Sniffing Out Gases

With ever more improved sensors, modern detection technology for flammable and hazardous gases enables the design of highly reliable gas detection systems. The foundation for planning any system is well-considered matching of the components, which are based on various technologies, to specific local conditions.

Gases are usually invisible to the unaided eye, and are often odorless. Still, they need not pose an invisible hazard to people or the environment, thanks to advanced gas detection technology that uses stationary as well as mobile solutions. Such systems offer a variety of possibilities for the measurement of flammable and hazardous gases in the atmosphere; they are applied to ensure employee health and safety in the workplace and in the prevention of incidents. To these ends, gas detection systems have established themselves as central elements of safety concepts in fields ranging from the chemical industry to mining. The high level of modern gas detection technology is illustrated by the multitude of techniques currently in use: electrochemical sensors and catalytic bead sensors are employed, along with infrared technology in point sensors and open path sensors, flame detectors, ultrasonic sensors, and gas cameras. New developments extend existing system capabilities, explains Gero Sagasser, Gas Detection Systems Portfolio Manager at Dräger. Continuous innovation has led to a differentiation of the available solutions, permitting very precise matching of measuring technology to the requirements at hand.

Gas Detection Systems Reduce the Risk

In operations that handle hazardous (in the sense of the European Seveso Directive) substances, gas detection systems play an especially important role. Such companies are obligated to evaluate the hazard potential of their production facilities, identify specific hazard areas, and evaluate the relationship of hazards to protection objectives. The resulting scenarios are based on the maximum possible release of a hazardous material. As a result of such planning, safety measures are defined and an integrated safety concept is developed. In Germany, the basis for this regulation is the Federal Immission Control Act ("Bundes-Immissionsschutz- Gesetz") and the resulting 12th Ordinance for the Implementation of the Federal Immission Control Act (12. BImSchV, the Major Accidents Ordinance). The economic significance of a gas detection system has been increased by the implementation of EC Directive 2004/35/EC, "Environmental Liability with Regard to the Prevention and Remedying of Environmental Damage" of May 10, 2007, implemented in Germany as the Environmental Damages Act - theUmweltschadensgesetz, or UschadG.

According to this directive, the operators of an installation are liable for damages resulting from incidents. Along with information regarding dangers or the occurrence of environmental damage, the law requires measures to prevent and limit damage, as well as its remediation. Application of a modern gas detection system reduces the risk of hazardous incidents as well as their consequences; accordingly, investment in measurement technology can be applied as an argument with insurers.

More than half of all incidents in chemical engineering installations are attributable to the release of materials. These are usually triggered by technical defects (35%) and operating errors (30%). Uncontrolled chemical reactions occur in only about 20% of recorded cases. Most incidents occur during production processes (45%), more rarely in storage (15%) and other processing stages. A typical incident that may be prevented by a gas detection system is the release of flammable or hazardous (toxic) gases. Critical points within the facility, where the release of gases could occur, include all types of storage vessels, including valves and pipes, as well as machinery and pumps. Dynamic loading as a result of temperature and pressure variations, corrosion, and material fatigue are important factors that may lead to leakage. Joints and seals as well as filling and tapping points deserve special attention. In addition, technical leakage must be considered, in which material is deliberately released by means of valves, rupture diaphragms, and overflows to prevent the progression of a malfunction.

Gas Dispersion

Not every release of chemicals counts as a hazardous incident. First among these is the category "unavoidable operational malfunction," which is in turn exceeded by the "major accident despite precautions" and ultimately by the "exceptional accident," which exceeds all previous experience and calculations. It is at the stage between "operational malfunction" and "hazardous incident" that gas detection technology offers the greatest opportunity: it can make a significant contribution to very early recognition of a malfunction. This provides sufficient time for intervention by automatic emergency systems and targeted countermeasures.

To ensure that a gas detection system is able to react quickly and precisely, its components must be carefully matched to the operating facility's particular circumstances. The placement of detectors is determined, among other things, by the physical and chemical properties of the substance to be measured, assumption of the most likely leakage points, as well as the location's constructional, geographical, and meteorological conditions. The bandwidth of these data corresponds to the range of available measurement methods, based on various technologies, to enable point, line, and sector measurement using sensors of various designs. The targeted matching of a gas detection system to the specific conditions at a production facility is therefore the primary objective of any plan. The public takes particular note of incidents in which dangerous substances are released. In this form of release, gases form a cloud around the free jet exiting from the point of leakage. The edge of the cloud is a turbulent boundary zone. The expansion of the cloud is determined by the physical and chemical properties of the released substance, process-related factors such as gas release velocity and the temperature of the immediate surrounding area, and more distant climatological, geographical, and structural conditions.

Once a gas has matched the temperature of its surroundings, it may be classified on the basis of its density relative to the density of air, as a light, heavy or neutral gas. Accordingly, the gas cloud will rise in the air (light gas), spread along the ground (heavy gas), or be entirely at the mercy of the wind (neutral gas). However, a gas often behaves differently from these general patterns immediately after its release, because its process temperature is often very different from ambient temperature. Besides gaseous escaping chemicals at slight overpressures, liquids, gases liquefied by means of pressure or temperature all of them may be the source of gas clouds. The most common of these are gases liquefied by means of pressure - often far in excess of 50 bars. Upon release, part of the liquid evaporates immediately (so-called flash evaporation); the remainder forms an aerosol and a boiling pool of liquid.
The extreme increase in gas volume as a result of the flash evaporation is especially dangerous, as it leads to the expansion of the dangerous substance at a high concentration over a large volume. As in the case of gases liquefied by cooling, at first a heavy gas cloud is formed as the liquid warms up to the boiling temperature, which is below the ambient temperature. Just as clouds of different flammable and hazardous gases differ in their behavior, so too there are distinct differences between gas detection in confined spaces and in the open air. The conditions for the formation of gas clouds in interior spaces are similar to those in open air, but a cloud behaves quite differently in an enclosed space. In such cases, factors such as architecture, convective flows, and active ventilation determine the spread of the gas. Pools of heavy gases are especially dangerous, as they can form on the floors of enclosed spaces such as cellars and tanks.

Detailed System Planning

The prediction of the gas dispersion and most likely behaviour based on the above Considerations forms the basis for planning a sophisticated stationary gas detection system. Above all, a system consisting of arrays of single-gas point sensors is based on detailed system planning, which can be assisted by computer simulations. In the event of an operational malfunction or a hazardous incident, such a system also provides emergency services personnel with important clues to guide their tactical approach, makes it possible to warn the general public in timely fashion, and enables the implementation of safeguards for personal protection.