Feature Gas detection & measurement


Controlling plant emissions


he chemical industry is a major player in the global economy, with 2011 sales reportedly worth some €2800 billion and a growth rate approaching 12%. Unfortunately, by their very nature, many chemicals are toxic, poisonous, flammable or explosive. As well as the intrinsically unattractive properties of the target material, the manufacturing process can also produce a wide variety of by-products, some of which have equally unpleasant attributes. So, how to square the circle? On the one hand the chemical indus-


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try’s products are essential; on the other, some can be unpleasant and dangerous. Legislation sets strict limits and quotas for the emission levels of different substances, and record keeping and monitoring by the appropriate regulatory authority is put in place to ensure that the limits are observed.


The gas sensing industry plays a key role in enabling the industry to meet its obligations.


Gaseous emissions analysis


Stack emission from chemical plants is the largest source of the gases produced; and a quota system manages emission production. To comply with its requirements, Continuous Emission Monitoring Systems (CEMS) provide real-time monitoring and recording of a range of gases. Typically, CEMS systems will monitor for sulphur dioxide, nitrogen monoxide, nitrogen dioxide, carbon monoxide, carbon dioxide, oxygen, hydrogen sulphide, total hydrocarbons and opacity. A chemical facility will normally contain a number of small power plants, fuelled by natural gas, LPG, light and heavy oils, biomass, wood pellets, coal, propane or butane. Clearly, for emissions control purposes, these boiler rooms will contribute to the plant’s overall emission levels. In addition, with today’s ever-increasing energy costs, keeping the combustion process operating at peak efficiency makes complete sense. Flue gas analysers are portable/transportable instruments that are used to take spot measurements of a range of gases including oxygen, carbon monox- ide, carbon dioxide, SOx and NOx present in the flue gases. They will normally be based on electro- chemical sensors, arguably the most versatile sensors available. Internal filtration in the sensors ensures that the readings for the target gas are immune to cross-contamination from other gases commonly present in the flue gases.


The latest generation of oxygen sensors use lead-free catalytic technology derived from toxic gas sensors, improving response times, reducing current consumption and offering a quicker start up from cold. Electrochemical sensors characterised to a large number of different gases are available, enabling monitoring in specialised applications to be imple- mented quickly and easily.


Leak detection Leaks are an ever-present potential hazard. An explosion or fire will damage plant and put workers at risk, while toxic gases can spread rapidly, putting the public at risk. Even a minor small leak has a cost impact, as material is being wasted and the fault has to be rectified. To monitor for leaks, fixed gas detectors are installed at key weak points such as valves, joints and pumps. For flammable gases, pellistors are widely used. These sensors work by burning the target gas, with the heat generated producing a change in the resistance of the detecting element of the sensor proportional to the gas concentration. To detect hydrocarbons, NDIR sensors are widely used.


Instrumentation NOVEMBER 2013 11


John Warburton, strategic marketing manager at City Technology, looks into how today’s advanced gas sensor technology can help the chemical industry control its plant emissions


Ultrasonic gas leak detectors measure the ultrasonic sound level, typically between 25kHz to 10MHz frequencies. Such detectors are mainly used for outdoor environments where weather conditions can easily dissipate escaping gas before allowing it to reach gas leak detectors that require contact with the gas in order to detect it.


Personnel protection Workers can wear small gas detectors that will normally respond to four or five potential atmospheric hazards including oxygen, com- bustible and toxic gases. These units are based on electrochemical sensors, which are extremely sensitive and give fast response times when gas is detected. As well as protecting the workers themselves, personal gas detectors also act as mobile leak detectors, a useful back up to fixed leak detectors.


Electrochemical sensors are also used in portable area monitors. Often complete with wireless communications to allow for remote monitoring, local area monitors are frequently deployed after an incident to provide perimeter monitoring around the location of a leak.


Open area protection – storage tanks


Infrared open-path gas detectors send out a beam of infrared light – up to a few hundred metres in length – detecting gas anywhere along its path. They are increasingly used in the petrochemical industry in the detection of leaks from storage tanks, mainly to achieve very rapid detection for flammable gases at concentrations comparable to the lower flammable limit, typically a few percent by volume.


Gas detection technology advantages


The gas sensor industry has developed a number of different technolo- gies, each of which has strengths and weaknesses for use in specific applications. All manufacturers share a common purpose in improving the effectiveness of life and property protection in potentially dangerous environments. Advances in sensor design are generally aimed at producing faster response, greater specificity, better stability, longer life and greater reliability. In capital-intensive industries such as the chemical sector, which produces toxic, corrosive, explosive and flammable gases as a matter of routine, plant protection rightly has a very high priority. Add in the duty of care to the workforce, a reduc- tion in emissions to reduce environmental damage and the need to prevent leaks that could affect the local population, and the benefits from using today’s gas detection advanced technology have never been more obvious.


City Technology T: +41 44 943 4300 Enter 655 www.citytech.com


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