[ safety ]
The design of a hydrogen storage vessel and the ancillary instrumentation used is primarily based on explosion prevention and protection , while the material of construction is based primarily on hydrogen quality , environmental conditions , operating conditions , and in particular the potential for embrittlement .
Many potential explosion protection and prevention strategies can be used , including deflagration prevention by oxidant concentration reduction or predeflagration detection and control of ignition sources . The reader can refer to the National Fire Prevention Association ( NFPA ) 68 and 69 standards for a complete list and detailed implementation strategies .
Due primarily to its small atomic size , hydrogen has the ability to damage materials that are in direct contact through a process called hydrogen embrittlement . Although the mechanism by which hydrogen causes embrittlement is not fully understood , the general understanding is that hydrogen atoms can diffuse easily ( due to their small size ) into the microstructure of the metal and cause it to become more brittle , lowering the stress required for cracks in the metal to initiate and propagate . This also causes a loss of ductility of metals and is likely the defining characteristic when choosing a material for a hydrogen storage system . To mitigate this effect , austenitic ( 300 series ) stainless steels are recommended as they are relatively immune to high-pressure hydrogen embrittlement ( 316 being the preferred grade ). Carbon and alloy steels can also be used for lowpressure hydrogen vessels (< 20 MPa working pressure ) above 29 ° C . However , grey , ductile , or cast irons and plastic should not be used for the purposes of storing gaseous hydrogen . 2 , 3 A complete list of acceptable materials for hydrogen service can be found in the American Society for Mechanical Engineers ( ASME ) B31.12 standard .
Purge systems
In addition to a rugged storage system , proper pressure-relief and purge systems should also be in place in the case of overpressurization or for maintenance purposes . A pressure relief valve made of 300 series stainless steel , carbon steel , or bronze should be located on the storage tank with a vent stack to atmosphere ; refer to ASME B31.12 for detailed information on material selection and the Compressed Gas Association ( CGA ) G-5.5 standard for detailed information on vent stack design . Vent stacks should be placed a minimum of 3 m ( 10 ft ) above grade , 0.6 m ( 2 ft ) above adjacent equipment , or 1.5 m ( 5 ft ) above rooftops , away from personnel areas , ignition sources , air intakes , or building openings / overhangs . 4 The vent stack design ( height , pipe size , location and discharge direction ) should be engineered to follow NFPA 2 and CGA G-5.5 and meet the specific relief pressure and relief valve size on the storage vessel . The engineered vent stack may be higher than the minimum height above grade , equipment , and rooftops . As hydrogen gas is lighter than air , the risk of asphyxiation is minimized if these protocols are followed .
Hydrogen has a lower flammability limit ( LFL ) of 4 vol % in air and an ignition energy of 0.019 mJ , approximately 10 times lower than that of common hydrocarbons , increasing the risk for explosions . 5 , 6 There are varying opinions on the efficacy of an inert gas dilution to prevent autoignition or using a flame arrestor to inhibit backwards propagation of a flame in the event of venting . The use of either or both is acceptable though not required . Dilution of pure hydrogen to below 25 % of its lower flammability limit ( 1 vol % hydrogen in air ) results in an unrealistic volume of inert gas to be used that itself has safety considerations for storage , however , a purge of the vent stack volume itself before venting could help in ensuring safe exit of the gas and would require
Hydrogen Tech World | Issue 8 | February 2023 13