The risks and rewards of catalytic and infrared sensors

By Ulf Ostermann

The ultimate goal of an explosive gas monitoring system can be summarized in a single word: safety. In order to achieve this goal, numerous factors must be assessed and analyzed. Sensor capabilities play a vital role in worker protection—fast response times and wide temperature ranges are must-haves, and so, too, is the understanding of when to use catalytic or infrared (IR) gas detector technology.

This assessment is based on each application’s unique environment. During the refining process, exposure to explosive hazards from either natural gas (methane/ethane) or oil vapors (higher hydrocarbons such as pentane or octane) is possible.

Referring to the typical refining process, explosive hazards at the gas/liquid separator may be encountered as the oil and water mixture is pumped into tank vessels or when the separator tank must be cleaned. Later in the refining process during the gas sweetening, dehydration and separation process steps, explosive hazards may be present due to a variety of factors including: leakages in processing vessels, process parameters (e.g., temperature, pressure, etc.), maintenance of processing vessels, transportation of CO2 and H2S by-products, and conducting work in confined spaces where oxygen-deficient atmospheres may be encountered.

Catalytic Sensors

One of catalytic sensor’s greatest differentiators is the flexibility it affords the user. This technology is capable of measuring known and unknown explosive gases and vapors in applications where oxygen is present. Flammable gases and vapors, from methane to nonane, oxidize at the surface of the catalytic bead or pellistor to produce heat energy that results in a change of resistance. This heat energy corresponds in linear proportion to the concentration.

Understanding Sensor Poisoning for Catalytic Sensors

Catalytic bead sensors are subject to poisoning of the pellistor, which means it will exhibit reduced reactivity. Substances that may contaminate sensors include:

  1. Silicone
  2. Tetraethyl lead
  3. Halogenated hydrocarbons
  4. Sulfur compounds
  5. Organophosphorus compounds

The impact of a poisoned pellistor on the methane sensitivity is much higher compared to other explosive gases or vapors. Methane is a more stable chemical compound and requires a greater activation energy to oxidize at the pellistor surface. A less active, poisoned pellistor has less capability to reduce the activation energy. As a result, the poisoned pellistor or contaminated catalytic bead sensor has a lowered sensitivity towards methane.

It is highly recommended to conduct an appropriate bump test and calibration concept to ensure proper performance of catalytic sensors. Consider counter strategies to deal with contamination:

Known Gas Hazard

  1. Bump test and calibration to target gas is preferred
  2. Vapor calibrations are complicated to handle, so cross-calibration with other known gases (e.g., with propane) is valid; regular check of target vapor should be done

Numerous Gas Hazards Expected

  1. Calibration to main risk, as vapor calibrations are complicated to handle; so cross-calibration with other known gases (e.g., with propane) is valid
  2. If methane risk, bump test on methane

Unknown Gas Hazard

  1. Calibration to highest sensitivity
  2. E.g., cross-calibration with propane to nonane sensitivity
  3. Bump test with methane

Infrared Sensors

Infrared (IR) sensor technology is based on the carbon-hydrogen bonds of hydrocarbons that absorb infrared light of a certain wavelength. It’s important to note that small wavelength differences between different infrared sensors can lead to different detection performance.

One of the greatest advantages infrared sensors present over catalytic bead sensors is that they are not subject to sensor poisoning, which means the lifetime of the sensor corresponds to the lifetime of the instrument. Furthermore, oxygen is not necessary for operation, which allows these sensors to be used for inert measurements.

While infrared sensors are applicable for lower explosive limit (LEL) measurements of hydrocarbons, it is not applicable for:

  1. LEL measurements of non-hydrocarbon components such as hydrogen
  2. Acetylene
  3. Measurement of unknown explosive gases

Conclusion

Education of the user is an essential component to every task, particularly in situations when a quick decision can lead to poor choices that not only impact device performance and maintenance, but also the end user’s safety. Appreciating the power behind a good sensor and understanding which applications it is most suited for should be used is needed to optimize the effectiveness of your safety program.

For more information on implementing gas monitoring systems, call 800-437-2437 or visit the Dräger website.

Register today for the free webinar How to Design a Wireless Safety Instrumented System (SIS) for Plant-Wide Gas Detection (Live February 15)

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