Efficient Placement of Sensors

 ‘How many detectors do I need?’ and ‘where should I locate them?’ are two of the most often asked questions about gas detection systems, and probably two of the most difficult to answer. Unlike other types of safety related detectors, such as smoke detectors, the location and quantity of detectors required in different applications is not clearly defined.

Considerable guidance is available from standards such as EN 60079-29-2 and others regarding the selection, installation, use and maintenance of apparatus for the detection and measurement of combustible gases or Oxygen. Similar international codes of practice e.g. National Electrical Code (NEC) or Canadian Electrical Code (CEC) may be used where applicable. In addition, certain regulatory bodies publish specifications giving minimum gas detection requirements for specific applications.

These references are useful, but tend to be either very generic and therefore too general in detail, or application specific and therefore irrelevant in most applications. The placement of detectors should be determined following the advice of experts having specialist knowledge of gas dispersion, combined with the knowledge of process/equipment engineers and safety personnel. The agreement reached on the location of detectors should also be recorded.

Detectors should be mounted where the gas is most likely to be present. Locations requiring the most protection in an industrial plant would be around gas boilers, compressors, pressurized storage tanks, cylinders or pipelines. Areas where leaks are most likely to occur are valves, gauges, flanges, T-joints, filling or draining connections etc.

There are a number of simple and quite often obvious considerations that help to determine detector location:

• To detect gases that are lighter than air (e.g. Methane and Ammonia), detectors should be mounted at high level and preferably use a collecting cone
• To detect heavier than air gases (e.g. Butane and Sulphur Dioxide), detectors should be mounted at a low level
• Consider how escaping gas may behave due to natural or forced air currents. Mount detectors in ventilation ducts if appropriate
• When locating detectors consider the possible damage caused by natural events e.g. rain or flooding. For detectors mounted outdoors it is preferable to use the weather protection assembly
• Use a detector sunshade if locating a detector in a hot climate and in direct sun
• Consider the process conditions. Butane and Ammonia, for instance are normally heavier than air, but if released from a process line that is at an elevated temperature and/or under pressure, the gas may rise rather than fall
• Detectors should be positioned a little way back from high pressure parts to allow gas clouds to form. Otherwise any leak of gas is likely to pass by in a high speed jet and not be detected
• Consider ease of access for functional testing and servicing
• Detectors should be installed at the designated location with the detector pointing downwards. This ensures that dust or water will not collect on the front of the sensor and stop the gas entering the detector
• When installing open path infrared devices it is important to ensure that there is no permanent obscuration or blocking of the IR beam. Short-term blockage from vehicles, site personnel, birds etc can be accommodated
• Ensure the structures that open path devices are mounted to are sturdy and not susceptible to vibration