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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 #PI-22-0006

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

Interpretation Response Details

Response Publish Date:

Company Name: CH-IV Regulatory

Individual Name: Mr. Juan Carlos Rivadeneira

Location State: TX Country: US

View the Interpretation Document

Response text:

Mr. Juan Carlos Rivadeneira
11700 Katy Freeway
Suite 1350
Houston, TX 77079

Dear Mr. Rivadeneira:

In a letter to the Pipeline and Hazardous Materials Safety Administration (PHMSA), dated February 28, 2022, you requested an interpretation of the federal pipeline safety regulations in 49 Code of Federal Regulations (CFR) Part 193 with respect to the applicability of § 193.2167 to buried liquefied natural gas (LNG) piping systems.

In your letter you quoted the applicable provisions for enclosed impounding systems found in the pipeline safety regulations, and the 2001 and 2019 editions of the National Fire Protection Association 59A Standard for the Production, Storage, and Handling of Liquefied Natural Gas (NFPA 59A-2001 and NFPA 59A-2019 , respectively). You noted that § 193.2167, NFPA 59A-2001, and NFPA 59A-2019 all recognize the hazards involved with the use of enclosed LNG impoundment systems and, therefore, prohibit the use of such systems or provide specific requirements for the safe design of such systems. You explained that enclosed LNG impounding systems can result in confined vapor explosions resulting from the mixture of LNG vapors and oxygen from the atmosphere. You opined that the prohibition on the use of covered impounding systems in § 193.2167 was to reduce the risk of confined vapor explosions due to the mixture of LNG vapors with air in enclosed volumes.

Therefore, you reasoned that § 193.2167 should only be applied to covered impoundment systems where air is expected or can be present under normal operating conditions. You asserted that buried LNG piping is not open to the atmosphere and does not have empty voids in between the piping and soil where LNG vapors from a leak could concentrate with ambient air leading to a confined vapor explosion. You stated that even if LNG or LNG vapors were to surface to grade, the LNG would then be unconfined and would not result in the kind of vapor explosion § 193.2167 was designed to prevent.

Asserting the same line of reasoning, you argued that a buried LNG pipeline surrounded completely by soil with a concrete slab directly placed on top of the soil, such as a buried piping installation crossing a road, is also not an enclosed impoundment system. As you explained, in this scenario, there is no empty void in between the soil and concrete slab that is open to atmosphere where ambient air and LNG vapors can form an explosive mixture in a confined volume.

You further contended that for a buried pipeline to become a closed impoundment system such as those prohibited by § 193.2167, the soil impoundment over the pipeline, exposed to the atmosphere, would need to be covered or congested in such a manner that LNG vapors could mix with ambient air in a confined or congested volume. You explained that for LNG vacuum jacketed piping, the soil surrounding the outer jacket will also serve as the impoundment system where 1) the outer pipe is not used for LNG impoundment, and 2) where the vacuum jacketed pipe does not affect the prescribed design spills, impoundment determinations, or other hazard calculations.

Concluding, based on the inherent differences between an enclosed impoundment system and buried piping, you asked if § 193.2167 applies to buried LNG piping.

Part 193 prescribes safety standards for LNG facilities used in the transportation of gas by a pipeline that is subject to the pipeline safety laws (49 U.S.C. 60101 et seq.) and Part 192 of this chapter. See § 193.2001(a). Section 193.2167 is specifically found in Subpart C, Design, of Part 193, which incorporates by reference NFPA 59A–2001. See § 193.2101. If there is a conflict between Part 193 and NFPA 59A–2001, the requirements in Part 193 prevail.

The applicable definitions and requirements in 49 CFR Part 193 and NFPA 59A-2001 are as follows:

§ 193.2007 - Definitions.

Impounding space means a volume of space formed by dikes and floors which is designed to confine a spill of hazardous liquid.

Impounding system includes an impounding space, including dikes and floors for conducting the flow of spilled hazardous liquids to an impounding space.

§ 193.2167 Covered systems.

A covered impounding system is prohibited except for concrete wall designed tanks where the concrete wall is an outer wall serving as a dike.

NFPA 59A-2001 Section

Enclosed drainage channels for LNG shall be prohibited.

Exception: Container downcomers used to rapidly conduct spilled LNG away from critical areas shall be permitted to be enclosed if they are sized for the anticipated liquid flow and vapor formation rates.

Part 193 does not require buried LNG piping to have an impoundment system. However, if an operator has an impoundment system, pursuant to § 193.2167 a covered impounding system is prohibited except for concrete wall designed tanks where the concrete wall is an outer wall serving as a dike. This requirement applies to all impoundment systems without consideration of atmospheric exposure. Therefore, a buried LNG piping system with a covered impoundment system or vacuum jacketed piping, where the secondary pipe serves as the impoundment system, is prohibited under § 193.2167.

If an operator desires to have buried LNG piping with a covered impoundment system or vacuum jacketed piping, the operator may seek a special permit to deviate from the prohibition against covered impoundment systems in § 193.2167.

If we can be of further assistance, please contact Tewabe Asebe at 202-366-5523.


John A. Gale
Director, Office of Standards
and Rulemaking

Regulation Sections

Section Subject
193.2167 Covered systems