New T/O Directory

T/O-Certified Organizations Added to
Manufacturer and Repair Directory and NB-18 Listings


The online National Board “Manufacturer and Repair Directory” is a free resource for those seeking:

  • Manufacturers of boilers, pressure vessels, or other pressure-retaining items who are authorized to register these items with the National Board.
  • Manufacturers of pressure relief devices who have had these items capacity-certified by the National Board.
  • Repair organizations holding National Board Certificates of Authorization for use of the R, VR, or NR stamps.

Recently, a forth category has been added to the directory: pressure relief valve testing organizations holding National Board Certificate of Authorization for use of the T/O Certification Mark.

For complete details and to access the directory, click here.

In addition, a listing of T/O testing organization certificate holders has been added to NB-18, Pressure Relief Device Certification. NB-18 also includes listings for assembler and manufacturer certifications, certified device types, rupture disk combination capacity factors, and VR certificate holders.

NB-18 can be viewed here.