This article discusses how a facility would need to get one of its coded pressure vessels repaired, altered, or re-rated. The article will cover pertinent code and jurisdiction requirements and the differences among the American Society of Mechanical Engineers (ASME) Code, the National Board Inspection Code (NBC), and American Petroleum Institute (API) requirements, as well as discuss the following: design of repairs; planning and approval; responsible organization; materials; replacement parts; welding; postweld heat treatment (PWHT); inspections and hydro test procedures; and documentation and nameplates.
A pressure vessel inspection program will, from time to time, reveal that some form of deterioration is occurring in a vessel. Analysis of the deterioration may indicate that the vessel must either be repaired under the original design conditions or that the vessel must be rerated for less severe design conditions. Rerating for new design conditions may also be necessary because of changes in operating requirements.
The size and location of a pressure vessel within a process can make repair or alteration considerably more difficult than the construction of a new vessel in a fabrication shop. Construction of a vessel in a fabrication shop permits the positioning of the vessel to obtain favorable access and orientation for welding and other necessary work. Access to a vessel within a process can be obstructed by adjacent equipment, resulting in welding and other work having to be performed in unfavorable orientations and/or with restricted access to tools and equipment. Therefore, it may not be possible to comply with all of the requirements of the ASME Code that were mandated for the original design and construction of the vessel. The inability to repair a pressure vessel while it is in the process may make it necessary to move the vessel to a fabrication shop. Repairs of this nature will normally require more time than if performed with the vessel remaining in place. Plans should be made well before the scheduled shutdown to ensure that the repair organization has room for the vessel on their shop floor and manpower available to complete the repairs on schedule.
Nondestructive examination (NDE) of repairs and alterations is very important to assure that high integrity and reliability have been obtained because of the unfavorable working conditions that might prevail. More extensive NDE than was required for the original construction is HIGHLY RECOMMENDED.
ASME Code
The ASME Code applies directly only to the “design, fabrication, and inspection during [the original] construction of pressure vessels”. This statement is normally interpreted to mean that the direct applicability of the ASME Code terminates when the authorized inspector authorizes application of the ASME Code Stamp to the nameplate on a new vessel. The ASME Code is formulated around design details and fabrication procedures (welding procedures and postweld heat treatment, etc.) that are obtainable with good shop practices, and that are known to provide a high level of quality.
Most jurisdictions have pressure vessel laws that require owner/operators to operate and maintain their vessels in a “safe condition”. The majority of these jurisdictions have established regulations that refer to either:
The National Board Inspection Code or The API Pressure Vessel Inspection Code (API 510) for the repair, alteration, and rerating of pressure vessels.
The facility will need to determine which code governs their repair, based on local jurisdictional requirements. Many flammable lqiuid processing plants and refineries prefer to use the API Code whenever possible, because it is specifically oriented to the needs of the hydrocarbon processing industry and provides greater flexibility for exercising “engineering judgment.”
It is the obligation of every facility to be aware of and comply with the pressure vessel laws of the governing jurisdiction. Many jurisdictions make the owner/operator responsible for obtaining approvals and filing the documentation for repairs, alterations, and rerating – although another organization performing the work may actually prepare the reports and submit them to an authorized inspector for approval. Regardless of the jurisdictional requirements, all repairs, alterations, and re-ratings of pressure vessels MUST be accomplished in a manner to assure the continued integrity and reliability of the vessels by properly exercising the best engineering judgment.
National Board Inspection Code vs. API 510
The technical requirements of the National Board Inspection Code concerning repair, alteration, and re-rating are similar to those in API 510. It is very likely that the technical details of accomplishing a repair, rerating, or alteration would be identical regardless of which code is used. However, the procedural and administrative aspects of these two codes differ considerably. The major differences are that the National Board Code:
Requires an authorized inspector to hold a commission from the National Board.
Restricts the authority of an authorized inspector employed by the owner/ operator.
Requires preparation and approval of an R-1 form, and attachment of a new nameplate for repairs and alterations that do not change the maximum allowable working pressure (MAWP) and design temperature.
The more elaborate procedural and administrative details of the National Board Code do not necessarily result in repairs and alterations that have higher integrity, but they can considerably increase the costs incurred, especially if they delay the return of a pressure vessel to service. Differences between the codes are discussed below, including specific differences concerning repairs, alterations, and rerating.
In general, to expedite the repair, API 510 permits greater flexibility through the exercise of “engineering judgment” by the owner/operator than is usually possible when following the National Board Code. The “owner/operator” has more responsibility for the integrity of a repair completed under API 510 than for repairs made under the rules and requirements of the National Board Code, which are subject to the review and approval of an authorized inspector.
References to the ASME Code
Both the National Board Inspection Code and API 510 refer to the ASME Code for making repairs, alterations, and re-ratings of pressure vessels. However, the wording used by the National Board and API conveys somewhat different implications.
The National Board Inspection Code requires ALL repairs and alterations to conform to the ASME Code whenever possible, whereas the API Code requires “following the principles of the ASME Code”. Both the National Board and API codes recognize that it may not always be possible to adhere strictly to the ASME Code when making repairs or alterations. However, the implication of the wording in the National Board Inspection Code is that the ASME Code must be complied with whenever possible. By comparison, API 510 permits more flexibility for deviating from the ASME Code by exercising “engineering judgment”. Strictly complying with the design details and fabrication requirements in the ASME Code may not always result in a repair or alteration with the greatest integrity and reliability because of the working conditions where the vessel is installed.
However, design details for repairs and alterations that deviate from the rules of the ASME Code should be justified by an appropriate stress analysis to verify that the maximum allowable stress permitted by the Code is not exceeded. Fabrication procedures that differ from the original construction must be properly qualified to verify that:
- the minimum materials properties (strength and CV-impact toughness) required by the ASME Code are obtained, and
- any other materials requirements specified for the service conditions (such as maximum hardness of the weld metal and heat affected zones) are achieved.
Reference to the ASME Code connotes the edition used for the original design and construction of the vessel. The current edition of the ASME Code can be used, when it is advantageous to do so, only if all details of the original design and construction comply with the current edition.
Authorizations and Approvals
There are significant differences between the National Board Inspection Code and API Code in granting authorizations for and approvals of repairs, alterations, and re-ratings. BOTH the National Board and the API Code REQUIRE obtaining authorizations and approvals from an “authorized inspector”. However, the National Board Inspection Code requires the authorized inspector to hold a commission from the National Board, whereas the API Code requires only that the inspector “be qualified to perform the inspection by virtue of his knowledge and experience”.
The National Board Inspection Code emphasizes compliance with its rules through the scrutiny of an authorized inspector. This requirement is consistent with its dictum to conform to the ASME Code whenever possible. The API Code relies to a much greater extent on the expertise of pressure vessel or materials engineers to assure the continued integrity and reliability of a pressure vessel and allows the authorized inspector to base his authorizations and approvals on consultations with pressure vessel engineers. This practice follows from its underlying concept of adhering to the principles of the ASME Code while allowing flexibility to use engineering judgment.
Both codes permit the authorized inspector to be an employee of the owner/operator, but the National Board Inspection Code prohibits an employee from approving work performed by his employer unless the governing jurisdiction (or National Board) has given its consent upon review of the owner/operator inspection procedures. The API Code contains no such restriction.
Reports, Records, and Nameplates
The National Board Inspection Code establishes a formal administrative procedure for documenting and recording pressure vessel repairs, alterations, and re-ratings. An “R-1 Form” must be completed by the company performing the work, and submitted to the Authorized Inspector for approval. Copies of the R-1 Form are subsequently sent to the owner/user, governing jurisdiction (dependent upon administrative procedures), and National Board for permanent record. In contrast, the API Code requires only that the owner/user maintain permanent records that document the work performed.
BOTH the National Board Inspection Code and the API Code REQUIRE attaching a new nameplate adjacent to the original nameplate when a vessel is altered or re-rated. It must be attached to the altered or rerated vessel by the company performing the work AFTER the R-1 form has been approved by the authorized inspector.
The National Board Inspection Code also requires attaching a new nameplate to a vessel that has been repaired (with the exception of routine repairs) in a manner similar to that for an altered or rerated vessel. The API Code has no requirement for attaching a new nameplate after repairs.
Repair
Repair of a pressure vessel is the work necessary to restore the vessel to a suitable condition for safe operation at the original design pressure and temperature, providing that there is NO CHANGE in design that affects the rating of the vessel. A vessel must either be repaired or replaced when deterioration renders it unsatisfactory for continued service.
It is generally more economical to repair a pressure vessel than to replace it, but the primary consideration is integrity and reliability for continued service. Some forms of deterioration, such as creep and hydrogen attack, may indicate that the useful remaining life of the vessel is too short to justify the expense of a repair. Furthermore, the detection of other forms of deterioration, such as H2S stress cracking, may indicate that the vessel is not satisfactory for the service environment and the deterioration will recur after repair (presenting a continuing maintenance problem).
Planning and Approval
Both the National Board Inspection Code and API 510 require obtaining authorization for making a repair from the authorized inspector before the work is initiated, except for “routine repairs” when prior approval has been given by the authorized inspector.
Authorization for making a repair that is not routine is obtained from an authorized inspector by preparing and submitting a repair plan. The repair plan should be prepared by an engineer at the facility in consultation with a maintenance coordinator, and it should include the following information:
- Areas of vessel to be repaired.
- Repair procedures to be used for each area specifying:
- Preparation for repair (removal of deterioration)
- Materials
- Welding procedures
- NDE of repairs
Repairs that a contractor will make should be discussed with the contractor to obtain agreement with the plan BEFORE it is submitted to the authorized inspector. Plans for complex repairs (i.e., beyond the experience of an engineer at the facility) should be discussed with a pressure vessel or materials engineer.
Obtaining authorization from an authorized inspector before making emergency repairs may not always be possible. Under these circumstances, the repair can be initiated prior to submitting the plan to the authorized inspector. Still, complete documentation should be preserved and submitted to the inspector for his acceptance as soon as possible. The vessel cannot be returned to service until acceptance of the repair has been obtained from the authorized inspector.
Organization Making Repair
The National Board Inspection Code requires the organization performing a repair to have either a Certificate of Authorization from the National Board for the use of an “R” stamp or a Certificate of Authorization from ASME for the use of a “U” stamp.
API 510 also accepts an organization having an ASME “U” stamp as qualified to make repairs but makes no mention of a National Board “R” stamp. In addition, API 510 permits owners/operators to repair their own vessels in accordance with its requirements, and to have repairs made by contractors whose qualifications are acceptable to them.
All repairs that are not routine should be performed by an organization that has a valid “U” stamp, regardless of code or jurisdiction requirements that might permit repair by other organizations.
Repair Materials
Both the National Board Inspection Code and API 510 REQUIRE that the materials used for the repair must be an acceptable material of construction in the ASME Code. In other words, the materials must conform to one of the specifications in ASME Code, Section II. The materials used for a repair should be the same as those used for the original construction whenever possible. When this is not possible, the selection of alternative materials should be discussed with pressure vessel and materials engineers.
Replacement Parts
A repair can involve replacing a deteriorated part with a new part of the same design that is manufactured in a shop. Manufacturing a replacement part generally requires welding. If the ASME Code requires inspection of the weld joints by an authorized inspector, the National Board Inspection Code requires the replacement part to be manufactured by an organization that has an ASME Certificate for a “U” stamp. A “U” stamp with the word “part” is applied to the part when it is accepted by the authorized inspector. Replacement parts that do not require inspection by an authorized inspector are not required to be manufactured by a holder of an ASME Certificate of Authorization.
API 510 requires replacement parts to be manufactured according to the principles of the ASME Code, but has no requirement concerning the qualifications of the manufacturer. A general recommendation, however, is that all replacement parts should be manufactured by an organization that has a Certificate of Authorization from ASME for the use of a “U” stamp.
Repair Welding
The National Board Inspection Code requires qualification of all welding procedures used for the repair of a pressure vessel, including the manufacture of replacement parts, according to ASME Code, Section IX. Furthermore, ALL welders working on the repair must pass a welder performance qualification for each welding procedure used. The repair organization must make the records of procedure and performance qualification available to the authorized inspector BEFORE the actual repair welding is started.
API 510 requires the repair organization to qualify all welding procedures and welders used for a repair according to the principles of ASME Code, Section IX. This wording (i.e., “according to the principles”) differs from that used in the National Board Inspection Code (i.e., “according to”), and allows more flexibility for deviating from a welding procedure acceptable to the ASME Code when necessary to expedite a repair. The welding procedures used for a repair should not deviate from what has been qualified according to ASME Code, Section IX, unless the proposed procedure has been reviewed by pressure vessel and materials engineers with regard to the design of a repair and service requirements of the vessel to assure adequate integrity and reliability for continued service.
Postweld Heat Treatment
Postweld heat treatment (PWHT) can be a very difficult aspect of the repair and, when improperly performed, can cause additional damage to the vessel. Repair welds should receive the same PWHT used for the original construction whenever possible. PWHT of a repair weld is especially important when it was specified for the original construction of the vessel to prevent stress corrosion cracking in the process environment.
PWHT of a repair weld is most often accomplished by the local application of heat to the repaired area while the remainder of the vessel is at ambient temperature. During PWHT high thermal stresses that can damage the vessel may develop because of severe temperature gradients and restrained thermal expansion. Nozzles, head-to-shell weld joints, attachment welds for vessel supports, piping connections, and internal components are particularly vulnerable to damage. Care should be taken to be certain that the vessel is free to expand when the local area is heated, and efforts should be made to keep temperature gradients less than 100°F per foot along the surface and 100°F per inch through the thickness at temperatures above 400°F. When it is not possible to perform a local PWHT within these guidelines, the risk of damage to the vessel should be carefully evaluated and alternatives for repair without postweld heat treatment should be considered.
Both the National Board Inspection Code and API 510 permit substituting a temper bead (or half bead) welding procedure for postweld heat treatment for the repair of carbon steel pressure vessels. However, neither of these documents requires a separate qualification of this welding procedure to demonstrate that the weld metal and heat affected zones of the repaired vessel will have the properties required to assure adequate integrity and reliability for continued service (minimum strength, maximum hardness, and CV-impact toughness).
ASME Code, Section IX, contains superior requirements for qualifying and performing this type of repair weld. The requirements of Section IX have been adapted into recommendations for repair welding with a temper bead technique. These procedures should not be used for the repair of a vessel unless they are discussed with pressure vessel and materials engineers, giving consideration to the design of the vessel and its service conditions. The justification for the use of these procedures should also be thoroughly discussed with the authorized inspector to obtain approval before the repair welding is started.
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. Subsequent 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 nameplate is unnecessary (unless required by the jurisdiction).
Alterations
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 (attributable 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 alterations 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 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 original 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 inconvenience 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 temperature are changed by the alteration.
Rerating
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.
Calculations
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 pressure 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 evaluated 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.
Information Required
A very thorough inspection should be made to assure that the vessel is in satisfactory condition for the new pressure and temperature. It is especially important to determine the minimum remaining thickness of every pressure-containing component 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.