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Comment PDF Automation & Control

Modern Temperature Sensors Increase Accuracy

By Joy LePree |

Both new and traditional technologies simplify and improve the temperature-sensing process

Temperature is cited as the most frequently measured variable in chemical processing plants. As a result, there is no shortage of instruments available and the offerings run the gamut from traditional thermocouples and resistance temperature detectors (RTDs) to more modern and intelligent versions of these common instruments straight on through to higher technology measurement tools. As a bonus, no matter which of the many options suit your chemical processing needs, most of today’s temperature measurement instruments are updated and re-designed to simplify the temperature sensing process, while making it more accurate and reliable.

“As with all measurement instruments, the higher the accuracy and reliability of the sensor, the more valuable it is to the user because tighter, more accurate process control results in better quality and utilization,” says Ted Johnson, director of global temperature sales with SOR Controls Group (Lenexa, Kan.; www.sorinc.com). “This is especially true in temperature measurement because temperature is the most measured process variable in the plant. And, when you have the most accurate temperature sensor possible, you have tighter control of the process, which means you are optimizing the amount of energy needed to heat and cool the process. Not only does this save energy, but tighter, more accurate temperature control means improved repeatability and consistency of the process, which is essential in chemical processing.”

 

Challenging applications

For the most challenging applications, which include taking multipoint measurements, continuous measurements or measurements in extremely harsh environments, technologies have been developed to bring simplicity, accuracy and reliability to traditionally difficult and costly measurement points.

For example, Siemens Industry (Hauppauge, N.Y.; www.siemens.com) has recently introduced the Sitrans TO500 multipoint measuring system (Figure 1) for evaluating a large number of temperature measuring sensors, which are arranged on a very narrow fiber-optic measuring lance. The system consists of a read-out unit, the transmitter and the measuring lance, which can be connected to up to 48 temperature sensors on the transmitter at four channels. Up to four measuring probes can be used to process 192 measuring points at the same time by one Sitrans TO500.

temperature sensors

Figure 1. The Sitrans TO500 multipoint measuring system evaluates a large number of temperature measuring sensors, which are arranged on a very narrow fiber-optic measuring lance Siemens Industry

The technology is based on fiber Bragg gratings (FBGs), which are arranged at individually defined points on the sensor probe. The transmitter sends light waves to the fiber-optic sensors and evaluates the reflected portions. In the transmitter, light is generated in the wavelength range from 1,500 to 1,600 nm and output to the sensors’ measuring probe by means of a continuously tunable laser light. Each fiber Bragg grating reflects light of a defined wavelength. The wavelength reflected by the grating varies depending upon the temperature. The reflection at the FBGs provides a measure of the temperature at the respective measuring point. A gas cell with a fixed absorption line serves as a reference in the Sitrans TO500, and the wavelength determination is continuously adjusted by it. The transmitter provides the determined values for analysis in control systems via a Profibus DP interface and makes them available for management of the assets and optimization of the process.

“We were approached by our chemical customers who were looking for a better way to do multipoint temperature measurement in tanks and vessels because the traditional method of placing a single-point measurement sensor every few feet and wiring that back to the control system is a very clunky and time-consuming way of getting multiple point measurements in a tank or vessel,” says Justin DiNunzio, product marketing manager with Siemens. “It was also very costly because it often required a very large thermowell to hold hundreds of these sensors, which sometimes necessitates the use of a crane to put the large and heavy thermowell into the tank and pull it in and out of service if one of the points fails.”

The fiber optic technology, he says, provides users with a lightweight, simple method to take multiple point measurements inside large reactors, tanks or vessels, alerts the user to any issues and allows them to take corrective action quickly. “Picture a reactor and our lance installed vertically from top to bottom,” says DiNunzio. “Chemical processors want to know what the temperature is every few inches or feet because if the reaction gets too hot at any of these points inside the reactor, it could bake or overcook the chemical or catalyst, which can be very costly. The fiber optic system gives a great indication of where hot spots are occurring and allows them take measures to cool it down.”

Another challenging chemical process industries (CPI) situation is achieving continuous, accurate temperature monitoring in critical applications, according to Thomas Fortinberry, business development manager for industrial gas with Ametek Land Instruments International (Dronfield Derbyshire, England; www.landinst.com). “Traditionally, an operator takes a temperature reading once every shift or every few days, but this leaves a lot of time when the equipment is not being monitored, so issues may be missed or a situation could arise where equipment overheats and causes damage between readings,” he says. “Also, it is not always the same operator and measurements are not taken in the same spot or under the same conditions, so you can’t use the acquired data for any type of trending.

“However,” he continues, “continuous temperature measurement becomes a very powerful tool for trending because with 24/7 monitoring, you get repeatable, reliable results that can be trended, allowing users to see how the equipment is operating over time. This allows them to tweak the process and prevent unwanted downtime.”

To provide users with these benefits, Ametek Land has introduced the NIR Borescope (NIR-B) 3XR (Figure 2). The instrument is a short-wavelength radiometric, infrared borescope imaging camera for steam-reformer and cracker-tube continuous temperature measurement and furnace optimization and monitoring. It provides a high-resolution thermal image with realtime continuous, high-accuracy temperature measurements of both the tube wall and refractory wall surface. The camera measures temperatures in a single range from 600 to 1,800°C and uses wide dynamic-range imaging technology, making it suitable for applications with high differential temperature in the filed of view, such as tube and furnace walls.

Figure 2.  The NIR Borescope (NIR-B) 3XR is an imaging camera for continuous temperature measurement and furnace optimization and monitoring  of steam-reformer and cracker tubes Ametek Land Instruments

Figure 2. The NIR Borescope (NIR-B) 3XR is an imaging camera for continuous temperature measurement and furnace optimization and monitoring of steam-reformer and cracker tubes
Ametek Land Instruments

And, for extremely challenging conditions such as the high-pressure, high-temperature and highly corrosive environments often found in the CPI, Emerson Automation Solution’s Rosemount division (St. Louis, Mo.; www.emerson.com) has developed a technology that “solves the problem of having the measurement point in the process,” says Ryan Leino, senior product engineer, marketing/business development with Rosemount. The X-well Technology (Figure 3) delivers accurate process temperature data without thermowells or process penetration. Measuring the ambient and pipe-surface temperature, this surface-temperature solution calculates the process temperature via a thermal conductivity algorithm. This calculation takes into account the thermal conductive properties of the assembly and pipe for reliable and accurate process temperature measurements, allowing the surface-temperature-sensor solution to accurately measure internal process temperature, simplify measurement point specification, installation and maintenance and reduce possible leak points.

Figure 3.  X-well Technology delivers accurate process temperature data without thermowells or process penetration Emerson Automation Solutions

Figure 3. X-well Technology delivers accurate process temperature data without thermowells or process penetration
Emerson Automation Solutions

“This non-intrusive measurement solution was designed to clamp onto the pipe so that there are no issues with materials, temperature and pressure compatibility of thermowells being placed in the harsh environment found inside the process,” explains Leino.

 

Improved traditional sensors

Challenging applications aside, the bulk of temperature measurements in the CPI are still made with thermocouples and RTDs. “The number of specialty applications in any given plant is a small minority compared to the bulk of measurements, which are standard pipeline measurement, process tank, storage tank and metering stations where temperature is needed as part of the flow calculations,” says SOR’s Johnson. For this reason, it is very important that traditional temperature measurement sensors are manufactured properly, he says. “The last thing you want is for a $100 RTD or thermocouple to fail and possibly shut down a $2-million reactor or ruin a batch of product,” he says. “So processors shouldn’t necessarily view these instruments as a commodity purchase. Quality temperature sensor fabrication is a highly skilled, intricate manufacturing process involving precise welding and soldering of junctions and attaching small-gauge lead wires. So it is important to find a supplier that pays rigorous attention to the manufacturing process, ensuring that the process is very repeatable and includes multiple quality checks.”

In addition to a high quality and reliable product offering, says Johnson, many temperature sensor manufacturers also provide engineering and technical support, particularly when it comes to thermowells (the protection tube that is installed in the pipeline or tank that protects and allows the temperature sensor to be replaced while the process is still running). “It requires a fair amount of engineering work based upon the velocity of the process to design thermowells so that vibration or process factors don’t cause the thermowell to break apart and cause damage to the equipment or process. And, there’s additional engineering work involved in specifying thermowells, which can be a challenge for many chemical processors because their knowledge base in this area is dwindling. This means as consumers of temperature measurement devices and thermowells, it’s important to find a reliable vendor who can collaborate with you to provide the latest requirements per industry codes and standards, advances in performance capabilities, application knowledge and support. Experienced vendors can help solve problems and ensure that only the most reliable and accurate instruments make it into the process.”

In addition to working toward achieving the highest manufacturing standards, many suppliers are developing variations on the standard temperature measurement offerings in an effort to meet the challenges of today’s CPI. For example, Endress+Hauser (E+H; Greenwood, Ind.; www.us.endress.com) set out to improve the performance of RTDs. “Although RTDs generally provide higher levels of accuracy and repeatability, the downside is that they are slower to respond to temperature changes than thermocouples,” says Ehren Kiker, product marketing manager for pressure and temperature products with E+H. “So we developed an RTD sensor that is still a three-wire RTD with the accuracy and level of performance associated with RTDs, but that has the response time of a thermocouple to give customers the flexibility of having both higher performance and faster response time.”

E+H’s iTherm QuickSens offers a short response time ( t 90 = 0.75 s) combined with precise, fast and stable temperature measurement. It measures from –50 to 200°C and is vibration resistant up to 60g (g = 9.8 m/s 2). And, for plants that are exposed to vibrations, the iTherm StrongSens (Figure 4) offers shock and vibration resistance of >60g. “The StrongSens is important because one of the main causes of RTD failure in the chemical industry is vibration, so if the user has a sensor that continuously fails because the pipe where they take the measurement shakes violently, we can provide this sensor because it is more robust in these difficult applications. It saves time and money in that it doesn’t have to be replaced every few months and, when there’s a few hundred of these measurement points in one facility, there is real value to the user,” says Kiker.

Figure 4: Shown here is Endress+Hauser's TM411 with QuickSens and StrongSens as an option

Figure 4: Shown here is Endress+Hauser’s TM411 with QuickSens and StrongSens as an option Endress+Hauser

Another improvement is the addition of digital displays with added outputs on standard temperature measurement offerings, says Christopher Smock, vice president of operations and manufacturing with Tel-Tru (Rochester, N.Y.; www.teltru.com). “As we move more toward the Industrial Internet of Things, driven by both technology and the regulatory environment, the automation of processes and process monitoring is a growing trend,” he says. “And, as a result, we have recently added a 4–20-mA output on our Digi-Tel line of thermometers.” Digi-Tel thermometers (Figure 5) provide digital display of key process temperatures in addition to reliable transmitter output to remote devices within the facility that can be integrated into expanded control systems to help processes run more efficiently. The new Digi-Tel series also includes a PC-based calibration software tool that allows users to quickly and easily adjust the number of significant digits on the digital display and to field calibrate Digi-Tel electronic thermometers to traceable or relative accuracy standards.

Figure 5.  Digi-Tel thermometers provide digital display of key process temperatures in addition to reliable transmitter output to remote devices within the facility that can be integrated into expanded control systems to help processes run more efficiently Tel-Tru

Figure 5. Digi-Tel thermometers provide digital display of key process temperatures in addition to reliable transmitter output to remote devices within the facility that can be integrated into expanded control systems to help processes run more efficiently
Tel-Tru

Taking it a step further, some suppliers are adding intelligence and diagnostics to temperature measurement devices. For example, the Foxboro division of Schneider Electric (Foxboro, Mass.; www.schneider-electric.com) offers Model RTT80 (Figure 6), which is a microprocessor-based temperature transmitter with HART communication protocol that receives input signals from thermocouples and RTDs. “One of the most important trends is diagnostic capability in a temperature measurement system,” says Steven West, temperature product manager with Schneider Electric. “Users are looking for a means to detect corrosion of wires, broken wires and shorts or open wires that would alert them to a problem in the temperature measurement loop. We have developed a way to provide diagnostics based on dual sensors.”

Figure 6.  RTT80 is a microprocessor-based temperature transmitter with HART communication that receives input signals from thermocouples and RTDs Schneider Electric

Figure 6. RTT80 is a microprocessor-based temperature transmitter with HART communication that receives input signals from thermocouples and RTDs
Schneider Electric

He says it is very common for a temperature probe to have two sensors and to have both monitored by the transmitter electronics. The RTT80 is capable of operating with dual sensors and has the ability to detect short or open circuits and has a “hot back up” or redundancy feature and can detect corrosion when used with a four-wire RTD. The transmitter is also rated for safety integrity levels (SIL) and the software has been vetted to comply with international standards for use in safety systems, so it can be used to turn on or off safety systems based on temperature measurements.

“Not only are these capabilities helpful for safety reasons, but also in a chemical-processing environment, the synthesis of a chemical or a chemical reaction is being monitored because these processes are very temperature sensitive,” he says. “They may occur only within a certain temperature window or there may be an optimal temperature to achieve the greatest yield, so intelligent diagnostic capabilities in temperature measurement can provide great peace of mind and great savings for the user.”

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