Interpretation Response #PI-25-0013

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

Interpretation Response Details

Response Publish Date: 12/09/2025

Company Name: FlexSteel USA

Individual Name: Kirk Francis

Location State: TX Country: US

View the Interpretation Document

Response text:

Mr. Kirk Francis, PE
Senior Vice President, Engineering
FlexSteel USA, LLC
1221 Transport Drive
Ameriport Industrial Park
Baytown, TX 77523

RE: Request for Interpretation – Determination of Outside Diameter for Classification of
Steel-Reinforced, Multilayer Pipe under 49 CFR Parts 192 and 195

In a letter dated December 9, 2025, FlexSteel USA, LLC (FlexSteel USA) asked the Pipeline and Hazardous Materials Safety Administration (PHMSA), Office of Pipeline Safety (OPS) for a written interpretation on the applicability of the Federal safety standards in 49 CFR Parts 192 and 195 to gathering pipelines constructed with FlexSteel. FlexSteel is a three-layer composite pipe product manufactured by FlexSteel USA that consists of (1) an innermost high-density polyethylene (HDPE) layer, (2) a steel-reinforcing layer, and (3) an outermost HDPE layer.

FlexSteel USA seeks confirmation from OPS that the second, steel-reinforcing layer should be used in determining the outside diameter (OD) of a gathering line constructed with FlexSteel under Parts 192 and 195. FlexSteel contends that this result is consistent with the approach used in determining the OD of traditional steel and plastic pipe designs and the risk-based framework for regulating gathering lines. For the reasons provided below, OPS agrees that the steel-reinforcing layer should be used in determining the OD of a gathering line constructed with FlexSteel.

Background

PHMSA has prescribed Federal safety standards for gathering lines in Parts 192 and 195. The applicability of these safety standards to onshore gathering lines depends, in certain cases, on the OD of the pipe.¹ For example, Part 192 uses an OD threshold of "greater than or equal to 8.625 inches" in determining if an onshore gas gathering line is subject to the requirements for Type C gas gathering lines. Part 195 also uses a threshold of "219.1 mm (8 ⅝ in) or less nominal" OD in determining if a pipeline qualifies as a gathering line and a "nominal diameter" threshold range of between "6 5/8 inches (168 mm) to 8 5/8 inches (219.1 mm)" in determining if a gathering line in a rural area qualifies a regulated rural gathering line.

¹ OD is the actual, measurable external width of a pipe. nominal diameter (or nominal pipe size - NPS/DN) is a non-dimensional, standardized designation for sizing. For pipes up to 12 inches, the OD is typically larger than the NPS, while for pipes 14 inches and larger, the NPS corresponds directly to the actual OD.

Pointing to these regulations, FlexSteel USA states that "PHMSA has not directly addressed how to measure the OD of composite pipe for purposes of making jurisdictional and regulatory determinations." FlexSteel USA further notes that "[w]ith respect to steel pipe, PHMSA generally bases the diameter of the pipeline on pipe mill records and does not include any coatings or additional layers that may protect the pipeline while it is in operation when determining the OD of the pipe." FlexSteel USA contends that "the same approach can be applied in determining the OD of steel reinforced, multilayer pipe" such as FlexSteel.

FlexSteel USA further contends that applying that approach to FlexSteel leads to the conclusion that the steel-reinforcing layer should be used in determining the OD. FlexSteel states that the steel-reinforcing layer "provid[es] the pressure containment capacity" for the innermost HDPE layer, which "determin[es] the throughput capacity." The outermost HDPE layer, on the other hand, "shields the steel from corrosion and abrasion, serving a function similar to coating," which is not considered in determining the OD of steel pipe.

Finally, FlexSteel USA contends that "[b]asing the OD of the multilayer pipe on the steel layer is . . . consistent with PHMSA's established risk-based criteria for regulating certain gas and liquid lines." FlexSteel USA states that "PHMSA has routinely recognized that the regulation of gathering lines and rural lines is based on the risk associated with the line, which is determined primarily on the throughput of line and its proximity to communities." Because the inner HDPE layer and steel-reinforcing layers determine the volume and capacity of a pipeline constructed with FlexSteel, FlexSteel USA states that using the steel-reinforcing layer to determine the OD is "consistent with PHMSA's established risk-based approach for regulating certain gas and liquid lines."

PHMSA received a comment in support of FlexSteel USA's position from the Plastic Pipe Institute. PHMSA received comments opposing FlexSteel USA's position from the National Association of Pipeline Safety Representatives (NAPSR) and Mary S. Friend, Director, Gas Pipeline Safety Division, West Virginia Public Service Commission.

Relevant Regulations

The following regulations are relevant in responding to FlexSteel USA's request for interpretation:

§ 192.3 - Definitions

§ 192.8 - How are onshore gathering pipelines and regulated onshore gathering pipelines determined?

§ 195.2 - Definitions

§ 195.12 - What is a regulated rural gathering line and what requirements apply?

Analysis and Response

Upon review of the question provided by FlexSteel USA, additional information received from FlexSteel after submitting its request, the public comments, and the relevant requirements in Parts 192 and 195, PHMSA's analysis and responses are as follows:

Question: For purposes of complying with the Federal safety standards for gathering lines in Parts 192 and 195, should FlexSteel USA use the steel-reinforced layer in determining the OD of FlexSteel composite pipe?

PHMSA Response: Yes. While not cited by FlexSteel USA or the public commenters, Part 192 defines "composite materials" as "materials used to make pipe or components manufactured with a combination of either steel and/or plastic and with a reinforcing material to maintain its circumferential or longitudinal strength."² PHMSA added this definition to Part 192 in the final rule that established the safety standards for Type C onshore gas gathering lines in 49 CFR § 192.9(e), including the provision in 49 CFR § 192.9(h) authorizing the use of composite materials through a 90-day prior notice and no objection process.³

² 49 CFR § 192.3.

³ Pipeline Safety: Safety of Gas Gathering Pipelines: Extension of Reporting Requirements, Regulation of Large, High-Pressure Lines, and Other Related Amendments, 86 Fed. Reg. 63,296 (Nov. 15, 2021).

The key criteria for determining whether something qualifies as composite material under the Part 192 definition are (1) the use of a combination of steel and/or plastic and (2) a reinforcing material to maintain its circumferential or longitudinal strength. According to the information provided by FlexSteel USA, FlexSteel pipe consists of three layers of material: (1) an innermost HDPE layer, (2) a steel-reinforcing layer, and (3) an outermost HDPE layer. The first HDPE layer determines the throughout (or volume) of gas that can be transported in the FlexSteel pipe; the second steel-reinforcing layer provides pressure containment for the gas being transported in the innermost HDPE layer of the FlexSteel pipe; and the third HDPE layer provides protection for the innermost HDPE layer and the steel-reinforcing layer of the FlexSteel pipe from corrosion or external forces.

In these circumstances, the first and second layers are the composite materials used to make FlexSteel pipe as defined in Part 192. These two layers are made with a combination of plastic and steel, and the second steel-layer is the "reinforcing material" used to "maintain its circumferential or longitudinal strength." The third layer, by contrast, is used to protect the first two layers from other threats and is akin to the use of a coating for cathodic protection or a casing to protect a pipeline from external force damage.⁴ As the latter is not being used as "a reinforcing material to maintain . . . the circumferential or longitudinal strength" of the FlexSteel pipe, the third layer is not an essential element of the composite material as that term is defined in Part 192. Therefore, the OD of a gathering line constructed with FlexSteel should be based on the steel-reinforcing layer, not the outermost HDPE layer, under Part 192.

⁴ See 49 CFR § 192.323 (prescribing requirements for “[e]ach casing used on a transmission line or main under a railroad or highway); PHMSA Interpretation Response #PI-75-036 (discussing requirements in Parts 192 and 195 that apply to casings). PHMSA notes that the thickness of the outer layer is not adjusted based on the internal pressure rating of the FlexSteel pipe but is instead constant for all pressure ratings of the same size pipe.

Though there is no comparable definition for "composite material" in Part 195, the OD of a hazardous liquid gathering line constructed with FlexSteel should be similarly based on the steel-reinforcing layer. Part 195 only prescribes safety standards for pipelines constructed with steel pipes and includes a special notification process for operators that want to transport hazardous liquid or carbon dioxide in pipelines constructed with materials other than steel.⁵ Nothing in Part 195 suggests there should be a difference between gas and hazardous liquid pipelines when determining the OD of composite pipe material. Nor is there anything in Part 195 to suggest that the third HDPE layer, which provides protection from corrosion or external force damage, should be included in determining the OD of a gathering line constructed with FlexSteel.

⁵ See PHMSA Statement of Policy for Transporting Hazardous Liquids or Carbon Dioxide in Non-Steel Pipelines (May 18, 2026), available at https://www.phmsa.dot.gov/news/phmsa-statement-policy-transporting-hazardous-liquids-or-carbon-dioxide-non-steel-pipelines.

The comments submitted by NAPSR and Ms. Friend do not justify a different result. NAPSR and Ms. Friend both acknowledge that the function of the outer HDPE layer of the FlexSteel pipe "is to resist mechanical damage and provide environmental protection” but contend that "[a]ll three layers appear to be required to make the product function." Noting that "[t]he steel layer has continuous helical gaps that create an annular space and natural capture layer for the gas which permeates the HDPE liner," NAPSR and Ms. Friend also contend that "[t]he outer HDPE layer appears to function as a gas containing layer due to the gas permeation of the inner HDPE liner." NAPSR and Ms. Friend believe that the outer HDPE layer should be included in determining the OD of a gathering line constructed with FlexSteel under Part 192 for these reasons.

NAPSR and Ms. Friend do not account for the definition of "composite materials" in Part 192 in their comments. That definition makes clear that the innermost HDPE layer and steel-reinforcing layer of FlexSteel pipe are the relevant composite materials. The outermost HDPE layer, which NAPSR and Ms. Friend both recognize is used "to resist mechanical damage and provide environmental protection," is not "a reinforcing material" used "to maintain" the "circumferential or longitudinal strength" of FlexSteel pipe. Indeed, the outermost HDPE layer would not qualify as a composite material even if, as NAPSR and Ms. Friend contend, that layer could be used to contain gas that enters the annulus in the event that the innermost HDPE layer and steel-reinforcing layer are breached. FlexSteel USA has not represented that the outermost HDPE layer is designed for that purpose, and PHMSA does not ordinarily include measures that are used to prevent or mitigate pipeline failures in determining the characteristics of the pipe itself.

In summary, OPS agrees with FlexSteel USA that the steel-reinforcing layer should be used in determining the OD of a gathering line constructed with FlexSteel under Parts 192 and 195.

If we can be of further assistance, please contact Joe Berry (720) 601-3577.

Sincerely,

Cameron H. Satterthwaite
Acting Director, Office of Standards
and Rulemaking

Regulation Sections