Specifying a fixed gas detection system for an industrial plant is one of those engineering tasks that looks simple on paper and becomes surprisingly detailed the moment you start writing the requisition. A fixed gas detection system is a permanently installed network of gas detectors, controllers and alarm devices that continuously monitors process areas for combustible or toxic gases, so plant personnel are warned before concentrations reach dangerous levels. This guide walks procurement engineers and EHS managers through the main decisions involved in specifying such a system: identifying target gases, choosing sensor technologies, planning detector placement, defining the controller and signal architecture, and preparing a specification document that suppliers can quote against accurately.
Start with a Gas Hazard Assessment
Every specification should begin with a clear list of the gases that may be present, where they can be released, and what concentrations matter. The hazard assessment typically draws on process flow diagrams, material safety data, and any existing hazardous area classification for the site.
For combustible gases such as methane (CH4), propane (C3H8) or hydrogen (H2), monitoring is usually specified as a percentage of the lower explosive limit (LEL). Common alarm setpoints in industry practice are in the range of 10 to 25 percent LEL for the first alarm stage, with a higher second stage that may trigger shutdown actions. For toxic gases such as hydrogen sulfide (H2S), carbon monoxide (CO) or ammonia (NH3), monitoring is specified in ppm, and alarm levels are commonly aligned with the occupational exposure limits that apply in your jurisdiction. Because exposure limits differ between countries and are periodically revised, the specification should reference the applicable national or EU limit values rather than fixed numbers copied from an old document.
The output of this step is a simple table: gas, source location, measuring unit (percent LEL or ppm), and preliminary alarm levels. This table drives every later decision.
Match Sensor Technology to Each Target Gas
No single sensor technology covers every gas well, so most plant-wide systems combine several types:
- Catalytic bead sensors are a long-established choice for combustible gases. They respond to a broad range of flammable compounds but require oxygen to operate and can be degraded by silicone or sulfur compounds, so the specification should note any poisoning risks present in the process.
- Infrared (NDIR) sensors are commonly used for hydrocarbons and CO2. They are not poisoned in the same way as catalytic sensors and can be considered for areas with low oxygen or known sensor poisons, though they do not detect hydrogen.
- Electrochemical sensors are the standard approach for toxic gases such as CO, H2S, NH3, Cl2 and SO2, as well as for oxygen deficiency monitoring. Cell life is finite, so a sensor replacement interval should be built into the maintenance plan.
- Photoionization detectors (PID) may be evaluated where volatile organic compounds must be monitored at ppm levels, for example in coating or solvent handling areas.
A practical specification lists the required technology per measuring point, or at minimum states the target gas and range and lets the supplier propose the sensing principle with justification.
Plan Detector Placement and Coverage
Detector location has more influence on real-world performance than almost any datasheet parameter. Three factors dominate:
Gas density. Gases lighter than air, such as methane and hydrogen, accumulate at ceiling level, so detectors for these gases are generally mounted high, near the roof or above potential leak sources. Gases heavier than air, such as propane and many solvent vapors, sink toward the floor, so detectors are typically mounted low, often within about half a meter of ground level. Gases with density close to air, such as CO, are commonly monitored at breathing-zone height.
Leak sources and airflow. Detectors should be positioned near credible release points – valve stations, compressor seals, flanges, filling connections – while taking ventilation patterns into account, since forced airflow can carry a gas cloud away from an intuitively obvious location.
Accessibility. Every detector needs periodic calibration and bump testing. Points that require scaffolding to reach tend to get tested less often, so mounting positions should allow safe access, or remote calibration provisions should be specified.
For plants in Europe and the Middle East, detectors installed in classified hazardous areas must carry explosion protection appropriate to the zone. The specification should state the zone classification for each area and require the supplier to confirm the certification status of the offered equipment for that zone, rather than assuming a generic rating.
Define the Controller and Signal Architecture
Fixed detectors report to a gas alarm controller that powers the sensors, evaluates alarm thresholds, drives sounders and beacons, and passes signals to plant systems. Key points to define in the specification include:
- Channel count and expansion. Count your measuring points, then add spare capacity – a margin of 20 percent or more is a common practice for future process changes.
- Signal type. Analog 4-20 mA remains the most widely used interface between detectors and controllers, while RS485 bus wiring can be considered where many detectors are distributed over long distances and cabling cost matters.
- Alarm outputs. Specify how many alarm stages you need per channel, which relays must drive ventilation fans, shutdown valves or plant-wide alarms, and whether volt-free contacts to the DCS or fire panel are required.
- Power and fault behavior. Define supply voltage, backup power expectations, and how sensor faults or cable breaks must be annunciated.
Single-zone controllers suit small boiler rooms or storage areas, while multi-channel systems are commonly used in larger process plants. An overview of typical configurations is available on our gas alarm controller product center.
Write the Specification So Suppliers Can Quote It
A quotable specification package usually contains: the gas and measuring point table from the hazard assessment, a site plan marking detector locations and zone classifications, the controller and output requirements, environmental conditions (temperature range, humidity, dust, washdown), and the documentation you expect – calibration certificates, wiring diagrams and user manuals. State your commissioning expectations explicitly, including on-site calibration with certified test gas and a witnessed alarm function test. Vague requests produce vague quotations; a one-page table per measuring point produces comparable offers. Typical detector options for combustible and toxic gas duty can be reviewed in our gas detector product center.
Why ASA
ASA supplies fixed gas detectors, dust monitoring devices and gas alarm controllers to industrial buyers across Europe and the Middle East. Our engineering team reviews your gas list, site plan and zone requirements before quoting, helps match sensor technologies to each measuring point, and responds quickly with configuration proposals and documentation support throughout the project. If you are preparing a specification for a new or upgraded fixed gas detection system, contact our team for a detailed review and quotation.
Disclaimer: Product availability, specifications, measuring ranges, alarm configurations and certification coverage vary by model and region. The information in this article is provided for general reference only and does not constitute a performance guarantee, safety assurance or compliance statement. Always refer to the official product documentation and confirm suitability, certification status and local regulatory requirements with our team before selection, installation or use.