Combustible gas safety monitoring: infrared vs. catalytic gas detectors

Combustible Gas Safety Monitoring: Infrared vs. Catalytic Gas Detectors

INTRODUCTION When designing a combustible gas safety monitoring system for oil/gas, petrochemical or other applications, how do you decide whether to use infrared or catalytic gas detector technology? Both sensing technologies have their advantages dependent upon your application’s specific requirements. A thoroughanalysis of your 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.

sensing technologies: (1) catalytic detectors and (2) infrared detectors. Both technologies reliably detect gas at orbelow the lower explosive limit (0-100% LEL). They are also suitable for use in a broad variety of field application environments (see Table 1 for a list of gases typically monitored). Determining which sensing technology is the best fit for your individual application is extremely important. Our field applications engineers will help you conduct a risk assessment audit to determine the requirementsfor your safety monitoring system. Typical Gases Monitored • Methane • Ethane • Propane • Butane • Hexane • Butadiene Other Gases Monitored • Isopropylamine • Propylene • Ethylene Oxide • Propylene Oxide • Ethanol • Methanol

Table 1: Gases Monitored
CATALYTIC SENSING Catalytic (or electrocatalytic) detectors (Fig. 1) are based on a highly responsive technology with over 39 years of provenfield experience. They are single-point detectors for combustible gas detection. Based upon the simple principle that as combustible

As a global leader and innovator in gas sensing technologies, General Monitors offers the industry’s two most popular and reliable combustible gas

Figure 1: S4000C Intelligent Combustible Gas Detector 1

gas oxidizes it produces heat and the sensor converts thetemperature change via a standard Wheatstone Bridge-type temperature transducer to a sensor signal. The sensor components consist of a pair of platinum heating coils embedded in a catalyst. Since the reactants are all gaseous, the reaction takes place on the surface of this element with the combustible gases reacting exothermically with oxygen in the air to raise its temperature. This results ina change of resistance within the embedded coil, which is linearly proportional to gas concentration. General Monitors’ catalytic detectors utilize a pair of catalytic beads that are identical except for a glass coating on one of them (Fig. 2). This method of providing an inert “reference” bead ensures it remains permanently non-responsive to gas, thereby acting as a stable “baseline” signalgenerator to compensate for environmental changes which would otherwise affect the sensor’s stability. Glass-coating is a unique feature of our catalytic detectors and the physical and electrical matching of the bead pairs eliminates the need for compensating resistors which cause drift in operation. The secret of a catalytic detector’s accuracy, longevity and reliability is in the design of thesubstrate and catalyst system. It is critical to maintain an abundance of active sites as some may become poisoned in service. We achieve this by using a highly porous substrate with catalyst deposition down to the core.
Reference Bead Mechanical Support Post Platinum Wire Teflon Thermal Barrler

Advantages The major advantages of catalytic detectors are that they are: • Robust • Simple to operate •Easy to install, calibrate and use • Long lived with a low life-cycle cost • Proven technology with exceptional reliability and predictability • Immensely flexible with application • Easily calibrated individually to gases such as hydrogen which cannot be detected using infrared absorption

Disadvantages The limiting factors in catalytic detector technology are that: • Catalysts can become...