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T/O Mark

T/O Certificate of Authorization Program

The National Board offers the Certificate of Authorization and T/O mark for the inservice testing of pressure relief valves. Requirements are described in NB-528, Accreditation of T/O Test Only Organizations.


Organizations seeking a National Board T/O Certificate of Authorization must complete:


  1. have a written quality system that complies with the requirements of the current edition of the National Board Inspection Code (NBIC); NB-528, Accreditation of T/O Test Only Organizations; and includes the expected scope of activities,
  1. have the current edition of all parts of the NBIC. Organizations have the option of using either a printed version or an electronic version, including a subscription from a National Board licensed reseller, to fulfill this requirement; and,
  1. have available a copy of the code of construction appropriate for the intended scope of work.

Prior to issuance of a National Board T/O Certificate of Authorization, the organization and its facilities are subject to a review of its quality system. An important part of that review will be the witnessing of the testing of several sample pressure relief valves.

An independent test for operation and capacity of these sample pressure relief valves will be conducted at a National Board-accepted laboratory. The test is to verify the tested valves meet applicable ASME Code requirements for performance.

  • The National Board Testing Laboratory is available to perform this testing, along with several other major valve manufacturers which operate National Board-accepted testing laboratories.
  • A National Board representative will witness the valve testing.

Return forms by e-mail to: or mail the forms to:

          National Board Test Laboratory
          7437 Pingue Drive
          Worthington, Ohio 43085

Pressure Relief Department staff will contact you to schedule a date for the review of the T/O Quality System and its implementation, and to witness the testing of the required sample pressure relief valves.

Estimated Fees

Certification Fees:

The fee for the T/O Certificate of Authorization is $660 USD for a three-year period.

Review Fees:

The review of an organization’s quality control system for T/O certification, conducted by the National Board, can be performed in conjunction with its ASME review, where applicable, or as a standalone T/O review. The National Board requires an advance deposit to schedule reviews for ASME and T/O certifications or the standalone T/O certification, as shown below:

          Within the Continental US – $5,500 USD

          Outside of the Continental US – See NB-564, Terms and Conditions.

Cancellation Fees:

Withdrawal of an application in-process will result in a $330 USD cancellation fee. If a review has been scheduled and is cancelled, other cancellation fees may apply.


Organizations holding a T/O Certificate of Authorization who change ownership and/or name must submit a revised application along with a copy of their quality control manual revisions. There is a fee of $50 USD for a change to a certificate.

If there are questions on the certification program, please Contact Us or email the Pressure Relief Department at

Payment Options

Payment of the certificate fee, stamp and advance deposit, if applicable, can be made by check or by credit card.

If paying by check, please remit to:

     The National Board of Boiler and Pressure Vessel Inspectors
     Attention: Accounting Department
     1055 Crupper Avenue
     Columbus, OH 43229-1183

For credit card payments, we accept MasterCard, Visa and American Express.  For security reasons, we do not accept credit card payments via email.  If you prefer this method of payment, please call the National Board Accounting Department at 614.888.8320 ext. 217 or fax your information to 614.985.4615.

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