H2-O2 Gas Cross-Over Safe Practice
Electrolysis cell membranes and diaphragms are good at keeping generated oxygen and hydrogen separate from each other during generation – but “cross-over” selectivity is not absolute. Analysis has shown that commercial electrolyzer product gases frequently include a very small percentage of H2 in O2 and O2 in H2 in the part per million range or lower – well below the flammable mixture limit of 4% H2 in O2 found in the literature. However, under transitory process conditions such as startup, intermittent power turn down, and cell stack pressure imbalance, or toward the end of life for the electrolysis cell stack, these concentrations may increase significantly. In the worst case, flammable mixtures may be generated and transported downstream into electrolysis gas system and storage volumes, with hazardous outcomes.[5]
Best practice for electrolysis system integrators includes thorough adherence to electrolyzer manufacturer startup, generation, transient, turn down, and shutdown operation limits. Integrators should also closely adhere to manufacturer-prescribed maintenance of safety controls and devices such as gas mixture detectors and catalytic recombiners.
For electrolysis system developers, a thorough hazard analysis must be conducted at each stage of development to identify and mitigate safety vulnerabilities, of which H2-O2 cross-over can have one of the most serious outcomes. During system verification and validation testing, best practice includes frequent testing of safety instrumentation and controls that monitor hazardous process deviations to gas purity, pressures, temperatures, and power. As informed by process hazard analysis, best practice includes interlock of the safety control system with safety shutoff valves to isolate significant system or storage gas volumes upon detection of hazardous process gas mixtures or conditions that may cause them to be generated.
[5]Hydrogen Plant Explosion and Fire
Water Electrolysis System Explosion
The Explosion at Laporte Industries, Ltd Ilford (pdf)
S.I. Kim. “Review: Hydrogen Explosion in Gangneung, South Korea.” 2019 Conference on Hydrogen Safety, The Center for Hydrogen Safety, 14 Oct 2019, Sacramento, CA.
References
CGA G5.5, Hydrogen Vent Systems
CHS Webinar: Safety of Water Electrolysis
CSA/ANSI 22.2 No. 22734 (to be published in 2023)
EIGA Doc 154 09 E, Safe Location of Oxygen and Inert Gas Vents
EIGA Doc 211 17, Hydrogen Vent Systems for Customer Applications
IEC 61010-1, Safety requirements for electrical equipment for measurement, control, and laboratory use - Part 1: General requirements
ISO 22734, Hydrogen generators using water electrolysis — Industrial, commercial, and residential applications (current version 2019)
ISO/TR 22734-2, Hydrogen Generators Using Water Electrolysis – Part 2: Testing guidance for performing electricity grid service (to be published in 2023)