The chemical processing industries (CPI) rely heavily upon the use of sensors to measure everything from weight to temperature to pH to pressure. Because sensors are used for measurements that affect critical processing elements, such as quality, efficiency and safety, it is important that sensors be placed almost everywhere in the facility, including in highly corrosive environments. Also, sensor readings from these and other operations must remain stable and reliable. Knowing this, sensor manufacturers continue to make improvements to their technologies so that chemical processors can have confidence that the sensors, as well as their operations, will not fail.
Smarter sensors for reliability
Keeping a system running reliably is one of the biggest challenges faced by chemical processors. And, depending on the sensing technology, regular maintenance and calibrations can consume a lot of time. However, if not done regularly, a bad batch or product can be the result. “This is especially true in the case of pH and ORP (oxidation reduction potential), where the electrodes are consumable devices, requiring periodic cleaning and calibration in order to provide stable and reliable process readings for proper control,” says David Vollaire, instrumentation product manager with GF Piping Systems (Tustin, Calif.; www.gfps.com).
For this reason, he says, smarter sensors are being developed that can give the operator an indication of any potential issues that may arise between maintenance and calibration procedures. “In the case of pH sensors and electrodes, they are now being outfitted with such technology,” says Vollaire. Self-diagnostics will detect broken glass and high glass impedance, alerting the operator to probe failure or maintenance needs. Built-in memory chips will allow for calibration of electrodes in laboratory or other settings and installation of pre-calibrated probes in the field, reducing system downtime and cumbersome use of buffers and cleaning solutions in the process environment. Memory-chip-enabled electrodes can also store operational data, such as minimum and maximum pH/mV readings, runtime and minimum/maximum temperature for troubleshooting and operations evaluation.
For example, GF Piping’s Signet 2751 pH/ORP Advanced Sensor Electronics featuring the DryLoc connector offers realtime monitoring of the health of the pH electrode. In conjunction with the 9900 SmartPro Transmitter, the 2751 will detect broken glass and high glass impedance, alerting the operator to probe failure or maintenance needs.
The company’s Signet 2734-2736 pH and ORP Electrodes feature a patented reference-electrode design and use the DryLoc connector. The large-area PTFE reference junction, salt bridge and reference electrode are constructed to increase the total reference effectiveness, resist chemical attack, and ensure long service life. The electrodes contain an embedded memory chip, which will store the probe calibration, allowing the user to calibrate in the laboratory and install in the field. The memory chip will record the electrode runtime, minimum and maximum readings for evaluation of performance over extended time periods (Figure 1).
Craig McIntyre, chemical industry manager with Endress+Hauser, Inc. (Greenwood, Ind.; www.us.endress.com), adds that quality and trust of the measurement information is a related challenge. “As increased process reliability and control improvements are sought, the demands for more precise and trustworthy measurement information from sensors increase,” he says.
“Vendors are driven to design in not only physical sensor reliability improvements, but also calibration/verification and sensor information-qualification improvements, both inside the sensors themselves and also in companion sensor performance management tools,” McIntyre continues. “For example, sensor improvements and available data are supporting sensor calibration and performance verification programs that enable process improvement initiatives that did not previously have the means to ensure necessary measurement quality and trust.”
For example, Endress+Hauser’s FMD71 and FMD72 loop-powered, 4–20-mA HART electronic differential pressure systems (Figure 2) eliminate errors from impulse tubing and provide additional sensor diagnostics that can be accessed through an FDT standards-based tool (like FieldCare), and integrated and managed in something like Endress+Hauser’s W@M Life Cycle Management Portal environment.
The FieldCare tool allows access to the information in an asset management system via mobile devices, meaning that from the field, a technician can call up the calibration history, diagnostic data, troubleshooting instruction and other information needed to properly diagnose a device problem.
Advanced sensor materials
Another challenge faced by chemical processors is finding sensors that can stand up to the corrosive materials found in the process environment. Liquid and gas compatibility and potential contamination are one of the more difficult issues they deal with, says Greg Montrose, marketing manager with American Sensor Technologies, Inc. (Mt. Olive, N.J.; www.astsensors.com). “Certain sensor technologies are limited in the material that can be used or the method in which it is sealed,” he says. “For example, ceramic pressure sensors are clamped to a metal process connection with an O-ring seal. While ceramic has good compatibility with various liquids and gases, O-rings need to be selected carefully and considered in the compatibility process. O-rings may also have a limitation in temperature.”
For this reason, he says, there is a trend to move toward Hastelloy C276 sensor material in chemical processing. “It has a good combination of media compatibility and material strength,” says Montrose. “With the presence of hydrogen sulfide and chlorides in many chemical processes, nickel alloys offer higher survivability than standard stainless steels. AST uses the thickest Hastelloy diaphragm (Figure 3) and a low-operating strain to create a sensor that offers longterm pressure measurement. With the diaphragm being the thinnest and most critical piece of a pressure transducer, a thicker diaphragm ensures it will withstand a longterm installation.
Using Krystal Bond Technology, AST designs pressure sensors as a monolithic piece of material with no welds, O-rings or fluid fills. Bulk silicon strain gages are mounted directly to the top of the metal diaphragm using a special glass firing process. With high raw output signal, inorganic materials and a thick diaphragm membrane, users benefit from complete isolation of the pressure of the fitting and long-term stability.
According to Endress+Hauser’s McIntyre, ceramic materials are increasingly finding use in place of metals in chemical applications. For example, ceramics that approach the purity of sapphire are being used in pressure, differential-pressure and, now, electronic differential-pressure sensors to address chemical corrosion, abrasion, vacuum, shock and stability challenges in place of traditional exotic metals and sensor constructions.
And on the horizon, according to Bob Karschnia, vice president of wireless with Emerson Process Management (Austin, Tex.; www.emerson.com), is a series of coatings of nanomaterials that can be used to help prevent corrosive problems. “Today we have different materials like Hastelloy, gold and stainless steel, depending on the process, but there might be clever ways to look at nanomaterials as a single coating that would solve all the problems in a variety of applications,” he says. “This would not only prevent the problems associated with corrosion, but it would also reduce inventory for the plant and reduce human error in the form of someone choosing the wrong sensor material for an application.”
Karschnia says Emerson is in the process of working with these types of coatings to figure out how to use them correctly so they function as Emerson would like them to in the process plant.
While wireless sensors and the benefits they provide — such as reduced cabling and labor costs and the ability to monitor remote locations — have been around for some time in the process industry, Karschnia says the use of wireless sensors is still growing dramatically. “Six years ago wireless sensors might have been used in a greenfield plant in niche applications, but now 30 to 40% of all I/O on a new project is wireless and many of these are new applications where sensors previously might not have been used at all,” he says.
As a matter of fact, industry is coming up with new types of wireless sensors that help solve problems that might have been ignored in the past. For example, Emerson’s Rosemount 708 Wireless Acoustic Transmitter & Steam Trap Monitor (Figure 4) provides acoustic event detection, including leaks in steam traps and pressure relief valves. The transmitter communicates acoustic level and temperature, as well as device and event status via the WirelessHART network for integration into host systems, data historians or energy management software.
“Prior to this sensor, there was no way to determine whether steam traps were working, other than via a manual process,” explains Karschnia. “The sensors can tell you if they’re failed open and you’re wasting energy, or failed closed and likely to damage equipment due to water hammer.”
He says Emerson developed this sensor, and others like it, for what they are calling “pervasive sensing applications,” which are new sensors coupled with strategic interpretation software that allow users to solve problems they haven’t been able to solve in the past.
Pervasive sensing via the use of new wireless devices, according to Karschnia, leads to measurable and significant improvements in worker safety, regulatory compliance, equipment reliability and energy efficiency. It also provides business-critical results that are achieved with incremental investments that acquire new insight without adding complexity, while increasing profitability and productivity. And, many of the solutions address essential asset-monitoring capabilities, gas leaks, steam-trap monitoring, safety-shower monitoring, mobile-worker and operator-round reductions that can help processors realize positive results.
While miniaturization of sensors may not be a priority in the chemical industry because most of the processes have enough room, there are several applications where smaller sensors with very high resolution may come in handy. For example, Mettler Toledo recently launched its WMC Ultra Compact High Precision Weigh Module, which has a width of 1 in. and features 2 million divisions (Figure 5). The stainless-steel design and the built-in overload protection ensure performance, and it interfaces to a variety of automation buses like Ethernet/IP or ProfiNet IO for high-speed data processing. It is suitable for check weighing and automated sample preparation, as well as fast and precise filling of active pharmaceutical ingredients (APIs).
Jeff Holcomb, marketing manager for Industrial Automation with Mettler Toledo (Naenikon, Switzerland; www.mt.com/apw) says the module is also being used in applications that wouldn’t have been considered for measurements before. “In the semiconductor industry, they use a print head to build up some of the materials, and now they can use this sensor to count the drops that come out of the print head as a method of calibration, which adds a quality step into the process. Also, it can be used in various coating processes to ensure that the right amount is being applied,” he says. “These applications are ideal because the sensor is small and highly accurate, which allows it to be built easily into a machine. Analog sensors this size exist, but experience much higher measurement uncertainty.”
Often in process plants there is difficulty in making the transition from a traditional analog communication protocol to a digital communication protocol, says Randy Brown, marketing manager with Fluid Components International (FCI; San Marcos, Calif.; www.fluidcomponents.com). “It is a huge investment to swap from a traditional analog output control system to a digital-based protocol. This expense is made even more significant when the current instruments become obsolete and processors have to start over with new instruments that support the digital protocol they’ve selected,” says Brown. “But, if an instrument were interchangeable between multiple protocols just by switching the I/O card, that would go a long way toward making digital less expensive.”
With that thought in mind, FCI introduced the ST100 Series, including the ST100 gas flowmeter (Figure 6), to meet current and future needs for outputs, process information and communications. Whether the output is traditional 4–20-mA analog, frequency/pulse alarm relays or advanced digital bus communications such as HART, Foundation FieldBus, Profibus or Modbus, the ST100 can be converted to any of these outputs with a simple card change, right in the field, says Brown. “This also ensures that if you decide to switch back to analog for any reason, you can keep the instrumentation system and just switch out the cards, once again.”
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