Fixed roof tanks containing volatile flammable liquid, even with an internal floating roof, will develop an explosive gas/air mix above the liquid level unless the oxygen content inside is kept below the lower flammable limit (LFL). Some operators rely on flame arrestors fitted to the roof vents but a safer option is achieved by gas blanketing. Without a vapor recovery system, this results in venting the gas mixture to atmosphere during tank filling.
Many operators use nitrogen as blanket gas, but nitrogen manufacture is energy intensive. A better solution is to use low pressure fuel gas, feeding all blanketed tanks from a common manifold running at ~5 mbar positive pressure. If the pressure rises then gas can be re-compressed into the fuel gas system. For safety, in case the compressor trips or is otherwise unavailable, excess gas can be routed (on split range control) to a dedicated low pressure ground flare.
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