ASME PRESSURE VESSELS
The scope of this presentation is to present basic information and understanding of the ASME code for the design of pressure vessels for the chemical and process industry as applicable in the United States and most of North and South America.
REPAIRS AND ALTERATIONS – Part 4
Now here is the conclusion of this short series on repairs and alterations.
National Board Inspection Code vs. API 510
Inspection and Hydrotest
Inspection of Repairs
BOTH the National Board Inspection Code and API 510 require the acceptance of repairs to a pressure vessel by the authorized inspector BEFORE the vessel is returned to service. The authorized inspector will normally require performing all of the nondestructive examinations for the repair that were required by the ASME Code during original construction. Alternative NDE methods can be proposed (such as the substitution of UT for RT) when it is not possible, or practical, to use the NDE method that was used during construction.
All repair welds to vessels should be subjected to essentially full coverage NDE, in view of the more difficult working conditions that will usually be encountered for repairs compared to the favorable conditions in a fabrication shop. UT is an entirely acceptable NDE method for verifying the quality of welds, and does not involve the hazards and obstruction of other work associated with RT.
Hydrotest after Repairs
Neither the National Board Inspection Code nor API 510 makes it mandatory to perform a hydrostatic pressure test following the repair of a pressure vessel, but agreement from the authorized inspector is required for it to be waived. The purpose of the hydrotest in the ASME Code is to detect gross errors in the design, or major flaws in the construction of a new vessel. Repair of a vessel restores it to a satisfactory condition without any change in design, and, therefore, there is no need to verify the design of the repaired vessel. Furthermore, full coverage NDE of all repairs will detect much smaller flaws than those that could cause failure during a hydrotest, and will, therefore, provide greater assurance of the quality of the repair than a hydrotest.
In-Service Inspection after Repair
In-service inspection of a repaired pressure vessel should be planned and scheduled after it has been returned to service, to assure that the repair is providing sufficient integrity for reliable service. In-service inspection is especially important when the repair has been made by deviating from some details of the original construction, or from ASME Code rules, through the exercise of “engineering judgment”. The same NDE methods should be used that were previously employed to detect the deterioration of the vessel that necessitated the repair, and that were used to verify the quality of the repairs.
Approval of Repairs, Documentation, and Nameplate
The National Board Inspection Code requires that the repair organization document the repair of a pressure vessel by completing an R-1 form that is submitted to the authorized inspector for approval. Subse¬quent to obtaining approval of the R-1 form, the repair organization must attach a new nameplate to the repaired vessel. This name plate is stamped with an “R” symbol if the repair organization has a Certificate of Authorization from the National Board. The repair organization can NOT stamp this nameplate with a “U” symbol, despite using its Certificate of Authorization from the ASME to qualify it for making the repair. The ASME permits using the “U” symbol ONLY for the design and construction of NEW pressure vessels or NEW replacement parts. Completion of an R-1 form and attachment of a new nameplate may not be required for routine repairs, dependent upon consent of the jurisdiction and approval of the authorized inspector.
API 510 requires that the documentation of repairs to a pressure vessel must be kept as permanent records, but does not prescribe using a standard form. Furthermore, it does NOT require attaching a new nameplate to a repaired vessel. A new nameplate should NOT be attached to a vessel after a repair unless the governing jurisdiction mandates following the National Board Inspection Code. The original nameplate of the vessel has the primary purpose of permanently displaying the maximum allowable working pressure and temperature rating of the vessel. A repair does not change the rating of the vessel, and, therefore, a new name¬plate is unnecessary (unless required by the jurisdiction).
Alteration of a pressure vessel is a physical change to any component of the vessel that affects the pressure-containing capability. An alteration can change the maximum allowable working pressure and temperature rating of a vessel from that given on the original nameplate with a “U” stamp applied by the manufacturer. However, alterations can usually be designed not to affect the original rating of a vessel, when the operating pressure and temperature of the process are not changed.
Alterations to a pressure vessel are usually made to accommodate changes in process design. The installation of new nozzles in the vessel shell is a common alteration. Occasionally, internal components have to be changed for the new process designs. Although the internals are not pressure-containing components, the loads that they transmit to the vessel shell can affect its pressure-containing capability. Therefore, the effect that the design loads on the new internals (attribut¬able to pressure drop, static weight, and liquid head, etc.) have upon the stresses in the vessel shell should be calculated to determine if the maximum allowable working pressure of the vessel has to be changed.
Planning and Approval
The National Board Inspection Code requires all alterations of pressure vessels to conform to the ASME Code, whereas API 510 requires adhering to the “principles of the ASME Code”. The wording of API 510 allows more flexibility for designing the alterations when it is not advisable or practical to strictly conform to the ASME Code, under the circumstances prevailing for making the alteration.
Both the National Board Inspection Code and API 510 require authorization from the authorized inspector prior to initiating an alteration on the vessel. The authorized inspector will normally
- verify that the design of the alterations and the calculations have followed ASME Code criteria,
- determine that acceptable materials will be used, and
- assure that the weld procedures and welders are properly qualified.
It is important to note that API 510 requires the authorized inspector to consult with an experienced pressure vessel engineer before giving authorization to proceed with the alteration. This requirement assures that the greater flexibility that API 510 permits for designing alter¬ations is properly exercised consistent with sound engineering practice.
Organization Making Alterations
The National Board Inspection Code requires that an organization performing an alteration has an ASME Certificate of Authorization covering the scope of work involved. API 510 does not contain specific requirements for an organization performing an alteration. Presumably, the same requirements would apply as for a repair organization. Alterations can be designed by qualified pressure vessel and materials engineers, but it is recommended that only organizations holding an ASME Certificate of Authorization perform the work on the vessel.
Materials, Replacement Parts, Welding, Postweld Heat Treatment, and Inspection
The requirements for the alteration of pressure vessels concerning materials, replacement parts, welding, postweld heat treatment, and inspection are identical to those for repairs.
Hydrotest after Alterations
Hydrotesting vessel alterations is a mandatory requirement of the National Board Inspection Code (Paragraph R-308.2). API 510 states that hydrotesting is normally required after an alteration, but permits waiving the hydrotest after consultation with a pressure vessel engineer if superior designs, materials, fabrication procedures, and inspections are used.
A hydrotest should be performed after an alteration whenever possible. An alteration, by definition, changes the design of at least one component of the vessel shell, and the validity of the design changes cannot be verified by comprehensive inspection. In this regard, an alteration differs significantly from a repair, which does not involve design changes.
The pressure for the hydrotest should be the minimum test pressure required by the ASME Code for the design pressure and temperature appearing on the vessel's nameplate. This test pressure will normally be lower than the recommended hydrotest pressure required for new vessels because it is likely that some of the orig¬inal corrosion allowance will have been consumed during service before the alteration is made.
Hydrotesting pressure vessels that have been altered by the installation of a new nozzle requiring reinforcement is occasionally accomplished by welding a cap to the inside of the vessel shell covering the nozzle. This circumvents the inconve¬nience of preparing the entire vessel for hydrotest by providing for a “local hydrotest” of the nozzle opening. However, a local hydrotest performed in this manner will not develop the same stresses in the nozzle reinforcement and the vessel shell component surrounding the opening as would be developed by hydrotesting the entire vessel. The cap will effectively change the shape of the vessel shell component to which it is welded, and, consequently, have a significant effect on the stresses developed in that component by internal pressure. Therefore, a local hydrotest is not a valid verification of the design of an alteration, and this practice is NOT recommended.
Approval of Alterations, Documentation, and Nameplate
The National Board Inspection Code requires the organization performing the alteration of a pressure vessel prepare an R-1 form, which must be submitted to the authorized inspector for approval. The organization performing the repair must then attach a new nameplate that displays the design pressure (or MAWP) and temperature for the altered vessel. Approval of the alteration and attachment of the new nameplate MUST be obtained from the authorized inspector BEFORE the vessel is returned to service.
API 510 requires that the documentation of alterations to pressure vessels must be kept as permanent records, but it does not prescribe using a standard form. Approval of an alteration by the authorized inspector is required before the vessel is returned to service, but attachment of a new nameplate is not mandatory unless the design pressure (or MAWP) and temper¬ature are changed by the alteration.
Rerating a pressure vessel consists of changing the design pressure (or MAWP) and/or temperature from those displayed on the vessel's nameplate. Rerating usually does not involve a physical alteration of the pressure-containing capability of the vessel, but can be required by alterations that are not designed for the original design pressure and/or temperature. Rerating is most commonly necessitated by:
- A change in operating conditions for the process.
- Deterioration (i.e., the occurrence of corrosion or cracking) that affects vessel integrity and reliability for the original design pressure and temperature, and a repair cannot be economically justified.
Organization Performing Rerating
The National Board Inspection Code requires the rerating of a pressure vessel to be performed by the original manufacturer whenever possible. The rerating can be performed, however, by a registered professional engineer if the rerating cannot be obtained from the manufacturer.
API 510 permits either the original manufacturer or an experienced engineer employed by the owner/operator to perform the rerating. Only engineers with appropriate experience with pressure vessel design, fabrication, and inspection should perform re-ratings. A consultant retained by the owner/operator is also acceptable.
Rerating a pressure vessel requires making calculations for every major pressure-containing component (i.e., shell, heads, nozzles, reinforcements, and flanges, etc.) to verify that they will be adequate for the new design pressure and temperature. The effect of all internal and external loads on the vessel shell must be considered in the calculations for rerating. Therefore, rerating involves repeating all the calculations that were made for the original design of the vessel for the new design pres¬sure and temperature. However, it can be thought of as designing a pressure vessel in reverse. Instead of calculating the minimum required thickness for each shell component for the prescribed design pressure and temperature, the calculations are made to determine if the actual thickness of each shell component is adequate for the rerated pressure and temperature.
Both the National Board Inspection Code and API 510 require making the calculations according to the edition of the ASME Code controlling the original design and construction of the vessel. Alternatively, the latest edition of the ASME Code can be applied, if all details of design and fabrication can be verified to comply with this edition.
Decrease in Pressure
Rerating of a pressure vessel for a lower pressure is usually required if:
- The operating temperature is increased for new process conditions.
- Corrosion has reduced the remaining wall thickness below the minimum required thickness for the original design conditions.
Increase in Pressure
Rerating of a pressure vessel for a higher pressure can usually be accomplished only if:
- The operating temperature is decreased.
- Thickness measurements of all pressure-containing shell components indicate that the original corrosion allowance was greater than necessary for the actual corrosion experienced, and, therefore, some of this corrosion allowance can be used to gain additional shell thickness. However, the portion of the corrosion allowance that can be applied to the additional shell thickness should be evalu¬ated with regard to the new process conditions.
Increase or Decrease in Temperature
An increase in the temperature will almost always require decreasing the pressure, unless the new temperature remains below 450°F. A decrease in temperature will almost always permit an increase in pressure, unless the original temperature was 450°F or below. Rerating for a lower temperature should never be allowed to violate the rules in the current edition of the ASME Code for low temperature operation to assure adequate resistance to brittle fracture, regardless of the rules in the ASME Code when the vessel was built. It is, therefore, essential to check the vessel being rerated for compliance with the current rules for low temperature operation when the new temperature will be 120°F or below. This may be very difficult to do when the vessel is old and the materials used for construction are now obsolete. Under these circumstances, it may be necessary to cut samples from the vessel for CV-impact testing to perform a satisfactory rerating.
A very thorough inspection should be made to assure that the vessel is in satisfac¬tory condition for the new pressure and temperature. It is especially important to determine the minimum remaining thickness of every pressure-containing compo¬nent of the vessel shell, and to detect any cracks that may have developed during service. This will usually require more NDE than normally performed during a routine in-service inspection.
Approval of Rerating, Documentation, and Nameplate
The National Board Inspection Code treats the rerating of a pressure vessel as an alteration with respect to the requirements for preparation of an R-1 form, approval by an authorized inspector, and attachment of a new nameplate displaying the new pressure and/or temperature. API 510 also requires approval of the rerating by an authorized inspector and attachment of a new nameplate. The new nameplate should be considered mandatory, because the pressure and/or temperature for the rerated vessel differ from those displayed on the original nameplate.
We hope this will give you a better understanding of the requirements and methods to extend the life of your pressure vessel. If you need any assistance in all with planning your repair, alteration or rerate, please contact us here at Boardman. It is a pleasure to serve you.
We encourage your feedback and your suggestions for topics which interest you and you business.
Source: National Board Inspection Code
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