Many industrial processes involve dangerous gases and vapors: flammable, toxic, or both. With the different sensing technologies available, and the wide range of industrial applications that exist, selecting the best sensor and locating them properly for the job at hand can be a challenge.
To ensure a high level of safety, know the latest sensing technologies, which technology is best for the application, and where detectors should be installed for maximum protection.
General Introduction to fixed gas detection
Portable gas detectors are small, lightweight, and move with the user; many are disposable. Fixed gas detection systems, on the other hand, are designed for installation at a stationary location and are expected to provide long-term service life and protection. This article addresses fixed gas detection only. Three main types are generally offered: point-type, open-path, and analytic or sampling detection systems.
Point-type gas detectors
Point-type gas detectors can be fitted with either combustible or toxic gas sensors. These detectors monitor a specific area or point within the facility and must be strategically located for early detection of gas. These detectors require calibration for the gas type to be detected. Point-type detectors also must be routinely inspected to ensure they are capable of performing as expected.
Open-path or line-of-sight gas detectors
Open-path, or line-of-sight, gas detectors monitor the presence of combustible hydrocarbon gases within a beam of infrared light projected between a pair of modules. To ensure that the gas/vapor hazard passes through the light beam, the modules must be strategically located and properly aligned.
As with point-type detectors, open-path detectors must be calibrated for the gas type to be detected. Typically, open-path detectors are self-monitoring in the case of a blocked light beam or similar trouble.
Analytic/sampling gas detection systems
Many point-detection and analytical instruments use a sampling system technique to extract an air sample, direct the sample to a sealed sensor where it is analyzed, and then exhaust or return the sample to a safe location. Sampling system components typically include a vacuum pump, sensor(s), flowmeters, filters, and flow control elements. They are generally mounted on a subplate installed within an enclosure with compression fittings for sample tubing connections.
Gas Alarm Threshold Settings
Fixed gas detection systems provide alarm output signals to alert people and initiate corrective action. The alarm settings must be low enough to ensure the safety of people and equipment, but should not be so low as to cause false alarms, sometimes caused by background gases, sensitivity to other gases or vapors, or sensor signal drift. If false alarms are a problem, one option is to use voting: two detectors must detect hazardous gas levels before the alarm activates. In determining optimum alarm levels for fixed gas detection systems, consider the following:
- Applicable industry standards or codes
- Fire/explosion risk of the gas(es)
- Toxicity of the gas or vapor
- Typical background gas levels
- Size and magnitude of the potential leak
- Whether the area is occupied or unoccupied
- Time required to respond to the alarm
- Corrective actions required
Selecting Gas Detectors Third-party approved
Fortunately for users of fixed gas detectors, a number of independent agencies now have documented safety and performance criteria for fixed gas detectors. Manufacturers must submit their instruments to these agencies, or affiliated test labs, for testing to ensure compliance with the standards and mark their product as “certified.” Independent product safety and performance certification benefits end-users by providing a level of assurance that the product being considered for purchase is actually fit for duty. Following is a summary table of agencies and criteria to which they require compliance:
The latest fixed gas detection instruments and systems often offer on-board digital intelligence, meaning they deliver diagnostic capabilities, historical data logging, digital communications protocols, and provide additional microprocessor-based functionality. The most dominant digital protocols include HART and RS-485 Modbus, although proprietary digital protocols are also available.
Mean time between failure
New gas sensing technologies are available that significantly improve the mean time between failure. Foremost of these new technologies are non-dispersive infrared (NDIR or simply IR) optical gas detection devices that sense the presence of flammable hydrocarbons based upon their tendency to absorb infrared energy in a certain wavelength. This nondestructive measurement technique results in overall excellent service life expectancies, as long as 10 years in some cases.
Sensing technologies for combustible gases
For detection of combustible gases, the most common choices are catalytic and infrared sensors.
Catalytic sensors detect a wide range of combustible vapors, including hydrocarbon, hydrogen, and acetylene. Catalytic sensors offer good repeatability and accuracy with fast response time and low initial cost. A catalytic sensor’s greatest weakness is that at high combustible gas concentrations, there might be insufficient oxygen to catalyze all of the combustible gas, resulting in a decreased signal of gas concentration less than 100% LEL. Catalytic sensing requires routine calibration (typically every three months or less). Catalytic sensors are susceptible to poisoning from exposure to a substances such as silicones, halogens, tetraethyl lead, acid, pvc vapors, and other corrosive materials. Sensors can fail without annunciation, hence the requirement for routine calibration or bump testing.
Infrared (IR) detectors are immune to poisoning from contaminants and require less maintenance than catalytic. They are unaffected by prolonged exposure to gas, high gas concentrations, and changes in oxygen level. Unlike catalytic sensors, some IR detectors are fail-safe, meaning that the instrument checks itself and reports any internal condition preventing detection capability. IR sensors can detect only hydrocarbon-based gases and vapors. IR sensors do not detect the presence of substances such as hydrogen (H2), carbon disulfide (CS2) or acetylene. Apply IR sensors in combustiblegas applications where hydrocarbons are present.