Authors: Matt Schaser, P.E., Senior Engineer II

The service life of a pressurized equipment item may exceed its intended design life by many decades.  Most equipment items are designed for a 20-year service life but remain in service for more than 50 years.  Over this extended service life, the equipment item’s process conditions can change due to a need to increase production or because of a service change due to repurposing.  These changes can include an increase in pressure and/or temperature.  Even if the process conditions don’t change over the service life, anticipated (recurring) operating excursions can result in a need to increase the design pressure and/or temperature.  When production requirements, service changes, or anticipated excursions identify operating conditions that result in increases beyond the stamped pressure and/or temperature rating, or a temperature reduction below the stamped minimum design metal temperature (MDMT), a rerate of the equipment item is required by most jurisdictions.

Rerates are performed to qualify the equipment item for its new intended operation conditions.  Procedures for performing rerates can vary depending on jurisdictional requirements.  Jurisdictions that have adopted the rules in the National Board Inspection Code (NBIC) as their governing guidance for in-service pressurized equipment require that the rules in Part 3 of the NBIC be adhered to regarding rerates.  The NBIC defines a rerate as an alteration; therefore, consistent documentation and procedures are required as for all alterations. Jurisdictions that have not adopted the NBIC as the governing guidance allow rerates to be performed in accordance with industry standards such as the American Petroleum Institute (API) standard API 510 “Pressure Vessel Inspection Code:  In-service Inspection, Rating, Repair, and Alteration.”  Often operators in non-NBIC jurisdictions will opt to follow the guidance and procedures in NBIC when performing rerates.  The NBIC method has the advantage of providing traceability as documentation is retained by the NBIC for equipment assigned National Board (NB) numbers at the time of fabrication and provides consistency in corporate standards for operators with site locations in both NBIC and non-NBIC jurisdictions.  Rerating in accordance with either code requires performing code calculations to confirm and document that the equipment item is justified to operate at the rerate conditions.  Additional requirements include the review of inspection records, creation of a new nameplate, and pressure test (if required).

There are many questions to consider when rerating an equipment item:

• Is the jurisdiction NBIC or non-NBIC?  
• Are there additional jurisdictional requirements?  
• Which edition of the design code should be used for the rerate?  
• Is original design documentation available, such as a data report, drawings, etc.?  
• Is current inspection data available?  
• Is the original nameplate still attached?  
• Is documentation available for all previous repairs and alterations?  
• Have undocumented repairs been performed?  
• Is the original corrosion allowance required at the rerate condition?  
• Can the equipment item be hydrotested in its current configuration?  
• Does the equipment item contain damage such as cracks, metal loss in excess of the design corrosion allowance, creep damage, fatigue damage, etc.?  

Depending on the answers to these questions, a rerate may or may not be feasible.  Note that performing rerates on equipment items where not all these questions can be answered with confidence can result in elevated risk when operating at the rerate conditions.

Rerates in accordance with the NBIC will need to be performed by accredited organizations holding an R-Stamp certificate.  The organization will have a quality program approved by the NBIC and must maintain an inspection agreement with an authorized inspection agency (AIA).  An authorized inspector (AI) from this agency will be required to review calculations and inspection records, witness the attachment of the nameplate and the hydrotest (if required), and certify the Report of Alteration form (R-2).  Consideration should be given to the time involved in these tasks when scheduling a rerate.  Rerates performed in accordance with API 510 do not require a third-party AI’s involvement; instead, only an owner-user AI is required, and the rerate can generally be performed more quickly.  API 510 rerates require certification of the “Statement of Compliance” section on the API 510 form by the organization performing the rerate calculations and certification of the “Statement of Inspection” section on the API 510 form by an API 510 inspector employed by the owner acknowledging that work has been satisfactorily completed in accordance with API 510. 

Due to changes in the codes over the years, consideration must be given to which edition of the code to use for the rerate calculations.  Code editions consistent with the fabrication date of the equipment item can be used for the rerate; however, there may be advantages to using later editions of the code, if acceptable to the jurisdiction.  Both NBIC and API 510 allow for use of a safety factor of 3.5 associated with post-1999 addendum of the American Society for Mechanical Engineers (ASME) Boiler and Pressure Vessel Code Section VIII, Division 1 (ASME VIII-1) for equipment items built to the 1968 and later editions of the ASME VIII-1 code where the safety factor was 4.0.  The use of post-1999 editions allows for higher calculated MAWPs or larger corrosion allowances.  In addition, later editions of the code only require a 1.3 hydrotest load factor compared to the 1.5 factor for earlier codes, which may be the difference between requiring a hydrotest for the rerated equipment item or not.  Neither NBIC nor API 510 requires additional hydrotests for rerates where the calculated hydrotest pressure does not exceed the original hydrotest pressure documented on the data report.  This can be advantageous for equipment items that cannot come out of service for the hydrotest or cannot be hydrotested due to hydrophobic internals, such as refractory.  Note that additional requirements accompany the use of later additions of the code, such as establishing an MDMT, which may prove to be limiting over the design MDMT at the time of fabrication.  Also, the option for later editions may be prohibited based on lethal service designation or high cycle operation.

Rerating equipment items that do not possess the required nameplate, records, or stamping (NRS) can involve risk.  Rerates in accordance with NBIC are not permitted for equipment items with undocumented repairs as there is no traceability to weld quality.  API 510 provides guidance for equipment items lacking documentation; however, its acceptance for establishing a rating may be dependent on the local jurisdiction.

Rerates may be performed on equipment items containing damage.  In this situation, a rerate would be performed on the equipment item assuming no damage was present.  Once rerate conditions were determined acceptable, a subsequent fitness-for-service (FFS) assessment could be performed to qualify the equipment item with damage for continued service at the rerate conditions.  Rerates of equipment items containing damage may be subject to approval by the jurisdiction.

The following case studies represent typical rerate requests and showcase some of the options available when performing a rerate assessment.

Case Study 1:

A pressure vessel built to the 1974 edition of the ASME VIII-1 code is stamped for 200 psig at 650°F/340°C with 0.125-inch/3.175 mm corrosion allowance.  The vessel was originally hydrotested at 300 psig.  The owner indicates that pressure excursions exceeding the design pressure of the vessel often occur and requests a pressure rerate to 225 psig at 650°F/340°C while maintaining original corrosion allowance.  Rerate calculations performed to the original edition of the code indicate the vessel’s limiting MAWP is 205 psig.  Since the vessel was constructed to an edition of the code 1968 or later, supplemental calculations using post-1999 addendum allowable stresses were used to rerate the vessel.  The calculations using the new allowables indicate the vessel’s limiting MAWP is 234 psig.  Therefore, the vessel can be rerated for 225 psig.  The required hydrotest for the rerate condition using the later edition of the code is 293 psig (1.3 x 225 psig).  Since the original hydrotest pressure exceeds the required hydrotest pressure for the rerate, no additional hydrotest is required.

Case Study 2:

An exchanger built to the 2004 edition of the ASME VIII-1 code is stamped for 250 psig at 650°F/340°C (tubeside) and 180 psig at 650°F/340°C (shellside) with 0.125-inch/3.175 mm corrosion allowance.  The owner wishes to avoid a pressure relief scenario for the shellside of the exchanger due to tube rupture and requests a shellside rerate to 193 psig.  Rerate calculations performed for the shellside components including the original corrosion allowance indicate the shellside is limited to an MAWP of 185 psig.  Supplemental calculations with a reduced corrosion allowance of 0.119 inches indicate the shellside components are limited to an MAWP of 193 psig.  The required hydrotest pressure for the rerate condition is 251 psig (1.3 x 193 psig).  Since the shellside components will be subjected to a hydrotest pressure equal to or more than the tubeside rated pressure, the relief scenario for tube rupture is no longer required, per API 521.

Case Study 3:

A pressure vessel built to the 1965 ASME VIII-1 code is stamped for 180 psig at 400°F/200°C and includes B16.5 Class 150 nozzle flanges.  The owner wishes to increase production, which requires running the equipment at 500°F/260°C.  A rerate is required to operate at this elevated temperature.  Rerate calculations performed to the original edition of the code of construction indicate that the vessel is limited to a maximum temperature of 400°F/200°C by the pressure rating of the Class 150 flanges.  Supplemental calculations using ASME VIII-1, Appendix 2 were used to qualify the flanges at the rerate temperature of 500°F/260°C.  The vessel, therefore, met the code requirements at the rerate temperature with engineered flanges. 

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