Ambient temperature, not lumens, is the first filter
Process areas near flares, crackers and turbine enclosures sit at 45–60 °C for long stretches. A fixture rated only to Ta +45 °C trips its driver's thermal cutoff and drops output or goes dark exactly when the plant is running hardest. Choose Ta −40 to +65 °C, with some enclosures rated to +75 °C for flare tips and heater fronts. Watch thermal sag: a 200 lm/W module at 60 °C can deliver 15–20% less than its 25 °C label, so read the lumen-maintenance curve at operating temperature, not the headline figure.
Hazardous-area classification drives the certificate
Refineries are mostly ATEX/IECEx Zone 1 (gas present in normal operation) or Zone 2, with Zone 21/22 where catalyst or product dust accumulates at loadout and silos. Match the Ex mark to the gas group (IIA/B/C) and temperature class (T4/T5/T6) for the specific hydrocarbon on site. Ex d (flameproof) enclosures contain an internal arc; Ex e (increased safety) or Ex mb (encapsulation) suit auxiliary gear. A certificate that simply says "LED high-bay" means nothing unless the zone, group and T-class match the unit you are lighting.
Lumen output versus where the light lands
High mounting wastes a wide beam on the ceiling. Pick a narrow, precisely aimed optical so 150–200 lm/W actually reaches the deck at 50–150 lux for walkways and 200+ lux for valve aisles. Optical control beats raw lumens: a tighter beam needs fewer fixtures and cuts glare spilling into control-room windows. Lay out the aim points before fixing the count, because adding fixtures to fix a dark aisle usually just adds glare elsewhere.
Glare and uniformity on the process deck
Workers read gauges and climb stairs directly under the lights. Keep UGR below 25 and floor uniformity above 0.6 so shadows don't hide a leaking flange. A narrow beam with a louvre or prismatic lens tames spill, and a lux map at the design stage prevents the hot spots that blind CCTV and wash out instrument panels. Treat the lux map as part of the spec, not a checkout afterthought.
Dimming and control from the safe side
DALI-2 or 1–10 V dimming is standard, but the control gear must sit outside the hazardous zone or carry its own Ex certificate. Centralized DALI lets one building controller ramp a whole unit at dawn/dusk or drop to a security level at night. Microwave presence sensors cut energy in low-traffic pipe racks, but the sensor head still needs its own Ex rating. Keep all switching and addressing in the non-hazardous panel so field changes never break the enclosure seal.
Build for corrosion and service
Coastal and sour-gas sites eat standard powder coat in a season. Specify a C5-M (or C5) corrosion rating, 316 stainless brackets and rigid conduit, with IP66/IK10 to survive washdown and impact from tools and ropes. A copper-free aluminium body stays light and non-sparking, which also simplifies the Ex certification. Plan the access path: a fixture nobody can safely reach for relamping never gets maintained.