4.5 Pressure vessels complying with 4.4.5 or cylinders used for stationary storage without temperature compensation of the storage pressure shall be protected with one or more spring-loaded pressure relief valves in accordance with the ASME Boiler and Pressure Vessel Code.
Best Industry Practice: Station ground storage is not usually temperature compensated. Storage is usually filled to 80 - 90 percent of rated pressure.
4.5.2.2 Pressure relief valves for CNG service shall not be fitted with lifting devices. The adjustment, if external, shall be provided with a means for sealing the adjustment to prevent tampering. If at any time it is necessary to break such a seal, the valve shall be removed from service until it has been reset and sealed. Adjustments shall be made only by the manufacturer or other companies having competent personnel and facilities for the repair, adjustment, and testing of such valves.
The organization making such adjustment shall attach a permanent tag with the setting, capacity, and date.
4.5.2.3 Pressure relief valves protecting ASME pressure vessels shall be repaired, adjusted, and tested
in accordance with the ASME Boiler and Pressure Vessel Code.
It is recommended that pressure relief devices should be sized to protect ASME pressure vessels against excessive pressure caused by exposure to fire or other sources of external heat, since this is the worst case condition. As noted in the ASME Boiler Pressure Vessel Code, Section VIII, Appendix M, API RP520, Sizing, Selection, and Installation of Pressure Relieving Devices in Refineries, Part 1, is a reference document for sizing of PRDs for fire conditions. Other useful documents include CGA S-1.1, Pressure Relief Device Standards - Part 1 - Cylinders for Compressed Gases, and CGA S-1.2, Pressure Relief Device Standards - Part 2 - Cargo and Portable Tanks for Compressed Gases.
Where tank designs are such that heat conduction from a fire could be insufficient to activate a thermally operated pressure relief device, the use of heat conduction devices or insulating noncombustible materials, or both, should be considered (e.g., a ceramic blanket can prevent rupture of a cylinder due to a localized fire).
Thermally operated pressure relief devices might not activate if the cylinder is in a fire that is localized and some distance away from the pressure relief devices.
It has been demonstrated that a 1-in. (25-mm) ceramic blanket can keep the surface temperature of the container below 200ºC (392ºF) under fire conditions for 45 minutes.
Manifolded installations of multiple containers or pressure relief vent lines should be designed after taking
the following into account.
If fire can be expected to act on some of the containers while others are unaffected, gas may flow through the manifold from the unexposed containers to the containers exposed to fire. This can reduce the rate of pressure relief to an unsafe level. Some considerations in evaluating this risk are:
(1) The flow capacity of the individual pressure relief devices. High flow devices can safely vent multiple containers through a single device.
(2) Automatic valves closed by a fire condition or check valves can be used to isolate containers or groups of containers to prevent flow through the manifold.
(3) Containers distributed over a large vehicle are more susceptible to partial fire exposure.
(4) Containers mounted in compartments might not be exposed to the same fire conditionsas other containers out-side of the compartment or in a different compartment.
(5) Container manufacturer will have data from the required container fire tests that cansupport the design of the manifolding.
Manifolds for vent lines of multiple pressure relief devices can be designed with a flow capacity less than, the sum of the flow capacities of all of the pressure relief devices. The following are some of the conditions in such a design:
(1) Containers can have pressure relief devices at each end for protection against partial fire exposures. Either pressure relief device will generally have sufficient flow capacity to vent the containers safely.
(2) Containers protected with high flow pressure relief devices can be expected to vent to a safe pressure level before the fire spreads to containers located elsewhere inthe vehicle.
(3) The individual pressure relief devices might have greater capacity than is required to perform safely in the container fire test. The container manufacturer will have data from the required container fire test that can support a manifold design with flow capacity less than the total pressure relief device flow capacities.
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