- Fixed sensors’ accuracy and reliability depend on optimal placement.
- Several characteristics of gases, such as point of origin, facility process, and equipment, must be considered to determine the location and placement of sensors.
- Industrial hygienists and safety managers should consult experts on gases, facility processes, and engineering to jointly determine the placement of fixed sensors.
Industrial hygienists and safety managers use gas detectors for early warnings of leaks and higher-than-recommended levels of hazardous gases for timely personnel evacuation and other corrective actions. Optimal placement and location of fixed gas detectors are crucial for ensuring accuracy and risk management. Find out how you can decide where to install gas detection sensors in your facilities.
Industry Best Practices
Industries increasingly use toxic and flammable gases for their processes and need gas detectors. Two types of detectors exist: fixed and portable. A facility could need both kinds. Portable gas detectors are used when personnel enter confined spaces, during rescue operations, etc. Fixed monitors work 24/7, without personnel involvement, for leak detection and area monitoring.
Detecting gas concentrations accurately through fixed monitors can be challenging as there are no guidelines from national and international authorities on the optimal number, spacing, and placement of gas detectors for around 95% of systems. In this scenario, industrialists only have industry-approved guidance for correct installations and collaborative decisions.
Optimal gas detector placement is challenging for various other reasons:
Gases are unique: Over 400 gases are used in different industries. Each gas has its unique properties and behavior and different safety limits, and a facility may produce or use multiple hazardous gases.
Workflow considerations: In any industry, it is not just the critical gases in a given area that must be considered, but also the workflow and placement of equipment and processes in adjacent areas where the gas can spread and reach to produce gas cocktails. For example, welding may occur next to areas with flammable gas emissions.
Formation of gas cocktails: Gas cocktails are also produced when toxic and flammable gases are emitted together. A combination of gases will reduce the threshold when each gas explodes due to the presence of other combustible gases. For example, such cases are expected in chemical, rubber, and wastewater treatment plants. Carbon monoxide produced by machines can mix with hydrogen or propellant leaks from fuel stations to cause fires.
Collaborate
In the absence of clear guidelines for individual sectors and facilities and given the various challenges of dealing with industrial gas emissions, industrial hygienists and safety managers are advised to collaborate to decide on placement. They should ask and consider advice from experts in gas dispersion, process plant equipment and systems, and engineers. A joint decision about placement, with the agreement of all parties, is necessary and should be recorded.
General Guidelines for Positioning Gas Sensors
Some general guidelines are available through cumulative experience applicable across industries, which safety managers and their advisory experts could consider before deciding on placements. The factors influencing gas detectors’ placement are listed below.
Identify High-Risk Zones
First, identify zones in the facility that can be hazardous or have leaks and where gas concentrations must be monitored. These would be the zones where fixed gas monitoring is required.
Leakage Monitoring: Discharge points like pipes, fittings, flanges, valves, gauges, joints, and ignition points need sensors close to potential leakage points for early detection.
Area monitoring: Zones without well-defined leakage sources and where gases can be spread over an ample space, such as confined spaces, storage, generation, or processing points of hazardous gases, like boilers, compressors, cylinders, etc., require area monitoring. The sensors must be uniformly distributed at head height.
The detectors must be close to the entrance for confined spaces where gas can accumulate.
The best placement ensures a slight overlap of the areas monitored by a sensor to avoid gaps through which gas can pass undetected. The device’s manufacturer provides recommendations for maintaining the distance between sensors.
Placement Close to Workers
Sensors for toxic gases can be placed close to breathing zones and heights, as gases mainly enter people through the nose and mouth during breathing. Monitoring the areas close to breathing areas can minimize risks through inhalation by early detection.
Monitor Airflow
All airflow must be monitored. Safety managers must know the direction and speed of the air to ensure they are placing detectors in the airflow. In some cases, sensors in ventilation ducts may be necessary to check storage conditions, rooms, and equipment.
Avoid Air-inlets
Sensors should not be placed near doors or windows, which let in fresh air that can dilute gas concentrations. Gas sensors are sensitive and will record and show lower readings of hazardous gases.
Check Gas Dispersion Characteristics
The gases have unique dispersion characteristics depending on density. Some gases are lighter, and some are heavier than air. The dispersion method helps decide the height at which the corresponding gas sensor should be placed.
- For gases lighter than air that rise, mount the fixed gas monitors high, near the breathing height of personnel. These include gases like ammonia, carbon monoxide, hydrogen sulfide, formaldehyde, chlorine, nitric oxide, and hydrogen chloride. Exceptions are hydrogen and methane, for which sensors are placed close to the ceiling.
- For heavier gases like sulfur dioxide, gas detectors should be lower where they tend to accumulate. Heavy hydrocarbon sensors are placed inches above the floor level. This height applies to gasoline, diesel, ethanol, carbon dioxide, benzene, and propane sensors.
- Place fixed detectors 2-8 feet above the ground for gases around the same density as air.
Consider Equipment and Process
However, none of the factors can be used alone to decide placement. The density of gases alone is insufficient to determine the sensors’ height. The process and equipment placement are also crucial. For example, heavier gases, such as butane, released under high temperatures or pressure will rise, not sink.
Also, gases released under high-pressure travel at high speed and can bypass sensors close to release points; therefore, the gas monitors should be placed at a distance to allow space for accumulation.
Provide Easy Access
The sensors must also be placed so that personnel can access them for bump tests, calibration, and maintenance. No elaborate dismantling or equipment should be necessary to work with them.
The central control unit, connected to the sensors and shows readings, must be located in a safe area outside hazardous areas for personnel checks. This is a vital advantage of fixed systems, as it protects personnel from risks.
Prevent Obstructions
There should be no obstructions between the sensors and the gases. Hence, the sensors should not be placed behind large equipment, structural elements, or storage containers that can block gas flow to sensors. Avoiding obstructions is particularly vital for Infrared and open path sensors.
Environmental Factors Are Crucial
Environmental conditions such as temperature, humidity, and airflow affect gas dispersion and concentrations, so consider these factors. The gas detectors should be installed in spots with stable airflow.
Sensors located outdoors need to be protected from sunlight and storms. If the area is exposed to sunlight, temperature can be an issue, so a shade can be added to protect the sensor. Weather protection can be used to prevent damage to sensors by rainfall.
Sensors’ performance and lifespan are sensitive to heat, humidity, vibrations, and mechanical force. Place sensors where no exposure to adverse environmental factors occurs. If that is not possible, the sensor should be suitably protected to minimize error and replacement frequency.
Appropriate Sensors
Besides placement and number of sensors, choosing the sensor most sensitive to gas should be selected. Interscan offers AccuSafe Fixed Point Gas Detection for 21 hazardous gases. Its central control panel can be wirelessly connected to sensors configured to detect up to ten gases simultaneously since a facility can be at risk from several hazards.
Learn more about Interscan’s Accusafe sensor technology to keep your personnel and facility safe.
Sources
Bose, P. (2020, May 27). Best Practices When Positioning a Gas Sensor in an Industrial Automation Facility. Retrieved from https://www.azorobotics.com/Article.aspx?ArticleID=378
Galman. D. P. (2009, May 1). Special Challenges with Gas Detection. Retrieved from https://www.ehstoday.com/industrial-hygiene/article/21903858/special-challenges-with-gas-detection.
