In the compressed gas industry, cylinders and tubes have long served as the backbone of safe gas transportation. Gases are moved domestically and internationally to support nearly every industrial sector. Traditionally, tubes are manufactured from steel, feature water capacities of up to 3,000 liters, and are commonly grouped into frames for use on trailers or in intermodal transport.

In recent years, advancements in tube design and materials have enabled the development of larger tubes with water capacities exceeding 3,000 liters, including designs utilizing composite materials to enhance performance. While these larger tubes represent an important technological step forward, there are currently no unified international regulations governing their transportation. As a result, their use is managed through national approvals in a limited number of countries—approximately thirteen to date. In the United States, for example, these large tubes operate under a special permit issued by the Department of Transportation, allowing the transport of gases such as helium, hydrogen, methane, and natural gas.

Recognizing this regulatory gap, work has been underway within the international standards community for several years. In 2019, the International Organization for Standardization (ISO) Technical Committee 58 (Gas cylinders), Subcommittee 3 (Cylinder design), developed and published ISO/TS 17519, Gas cylinders – Refillable permanently mounted composite tubes for transportation, as an initial step toward addressing the issue. Efforts are now in progress to advance this document—under a revised title, Gas cylinders – Refillable permanently mounted composite large tubes for transportation—from a Technical Specification to a full International Standard. The long-term objective is to incorporate these requirements into the United Nations’ Recommendations on the Transport of Dangerous Goods – Model Regulations.

Also in 2019, the Compressed Gas Association (CGA) Cylinder Specifications Committee opened a corresponding work item, and ISO/TC 58/SC 3 established Task Force 1 – Large Tubes, chaired by CGA staff. Through the work of this Task Force and related activities within ISO/TC 58 and its subcommittees, it became clear that input from the United Nations is required to guide further standards development. A foundational gap was identified: the absence of a formal definition for “Large Tubes” within the UN Model Regulations. Without such a definition, ISO standards addressing large tubes cannot be incorporated into the UN framework.

To address this, CGA—working with partners across the international community—authored a formal paper proposing a definition of “Large Tubes” and related language for inclusion in the UN Model Regulations. The proposed definition states:

“Large Tube means an element of a MEGC of seamless steel or composite construction having a water capacity exceeding 3,000 litres but not more than 15,000 litres.”

In addition to requesting consideration of the proposed regulatory language, the paper called for the establishment of an Intersessional Working Group (IWG). The purpose of the IWG is to bring together a broader group of international experts, both within and outside the compressed gas industry, to further refine the proposal and build consensus among members of the UN Sub-Committee.

The paper was submitted to the United Nations and presented at the 67th session of the UN Committee of Experts on the Transport of Dangerous Goods and on the Globally Harmonized System of Classification and Labelling of Chemicals, Sub-Committee of Experts on the Transport of Dangerous Goods, held in Geneva, Switzerland in late November 2025. The response was overwhelmingly positive. Representatives from seven countries, along with partners from the European Industrial Gases Association (EIGA), spoke in support of advancing the work. No opposition was raised to the formation of the proposed IWG, and CGA staff, together with CGA members, will administer the group moving forward.

The IWG will begin its work shortly. Interested parties are encouraged to contact Jason Bombard to express interest in participating. To date, participants have been identified from CGA, the United States, EIGA, the United Kingdom, and Germany. The outcomes of this work will be submitted to the UN Sub-Committee of Experts for consideration in a future revision of the UN Model Regulations.