Interpretation Response #25-0066
Below is the interpretation response detail and a list of regulations sections applicable to this response.
Interpretation Response Details
Response Publish Date:
Company Name: The Hartford Steam Boiler Inspection and Insurance Company
Individual Name: Timothy Nuoffer
Location State: CT Country: US
View the Interpretation Document
Response text:
January 22, 2026
Timothy Nuoffer
Field Services Manager
The Hartford Steam Boiler
Inspection and Insurance Company
One State Street
Hartford, CT 06102
Reference No. 25-0066
Dear Mr. Nuoffer:
This letter is in response to your May 14, 2025 letter requesting clarification of the Hazardous Materials Regulations (HMR; 49 CFR Parts 171-180) applicable to the repair and modification of MC 331 specification cargo tanks. You explain that your company has received questions regarding repairs to Department of Transportation (DOT) MC 331 specification cargo tanks that underwent postweld heat treatment (PWHT) during their construction. You ask several questions and seek confirmation of your understanding as it relates to § 180.413, the Compressed Gas Association (CGA) Technical Bulletin TB-2, Guidelines for Inspection and Repair of MC 330 and MC 331 Cargo Tanks, and the NB-23, National Board Inspection Code (NBIC), A Manual for Boiler and Pressure Vessel Inspectors.
We have paraphrased and answered your questions as follows:
Q1. What edition of the NBIC must be used for the repair or modification of an MC 331 specification cargo tank?
A1. The 1992 edition of the NBIC—incorporated by reference in the HMR—is necessary to perform what makes up a "repair" for an MC 331 specification cargo tank (see § 180.413(b)(6)). By contrast, performing a "modification" on an MC 331 specification cargo tank is prescribed in § 180.413(a)(1) and (d). Please note if any modification involves welding on the cargo tank wall, it must meet all requirements for "repair," as defined in § 180.403 and the use of the 1992 edition of the NBIC.
Q2. For instances where PWHT is required for "non-minor" weld repairs, the 1992 edition of NBIC NB-23 states that alternative methods for PWHT may be used if it involves a "special welding method" that is acceptable to the inspector. It also emphasizes that, where necessary, competent technical advice should be obtained from the manufacturer of the object or from another qualified source. You ask, since the 1992 edition of NBIC NB-23 does not provide examples of "special welding" methods for P-No 11A and P-No 11B materials, may the alternative welding methods without PWHT, as described in the 2023 edition of NBIC NB-23 Part 3, Repairs and Alterations—which includes methods for P-No 11A and 11B materials—be considered as "another qualified source" per the 1992 Edition and not require a special permit?
A2. Yes. Welding methods authorized by the applicable edition of the NBIC may be used for an MC 331 specification cargo tank weld repair, provided the requirements of § 180.413(b)(6) are met. Section 180.413(b)(6) requires that each cargo tank having cracks or other defects requiring welded repairs must meet all inspection, test, and heat treatment requirements of § 178.337-16 in effect at the time of the repair, except that PWHT is not required after minor weld repairs. Accordingly, MC 331 specification cargo tanks undergoing non-minor weld repairs remain subject to the applicable heat treatment requirements. Please note that alternative welding methods described in external publications (i.e., reference materials) do not replace or waive the requirements; therefore, waiving any HMR mandated heat treatment requirement would require issuance of a special permit.
Q3. As it relates to § 180.413(b)(6), which references minor weld repair, the CGA TB-2 appears to describe a "minor repair" in section 3.4. What is the definition of "minor repair"?
A3. A "minor weld repair" as referenced in § 180.413(b)(6) is not defined in either § 171.8 or § 180.403 of the HMR. However, CGA TB-2 describes what constitutes a "minor repair" in section 3.4. Additionally, section 3.7 of CGA TB-2 states that no PWHT is required after a minor repair, which is referenced in § 180.413(b)(6) for "minor weld repairs." Thus, it is the opinion of this Office that, in this context, weld repairs as described in CGA TB-2 and performed in accordance with requirements of § 180.413(b)(6) and CGA TB-2 may be considered a "minor repair."
Q4. Does PHMSA permit a repair facility to forgo PWHT or any alternative methods on an MC 331 specification cargo tank after performing a non-minor welded repair?
A4. No. A repair facility is not allowed to forgo PWHT on an MC 331 cargo tank after making a non-minor welded repair. PWHT is excepted only for minor weld repairs, as referenced in § 180.413(b)(6).
Q5. If a repair facility is adding non-pressure retaining and non-structural pads to the inside of an MC 331 specification cargo tank that is welded directly to the shell or head by fillet welds, would PWHT be required?
A5. Yes. Direct welding to the shell or head(s) of a cargo tank would affect the structural integrity by introducing uneven heating and cooling, as well as residual stress. Therefore, installation of mounting pads by welding would be considered a modification. Note, § 180.413(b)(6) of the HMR requires non-minor weld repairs to an MC 331 specification cargo tank be accompanied by PWHT.
I hope this information is helpful. Please contact us if we can be of further assistance.
Sincerely,
Dirk DerKinderen
Chief, Standards Development Branch
Standards and Rulemaking Division
§ 171.8, § 178.337-16, § 180.403, § 180.413, § 180.413(a)(1), § 180.413(b)(6), § 180.413(d)