Requirements for Marking Pressure Relief Devices

This is in response to your letter dated June 24, 1991 on the subject of requirements for marking pressure relief devices on DOT 400 series cargo tank motor vehicles, as found at 49 CFR 178.345-10(h).

The June 12, 1989 final rule used the terms "set pressure" and "set-to discharge pressure" more or less interchangeably. In order to reduce confusion, only the term "set pressure" was used in the September 7, 1990 amendment to the final rule. It is defined at § 178.345-10(d) as follows:
"The set pressure of each primary relief system is the pressure at which it starts to open, allowing discharge."

References to "set-to-discharge pressure" were deleted.

You asked the question:
"What pressure marking would be required on a vent attached to a DOT 407 tar, marked with a MAWP of 25 psig?"

The answer is found at § 178.345-10(d)(1) and (h)(3). The set pressure mark on each pressure relief device must be no less than 120 percent of the MAWP, and no more than 132 percent of the MAWP. In the case of a DOT 407 cargo tank motor vehicle with a MAWP of 25 psig, the set pressure marked on the pressure relief device must be no less than 30 psig and no more than 33 psig.