Print This Page

T/O Mark

Print Date: 7/24/2024 9:07:35 PM

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 and submit the following:


  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 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.

Return forms by email to: or mail the forms to:

          National Board Pressure Relief Laboratory
          7437 Pingue Drive
          Worthington, Ohio 43085

Pressure Relief Laboratory 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 certificate 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:

US, Canada, Mexico, Caribbean $6,000 USD
All Others $9,000 USD


Cancellation Fees:

Withdrawal of an application in-process will result in a $330 USD application withdrawal fee.

Shop review cancellations after having been scheduled by the National Board will result in a $360 USD review cancellation fee, plus any loss of incurred expenses.


Organizations holding a T/O Certificate of Authorization who are having a name change prior to renewal must submit an NB-519, National Board VR, T/O, and/or NB Certificate of Authorization Name Change Application, along with a copy of their Quality Control Manual revisions. There is a fee of $50 USD for a name change to a certificate.

NOTE:  If the name change is being done at the time of renewal, an NB-519 does not need to be submitted, and the $50 revision fee is not needed. The name change should be noted on the NB-550 application for renewal.

Organizations holding a T/O Certificate of Authorization who are having a location change or scope change, must submit an NB-550, Application for the National Board VR and/or T/O Certificate of Authorization.  A review of your facility may be required prior to issuance of a revised certificate. Cost will be determined upon receipt of the NB-550.

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


Payment Options

Payment of the certificate fee and advance deposit, if applicable, can be made as follows: