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U.S. Department of Transportation U.S. Department of Transportation Icon United States Department of Transportation United States Department of Transportation

Interpretation Response #17-0065

Below is the interpretation response detail and a list of regulations sections applicable to this response.

Interpretation Response Details

Response Publish Date:

Company Name: Truck Trailer Manufacturing Association

Individual Name: John Freiler

Location State: VA Country: US

View the Interpretation Document

Response text:

June 15, 2018

Mr. John Freiler
Engineering Manager
Truck Trailer Manufacturing Association
7001 Heritage Village Plaza, Suite 220
Gainesville, VA 20155

Reference No. 17-0065

Dear Mr. Freiler:

This letter is in response to your July 6, 2017, letter requesting clarification of the Hazardous Materials Regulations (HMR; 49 CFR Parts 171-180) applicable to the installation of U.S. Department of Transportation (DOT) 400-series cargo tank vents on Motor Carrier (MC) 300-series cargo tank motor vehicles (CTMV). Specifically, you include in your letter two scenarios and enclose guidance documents and a technical bulletin from your organization; one driver/vehicle examination report; and a clarification letter on this topic issued by the Research and Special Programs Administration, the predecessor agency of the Pipeline and Hazardous Materials Safety Administration (PHMSA).

We have paraphrased and answered your questions as follows:

Scenario 1:

An MC 307 CTMV fitted with a 30-pounds per square inch gauge (psig) pressure relief device (PRD) has a set discharge pressure of 30 psig (or maximum allowable working pressure (MAWP)) to 33 psig (110% of MAWP); see superseded §§ 178.342-4(c) and 180.407(j)(1)(ii)(A). However, per the requirements in § 180.407(j)(1)(ii)(B), a vent upgraded to conform to DOT 407 specifications on a 30 psig MAWP CTMV would be required to open between 36 psig (120% of MAWP) to 39.6 psig (132% of MAWP); see §§ 178.347-4(c) and 178.345-10(d)(1).

Q1. Based on Scenario 1, when upgrading an MC 307 CTMV PRD to a DOT 407 CTMV PRD, is the correct set-to-open pressure 120–132% of MAWP?

A1. An MC 307 PRD modified to conform to DOT 407 specifications on a 30 psig MAWP CTMV would be required to open between 36 psig (120% of MAWP) and 39.6 psig (132% of MAWP), as prescribed in §§ 180.407(j)(1)(ii)(B), 178-347-4(c), and 178.345-10(d)(1).

Scenario 2:

An MC 307 CTMV with a 30 psig MAWP originally had a PRD flow rated at 130% or 39 psig. The original MC 307 design specification limited the flow to 130% per superseded § 178.342-4(b). In this case, the upgraded PRD would flow at 150% per the test pressure requirements prescribed in § 178.347-5; it would negate the requirement in paragraph (b) of superseded § 178.342-4 and conform to the PRD requirements stated in § 178.345-10.

Q2. Based on Scenario 2, when upgrading an MC 307 CTMV PRD to a DOT 407 CTMV PRD, is the correct flow pressure equal to the CTMV's test pressure?

A2. As prescribed in § 178.345-10(e), a DOT 400-series CTMV's flow rating is determined at a pressure not exceeding the CTMV's test pressure. Per § 178.347-5(b), the test pressure of a DOT 407 CTMV must be the greater of 40 psig or 1.5 times (150%) its MAWP. Thus, the flow pressure for Scenario 2 must be no more than 45 psig, provided the minimum venting capacity is met.

I hope this information is helpful.  Please contact us if we can be of further assistance.


T. Glenn Foster
Chief, Regulatory Review and Reinvention Branch
Standards and Rulemaking Division

Regulation Sections

Section Subject
178.345 General design and construction requirements applicable to Specification DOT 406 (178.346), DOT 407 (178.347), and DOT 412 (178.348) cargo tank motor vehicles
178.347 Specification DOT 407; cargo tank motor vehicle
180.407 Requirements for test and inspection of specification cargo tanks