Open Path Combustible Gas Detectors

What Is an Open Path Gas Detector?

There are two basic approaches to combustible gas detection: point measurement, which waits for the gas to reach the location of the detector, and open path measurement, which monitors for gas along a defined line. Point detectors measure concentration locally in %LEL; on open, windy sites, however, a gas cloud can drift past the detector or become so diluted that it stays below the detection threshold.

An open path combustible gas detector closes this gap with two separate units: a transmitter that houses the infrared light source and a receiver that picks up and analyzes the beam. A hydrocarbon cloud forming at any point along the beam path absorbs light at certain wavelengths and causes a measurable change in the signal at the receiver. The result is reported in LEL·m, a unit that expresses the total effect of the cloud along the path.

On its own, this method does not show exactly where on the line the gas is or at what concentration; in return, it watches a large area with a small number of measuring lines and contributes to the early recognition of leaks. For this reason open path detectors are usually installed around process units, along pipe racks and loading islands or at plant boundaries, and used together with infrared point gas detectors placed close to critical equipment.

How Open Path Infrared Gas Detection Works

Open path measurement applies optical absorption, as described by the Beer–Lambert law, over a long beam path. The measuring chain consists of the following steps:

1.      Infrared flash emitted by the transmitter: The transmitter unit produces infrared flash pulses aligned toward the receiver. Pulsed operation helps the receiver distinguish the signal from its own light source from sunlight and other continuous IR sources.

2.      Gas absorption along the beam path: When the beam passes through a hydrocarbon cloud on its path, it is attenuated at the wavelengths absorbed by C–H bonds. The degree of attenuation depends both on the gas concentration and on the thickness of the cloud along the beam path.

3.      Comparing measurement and reference bands: The receiver evaluates a measurement band affected by the gas together with a reference band that the gas does not affect. Dust, rain, fog or window contamination attenuate both bands in a similar way, so the ratio calculation compensates for a large part of these effects; automatic gain control also helps to keep detection going with up to 95% signal obscuration.

4.      Calculating the LEL·m value: Based on the factory calibration, the change in ratio is converted into the total amount of gas along the path, expressed in LEL·m. A reading of 1 LEL·m, for example, can represent a cloud 1 m thick at 100% LEL just as well as a cloud 5 m thick at 20% LEL.

5.      Reporting gas and beam status: The gas reading is transmitted as a 4–20 mA signal, while beam block, maintenance call, zero calibration mode and fault states are reported at separate current levels. The same information can also be read over HART and RS-485 Modbus.

Alignment and Mounting Rigidity

The alignment tolerance between transmitter and receiver is in the order of ±0.5°. Over long distances even this small angle can shift the beam noticeably on the receiver, so the units must be mounted on rigid structures that do not move under vibration, wind load or thermal expansion. Tilt, wall, pole and duct mounting options adapt to different site structures.

Beam Blockage and Visibility Conditions

Vehicles, personnel, cranes or dense fog that interrupt the beam path are recognized as a beam block rather than as a gas reading, and are reported at a separate level on the current output. This is not a fault but a warning that monitoring is temporarily not possible; the control system must handle it separately from a gas alarm. Heated optics limit loss of visibility by preventing condensation and icing on the window surface.

Because open path measurement delivers a path integral in LEL·m, the same reading can come from a small, dense cloud or from a large, diluted one. When alarm thresholds are defined, the expected cloud size, beam path length and leak scenarios should be assessed together; detection performance depends on how the gas disperses and on the position of the beam path relative to the leak source.

Key Features

·         Transmitters in four range classes: Transmitter options for 7–20 m, 15–40 m, 35–100 m and 80–200 m; with range adapters used on the 15–40 m transmitter, short paths between 1 and 7 m are also supported.

·         C1–C8 hydrocarbon selectivity: Measuring ranges of 0–5 LEL·m for methane and propane and 0–8 LEL·m for ethylene are available; the default calibration is methane with a 5 LEL·m full scale.

·         Low detection threshold: A minimum detectable level of 0.15 LEL·m helps to reveal small or diluted clouds that cross the beam path.

·         Fast response: A response time of under 2 seconds helps the alarm chain act quickly once a gas cloud crosses the beam path.

·         Automatic gain control: Detection is maintained with up to 95% signal obscuration caused by fog, rain, snow or dust.

·         Immunity to external IR sources: The measurement is designed so that external IR sources such as sunlight or hydrocarbon flames do not influence it.

·         Heated optics: Heating the optical windows prevents condensation and icing and preserves measurement continuity in harsh climates.

·         Event recorder with real-time clock: Time-stamped storage of the last 375 events allows alarm and fault history to be reviewed afterward.

·         Three-color status indicator: Green (power on), amber (fault) and red (alarm) LEDs on the front and rear of the receiver show the status quickly on site.

·         Factory calibration and simple installation: The factory-calibrated units are designed for installation by a single person and for low maintenance requirements.

Technical Specifications

Typical technical data for the open path combustible gas detector described on this page are listed below. Transmitter range class, housing connection, calibration gas and approval option are defined at ordering; final values should be confirmed for the selected configuration.

The 1–2.5 m range adapter is suitable only for duct installations. Accuracy and detection threshold are defined for the calibration gas; because the reading can differ for other hydrocarbons, alarm levels should be set for each project.

Key Advantages

·         Large areas monitored with few lines: A single transmitter–receiver pair can watch a line of up to 200 m depending on the transmitter class, so the number of measuring points needed for perimeter monitoring can be reduced.

·         Detection independent of leak location: As long as the gas cloud crosses the beam path, the position of the leak along the line does not affect the measurement; this raises the chance of catching clouds carried by the wind.

·         Continuity in adverse weather: Automatic gain control and heated optics help monitoring continue under fog, rain, snow and condensation.

·         Clear status diagnostics: Separate current levels for beam block, maintenance call and fault let operators easily tell a real gas alarm apart from a loss of monitoring.

·         Low maintenance requirement: Factory calibration, conformal-coated electronics and a stainless steel housing aim to keep field maintenance limited to periodic cleaning and function checks.

·         Suitability for offshore and corrosive environments: A 316L stainless steel housing, IP66/IP68 protection and DNVGL-CG-0339 environmental conformity provide a construction suited to long service on platforms and coastal facilities.

·         Functional safety suitability: SIL2 approval to IEC 61508 means the detector can be assessed for use in safety instrumented functions; loop-level verification remains a separate step.

Application Areas

Open path gas detectors are used for perimeter and line monitoring across large areas where combustible hydrocarbons are produced, processed, stored or transported:

Refineries and Petrochemical Plants

Beam paths placed around process units, along pipe racks and past pump packages help to reveal gas clouds that may spread beyond the unit at an early stage.

Offshore Platforms and FPSOs

Used on fixed platforms and on floating production, storage and offloading (FPSO) units to monitor the open spaces between process modules.

LNG and LPG Systems

Monitors the dispersion routes of heavier-than-air or cold gas clouds in liquefied gas storage, filling and transfer areas.

Compressor and Pumping Stations

Used at pipeline stations and around compressor buildings to monitor leaks that may originate from seals.

Pipelines, Refueling Stations and Fuel Storage

Offers long-distance monitoring along pipeline routes, at valve stations, filling points and tank farms.

Loading Docks, Transportation Depots and Warehouses

Provides wide-area surveillance at tanker loading jetties, hazardous goods loading ramps and shipping warehouses, despite vehicle and personnel movement.

Chemical and Pharmaceutical Production and Storage

Used for perimeter gas monitoring at sites where flammable chemicals are produced and stored, and in hazardous waste disposal areas.

Engine Rooms and Test Cells

With duct mounting options and short-range adapters, the detector can also be applied in enclosed volumes such as engine rooms and engine test cells.

How to Select an Open Path Gas Detector

The effectiveness of an open path system depends on relating the beam path correctly to the leak scenarios. Before a technical assessment, please clarify the following points:

Target Gas and Leak Scenario

·         Hydrocarbons to be monitored and choice of calibration gas (methane, propane or ethylene)

·         The fact that gases without infrared absorption, such as hydrogen, cannot be detected with this method

·         Height of the beam path depending on whether the gas is lighter or heavier than air

·         Expected leak rate, cloud size and prevailing wind direction

Beam Path and Distance

·         Correct transmitter class for the distance between transmitter and receiver

·         Structures, vehicle and personnel traffic and crane movements that could interrupt the beam path

·         Need for range adapters in duct and very short-distance applications

·         A rigid mounting structure able to hold the ±0.5° alignment tolerance

Ambient Conditions

·         Intensity of fog, rain, snow, dust and salt spray

·         Whether the ambient temperature stays within –55 to +65 °C

·         Access for window cleaning; need for an air shield and protective cover

Alarm Philosophy and System Integration

·         Warning and alarm thresholds in LEL·m defined according to the leak scenario

·         Separate handling of beam block, maintenance call and fault levels in the control system

·         Separate power supply for transmitter and receiver; typical consumption of 220 mA and 240 mA included in the power budget

·         Need for diagnostics and configuration via a HART handheld and RS-485 Modbus

Approval Requirements

·         ATEX/IECEx, FM/FMC, INMETRO or EAC approval depending on the project country

·         Target safety integrity level for safety instrumented functions

Open path lines give more meaningful protection when they are planned together with point gas detectors near critical equipment and, where needed, within the same safety concept as UV/IR flame detectors or IR3 flame detectors. Safety-related gas detection does not replace process measuring instruments such as process gas chromatographs or oxygen and combustibles transmitters; the two needs should be assessed separately.

Hazardous Area Approvals and Conformity Standards

The approvals that can be selected at ordering and the conformity standards defined for the open path combustible gas detector are summarized below.

Hazardous area marking details (protection type, gas group, temperature class) and the scope of gas detection performance approvals should be confirmed for each project from the valid certificate copies.

Open Path Gas Detection Solutions from TLY Enerji

The key to successful open path gas detection is relating the beam path correctly to real leak scenarios. When TLY Enerji reviews project data, we consider potential leak sources, the density of the gas relative to air and its tendency to disperse, the prevailing wind direction, structural obstructions and the rigidity of mounting structures together, and base our recommendations for transmitter range class, calibration gas and alarm threshold on this review.

Our team supports power supply and cabling planning for transmitter and receiver, the transfer of 4–20 mA and digital communication signals to the fire and gas control system or PLC/DCS infrastructure, the incorporation of states such as beam block and maintenance call into the alarm philosophy, and the planning of alignment and function checks during commissioning. We also offer an engineering opinion when layout alternatives combining open path lines with point detectors are reviewed.

Frequently Asked Questions

What is an open path gas detector?

An open path gas detector is a gas detection system consisting of a transmitter and a receiver installed opposite each other, which monitors combustible hydrocarbon gases along the beam path between them. The transmitter sends infrared light; when a gas cloud crossing the beam path absorbs certain wavelengths, the receiver measures this change and reports the result in LEL·m. It is used to monitor large areas and linear routes.

What does the unit LEL·m mean?

LEL·m is the product of the gas concentration as a fraction of the lower explosive limit (LEL) and the thickness of the cloud along the beam path. For example, a cloud 1 m thick at 100% LEL and a cloud 5 m thick at 20% LEL give the same reading of 1 LEL·m. An open path reading therefore does not express the local concentration directly, but the total amount of gas along the path.

What is the difference between open path and point gas detectors?

A point gas detector measures the concentration in %LEL at the spot where the gas reaches it and is placed close to the leak source. An open path detector monitors the total amount of gas along a line and detects clouds over large areas regardless of their position. The two methods complement rather than replace each other; many plants use point detectors for critical equipment and open path detectors for perimeter monitoring.

How do fog, rain or snow affect the measurement?

Fog, rain, snow and dust attenuate the beam in a similar way in both the measurement and the reference band. Because the receiver uses the ratio of the two bands, a large part of these effects is compensated, and automatic gain control helps detection continue with up to 95% signal obscuration. If attenuation exceeds this limit, the detector reports a beam block state at a separate current level and informs the control system that monitoring is temporarily not possible.

What happens if the beam path is blocked?

When a vehicle, a person or equipment interrupts the beam path, the detector does not interpret this as gas; it drives the current output to the beam block level (2 mA). This lets the control system distinguish a real gas alarm from a loss of monitoring. For prolonged blockages, an operator warning should be defined in the alarm philosophy, and beam routes should be kept away from traffic areas during planning.

Which gases can an open path detector detect?

The detector is selectively sensitive to combustible hydrocarbons in the C1–C8 range. Measuring ranges of 0–5 LEL·m for methane and propane and 0–8 LEL·m for ethylene are defined. Gases that show no infrared absorption, such as hydrogen, cannot be detected with this method, so other detection technologies must be considered for such risks. For hydrocarbons other than the calibration gas, the reading may differ.

How is the distance between transmitter and receiver selected?

The transmitter is chosen from the 7–20 m, 15–40 m, 35–100 m or 80–200 m classes according to the installation distance. For paths shorter than 7 m, the 15–40 m transmitter is used together with range adapters; the 1–2.5 m adapter is for duct applications only. When setting the distance, obstructions, the rigidity of the mounting structures and the position of the leak scenario relative to the beam path should be assessed together.

Does an open path gas detector need calibration?

The units are shipped factory calibrated. During commissioning the units are aligned and, where necessary, a zero calibration is performed; in this mode the current output drops to 1 mA. During operation, window cleaning, alignment checks and function tests in line with plant procedures should be planned. Diagnostics and configuration can be carried out with a HART handheld or over an RS-485 Modbus connection.