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April 2015 BSJ

Hydrogen and Fuel Cells: Focusing on Facility Safety Requirements continued 23, 50, 53 and 58, which require compliance with NFPA 2 and NFPA 55 (both NFPA codes reflect the same requirements). The bulk system definition in NFPA includes storage containers, pressure regulators, pressure relief devices, vaporizers, liquid pumps, compressors, manifolds, and piping and terminates at the source valve. Separation distances apply to all parts of the bulk system. It is important to note that unconnected storage cylinders and tanks, even in quanities exceeding the bulk definition, would not constitute a bulk system. NFPA 55 and NFPA 2 base separation distances for bulk gaseous hydrogen systems on the prediction of the characteristics of unignited jets or ignited jet flames from hydrogen leaks. Because the characteristics of hydrogen jets and jet flames depend on the source pressure and effective diameter of the leak, the NFPA separation distance table is divided into four pressure ranges. The effective leak diameter for each pressure range is based on a characteristic pipe inside diameter, where the leak flow area is 3 percent of the flow area of that diameter.1 Table 1 identifies the formulas for four different pressure ranges with the exposures categorized for three exposure groups. Reduction of distances through active or passive mitigation and additional requirements for separation from flammable liquids can be found in the NFPA codes. More information on this risk-informed methodology can be found in Annex E of NFPA 2 or 55. The separation distance requirements for bulk liquid hydrogen systems are based on legacy values and are provided in NFPA 2, Chapter 8, and NFPA 55, Chapter 11. Fourteen exposure types and three quantity categories determine the required distances. The values range from 5 fee (for separating bulk systems from buildings with sprinklers or buildings having a three-hour fire barrier separation wall) to 100 feet (for separating large bulk systems having greater than 15,000 gal from unsprinklered combustible buildings, flammable liquids storage area and slow-burning combustible solids). Many exposures require a minimum of 75 feet of separation for even the lowest quantity threshold. Those include air intakes, operable openings in buildings, places of assembly and hazardous materials. Further work remains for the NFPA committees. A liquid hydrogen task group is considering how risk-informed approaches can be implemented to produce separation distances (and possibly reduce some distance requirements), similar to the approach for gaseous hydrogen. The industry is also discussing how additional mitigation features (flow-restricting orifices, isolation of more vulnerable equipment, etc.) could be considered for reduction in separation distance requirements for gaseous systems. ELECTRICAL SAFETY REQUIREMENTS Because hydrogen requires only minimal ignition energy (0.02 millijoules) and has a very large flammable range (4 percent to 74 percent by volume in air), ignition sources must be controlled in areas of where hydrogen is stored or dispensed. The energy associated with static electrical discharges and electrical arcs can easily ignite hydrogen gas clouds. To address this risk, hydrogen systems are required to be bonded and grounded and electrical sources of ignition near hydrogen equipment are prohibited unless they are properly classified or protected. Table 1. Calculations for Outdoor Bulk Gaseous Hydrogen System Separation Distances (m) (NFPA 55) Pressure (psig) > 15 to <– 250 > 250 to <– 3000 > 3000 to <– 7500 > 7500 to <– 15,000 Exposure Group 1 Lot lines, air intakes, operable openings in buildings and ignition sources 0.231d* 0.096d 1.105d 1.448d Exposure Group 2 Exposed persons other than those servicing the system and parked cars 0.12584d− 0.47126 0.43616d− 0.91791 0.68311d− 1.3123 0.92909d− 1.6813 Exposure Group 3 Buildings, flammable gas storage, hazardous materials storage, slow-burning combustible solids, ordinary combustibles and overhead utilities 0.096d 0.307d 0.459d 0.602d * d – Pipe internal diameter (mm). To convert the calculated distance to feet, multiply the calculated separation distance in meters by 3.2808 and round to the nearest whole foot. Derived with permission from NFPA 55-2013, Compressed Gases and Cryogenic Fluids Code, Copyright © 2012, National Fire Protection Association, Quincy, MA. APRIL 2015 | 46


April 2015 BSJ
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