An introduction to Combustible gas safety monitoring

The two main technologies used today when designing combustible gas safety monitoring systems are infrared gas detection and catalytic bead gas detection. Both technologies reliably detect gas at or below the lower explosive limit (0-100% LEL).Each one of these sensing technologies has specific advantages, depending on the application. A thorough analysis of an application’s unique field environment is needed to ensure optimal performance, safety, reliability, and cost-effectiveness. A quick decision, of course, can lead to poor detector choices as well as safety, performance, maintenance, and life-cycle cost consequences.Catalytic sensing With over 50 years of proven field performance, catalytic bead (or pellistor) detectors are based on a highly responsive technology. They are single-point detectors for combustible gas detection applications, based upon the simple principle that as combustible gas oxidizes, it produces heat and the sensor converts the temperature change via a Wheatstone Bridge-type temperature transducer to a sensor signal.The secret of a catalytic detector’s accuracy, longevity, and reliability is in the design of the bead and catalyst system. It is critical to maintain an bundance
of active sites as some may become poisoned in service.Catalytic detector advantages The major advantages of catalytic detectors are:
• Robust
• Simple to operate
• Easy to install, calibrate and use
• Offer long life and a low life-cycle cost
• Proven technology – exceptional reliability and predictability
• Ideal for multi-gas applications
• Ease of individual calibration to gases
such as hydrogen Catalytic detector disadvantages There are two primary limiting factors in catalytic detector technology: Catalysts can become poisoned or inactive due to contamination; The only means of identifying detector sensitivity loss is by checking with the appropriate gas on a routine basis and re-calibrating as required.

Infrared (IR) gas detector principles Infrared gas detectors rely on two wavelengths in order to perform, one at the gas-absorbing wavelength and the other at a wavelength
not absorbed by gas.If a gas intervenes between the source and the detector, the level of radiation falling on the detector is reduced and can be continuously monitored.
Gas concentration is determined by comparing the relative values between the two wavelengths.IR gas detector advantages Major advantages of IR gas detectors are:
• Fail-to-safe operation
• Immunity to contamination and poisoning
• Ability to operate in continuous presence of gas
• Ability to operate in the absence of oxygen or in enriched oxygen IR gas detector disadvantages The limiting factors in IR technology are:
• The gas to be measured must be infrared active
• Gases that do not absorb IR energy (such as hydrogen) are not detectable
• Does not perform well for multiple gas applications
• Cannot replace the IR source in the field Site location and experience factors
A key advantage of both IR point detectors and catalytic detectors is that they have demonstrated long life performance, including in severe environments.In the harsh nvironment of refineries, IR detectors offer fail-to-safe operation, but still should be checked with gas periodically to verify that gas is free to enter the optical path.Experience has shown that users of both IR and catalytic technology do prefer to check the detectors with gas and as such, perhaps there is no significant difference in the overall maintenance
requirements. In climates with low and high temperature extremes, very humid conditions, and around hot or vibrating machinery, catalytic detectors are regarded as the best choice.Conclusions There is clearly a requirement for both IR and catalytic detectors. While IR detectors do offer enhanced reliability due to their fail-to-safe style, immunity to poisons and ability to function without oxygen, catalytic detectors offer application flexibility, simple maintenance, and low replacement costs.Both technologies are reliable, fast detecting, and accurate. When making a choice, consider the field environment and variables in detector design from manufacturer to manufacturer. Life-cycle
cost assumptions will not hold true in all environments; the same can be said for detector mean-time-to-repair or failure data among various manufacturers.Careful analysis of detectors, suppliers and field experience will help in selecting the best detector for a given application.

2014-3-7 15:53:23

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