Fixed-point combustible gas detectors remain the primary safety layer at every flanged connection, pump seal, and compressor within a tank farm, while open-path IR or laser units are layered above the same area to catch drifting hydrocarbon clouds the point sensors can miss [S1][S5].
Tank farms typically mix pressure tanks operating up to 25 bar, cryogenic tanks at atmospheric pressure storing LNG or argon between -186 °C and -161 °C, and atmospheric bulk storage, which forces a mixed detector strategy because each tank type releases gas of a different density, temperature, and dispersion behaviour [S1].
What Each Detector Type Actually Measures
Fixed-point detectors output a local concentration at the sensor head, typically in %LEL for catalytic-bead and point-IR combustible units, and are placed at the source of a credible release such as a valve, flange, or pump [S5][S6].
Open-path detectors output a path-integrated reading, expressed in LEL·m or ppm·m, which is gas concentration multiplied by the length of cloud intersecting the beam, and the measurement therefore represents gas encountered across the beam path, not concentration at a single point [S4]. A 10 m cloud at 10 ppm produces 100 ppm·m; a narrower cloud at higher concentration can produce the same integrated value, which is why alarm thresholds on open-path units cannot be set with point-detector logic [S4].
Coverage Geometry and Path Lengths
Open-path hydrocarbon sets such as the Spectrex Quasar 900 family cover four bands: 7-20 m (901), 15-40 m (902), 35-100 m (903), and 80-200 m (904), all FM performance-tested with ATEX, IECEx, SIL2, FM, Inmetro, and CU TR EAC approvals [S3].
Open-path toxic sets for H2S, NH3, and SO2 in the Quasar 950 and 960 series stop at 60 m, because toxic gas clouds of engineering concern rarely extend that far before dilution drops them below alarm thresholds, and the shorter path length preserves sensitivity for low-ppm targets [S3].
Fixed-point catalytic and point-IR sensors cover a radius on the order of a few metres around the head; covering a 200 m tank berm with point units alone would require a substantial number of detectors, with cabling, calibration, and ongoing maintenance scaled linearly to that count [S2][S5].
Tank-Farm Application Matrix
Pressure-tank truck-loading bays handling LPG, ammonia, chlorine, or vinyl chloride are best served by fixed-point detectors at the hose coupling, because the credible release is small, localised, and immediately at the worker [S1].
Cryogenic LNG storage requires both layers: a point combustible gas detector at the boil-off vent and a perimeter open-path beam across the bund, because LNG vapour is odourless, invisible, and cannot be odorised like natural gas, so the first reliable cue is a remote optical reading [S1][S4].
Atmospheric bulk-storage tank berms and tank-farm pipe racks are the natural fit for open-path units, where one transmitter-receiver pair can replace multiple point heads and a drifting vapour cloud from a flange or roof seal is the dominant release scenario [S1][S2][S5].
Sensor Technologies and Cross-Sensitivity
Point combustible units use catalytic bead, point-IR, or electrochemical cells depending on the gas; catalytic bead poisons in silicone or lead atmospheres and will not respond to high-flash-point hydrocarbons above its heated-element operating range [S5][S6].
Open-path units are dominated by line-of-sight IR absorption and, in higher-end sets such as the MSA Senscient ELDS, Enhanced Laser Diode Spectroscopy with Harmonic Fingerprint reference cells to suppress false alarms from interferent gases that share absorption bands with the target hydrocarbon [S4].
For toxic gases such as H2S, NH3, and SO2 the same open-path platform is used but with shorter path lengths of 5-60 m because toxic alarm thresholds sit in the ppm range and require higher optical sensitivity per metre of beam [S3].
Failure Modes, Weather, and Blind Spots
Open-path detectors lose signal in fog, heavy rain, or snow, because the optical beam is attenuated by water droplets before the gas absorption can be resolved, so siting must avoid known plume paths and accept a higher nuisance-alarm rate in coastal or humid climates [S2].
Point detectors fail for the opposite reason: a small but concentrated release at a flanged joint that the wind carries away from the sensor head is invisible to a single catalytic bead, and the same gas cloud could be caught by a properly aligned open-path beam crossing the predicted drift direction, which is why designers spec both rather than choose one [S5].
Path-integrated readings also mean an open-path unit can completely miss a small concentrated cloud that crosses only a short fraction of the beam, an edge case discussed in detail in why an open path gas detector can miss a small concentrated gas cloud, so the point layer remains non-negotiable for personnel protection at the leak source.
Standards, Calibration, and Lifecycle
Open-path IR units are commonly FM performance-tested and carry ATEX, IECEx, SIL2, FM, Inmetro, and CU TR EAC explosion-safety approvals, with factory calibration on real gas rather than surrogate surrogate span gas to keep the LEL·m reading traceable [S3].
Fixed-point units are field-calibrated with a known methane or propane span gas at a defined flow rate, and their expected service life runs to several years between sensor replacements; open-path units share that maintenance interval but require periodic beam-alignment checks and window cleaning, which is covered in open-path gas detector alignment: peak signal procedure and commissioning checks [S2].
For the broader layer of toxic coverage that surrounds a tank-farm perimeter, a toxic gas detector or a multi-gas detector head feeds the same gas alarm controller, and the HART diagnostics ride over the standard 4-20 mA loop as described in fixed gas detector HART diagnostics over the 4-20 mA loop: wiring, signal, and selection.
Next signal to track: IEC 60079-29-4 performance verification of open-path detectors in the field, which remains the main document that procurement engineers cite when refusing a beam-only layout in favour of a layered point-plus-path design.