Progress: Addressed, Not Adequately
DOT has previously required that cylinders use pressure relief devices that conform to CGA S-1.1 (1994 release). There has been discussion within the industry as to whether pressure relief devices, which are non-reseatable, should be required or prohibited. Many of the problems with pressure relief devices have been related to adequacy of the standards that have been referenced. Pressure ratings of S-1.1 devices have generally been limited to 41.4 MPa (6000 psi).
Pressure relief devices for steel cylinders are often pressure activated, such as a rupture disk. The increased pressure due to exposure to high temperatures or a fire will generally cause the rupture disk to activate before the cylinder loses strength due to high temperature exposure. However, pressure activated devices may not work if the cylinder is not at service pressure when high temperature exposure occurs.
Composite reinforcement tends to insulate the contents of a cylinder, preventing buildup of internal pressure. In addition, composite reinforcement is degraded when exposed to fire. Therefore, composite cylinders are more likely to use thermally activated pressure relief devices.
Thermally activated pressure relief devices have been used to protect cylinders up to 70 MPa (10150 psi), but there are a limited number of applications. Pressure relief devices for higher pressures are being developed.
If a cylinder does not have protection from a thermally activated relief device, there is a significant risk of rupture in a fire. This would create an explosive release of energy initially, and the possibility of a second high rate energy release if the escaping hydrogen burns or detonates.
The industry has developed the PRD1 standard for compressed natural gas, which was ANSI approved in 1998. PRDs qualified to this standard have demonstrated high reliability. The requirements of PRD1 were incorporated into a later revision of S-1.1 as a CG-10 device and into similar ISO standards. PRD1 is the de facto standard for high pressure hydrogen applications in North America. Efforts are currently underway to update PRD1 coverage to address use in hydrogen applications and use at higher pressures.
The industry should continue to develop standards for use with high pressure hydrogen cylinders. The industry and DOT should review field experience using PRDs built to standards that are current or in development. A determination should be made as to whether the industry has sufficiently addressed PRD safety, if special permits should be required that address the use of PRDs, or if reference to industry standards should be incorporated into the federal code.