In factories producing batches of tablets, drinks or virtually any other market good, precise use of heat and cold is often critical.
In a large processing plant, several temperature changes can be required at different stages, and getting those wrong can be commercially disastrous as well as physically dangerous.
But of course to set processes to the correct mark on the thermometer, the thermometer itself has to be accurate in the first place.
“Temperature is the most widely measured variable in the process industries,” says Martin Adshead – product manager, temperature, at Emerson.
“If a temperature measurement is not accurate or reliable for any reason, it can have a detrimental effect on such things as process efficiency, energy consumption, and product quality.”
In a safety instrumented system, poor performance could be deadly, costly or both
Martin Adshead – product manager, temperature, Emerson
Even seemingly minor errors can be disruptive and costly in some processes. “Pharmaceutical processing is an example where an inaccurate temperature measurement might ruin a batch of product worth hundreds of thousands of pounds,” says Adshead.
“In a safety instrumented system, poor performance could be deadly, costly or both. An example might be a process that can go exothermic and possibly explode if temperature is not measured and controlled accurately.”
Feeling the pressure
Sensors are commonly used to detect temperature at various points in a manufacturing environment. However, these sensors themselves need protecting from the conditions inherent in many of the processes they are installed within.
Thermowells are one solution to this challenge, essentially acting as a protective casing covering the sensors while allowing them to work.
Measurement product maker Omega says thermowells are used to guard temperature sensors such as thermocouples, thermistors and bimetal thermometers against damage from excessive pressure, material velocity and corrosion.
“They also increase the longevity of the sensor, allow sensor replacement without draining the system, and reduce the probability of contamination,” says an Omega white paper on the subject.
Selecting the right material is crucial to the longevity of a thermowell. “The type of chemical, temperature and flow rate the thermowell will be exposed to should be considered when specifying the material,” says Omega. “The corrosive effects of chemicals are increased at higher concentrations and temperatures. In addition, particles suspended in fluid can cause erosion.”
Insertion technology is the best way to monitor process temperature but it is not always possible
The white paper concludes that thermowells “protect the measuring sensor from the damaging effects of the process environment to prevent measurement drift”.
Adshead says thermowells allow sensors to be put directly into a process at the point where the temperature measurement is needed. But he adds that this can come with drawbacks.
As such, Emerson has developed Rosemount X-well Technology, which uses a set of mathematical rules to measure process temperature without requiring any intrusions or penetrations into the process.
“This eliminates possible leak points, and allows quicker and easier installation along with simplified long-term maintenance,” says Adshead. An Emerson white paper indicates that the technology uses a heat flux algorithm to accurately record internal temperature from outside a process.
“There are many factors that can impact a traditional surface temperature measurement reading,” says the paper. “This makes it difficult to use as a simple point of inference when determining the temperature of the associated internal process.
“However, by implementing an algorithm with an understanding of the thermal conductive properties of the temperature measurement assembly and corresponding piping or vessel, a surface temperature sensor solution can be utilised to accurately calculate internal process temperature.”
Benefits of the system include the ability to fit it using hand tools and the opportunity to add measurement points while a process is running. Drawbacks can include the need to have the inputs right, and the time taken to measure.
“Insertion technology is the best way to monitor process temperature but it is not always possible,” says Adshead.
“Surface measurement may exhibit an increase in time response in comparison to an intrusive thermowell. Time response in this comparison, however, is dependent on many factors including pipe material, pipe wall thickness, fluid type and flowrate.
“Temperature is one of the slowest moving process variables and therefore time response is not an issue in the vast majority of applications.”
The Rosemount X-Well system is not currently intended for safety loops, fast control applications or custody transfer applications. It is being used for small line sizes, heating loops, oil and sand separators, water lines and more.
Adshead says many process plants use resistant temperature detectors with transmitters to monitor process temperatures.
Thermocouples are more commonly used for high temperature applications based around furnaces, such as the float glass industry.
“Infrared measurement is often used to measure the temperature of cement, lime, aggregate and other materials in rotating cement kilns.”
There is also a role for thermowells to protect sensors inserted into processes, and new designs are emerging, such as Emerson’s twisted square thermowell, which it says reduces the stress on insertion technology.
“Continuous vibration monitoring of a complete temperature assembly to predict the health of the thermowell is a concept being tested,” adds Adshead.
Collecting the data is one thing, reading it is another. Rotronic Instruments has created a system that allows process managers on the move to view temperature information from all kinds of sensors in their factories.
“The Rotronic Monitoring System (RMS) is suitable for almost any application across industry,” says sales and marketing manager Chris Fox.
“For some companies, having remote access to measurement data 24/7 is useful, but for many applications it is crucial with defined alarm conditions set, and not just for regulated industries like pharmaceutical and food – also materials manufacturing for example.”
The company says it is necessary to react immediately to incorrect temperatures that can result in expensive damage to products if not arrested.
The RMS can be installed to be compliant with Food and Drug Administration requirements and can guide the location of sensors.
“It is first necessary to investigate where and how many measuring points should be set up to monitor the production or storage rooms and cabinets,” says the company. “This is done with the help of a temperature and humidity mapping procedure. It provides information on how many different climate zones there are in the rooms.
“Using the measured data, it is possible to define the optimum storage positions for products or even to initiate changes in the room climate.”
Mapping also takes into account factors such as sunlight, air conditioning, insulation, heat sources and outdoor temperature.
Using the measured data, it is possible to define the optimum storage positions for products or even to initiate changes in the room climate
It is typically performed at very hot and cold times of the year and takes up to a fortnight. “We recommend renewed mapping if a production or storage area is modified or if there are other significant changes to the room.”
Fox adds that Rotronic is continuing to invest in forwardlooking digital technology.
“The Industrial Internet of Things (IIoT) is the billion-dollar business of the future,” he says.
“The most important elements in this are the sensors, which provide the necessary information for new business models.”
Fox believes the way process managers monitor temperature and other variables will shift.
“In the case of RMS, the device, or rather the individual component, fades into the background. The focus is on the system, and the customer’s requirements.”