Vacuum Insulated MC-338 Cargo Tanks

This is in response to your October 19, 2006 letter and enclosed video regarding the use of MC-338 cargo tank motor vehicles as specified under the Hazardous Materials Regulations HMR; 49 CFR Parts 171-180). Your scenarios revolve around the shipment of hazardous materials in vacuum insulated MC-338 cargo tank motor vehicles. Your questions are paraphrased and answered as follows:

Q1: Section 173.33(g) requires each liquid filling and liquid discharge line in an MC-338 cargo tank to be provided with a remotely controlled internal self-closing stop valve except when the MC-338 cargo tank is used to transport argon, carbon dioxide, helium, krypton, neon, nitrogen, and xenon. Section 178.338-11(b) specifies that for MC-338 cargo tanks, each liquid filling and liquid discharge line must be provided with a shut-off valve located as close to the tank as practicable and, unless the valve is manually operable at the valve, the line must also have a manual shut-off valve. This provision allows for either an internal or an external stop valve placed as close to the tank as possible. We have found that most vacuum insulated MC-338 cargo tanks operate at temperatures below the reliable operating temperature of available internal self-closing stop valves, and currently no manufacturer builds an internal self-closing stop valve that will operate reliably at temperatures that may reach minus 452 °F. Therefore, is a MC-338 required to have a remotely controlled internal self-closing stop valve as specified in § 173.33(g)?

A1: The answer is no. MC-338 cargo tanks that utilize an external self-closing stop valve for compliance with the requirements in § 178.338-11(b) may be used to transport hazardous materials in commerce, notwithstanding the provisions in § 173.33(g). This issue will be clarified in a future rulemaking.

Q2: Section 177.834 requires cargo tanks to be transported with all valves and other closures in liquid discharge systems to be closed and free of leaks unless transported in accordance with § 173.29(b)(2). This provision was added as a result of HM-222B (May 30, 1996; 61 FR 27166) to consolidate the closure requirements for cargo tanks transporting Class 3 (flammable liquid) materials, class 8 (corrosive) materials, and Division 6.1 (poisonous) materials. This rule did not consider the impact this provision would have on MC-338 cargo tanks that transport cryogenic liquids. These tanks have external self-closing valves that are normally transported in an open position. These valves are designed to close with a tremendous amount of force to ensure proper closure. Subsequently, these valves require a large amount of force and effort to open. As a result, the potential for physical injury to employee personnel is increased and the ability of the valve system to operate is potentially compromised as a result of repeated cycling opening, closing, and testing). Does § 177.834 permit emergency self-closing valves on MC-338 cargo tanks to be either open or closed during transit?

A2: The answer is yes. External emergency self-closing valves on MC-338 cargo tanks containing cryogenic liquids may remain either open or closed during transit. We will clarify this issue in a future rulemaking.