How Many and Where to Put them?
 

Area monitoring for hazardous Gases and Vapors has several meanings. Gas sensors are often mounted near process equipment such as valves, pumps and tanks for process leak detection. Examples of these applications and the monitored gases are Black Liquor (H₂S), Pipeline Compressor Stations (Methane, CO₂ et al) and Specialty Chemicals (Solvents et al). In such applications the gas sensor is a process leak detector. It protects property and ultimately people from the leaking chemical, but its information is also used by maintenance and process control.

For personnel protection from toxic chemicals such as CO, HF or H₂S, gas sensors are usually mounted in the breathing zone. When monitoring for flammables, sensors are mounted high for lighter than air gases / vapors (Hydrogen), and low for heavier than air compounds (Solvents). 

There are some variations on these applications. Gas sensors are often positioned between personnel and the hazard source. Sample draw may be used for difficult access areas such as wet wells. Duct mount sensors might be employed to protect ventilation or thermal oxidizer ducts from having a flammable mixture. Remote mounted sensors are often in another room or area.

Caution must be observed in all these sensor mounting situations. Class I, Division 1 applications require rigid conduit and poured seals when the sensor is remote mounted, unless the sensor head is approved Intrinsically Safe[1]. In that case, cable only may be used with a seal at the transmitter. Many gas detection suppliers do not have hazardous area approval for remote sensors. Sample draw systems must have a sample flow fault to warn that the safety system is not functioning[2]. Sample draw is maintenance intensive as there will be dirt and condensation in the tubing. Consider remote mounting an intrinsically safe sensor first. Duct mount sensors are air velocity limited so observe the maker's recommendations.

The functional testing (calibration) of remote mounted sensors should be via a gassing fixture and tubing run back to the transmitter, to simplify maintenance. Avoid excessive length runs of tubing.

Oxygen deficiency gas sensors are always mounted in the breathing zone. They may be remote mounted but are seldom on a duct and almost never on a sample draw system. The usual application is sensor and transmitter together. Annunciation inside and at the entrance to the monitored area is required. Supervisory notification is mandatory if the space is occupied and strongly recommended otherwise[3].

Good planning and observing best practices when installing a gas detection system will yield a highly functional, protective and low cost of maintenance system.

[1]  An electrical protection method such that a device will not produce any spark under any conditions that would ignite any flammable mixture.

[2]   ANSI/ISA-RP12.13.02-2003 (IEC 61779-6 Mod) Recommended Practice for the Installation, Operation, and Maintenance of Combustible Gas Detection Instruments

[3]   Alarm notification to a remote location, manned 24/7/365 and capable of response