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Table 4 Results from risk assessment of production concept 4: Hydrogen production by water

electrolysis.

Process unit Electrolyser unit

Identified hazards/risks H2 and O2 gas mixture in electrolysis cell, local ignition causing pressure wave through electrolyser, material damages. Imbalance in liquid level in separator. Lye escaping through vent line, may expose persons and e.g vehicles outside the production unit.

Suggested risk reducing measures Continuous measurement of H2 in O2, critical H2-concentration <LFL; controlled shutdown of installation.

Shutdown, separators min 50% water filled, level switches. Expanding vent line with water trap. Vent stack kept frost free by heating elements in container.

Lye splash/exposure on personnel during maintenance.

Transparent cover for monitoring, full facial cover, including safety glasses) and personal protective equipment required.

Identified risk at gaseous hydrogen filling station downstream production unit

In the following scenarios identified to represent unacceptable risk are described and suggestions to risk reducing measures are briefly discussed. Aspects coupled to safety distances and protection against sabotage is also discussed.

Compressor unit

Ingress of air in suction side of compressor implies risk for internal fire or explosion and significant material damages. Special design for hydrogen compressors to prevent ingress of air (coupled to temperature and pressure indicators) will reduce the risk.

Compressors are units with high leak frequencies, and if located in confined areas, the risk of gas accumulation should be addressed and measures taken for control of such situations.

Leaks from high pressure

The high pressures in storage tanks and equipment downstream the compressor will, in case of a leak, lead to large release rates compared to leaks from systems with low pressure. Even if such releases are to take place outdoors in unconfined areas, and the conditions for dilution of hydrogen are good, this may lead to significant flammable gas clouds and hazard distances of several meters. The reason is that the impulse forces dominate above the negative gravity forces at a significant distance downwind the release source. The release direction plays an important role see ref. /10/. Since the ignition probability of hydrogen is high, see ref. /11/, and hydrogen fires are nearly invisible in daylight this scenario is a significant risk.

High pressure leaks in confined areas

If high pressure hydrogen releases occur in a confined area (e.g. inside a container for compression or inside a confined storage area) impulse and buoyancy effects will influence on the dispersion. The confinement will trap the gas, the gas jet will impinge on walls, floor or other objects present, and loose velocity, and thus the impulse will be reduced. There may be special conditions related to the flow pattern inside the confined area, dependent of

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