Sensors are the eyes and ears of a chemical plant. They are traditionally used for process control and optimisation. But in recent years, sensors have expanded their sphere of influence and they are getting increasingly integrated into new applications like predictive analytics and predictive maintenance. Sensors are helping to improve the efficiency, reliability, safety and security of chemical plants.
Sensors play a crucial role in process control in the chemical industry. Classically the sensors have been limited to detect the five major process parameters – pressure, temperature, flow, level and composition. But increasingly, the scope of sensors has been expanding away from traditional process control to a whole gamut of new areas including predictive analytics, asset monitoring and predictive maintenance, leak detection and corrosion monitoring. Many bigger industries have installed a variety of sensors to improve the efficiency, safety, reliability and security of their operations. The deployment of sensors for these unconventional applications are growing exponentially.
Thermography
Thermography uses an infrared camera to detect the heat emitted from various equipment and is an invaluable diagnostic tool for predictive maintenance. It is a quick, real-time, and non-invasive method of collecting valuable information about the health of plant assets. Thermography is used for the early detection of material wear and tear and thus avoid unscheduled plant outages and loss of productivity. It can detect impending insulation failures and thus reduce energy losses through taking corrective actions in advance. It has been used for troubleshooting performance loss in heat exchangers. Thermography can complement traditional methods to provide valuable insights about the plant equipment and improve their performance, efficiency and reliability.
Steam Traps
The ubiquitous steam traps play an extremely crucial role of conserving energy and water in the chemical industry. Yet they don’t usually get the attention they deserve as they are small and notoriously difficult to access. Steam traps are often prone to malfunction and failure. They can get plugged by solids and corrosion debris in condensate. They are also vulnerable to vibration induced by water hammer. A 2014 survey by US Department of Energy reported that approximately 20% of steam generated in a boiler is lost due to faulty and malfunctioning steam traps in the consumers. A good way of understanding steam trap’s performance is to listen to them. Steam traps open intermittently to discharge condensate in slugs and this creates a characteristic noise in the condensate piping. Tracking the signature of this noise will reveal a good deal of information about the health of the steam trap. Installing an acoustic transmitter in the steam trap discharge piping will help to monitor the health of steam traps and plan their predictive maintenance.
Fugitive Emissions
Chemical industry is plagued by fugitive emissions, which occur due to a plethora of complex reasons that are either accidental or unplanned. Faulty equipment is the most common reason. Evaporation losses from storage tanks are classified under fugitive emissions. Fugitive emissions are difficult to monitor and measure as they are small and scattered over a wide area of the plant facilities. Besides creeping loss of material, they are also a serious threat to health and environment. There is a rising clamour to curb fugitive emissions through design and engineering solutions. But their effectiveness can be understood only if there is a robust method of detecting these emissions in the first place. Also, if the exact sources of these emissions are not identified, it is impossible to fix the problem. Portable leak detection instruments have been used for monitoring fugitive emissions. Flame Ionisation Detectors (FID) are commonly used for this purpose. The sample is ionised in a hydrogen flame and the positively charged ions are attracted to a negatively charged collector to produce a current that is proportional to the concentration of ions. FID is a highly sensitive instrument to detect hydrocarbons. Another commonly used detector is the Photo Ionisation Detector (PID) which uses UV light for ionising the sample. FID can detect VOCs more accurately than PID. PID is more useful to monitor a single compound in low concentrations. Infrared cameras have recently been pressed into service for detecting the thermal signatures of emissions from large industrial sites.
Corrosion Monitoring
Corrosion is a pernicious problem in the chemical industry. It can cause catastrophic failure of equipment and piping if left unattended. Round the clock monitoring of corrosion is desirable to maintain the mechanical integrity of plant assets. Monitoring is essential to establish the efficacy of corrosion management and inhibition programmes and fine tune them. Corrosion monitoring can be offline through weight loss measurements in the laboratory or online by inserting probes into the process streams. Probes may be mechanical, electrical or electrochemical. A simple way of monitoring corrosion is to measure the change in ohmic resistance of a corroding probe inserted into the process stream. Another common method is to insert two electrodes of dissimilar metals and measure the current flowing between them. The current will change when one of the electrodes starts corroding. This technique is particularly useful for monitoring systems where the corrosion is due to dissolved oxygen. Asset failures typically occur due to wall thinning below a critical thickness due to corrosion or erosion. Ultrasonic thickness measurement is the most accurate and reliable method of assessing corrosion. Online ultrasonic probes can be installed in a specific part of the equipment where high degree of corrosion is anticipated to closely monitor the thickness during the asset’s entire service life.
Biosensors
Biosensors use biological molecules like enzyme or DNA to detect the presence of chemicals in trace amounts. They are used to measure pesticide residues and other toxic substances in the environment. Biosensors are made up of bioreceptors, like DNA or enzymes, which interact with the sample that is being tested. The resulting biological response is translated by transducers into optical or electrical signals. Biosensors have a wide range of applications, especially in medicine. In the chemical industry, they are used to control fermentation processes and also to monitor industrial effluents for trace contaminants.
Soft Sensors
A soft sensor uses process data measured by “hard” sensors and a model to predict a target output. Soft sensors thus provide indirect measurement. They are used to predict process parameters that cannot be measured at all or only through sophisticated instrumentation. For example, biomass concentration in a bioreactor. Soft sensors can also be used to detect sensor faults. A hard sensor reading that diverges widely from that predicted by soft sensor can be quickly red-flagged as an outlier and ignored. Soft sensors are expected to come into prominence as more and more mainstream chemical processes are replaced by biotechnological routes. Soft sensors will play a crucial role in the inferential control of biotechnological processes.
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