By Mark Rehak, Divisional Sales Director, US, Draeger, Inc.
When hazardous gases and vapors build up and reach dangerous concentrations, our sensory organs are often unable to detect these airborne hazards. As a result, we seek and develop solutions that maximize our safety and protect against these risks before they escalate and become critical health and infrastructure threats. Enter: Sensors.
The sensor is an important component that resides inside a gas detector and serves as a mini-alarm system. It reacts with the gas present in the environment to produce an electrical signal that corresponds linearly with the gas’s concentration level. Furthermore, the reliability in which harmful airborne substances can be detected largely depends on the sensors used and the substance to be detected. For a stable, flammable gas such as methane, one must consider the different scenarios where a catalytic or infrared sensor is the wisest choice, and with the help of Dräger’s methane gas detection systems, facilities can maximize safety while minimizing maintenance.
Infrared (IR) sensors are a sophisticated breed of sensors that are ideal for applications in which demands for functional safety are high. Unlike catalytic bead sensors that rely on oxygen for the proper reaction to determine gas concentrations, IR technology does not. This allows for its use as a reliable tool in atmospheres that could become oxygen deficient from large hydrocarbon releases in enclosed spaces.
When substances contain hydrocarbons, their C-H bonds absorb part of the infrared radiation emitted from the sensor. As a result, this slightly weakens the intensity of the emitted light in a specific frequency, which light-sensitive pyro-electric detectors register while downstream electronics generate a signal that indicates whether or not a gas concentration is present.
IR sensors have the capability to detect gas concentration through infrared absorption ranging from a few hundred parts per million (ppm) and up to 100 percent by volume. This means IR sensors are able to detect flammable gases and vapors within their 0-100 percent LEL ranges, and they can be used for the early detection of even the smallest leaks, so serious damage can be swiftly avoided.
When it comes to IR sensors, there are several options available that can help optimize safety in the workplace. For example, open path detectors such as the Dräger Polytron® Pulsar detector consist of a transmitter and a receiver that are placed on opposite sides of the facility. A continuous beam runs between the two, spanning across the length of the room, and when gas leaks occur, any gas clouds that form and intercept the beam are automatically detected. In outdoor facilities where external factors cannot be controlled, continuous communication between the transmitter and receiver in the Dräger Polytron Pulsar allows the system to adapt to harsh environmental conditions. The Dräger Polytron Pulsar contains four high-power xenon lamps that are paired with complex algorithms that vary in intensity and frequency in accordance to the ambient conditions. This combination, along with the digital link between the transmitter and receiver, allows the device to be easily and properly aligned, while protecting its measurements from external influences such as cross talk, solar radiation or resonance effects associated with vibration from machinery, as well as environment changes along the beams such as fog, mist and snow.
Open path IR sensors can also be beneficial based on the type of facility that needs gas detection. While point gas detection devices measure the exact concentration of the gas at that particular location (further stressing the importance of sensor placement), open path detectors take the gas’s average concentration over the entire beam path, which can greatly benefit and compliment point detectors for an overall safety level. In liquefied natural gas (LNG) facilities, for example, if LNG is spilled or a leak occurs, initially, the cold, dense vapors can stay at ground level near the release point. But as the vapors warm, they will disperse and dilute with the ambient air quickly. Therefore, the gas can be presented as a very high concentration over a small area or a very large volume of low concentration gas over a large area. Depending on the placement of the point detectors, the detectors could read a very high alarm, a very low alarm or no alarm. In the case of open path, only the number of gas molecules in the entire beam path are considered, and it does not differentiate between a small cloud of high concentration or a large cloud of low concentration. Either scenario can present the same danger. Taking an average concentration over a wider area can provide a much clearer picture of what airborne substances are lurking and placement concerns are not as critical.
With the new integration of wireless technology, IR sensors have broadened their capabilities. The GasSecure GS01 combines a single-beam, triple-wavelength IR technology offering reliable detection for demanding industrial applications. Using wireless communication that is based on the open ISA100 Wireless standard, it can seamlessly be integrated with other commercially available field wireless devices. This not only allows users to customize their gas detection systems based on their needs, but also greatly improves installation flexibility. Furthermore, its SafeWireless communication system provides reliable, yet fast, 5-second response time with full control of network traffic all while providing a very long life battery. By adapting this no-cable system, facilities can streamline workflow, reducing total project costs by up to 60-80 percent.
An IR sensor’s advantages over a catalytic sensor are that it is not subject to sensor poisoning, has a much more stable reading over a longer period of time, greatly reducing the calibration requirements, can reliably monitor in low oxygen atmospheres and has no mode of failure that can go undiagnosed. While IR sensors generally add to the upfront cost of the device, the cost is easily offset by offering a much better long-term cost of ownership. Furthermore, all hardware needed to perform the gas measurements, such as light source detectors, signal amplifiers, processors, memory chips, etc., are protected against external influences. The Dräger Polytron 8700, for example, is built with harsh environments in mind. Using a solid, explosion-proof stainless steel housing, the enclosure acts as a seal to keep the interior components safe from dirt, moisture and corrosive gases to prolong the instrument lifetime, and protect it from any factors that may impair gas measurement.
Dräger IR sensors are continuously monitored internally, and offer full diagnostics. Any component’s failure immediately triggers a fault alarm—providing a true, fail-safe operation.
Historically, catalytic sensors have been favored due to their lower initial cost and their ability to detect most combustible gases. In order to measure flammable gases such as methane, the gases are burned through a process called oxidation. Oxidation requires oxygen, fuel and a catalyst that is attached to a ceramic body called the pellistor. The greater the energy generated during the oxidation process, the higher the temperature will rise on the measurement pellistor. The temperature is measured as resistance and compared to a reference pellistor. The delta between the reference and measurement pellistors is then output as a proportional gas concentration.
It is important to have a holistic understanding of a sensor’s advantages and limitations. As previously noted, catalytic sensors rely on a heated oxidation process to accurately detect gas concentrations. Because of the heat generated on the pellistors, it is necessary to employ a flame arrestor (commonly called a “frit”) on the face of the sensor to keep any possibility of ignition from happening outside of the sensor. This flame arrestor creates a barrier and torturous path that the gasses must migrate through to reach the sensor. This can create a bit of a lag in the response time of most sensors. Dräger has designed a unique flame arrestor that greatly reduces this effect, affording a much shorter time for the gas to migrate to the sensor and offering a much faster response time.
Second, a catalytic sensor’s ability to accurately and reliably detect gases can be weakened by poisoning through exposure to substances such as: silicones, tetraethyl lead, halogenated hydrocarbons, sulfur compounds and organophosphorus compounds. Sensor poisoning is a potential issue that can go undiagnosed without frequent testing. To help identify and safeguard against catalyst contamination, clogged frits, as well as to ensure all around proper working functions, it is necessary to frequently conduct appropriate bump tests and calibrations to ensure a reliable measurement. In addition to staying compliant with OSHA’s guidelines, always review the manufacturers’ instruction manuals to identify the best maintenance method for your equipment.
At the end of the day, open path and point detection both serve their purposes, and both catalytic and IR sensors suit specific demands.
To maximize safety, pursuing a comprehensive understanding of the technology ins-and-outs of your sensors is critical when tailoring a gas detection system to suit your needs.
For more information, please visit www.draeger.com.