Gas detector selection is a critical task for fire-and-gas-system engineering design at chemical facilities. Correct detector choice, coupled with proper placement and allocation, underpins fire-and-gas-system effectiveness. A poorly selected gas detector, however reliable and adequate in number and detection coverage, may fail to provide early warning of a hazardous material release.
End users continue to rely on conventional design, using traditional technologies like catalytic, electrochemical and solid-state toxic gas detection. Yet advances in sensor design and the advent of new methods like ultrasonic and open-path detection suggest the need for a reassessment as solid-state combustible, thermal conductivity and calorimetry gas detection have fallen into disuse.
Equally important, the demands placed on gas detection equipment have changed over the years. Much larger process facilities have placed a premium on area monitors, which offer substantial cost savings in installation and maintenance per unit area covered. Similarly, the need to develop safety devices suited for safety integrity level (SIL) 1 and 2 environments has spurred the development of diagnostics, which has favored optical gas detection over passive systems. (Optical gas-detection methods not only provide effective self-checks but also information on sensor degradation that enables predictive health intelligence.)
Any updated guide on gas detector selection should consider field experience. Indeed, end users weighing the merits of new methods should take a measured approach: incorporating them only if they offer substantial benefits over legacy devices, and evaluating their service after installation. This article provides a practical list of available gas detectors as well as selection criteria. In developing this, I acknowledge the contributions from the U.K.’s Health and Safety Executive, AIChE’s Center for Chemical Process Safety, and the Council of Gas Detection and Environmental Monitoring.
As shown in the selection decision tree (Figure 1), catalytic and infrared (IR) technologies can detect combustible gases. Of the two techniques, only the optical method lends itself well for point and area monitoring. Choose IR devices for detecting a single type of hydrocarbon or simple mixtures. Because they are unaffected by oxygen levels, such detectors are ideal for anaerobic processes, operations with high concentrations of corrosive agents, and areas with constant background of combustible gases. On the other hand, catalytic gas detectors are a better choice for monitoring hydrogen or hydrocarbon mixtures. Because catalytic detectors require oxygen gas concentrations in excess of 10% by volume to operate, they only suit situations where oxygen always is present. Moreover, catalytic gas detectors perform best in applications where the target gas normally is absent.
For monitoring toxic gas leaks in congested spaces, electrochemical and solid-state gas detectors are the instruments of choice. Use electrochemical detectors in applications where the target gas is not normally present and in environments with relative humidity (RH) above 15%. By contrast, solid-state detectors should be used for extreme high temperatures or low humidity applications or where ambient conditions are stable. Oxygen deficiency is best monitored using electrochemical gas detectors sited near the breathing zone. As for open or uncongested spaces, open-path near infrared or ultraviolet detectors offer the widest coverage per device.
Hazardous material releases can take many forms. For pressurized gas releases, choose ultrasonic gas leak detectors to provide the fastest response to a leak. They excel at detection in open, well-ventilated areas where other detection methods may not be wholly effective. For pressurized liquid releases, select air particle monitors.
Gas Detection Options
Plants can turn to a variety of technologies to detect gas leaks. So, let’s briefly look at these options and their pros and cons.
Catalytic and electrocatalytic detection. Traditional detection technologies include point detection systems and area and perimeter monitoring devices. Catalytic detectors often serve as point instruments to measure combustible gases in air at low concentrations. As the combustible gas oxidizes in the presence of a catalyst, the heat of combustion causes the temperature to rise, which increases sensors resistance. A standard Wheatstone bridge circuit transforms the offset voltage caused by the temperature rise into a sensor signal.