Infrared Combustible Gas Detectors Provide Improved Performance and Versatility

Summary

Facilities using large volumes of hydrocarbons as fuels or feedstocks or making these types of products, must have comprehensive safety systems designed with layers of protection to prevent or mitigate safety incidents. One critical layer is combustible gas detection to determine when gaseous hydrocarbons (propane, methane, etc.) or vapors from liquid hydrocarbons (gasoline, ethanol, etc.) are present, as these can potentially form explosive clouds.
 
This type of gas cloud formation indicates containment has been breached, and the next likely incident will be a fire or an explosion if a source of ignition is in the area. An effective detection system warns operators that they must take appropriate action, and/or triggers safety instrumented functions (SIFs) designed to shut down product flow at strategic points.
 
These basic safety concepts are nothing new and should be well understood by anyone working in such a plant or facility. However, improving upon these systems calls for a higher degree of detailed knowledge, including an understanding of specific gas detection technologies.
 

Detecting flammable gases

The traditional technology for detecting flammable gases is a catalytic bead sensor (Figure 1). When the target gas comes into contact with the sensor, it reacts with the catalyst on one of the beads and combines with atmospheric oxygen. This reaction produces heat which can be detected by a resistance temperature detector (RTD) within a pellistor. If the temperature of the active pellistor with the catalyst begins to climb above the inactive pellistor, the difference signals the presence of combustible gas. If the level reaches the sensors set point, an alarm is activated.

A more versatile sensing technology

One characteristic of flammable hydrocarbon gases and vapors is their ability to absorb specific wavelengths of infrared (IR) radiation. This is beyond human visual perception, but electronic sensors can be fine-tuned to capture and quantify these changes. This absorption is detectable using differential optical absorption spectroscopy. If the sensor detects an attenuation in the frequency for a target gas that does not correspond to an overall ambient change, it can signal the presence of the target gas. This technology can provide a very fast response time which is highly beneficial, and it can also be configured for a wide variety of common use cases.
 
These types of detectors share two common elements: the IR source and the electronic sensor, placed such that the source shines its light on the sensor and there is space in between where air mixed with the target gas can circulate. The sensor must be able to measure wavelengths related to the target gases, plus some other part of the spectrum which remains unaffected and serves as the ambient light for a basis of comparison. The electronics in the transmitter compare these two wavelengths and look for deviations.
 
Let’s look at some ways in which this basic concept has been adapted and used to solve specific flammable gas detection applications.
 

Point source

On one end of the size spectrum, IR sensor technology can be used on a very small scale to provide a point source measurement within the same sensor and transmitter form factor as the traditional catalytic bead version (Figure 2).

• Far less testing, calibration and maintenance.
• More durability without the potential for catalyst poisoning.
• Higher immunity to false alarms.
• Easier to tune to detect specific target gases.

 While these represent significant improvements over traditional catalytic bead sensors, point source sensors are limited to detecting gas concentrations wherever the sensor is located, so detector placement is crucial. This is ideal for some applications, while other situations call for a different technology.
 

Open path

Open path hydrocarbon gas detectors separate the light source and sensor completely, sending a focused beam of IR across an open space up to 200 meters in length. This means perimeter monitoring is possible with relatively few detectors, which are usually installed around clusters of equipment where there may be multiple potential leak sources. If the target gas drifts into the beam anywhere along its coverage area, the detector will respond.
 
The source and sensor are deployed as a pair (Figure 3), looking from one to the other across the area of coverage by sending a beam of IR with consistent wavelength characteristics from the source to the receiver. Lenses and shading are used to minimize potentially confusing effects from ambient light since these systems must operate equally well in full sunlight and night-time darkness.

Hybrid bench designs

One of the most versatile aspects of IR sensor design is the flexibility available when the two components can be separated, but on a smaller scale than with an open path detector. Many designs emerged from original use in laboratory environments, and the name “bench detector” has stuck, even though these have moved out into the plant environment. A bench detector has very good accuracy and repeatability, quick response time, fast recovery from moisture issues on the sources and receivers, easy-to-clean optical surfaces and long maintenance and calibration intervals.
 
Typically, these are short-path detectors, where the beam traverses a distance between 1 to 40 centimeters, and this technology captures target gases moving through this space. These detectors are built as single-piece assemblies, with the sources and receivers located in the same compartment.
 
These detectors are used to create solutions where conventional point gas detector designs are not suitable for arduous applications, such as offshore where the environmental conditions are very challenging. One typical configuration uses an extension from the transmitter body (Figure 4), with slots or perforations to allow easy gas flow into sensing area. The transmitter body contains both the source and detector, with a reflecting surface at the end of the duct to send the beam back. Since the beam is passing through the gas twice, this effectively doubles the amount of IR absorption, boosting sensitivity.

• Very high sensitivity.
• Very fast response.
• Tunability for specific gas sensitivity and identification.
• High false-alarm suppression.

While point source and open path designs continue to be the primary solution for environmental flammable gas monitoring for safety purposes, bench designs are normally assigned to specific tasks where their special configurations are particularly useful. For example:

• Gas turbines—fuel leaks in enclosures.
• Gas compressors and pumps—leaks around valves and gasketed connections.
• Valve clusters in tank farms—leaking valve stems and fittings.
• Ductwork—fuel drawn into airstreams.

Detectors in these applications can be fine-tuned for whatever characteristics are most important. For example, the ability to be scaled for butane or ethylene in the explosive range means operators can use the same model of detector in different locations.
 

Rugged and reliable

While highly sophisticated equipment often suggests delicate mechanisms, these types of flammable gas detectors are very rugged and able to stand up to the challenges of plant environments (Figure 5). Many are suitable for Class 1, Div. 1 and Zone 1 & 2, hazardous environments, so they can be installed where the problems are, regardless of the potential hazards. Optical elements are protected, and even heated if necessary, to ensure readings are not compromised by dirt or deposits. Internal diagnostics continuously monitor detector health status and warns users if unusual events are occurring.

Suitability delivers safety

Choosing a flammable gas sensor designed specifically for a critical application can make all the difference. Settling for a detector that should work can be replaced with one that will work. New capabilities make it practical and even economical to customize operational characteristics, while minimizing maintenance and lifecycle costs.

<sub>All figures courtesy of Emerson

Product references:</sub>

1. _SC310
2. _{Net Safety Millennium II SC311 Infrared Combustible Gas Sensors}
3. _{Flame & Gas Safety Solutions Brochure}
4. _{Rosemount 935}
5. _{Rosemount 625IR Infrared Gas Detector | Emerson US}