:
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 are in this Group. For example, ASME Code section VIII for welding requirement such as WPS (Welding Procedure Specification), PQR (Procedure Qualification Record), Welder Performance Qualification, etc. refer 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. 3. National Board Inspection Code(NBIC):
We have assigned a separate article for the NBIC, but as required for this article, the NBIC is making certification for ASME Authorized Inspectors and is also certifying R stamp for Repair services for stamped pressure vessels. ASME Code Section 8 Content:
See following Fig; it shows ASME Code Section 8 Content:
This section 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 covers specific requirements that are applicable to the various methods used in the fabrication of pressure vessels. It consists of Parts UW, UF, and UB dealing with welded, forged, and brazed methods, respectively. Subsection C covers specific requirements applicable to the several classes of materials used in pressure vessel construction. It consists of Parts UCS, UNF, UHA, UCI, UCL, UCD, UHT, ULW, and ULT dealing with carbon and low alloy steels, nonferrous metals, high alloy steels, cast iron, clad and lined material, cast ductile iron, ferritic steels with properties enhanced by heat treatment, layered construction, and low temperature materials, respectively. For example, if you need to manufacture a pressure vessel with SA 516 Gr.70 material (Carbon Steel), then you need to meet the marked items in above Fig. Please note that ASME Code Section 8, does not provide you fabrication tolerances except for misalignment and weld reinforcement. For example, for nozzle orientation, projection, elevation and other required tolerances there are no values in the code, and you may refer to pressure vessel handbooks for such information. Review the Pressure Vessel Dimension Inspection article for such tolerances. If you review 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.” For example, ASME Code Section 8 Div 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.
For example, the formula for wind or earthquakes is not provided in the ASME Code Section 8, and these items and other similar loading considerations need to be designed by using the information provided in the pressure vessel handbooks. For ASME pressure vessel manufacture certification, as well as Authorized Inspection Agency certification, Review the Pressure Vessel Certification article. We Provide TPI Inspection Services in Most Industrial Areas Around the Globe. Click Here for More Information
What is the Summary of Important Points in ASME Code Section 8 ?
1. ASME Code Section 8 edition is issued once every 3 years and addenda, once a year – both on July 1st. Edition and addenda become effective on the 1st of January of next year (i.e., 6 months after issue). 2. Thickness of cylindrical shell t = PR/(SE-0.6P) + C 3. Longitudinal weld is more critical because it is subjected to double the stress than Circ. Weld. 4. “Weld joint categories” A, B, C, D – are based on joint locations in the vessel and stress levels encountered. “Weld Types” (type 1, 2, 3, etc.) describe the weld itself. 5. Depths of 2:1 Ellip. and hemisph. heads are D/4 and D/2 respectively. (D= Head diameter.) 6. Weld Joint categories: 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. 7. 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) 8. For full penetration welds (type 1): Joint efficiency, E = 100%, 85%, 70% (For the radiography = Full, Spot, Nil respectively) 9. Radiography marking on name plates (typically for Type-1 welds) RT-1: (E=1) All butt welds – full length radiography RT-2: (E=1.0) All Cat. A Butt welds Full length, Cat B, spot RT-3: (E=0.85) Spot radiography of both Cat A and B welds RT-4: (E=0.7) Partial/No radiography 10. For Welded Heads for E=1, all welds within the head require full length radiography (since they are all Cat. A welds) 11. For seamless heads, E=1, If a) head to shell weld is fully radiographed (if Cat. A), and at least spot radiographed (if Cat. B) 12. Compared to Cylindrical shell, thickness of 2:1 Ellipsoidal head is approx. same as shell, Hemisph. head approx. half and Torisph head is 77% higher. 13. MAWP is calculated for: Working condition (Hot & Corroded). Vessel MAWP is always taken at the Top of the Vessel and is lowest of all part MAWPs adjusted for static pressure. 14. Hydro-Test is Standard Pressure test on Completed Vessels. Hyd. Test Pr. = 1.3 x MAWP x stress ratio Insp. Pressure (hydro) = test pr. / 1.3 Min. Test temp. = MDMT + 30°F Max. Inspection temp. = 120°F 15. Pneumatic test is performed if hydro is not possible due to design or process reasons. Prior to the test, NDT as per UW-50 is mandatory.
Pneumatic test pressure = 1.1 x MAWP x stress ratio, Pressure should be increased in steps (Total 6). 1st step – 50% of test pressure 2nd to 6 step – 10% of test pressure Insp. Pr. (pneumatic) = test pressure /1.1 16. 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. 17. Vessel MAWP represents the maximum safe pressure holding capacity of the vessel. Vessel MAWP is measured at top-most point and is lowest of vessel part MAWPs, adjusted for hydrostatic head. 18. For vertical vessels, hydrostatic pressure caused due to liquid with specific gravity = 1, 1ft of height = 0.43 psig. Or 1 mtr of height = 0.1 Bar 19. Total pressure at any point of Vertical vessel is given by: Total Pr. = Vessel MAWP + h x 0.433. (h = height from top in ft.) 20. If part MAWP and elevations are known, Vessel MAWP can be calculated by the deducting hydrostatic head from part MAWP. 21. Ext. 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, ) Allowable Ext. Pressure, Pa = 4B/(3(Do/t)) 22. For values of A falling to the left of material line in the material chart: Pa = 2AE/(3(Do/t)) 23. Name plate shows The Code stamping, MAWP, design temp., MDMT, and Extent of Radiography. 24. ASME materials (SA) shall be used for code stamped vessel fabrication instead of ASTM (A) materials. 25. Reinforcement pad is not required, if the size of finished opening is (UG 36)
Not exceeding 2-3/8” for all thicknesses of vessel
Not exceeding 3-½”, if vessel thickness is ≤ 3/8‟‟
26. Reinforcement pad with OD = 2d and thk = vessel thk is always safe (d = diameter of finished opening) 27. Reinforcement limit along vessel wall = 2d 28. Reinforcement limit normal to vessel wall = smaller of 2.5 t or 2.5 tn 29. In reinforcement pad calculations, credit can be taken for area available in shell and nozzle. 30. Fillet weld throat dimension = 0.707 x leg of weld 31. Adequacy of weld sizes shall be checked as required by UW-16. The nozzles construction shall be one of the Code acceptable types. 32. The maximum permitted ovality tolerance (D max – D min) shall not exceed 1% of nominal diameter of vessel. If there is opening, then the tolerance can be increased by 2% x d (d = diameter of opening) if measurement is taken within a distance of „d‟ from axis of opening. 33. The mismatch tolerances and the maximum allowable weld reinforcement is more strict on longitudinal welds compared to circumferential welds (UW-35). 34. Principle of reinforcement: Area removed = Area compensated. Compensation area shall be within reinforcement limits. 35. For use as pressure parts, the plates shall be fully identified. Maximum permitted under tolerance on plates is 0.01” (0.3 mm) or 6% of ordered thickness, whichever is less. 36. All welding (including tack, seal, etc.) shall be done using qualified procedures and welders. 37. Mandatory full radiography in ASME Code Section 8 is required for all welding with thickness exceeding Table UCS-57, and also for lethal service vessels and unfired boilers with Design Pr. More than 50 psig. 38. PWHT is ASME Code Section 8 requirement if thickness exceeds those given in tables UCS-56 (given in notes under the tables). These tables also give min. PWHT temperature and min. holding time (soaking period) based on P-Nos. and thickness respectively.
39. For Furnace PWHT in ASME Code Section 8 , Loading Temperature shall not exceed 800°F, heating rate 400 deg F/hr/inch of thickness, cooling rate 500°F /hr/inch of thickness. Still air cooling permitted below 800°F. During soaking period, temp difference between hottest and coldest part shall not exceed 150°F. 40. Minimum overlap for PWHT in multiple heats = 5 ft. 41. For the ASME Code Section 8 impact test requirement, UCS 66 curve. If MDMTthickness combination falls on or above the curve, impact testing is exempted. Additional exemptions are given as per UG-20(f) and UCS=68 (c).
Pressure Vessel Heads The Pressure Vessel Heads article provides you with information about different types of ASME heads and related points in pressure vessel inspection. How many standard Pressure Vessel Heads are in the ASME Code? What are their characteristics? This article introduces you, different type of Heads, applications and dimensional features. The Ellipsoidal Head, Hemispherical Head and Torispherical Head are three types of ASME Pressure Vessel Dished Heads. See the following Fig. for these heads sketches:
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Ellipsoidal Head Dimensional Characteristics:
In the same design condition, such as design pressure, design temperature and material, your calculated wall thickness under internal pressure will be approximately equal to shell thickness. For example if you have calculated your shell thickness under internal pressure and obtained 12 mm, your ellipsoidal head thickness also will be approximately 12 mm. The inside depth of your ellipsoidal head (h in above fig.) will be one-fourth of the head inside diameter (h = D/4). Your ellipsoidal head knuckle radius is 0.17D and the spherical radius is 0.9D. Torispherical Head Dimensional Characteristics:
In the same design conditions within the same design pressure, design temperature and material, your calculated wall thickness under internal pressure will be approximately equal to 1.77 times the shell thickness.
For example, if you have calculated your shell thickness under internal pressure and obtained 12 mm, your torispherical head thickness will be approximately 21.24mm. Your torispherical head knuckle radius is 6% of the inside crown radius, and your crown radius is equal to outside the diameter of the head skirt.
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Hemispherical Head Dimensional Characteristics:
With the same same design conditions with the same design pressure, design temperature and material, your calculated wall thickness under internal pressure will be approximately half of the shell thickness. For example, if you have calculated your shell thickness under internal pressure and obtained 12 mm, your hemispherical head thickness also will be approximately 6 mm. The inside depth of your hemispherical head will be one-half of head inside diameter (h = D/2). Dimensional Inspection of Pressure Vessel Dished Heads
You need to make a dimensional inspection on the pressure vessel heads for any effort for the fit-up process. The dimensional inspection is done on the knuckle radius, crown radius, skirt length, depth and thickness. Review the Pressure Vessel Dimensional Inspection Article for a detailed description for heads inspection. Pressure Vessel Dished Head Joint Efficiencies:
If your dished head is welded and the joint efficiency is 1, then all of your head welds need full length radiography.
If your dished head is seamless and your joint efficiency is 1, your head to shell needs full radiography and spot radiography for your ellipsoidal or torispherical heads. For a more detailed description, please review the ASME Pressure Vessel Joint Efficiencies article.
Pressure Vessel Plate The Pressure Vessel Plate article provides you information about the ASME Code requirement regarding plate material and related points in the pressure vessel inspection. Global TPI Services We Provide Pre-Shipment Inspection, Vendor Inspection and Commodity Inspection Services. Click Here for More Information
You need to take care about your Pressure Vessel Plate Materials, there are lots of requirements and specific prohibitions in the ASME code. Some of these requirements are ASME and Non ASME plate materials, plate specifications, inspection requirements and material test reports. This article describes these requirements for you. What is ASME Material?
ASME Code Sec VIII DIV 1 requires the materials that are used for pressure containing parts to be one of:
Material specified in ASME section II materials but with some restrictions, which are stated in ASME Code Sec VIII Div 1 in subsection C e.g. UCS, UHF, UNF. For example, SA 283 plate material is listed in ASME Sec II, but when you refer to subsection C in Sec VIII Div 1, you see it is not allowed to be used for lethal substance services as well as for unfired steam boilers. Also, you cannot use this material when you need your thickness to be greater than 5/8 inch.
What is Difference Between ASTM Material and ASME Material?
When you refer to ASME Section II Part A, you see all material with SA prefix identification and on one side there is an ASTM logo, and on the other side is ASME logo. But the ASTM prefix is single A.
The material specification in ASME Section II Part A was originally developed by ASTM and then reviewed and adopted by ASME Code. That is the reason you see both societies' Logos. For example, if you open the SA 516 material specification in ASME Section II, material Part A, edition 2004, you will see this statement below the page title: “Identical with ASTM specification A 516/ A516M-90”; This means you can use A 516 Edition 1990 instead SA 516 2004. But if your ASTM material is A 516 and not manufactured based on ASTM Edition 1990, you cannot use this material unless you do the assessment based on clause UG10 in ASME Section VIII Div 1, and your assessment result will determine either it is permissible, or you need to do more tests to recertify this material. For some materials, the specifications are not identical or may be identical with some exceptions, so again we cannot use these ASTM material unless we make an assessment based on the above mentioned clause (UG-10) Is it possible to use Non ASME material instead of ASME Section II material?
The answer is “Yes,” but with some conditions. These conditions ar addressed in UG10(a) as following items:
Melting method, melting practice, deoxidation, quality and heat treatment to be same between suggested non-ASME material and equivalent permitted ASME section II material. Material certificate proves there is no conflict in chemical analysis between suggested non-ASME material and equivalent permitted ASME material. Material certificate proves the suggested non-ASME material was produced and tested in accordance to equivalent permitted ASME material. The material certificate to be recertified by the equivalent permitted ASME material with a notation of “ Certified per UG-10”
Example:
A manufacturer received an order for manufacturing a pressure vessel with an SA 516 Gr.60 pressure vessel plate normalized material, but apparently this material is not available in the market... So the material supplier recommended to the pressure vessel manufacturer to purchase an available P265GH material instead. The manufacturer requested a copy of the material certificate for P265GH to verify for recertification per UG-10.
Pressure Vessel Manufacturer Assessment: Step 1: Checking of melting method, melting practices, deoxidation, and heat treatment
P265GH is produced based on the European EN-10028 Standard, manufacturer controls plate material certificate and confirms: We Provide TPI Inspection Services in Most Industrial Areas Around the Globe. Click Here for More Information
Melting method, practice, deoxidation and heat treatment are consistent with SA 516 Gr.60 specification, the current available P265GH material produced by basic oxygen process and secondary vacuum process refinement and both normalized. Step 2: Plate material certificate prove there is no conflict in chemical analysis between the suggested non-ASME material and the equivalent permitted ASME material.
The manufacture controlled chemical analysis in pressure vessel plate material certificate of P265GH with SA 516 Gr.60 specification and confirm everything is in specific minimum or ranged limits. Step 3: Plate material certificate prove the suggested non-ASME material produced and tested in accordance to equivalent permitted ASME material.
Manufacturer the first control with the required number of test specimens for each mechanical test, as well as the size of specimens and confirm that both standards are the same; Then check the plate material mechanical test result for yield stress, tensile strength and elongation and confirm all are in specific range of SA 516 Gr.60. Manufacturer control for plate material impact test requirement and notice impact test carried out by ISO V method and in - 53 degree F; But SA 516 Gr.60 requires to be impact tested in -60 degree F with Charpy V-notch test method based on SA 370 Specification so there are two conflicts here. First is the test method, and seconder is in the test temperature. So pressure vessel manufacturer requests that the plate material supplier provide a test plate and send the test piece to a laboratory for impact testing based the SA 516 Gr.60 specification. The lab test report confirms that the P265GH plate material covers the SA 516 Gr.60 impact testing requirement.
Step 4: The material certificate is to be recertified by the equivalent permitted ASME material with a notation of “Certified per UG-10.”
The manufacturer purchases the P265GH plate material and changes the marking on the plate and also rectifies the material test report with SA 516 Gr.60 material. This material identification will be SA 516 Gr.60 In the above example, a vessel manufacturer with doing extra tests recertified a non ASME pressure vessel plate material. In some other cases, this might not be possible even by extra testing for example if the tensile strength is less than the ASME permitted material. Sometimes it might be recertified even without any extra test when everything falls in the limitation range of the ASME permitted material. What is the Other Requirement for Pressure Vessel Plate?
UG-93 in ASME Code Sec VIII Div 1 specifies requirements for plate material inspection. Based on this clause, only plate material is require to have a material test certificate(MTR). It means this requirement only can be applied for plate material; for other material such as pipe and flange, MTR's are not required and marking on the materials would be acceptable. You need to inspect the plate material per specification of ASME Sec II Part A. For example, for SA 516 Gr.60 refer to this specification and check the chemical composition, mechanical property with your MTR. You need to check that the Heat Number stated in MTR is the same stenciled on the plate material marking and make sure this MTR belongs to this material. You need to check your pressure vessel plate dimensions, such as thickness, width, length, weight as per tolerances stated in specification SA 20. Visual inspection for surface defects also need to be done based on SA 20 specification.
SME 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. Global TPI Services We Provide Pre-Shipment Inspection, Vendor Inspection and Commodity Inspection Services. Click Here for More Information
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 (UW-11(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 UCS-57, 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 (UW11(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( U11(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:
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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)
ASME Impact Test Requirement The ASME Impact Test Requirement article provides you with information about impact test requirements in pressure vessel design and construction. Global TPI Services We Provide Pre-Shipment Inspection, Vendor Inspection and Commodity Inspection Services. Click Here for More Information
Let's say, you have a pressure vessel under design, process and construction has not started yet. Based on the ASME impact test requirement, you need to make assessment to see that either your pressure vessel is exempted from impact testing, or you need to carry out the test. There are 4 steps for impact test exemption assessment. You need to know these steps. You might be exempted in the first, second or third steps and might not be exempted even in step 4. So if you are in step 4 and you have not exempted, then you need to carry out the test. I will explain the process for exemption in this article. Basic Concept:
You may know carbon steels and low alloy steels exhibit a drastic change in their room temperature ductility at sub-zero service temperatures. Different types of materials exhibit different types of transition behavior. We can see there is a sudden, phenomenal drop in their notch-toughness properties below the "transition" range of temperature, which should be a matter of concern for us. Body centered cubic or Ferritic alloys exhibit a significant transition in behavior when impact tested over a range of temperatures. Above transition temperature range, impact specimens fracture in a "ductile" manner, absorbing relatively large amounts of energy.
At lower temperatures, i.e. below the transition temperature range, the impact test specimens are found to fracture in a brittle (cleavage) manner, absorbing less energy. And within the transition temperature range, the fracture is a mixture of ductile and brittle nature. A material would be invulnerable to a sudden drop in notch-toughness at the lowest specified service (or design) temperature, if it is proved by conducting Charpy Vnotch Impact tests on representative test samples, at reference (the lowest service) temperature. Grain refined carbon steel forgings and wrought materials (thoroughly worked and normalized) generally exhibit good notch toughness. ASME Code Section VIII Div 1 exemption rules for ASME Impact Test Requirement:
There are specific rules in ASME Code for exemption from ASME Impact Test Requirement. This test is very expensive, so pressure vessel manufacturers are trying to be exempted for this costly test. You need to follow the following clauses to make exemption assessment for ASME impact test requirement: UG-20(f) →→→UCS-66(a) →→→ UCS-66(b) →→→UCS-68(c)
First you have to keep your pressure vessel design data available and then refer to UG-20 (f). If you are exempted from this clause, you do not need proceed further. But if you are not exempted by UG-20 (f), you have to proceed to UCS-66(a), but again if you are exempted, there is no need for more assessment. But if not, you have to proceed to UCS-66(b). If you are exempted now, there is no need for more assessment; otherwise, you have to proceed to UCS-68(c), and again if you are still not exempted, you have to carry out impact testing. For some cases, You might be exempted from the ASME impact test requirement in the first stage in UG-20 (f). In others, You might be exempted in UCS-66(a) or UCS66(b) or UCS-68(c). If you are not exempted, you must prepare yourself for doing this costly test. This test would be more costly out of the US because of Laboratory Accreditation requirements. Also, there are fewer accredited labs in Europe and the Middle East, and their price is high as well. UG-20(f)
We will start with UG-20(f) for the ASME impact test requirement. If your MOC (Material of Construction) is categorized in P-No. 1 or 2 (Refer to ASME Code Section IX for P-No Definition) and your MOC thickness has the limited value defined in this clause, then you might be exempted from impact testing. But you need to refer to Fig UCS-66 in ASME Code Section VIII Div 1 and see in which A, B, C or D curves your MOC is listed. All ASME carbon steel and low alloy steel material is distributed in these 4 groups (Curves) of materials. You need to know that the materials listed in curve D have the best toughness property, better than the materials listed in curve C. Similarly materials listed in curve C have better toughness properties compared to materials listed in curve B and materials listed in Curve B have better toughness than materials listed in Curve A . See Following Fig UCS-66(a): We Provide TPI Inspection Services in Most Industrial Areas Around the Globe. Click Here for More Information
When you determine your MOC curve, then you have to review UG-20(f) and look for the possibility of exemption from the ASME impact test requirement. There are some other conditions in this clause, which you should consider for exemption. For instance, the vessel should be hydrostatically tested after completion, and the thermal and mechanical loading can not be a design controlling factor. For example, if your MOC is a normalized SA 516 Gr.70 with 0.75 inch thickness you will be exempted from ASME impact test requirement.
Your thickness, in this example, is 0.75 of an inch, and is listed in curve D which is up to 1 inch, you are exempted by this clause. Of course, you will be carry out hydro-static test and ensure that the mentioned loadings are not a design controlling factor in your considered pressure vessel. UCS-66(a)
So assume that in the above example, your MOC thickness is 1.125 inch instead of 1 inch, you will not be exempted by UG-20(f) and you have to refer to UCS-66(a); But for assessment, based on this clause, you need to know your pressure vessel MDMT (Minimum Design Metal Temperature). Assume that is -20 degree F, so you should now go to Fig. UCS-66 and locate 1.125 inch in the horizontal axis and draw a vertical line. In a similar way, locate -20 degree F in the vertical axes and draw a horizontal line. These two lines will cross each other. See above Figure, the lines are identified in red. If the cross point falls above the curve D (because your MOC is listed in curve D) you are exempted. Otherwise you are not, but for the current example, you are above the curve D so you are exempted from impact testing. To simplify your assessment for the ASME impact test requirement, the Fig-66 has been converted to the table(table UCS-66). For any MOC with specific thickness you can go to this table and see what is the minimum permissible temperature without impact testing. See following UCS-66(a) Table:
In the above example (normalized SA 516 Gr.70, Curve D, 1.125 inch thickness), the minimum permissible temperature without impact test is -26 degree F. This means that, if in the above example your MDMT changes from -20 degree F to -27 degree F, then you cannot be exempted from the ASME impact test requirement by UCS-66(a), and you have to proceed to UCS-66(b) UCS-66(b)
Let us explain this clause with the above example. Your MDMT from above is -27 degree F, nominal thickness is 1.125 inch, normalized SA 516 Gr.70 listed in curve D and you are not exempted by UCS-66(a) So you are here to continue your assessment to find a chance for exemption. You have to refer to Fig UCS-66.1 and calculate the following formula: Ratio= tr E / (tn – c) tr is the required design thickness for all applicable loading. We assume for the above example that is 0.95 inch. E is your joint efficiency, and we assume for this vessel it is 1. This means your vessel is RT2, tn is your nominal thinness, which in the example from above it is 1.125 inches, and C is corrosion allowance, and we assume it is 0.125 inches; so let calculate: Ratio = 0.95x1/(1.125 – 0.125) Ratio= 0.95 See following Fig UCS-66(b):
Then go to the Figure UCS-66(b) and in the vertical axes locate Ratio and draw a horizontal line. Then locate the cross point with the graph and draw a vertical line to cross the horizontal axis. You will be able to reach a value of 8 in the horizontal axes. This 8, is your 8 degree F bonus from table UCS-66, which you can reduce by 8 degrees F minimum permissible temperature in table without impact testing. In the above example, your MDMT is -27 degree F, and in the UCS 66 table, the minimum permissible temperature without impact testing designated -26 degree F. So with this clause you can reduce it to -36 degree F(-26 -8 = -34). Your MDMT is -27 degree F, so you are exempted from impact testing with this clause. UCS-68(c)
Let us change one variable in the above example. Let's assume you need to have -45 degree F for your MDMT. Other variable are the same; it means normalized SA 516 Gr.70 listed in curve D, thickness 1.125, so you can see you are not exempted by UCS-66(b); This is because the minimum permissible temperature is -36 degree F, but your MDMT is -45 degree F, so UCS-68(c) might be helpful. It says that if post weld heat treatment is not a code requirement and your P-No is 1 and you carry out post weld heat treatment, a 30 degree F bonus will be granted to you to reduce the minimum permissible temperature in table UCS-66. So when post weld heat treatment is code requirement?
