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.
But if the same plate has an actual thickness 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 106 Gr.B nozzle to be attached 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.
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 – D min) shall not exceed 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 normalmeasuring 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:
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 isOK, 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 =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.
Pressure Vessel Inspections
The Pressure Vessel Inspections article provides you information about the inspection of pressure vessels and pressure vessel tests in a manufacturing shop.
This article provides you lots of invaluable information about the following items:
Inspection and Testing During the Manufacturing Phase Inspection and Testing During the Operation Phase Pressure Vessel Codes and Regulations
Maintenance and Repairs
The Pressure vessel inspections article, alongside other linked articles are useful for Manufacturer's Quality Control personnel, Second and Third Party Inspectors, Purchasers, Sellers, Plant Inspectors, HSE Engineers, Integrity Engineers, Operation and Maintenance Engineers and any Other Interested Individuals.
Pressure Vessel Definition - Based on the ASME Code Section VIII, pressure vessels are containers for the containment of pressure, either internal or external.
This pressure may be obtained from an external source, or by the application of heat from a direct or indirect source, or any combination thereof.
Click on the above link for detailed information about pressure vessel definition, scope, and boundaries based on ASME and API codes which specifically focus on pressure vessel inspections.
ASME Code Section 8 - ASME Code Section 8 is the construction code for Pressure Vessels.
This Code section addresses mandatory requirements, specific prohibitions, and non-mandatory guidance for pressure vessel materials, design, fabrication, examination, inspection, testing, certification, and pressure relief.
You may know that ASME Code Section 8 has three divisions. Division 1 covers pressure up to 3,000 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.
This section is divided into three parts: subsections, mandatory appendices, and non-mandatory 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, and deals 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.
Click on the above link for detailed information about ASME code section 8, which specifically focuses on pressure vessel inspections.
Pressure Vessel Plate Material - You have to be careful when choosing 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 material, plate specification, inspection requirement and material test report. ASME Code Sec VIII DIV 1 requires that materials that used for pressure containing parts are one of the materials specified in ASME
section II, 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 of an inch.
Click on the above link for detailed information about pressure vessel plate materials which are specifically focused on pressure vessel inspections.
ASME Pressure Vessel Joint Efficiencies - You may know ASME Pressure Vessel Joint Efficiencies are linked to the radiography testing grades, and there is concession for full radiography testing per the UW-11(a) (5) (b) clause, which it is a little bit confusing.
Based on ASME Code requirements, manufacturers have to mark the type of RT, e.g. RT1, RT2, RT3 and RT4, in the pressure vessel name plate and state the same in the Pressure Vessel Data Report.
We have seen many professionals including inspectors and quality control engineers who are confused between RT1 and RT2, specifically when they see that the ASME Pressure Vessel Joint Efficiencies for both RT1 and RT2 is the same and is 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 spot? We are doing spot radiography, but it is categorized in full radiography!!!
Click on the above link to answer to this question, which specifically focuses on pressure vessel inspections.
ASME Impact Test Requirement - You need to take care about ASME Impact Test Requirement. Suggest you have pressure vessel in the design process and construction has not started yet.
Based on ASME impact test requirements, you need to make an 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 go through these steps, and you might be exempted in the first, second or third steps, and might not be exempted in step 4;
So if you are in step 4 and you have not exempted, then you need to carry out the test. This article explains you this assessment process. 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 proceedanymore.
But if you are not exempted by UG-20 (f), you have to proceed to UCS-66(a). 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, no need for more assessment, otherwise you have to proceed to UCS-68(c). Again, if you are still not exempted; you have to carry out impact testing.
It means for some cases we might be exempted from ASME impact test requirement in first stage in UG-20 (f), in others in UCS-66(a) or UCS-66(b) or UCS-68(c) or might not be exempted and must prepare for doing this costly test.
This test would be more costly out of the US because of the Laboratory Accreditation requirement. The Accredited Laboratory based US accreditation system is not used too much in Europe, the Middle East and other locations.
Click on the above link for detailed information about the ASME Impact Test Requirement, which specifically focuses on pressure vessel
inspections.
Pressure Vessel Dimension Inspection - Do you know what the Pressure Vessel Dimension Inspection requirement is? 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. Most dimensional controls of Pressure Vessels are either addressed or not addressed in the ASME Code. They consists of the following items:
Pressure Vessel Inspections - Dimension 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
Click on the above link for detailed information about Pressure Vessel Dimension Inspection, which specifically focuses on pressure vessel inspections.
Pressure Vessel RT Test - Do you know what your Pressure Vessel RT Tests Requirements are? Is full radiography mandatory for your vessel? When the full radiography is mandatory? What the acceptance criteria are? What the RT symbols are?
When one of following condition is existing, you need to do full radiography:
1. All butt welds in vessels used to contain lethal substances
2. All butt welds in vessels in which the nominal thickness exceeds specified values
3. All butt welds in unfired steam boilers with design pressure > 50 psi
4. All category A and D butt welds in a vessel when “Full Radiography” is optionally selected
As you see, the item numbers 1, 2 and 3 are really mandatory for the full RT test;
But the pressure vessel manufacturer may make an optional decision for full radiography in item number 4.
Why would pressure vessel manufacturers want to spent more money for full radiography in item Number 4?
Because joint efficiency in full radiography condition is 1, and the higher joint efficiency in the pressure vessel wall thickness formula causes less wall thickness, so the manufacturer might save lots of money with a lower thickness plate material.
But code has given some bonuses to the manufacturer in item 4, because it is not mandated to do really full radiography in all butt welds. The manufacturer can do spot radiography in B and C joints with the same joint efficiency of 1.
Click on the above link for detailed information about Pressure Vessel RT Tests which are specifically focuses on pressure vessel inspections.
Vessel Pressure Testing - You need to do a hydro-static test after the completion of construction process, but before the internal parts assembly, and also before the painting process.
Please note that performing the pneumatic test instead of the hydro-static testing is not allowed and it can be replaced only when it is not possible due to design and process.
Vessel Pressure Testing requirements have been addressed in UG-99 and UG-100 in ASME Code Section VIII Div. 1.
The activities are done in 3 stages; the activities before start of the test, the activities during test and the activities after the test.
Click on the above link for detailed information about pressure vessel hydro-static testing which is specifically focuses on pressure vessel inspections.
Pressure Vessel Certification - Do you know what the pressure vessel certification process is? 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?
You may know pressure vessel manufacture certification is the same as the Authorization for the ASME Stamp.
The pressure vessel manufacturers can implement the ASME Quality Control System and then apply for ASME Stamp.
It means that if a manufacture accredited by the ASME organization for pressure vessel per ASME Code Section VIII Div. 1 can stamp the letter “U” in the pressure vessel nameplate. These are the processes for ASME Stamp Accreditation:
Obtain Application Forms from ASME
Sign a Service Agreement with an Authorized Inspection Agency (AIA) -Authorized inspection Agencies are Third Party Inspection Companies that have been accredited by the ASME organization and their inspectors certified by the National Board Inspection Code (NBIC).These Inspectors are named Authorized Inspectors and hold Commission Cards issued by the NBIC
organization. Summary: AIAs are accredited by ASME, but the inspectors are certified by NBIC.
Submit Application Forms to ASME and Transfer Fees Purchase ASME Code Books
Describe a QC-System according to the ASME Code Quality Control manual and have the procedures prepared by the manufacturer.
Prepare a Demonstration Item - A representative demo pressure vessel needs to be constructed and all drawings, calculations, parts lists, purchase orders, material test reports, fabrications, inspections, tests and reports should be based on the ASME code section VIII requirement.
Qualify Procedures and Personnel - Quality Control Procedures and Personnel also need to be approved by manufacture.
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 the authorized inspection agency, which is under contract with the manufacturer. This auditor would report non-conformities found in the audit process, and then manufacturer would have some time to correct them.
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 called a joint review.
Issuance of Certificate and Stamp by ASME
If the result of the audit was satisfactory, the certificate would be issued by ASME, and then the manufacturer would be authorized to stamps nameplates with the “U” Stamp.
Click on the above link for detailed information about Pressure Vessel Certification, which is specifically focuses on pressure vessel inspections.
Pressure Vessel Heads - How many standard Pressure Vessel Heads are in the ASME Code? What are their characteristics?
Ellipsoidal Head, Hemispherical Head and Torispherical Head are three types of ASME Pressure Vessel Dished Heads.
Under the same design conditions, such as design pressure, design temperature and material, your calculated wall thickness under internal pressure for ellipsoidal head will be approximately equal to the shell thickness. For a torispherical head, the thickness is equal to 1.77 times that of the ellipsoidal, and the shell thickness and for a hemispherical head is equal to half the shell thickness.
For example, if you have calculated your shell thickness under internal pressure and obtained 12 mm, your thickness for an ellipsoidal head will be approximately 12 mm; for a torispherical head, 20.4 mm; and for hemispherical head, 6 mm.
Click on the above link for detailed information about Pressure Vessel Heads, which specifically focuses on pressure vessel inspections.
Third Party Inspection for Pressure Vessel - What is the third party inspection requirement for pressure vessel inspection in a manufacturing shop?
This article provides information about pressure vessel inspection from material inspection to final inspection and dispatch to site.
Inspection and Test Plan for Pressure Vessel - The Inspection and testing requirement is distributed in a different part of the construction code.
The inspection and test plan have a tabulated format and collect all these requirements in a simple table and determine the responsibly of each party, i.e. the manufacturer, third party inspector and purchaser.
Pressure Vessel Handbook - When there is ASME Code Section VIII, why do we need the Pressure Vessel Handbook? What are the applications? All design formulas and calculations methods have not been addressed in ASME Code, and also, there are no fabrication tolerances in ASME Code.
So the Pressure Vessel Handbook assists us in covering all these requirements.
The Pressure Vessel Handbook supports pressure vessel inspections engineers, designers, pressure vessel manufacturer quality control technicians and engineers, and any other people who deal with pressure vessels.
Click on the above link for detailed information about the Pressure Vessel Handbook, which specifically focuses on pressure vessel inspection.
Spherical Pressure Vessel - What is the construction Code for a Spherical Pressure Vessel? What is the In-Service Code for Spherical Pressure Vessel? What are the Spherical Vessel applications? What are the advantages?
Click on the above link to see the answer to the above questions, which is specifically focuses on pressure vessel inspections.
The ASME Code Section VIII refers to the ASME Code Section IX for welding procedure specification, procedure qualification record and welding performance qualification.
Essential variables are variables (such as P number, A number, F number) that directly affect the mechanical property of material so need the re-qualification of procedure qualification.
The nonessential variable is the variables that are not affecting the mechanical property. The WPS must be revised and no need to be re-qualified. The supplementary essential variable will be essential variable when construction code requires impact testing. An example of the supplementary essential variable is Group number.
