You should review the requirements of the National Board Inspection Code for guidance on the subject of inspection and repairs. Compliance with the NBIC is required if the owner needs to keep the vessel's NB (code) registration.

Al

An available reference could be ASME PCC-2, 2011.
Article 2.2 - External Weld Overlay to Repair Internal Thinning
Article 2.10 - In-Service Welding onto Carbon Steel Pressure Components or Pipelines
Article 2.11 - Weld Build-up, Weld Overlay and Clad Restoration

Welding procedures, welders, and welding operators
shall be qualified in accordance with the current requirements
of the applicable code of construction or
post-construction code. If not otherwise specified,
ASME BPVC Section IX may be used for procedure and
performance qualifications.

1. is pressure vessel stamped?
2. is pv located in USA?
3. are you performing the welded work as an Owner or Subcontractor?

Assuming this is a stamped vessel and subject to NBIC & jurisdicational requirements, you (performing welded work) have an (required) R stamp then you must have Part 3 NBIC Repairs & Alterations standard.

This appears to be a routine repair as stated in Part 3 NBIC 2011 edit. 3.3.2 (4) Weld build up of wasted areas in heads and shells not exceeding an area of 100 sq. inches or a thk of 25% of nom. wall thk or 1/2 inch, whichever is less.

Routine repairs are repairs for which the requirements for in-process involvement by the Inspector and stamping by the R Cert Holder may be waived as determined appropriate by the Jurisdication and the Inspector.

There never seems to be enough information to give advice or suggestions.  Please contact your A.I. for asssistance & guidance.

QCRobert