Road Valves for Portable Cryogenic Vessels

“Road Valves” were mentioned in the last article without defining them. A Road Valve consists of a 25psi or less relief device and an isolation valve installed as a bypass to the main safety relief on a portable cryogenic vessel. The purpose of this bypass system is to assure that pressure does not exceed 25psi while the vessel is transporting cryogenic liquid oxygen, nitrogen or argon over the road. The 25 psi mark is noted in the DOT exception found in 49CFR 173.320 and also outlined in specific exemptions established for a variety of cryogenic transports and portable vessels.

Compliance with the applicable exception or exemption typically requires that the vessel pressure be maintained at a level below 25.3 psi so most valves are set at 22-24psi for added assurance. It is the responsibility of the transporting driver to vent the vessel down below 25psi and open the road relief isolation valve prior to traveling with contents. Not all portable cryogenic vessels are fitted with road valves. For instance- Those built under DOT-4L or MC-338 specifications are typically able to travel at their full operating pressure.

The critical point here is that it is very important to understand the applicable guidelines and legalities for transporting cryogenic liquids with whatever vessel you are using. Ignorance of the law is no defense and fines related to DOT violations can be enormous. Most importantly- the guidelines were established not only for the safety of the operator, but also for those around them.