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. For more information about our products, heavy plate & custom fabrication services or fabrication capabilities contact us today!
REPAIRS AND ALTERATIONS – Part 3
National Board Inspection Code vs. API 510
This article discusses the steps in how a facility would need to go about getting one of their 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 organi¬zation; materials; replacement parts; welding; postweld heat treatment (PWHT); inspections and hydrotest 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 construction of a new vessel in a fabrication shop. Construction of a vessel in a fabrication shop permits 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 for 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 within the process may make it neces¬sary 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 in advance of the scheduled shutdown to make certain that the repair organiza-tion 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 unfavor¬able working conditions that might prevail. More extensive NDE than was required for the original construction is HIGHLY RECOMMENDED.
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 obtain¬able 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 liquid 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 pres¬sure 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 iden¬tical 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 allow¬able 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 prop¬erly 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 hard¬ness 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 authoriza¬tions 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 engi¬neering judgment.
Both codes permit the authorized inspector to be an employee of the owner/oper¬ator, 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 proce¬dures. 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 repairs, alterations, and re-ratings of pressure vessels. 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 proce¬dures), 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 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)
- Welding procedures
- NDE of repairs
Repairs that will be made by a contractor 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.
It may not always be possible to obtain authorization from an authorized inspector before making emergency repairs. Under these circumstances, the repair can be initiated prior to submitting the plan to the authorized inspector, but 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 owner/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.
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.
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.
The National Board Inspection Code requires qualification of all welding proce¬dures used for the repair of a pressure vessel, including the manufacture of replace¬ment 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 orga-nization 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 proce¬dure 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 mate¬rials 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.
We will continue with the procedure to repair your vessel in Part 4. If you have questions or comments, please contact us here at Boardman. We are here to assist in the repair or alteration of your equipment.
Source: National Board Inspection Code
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