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Tech Series on High-Pressure CNG Fuel Systems: Part 3- Pressure Relief Devices

By Kasia McBride, Marketing Manager, NGVi 


While CNG fuel systems are extremely safe, due to stringent manufacturing requirements, it is essential that NGV technicians fully understand the specific standards that apply, and know how each component within the fuel system should operate. This article explores pressure relief devices (PRDs), and is the third in our series on the high-pressure side of the CNG fuel system.

Each CNG cylinder must have at least one PRD. PRDs are mandatory safety devices located at one or both ends of a CNG fuel cylinder. They are designed to protect a CNG cylinder from catastrophic rupture by activating (opening) at a specified temperature or pressure, and thereby relieving the CNG cylinder of its contents if the pressure becomes excessive.

The PRD type and location, determined by the cylinder manufacturer, is part of the certification process for the cylinder, and cannot be changed without the manufacturer’s approval.

PRDs can be activated by pressure, temperature, or a combination of both. The most common type of PRDs used in North America are thermal PRDs. These PRDs use a fusible alloy material that melts at a specific temperature (the alloy itself has a set melting point between 212°F-230°F). In the event of heat build-up, the material melts, opening a path for natural gas to release safely through a vent. CNG cylinders must be installed in such a way that their PRDs are not thermally shielded in likely fire scenarios.

The key point to remember is that PRDs are
one-time use only devices. If they open due to excessive temperature or pressure, they will stay open. They are also not serviceable in any manner and if they are activated, they must be replaced. If a PRD is damaged (bent, corroded, etc.) it must be removed from service, destroyed, and a new approved PRD installed in its place. If for any reason a PRD is removed, but is not damaged, it may be reinstalled–but on the same exact cylinder it was removed from (no swapping).

Because ensuring the integrity of the CNG fuel system is the core of NGV safety, routine inspections of the CNG fuel system, including examining all PRDs, are required at least every three years or 36,000 miles, whichever comes first, and after any accident or fire. 

During the CNG fuel system inspection, NGV technicians should check PRDs for damage, including gouges, scratches, corrosion, rust, bulging, and plugged channels. They should verify the PRDs are properly attached to the cylinder and are not deformed or corroded. Also during the inspection process, NGV technicians should inspect the interface between the PRD and cylinder valve and make sure it is tightly seated with no gaps or evidence of being loose.

One of the biggest causes of PRD failure is moisture. According to NFPA 52, PRDs must be designed in such a way that moisture cannot collect and freeze in a manner that would affect their performance. 

During CNG fuel system inspections, technicians should visually determine whether the vent tube is routed or bent in such a way that water hasn’t accumulated and filled the PRD. Technicians also should verify and record whether there is a cap on the end of each PRD vent line. These caps are installed to prevent water or debris intrusion. If caps are missing, technicians should check for evidence of water, including water marks in the vent tube or the PRD, for loose fittings on the outlet side, or loose or stretched PRDs.

Finally, PRDs should be carefully checked for leakage, using a liquid leak check solution and possibly a gas detector. This includes leak checking each PRD connection to each cylinder valve, between the PRD and cylinder port, and the vent side of the PRD.

Assuring the CNG fuel system’s integrity is crucial for NGV fleet safety and requires specialized training on the entire CNG fuel system, including all high-pressure components such as PRDs. For more information about NGVi’s CNG Fuel System Inspector Training, click here.