Battery energy storage trailer inside a gas wellsite Zone 2 boundary
A workover crew is mobilising to an onshore gas wellsite with a 250 kWh lithium-iron-phosphate battery energy storage trailer to run a 60 kW electric hydraulic power pack, replacing the diesel engine that normally drives it. The wellhead area is classified Zone 2, gas group IIA, temperature class T3, with a 20 m hazardous-area boundary drawn around the wellhead; the trailer would sit 17 m from it. Site gas detection alarms at 10% LEL and trips at 20% LEL, and the gas stream is expected to carry up to 15 ppm hydrogen sulphide. The trailer's inverter sits in a forced-air-ventilated non-Ex enclosure, and the battery modules vent through a pressure-relief panel. The hot-work permit requires ignition-source control inside the classified area. The supervisor argues that because the unit is electric it is inherently safer than the diesel set and can therefore sit inside the 20 m line.
1. Assess whether the proposed location and equipment are acceptable. State the standards basis and what must change.
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Not acceptable. At 17 m the trailer is inside the 20 m Zone 2 boundary and nothing on it is certified for that location. Under IEC 60079-10-1 and 60079-14, Zone 2 equipment must be at least category 3G, EPL Gc, gas group IIA and temperature class T3 (maximum surface temperature 200 °C).
The inverter in a forced-air-ventilated non-Ex enclosure is an ignition source. The battery's pressure-relief vent is a potential release of hydrogen, carbon monoxide and electrolyte vapour, so the enclosure itself can become a source of release and its own zone must be considered - ventilation alone does not make that argument go away without assessment.
Change one of two things: relocate the trailer outside the boundary with a documented risk assessment, allowing a margin beyond 20 m; or procure a container certified for the zone, including the inverter. Separately, the hydraulic power pack's motor, starter and instrumentation inside the boundary would need 3G IIA T3 certification.
2. Redesign the gas detection and shutdown philosophy for the unit as it stands.
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With a non-Ex ignition source near the boundary, tighten rather than relax the philosophy: pre-alarm at 5% LEL, alarm with automatic trip of the power pack at 10% LEL, and retain 20% LEL for the wellhead emergency shutdown. Any trip must open the battery contactor or inverter enable outside the hazardous area; making or breaking a circuit inside the non-Ex enclosure is itself the event you are trying to prevent.
Add H2S detection at 5 ppm alarm and 10 ppm action with escape sets available, since 15 ppm is expected. Interlock container ventilation to an airflow switch and inhibit charging on loss of ventilation. Include the battery's own off-gas detection in the philosophy, because an internal cell fault is a release that the wellsite detectors will not see.
Ensure the emergency plan covers a 250 kWh LFP thermal event: approach distance, water application, runoff containment and re-ignition risk after apparent extinction.
3. The crew is ready to mobilise. What permit and authorisation steps would you insist on before the unit is energised?
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Insist that "electric means safe" is not an argument in a permit. Electric removes the diesel ignition source but adds stored energy, an 800 V DC arc-fault hazard and a vent-gas release. Required before energisation: the unit positioned outside the zone; a hot-work permit with gas testing before and during the task; an exclusion zone with signage and a briefing; DC lock-out/tag-out with a verified isolation point; documented arc-flash boundaries and appropriate PPE for anyone opening the inverter, which is not permitted inside the zone; and an emergency plan agreed with the wellsite supervisor.
Record the change from the original plan as a formal deviation, signed by the site's authorised person and the contractor's appointed person. Brief the whole crew, because operators who believe the electric unit is inherently safer will not maintain the distance that the assessment depends on.