Getting sufficient numbers of temperature sensors in the right places can make all the difference. That’s the message of Shane’s article in the August 2019 issue of Process Heating magazine titled Instrumentation Solves Temperature Measurement and Control Problems. The discussion also extends into the characteristics of temperature loops, and why they’re different than pressure and flow.
Changing the temperature of a process requires transferring heat into or out of the system. Many factors —burner flame temperature, heat transfer surface area, temperature differential and so forth — affect the speed of the process. Flow or pressure changes, by contrast, can be realized and measured virtually instantaneously. With temperature, heat has to move through the process and, depending on the heat transfer characteristics, heat may not circulate easily through the mass of fluid.
This concept of thermal inertia comes up with reference to the process itself and how heat moves through a body, but also with temperature sensors and the way they respond to changing conditions.
A sensing element usually is encased in a stainless-steel sheath. Heat from the process must be transferred through the sheath — and whatever insulation may be packed inside the sheath — before it reaches the sensor. Putting a sensor in a thermowell adds to the delay. The thermowell material and thickness, together with the sensor, will determine the assembly’s response time. This characteristic extends the effect of thermal inertia by slowing the recognition of a temperature change. So, choosing the right sensor configuration and its accompanying thermowell has a major effect on response time.
Where the measurement is taken is also critical. Since heat may not be distributed evenly, it is not a good idea to look at one reading and conclude it applies to the entire mass.
Consider a batch process where a tank is filled with liquid that must be heated and held at a specific temperature for some period of time. If heated from the bottom and the liquid flows easily enough to allow free convection, where should the sensor be placed? If there is just one measurement point, choosing the location could be a challenge. The natural flow currents will reflect the differentials: Near the heater, the fluid will be hot while at the top, it will be cold.
This suggests having more temperature sensors is better, which is frequently the case. But the reason many processes have as few sensors as they do is often because of the cost and complexity of adding sensors with associated process penetrations, which is compounded by adding the required wiring. Fortunately, there are alternatives able to solve both problems.
One available technology infers the process temperature from the pipe-wall temperature corrected by other factors. The sensor is clamped to the pipe and insulated without a process penetration. The pipe size, material and schedule are configured into the transmitter, which then infers the process temperature. Such an instrument can be left in place permanently or moved as needed to take readings in new locations. Once the transmitter is joined to a WirelessHART network, it can be placed anywhere within the overall coverage area.
Here Shane is describing Emerson’s Rosemount X-well Technology, which provides accurate temperature measurements without the complexities of a thermowell assembly. And since it operates using WirelessHART, it avoids the need for any additional wiring. In fact, once joined to the network, an X-well transmitter can be moved as needed to check various measurement locations. This is the kind of flexibility needed for dealing with those troublesome temperature loops.
You can find more information like this and meet with other people looking at the same kinds of situations at Emerson Exchange and in the Emerson Exchange365 community. It’s a place where you can communicate and exchange information with experts and peers in all sorts of industries around the world. Look for the Temperature and Wireless Groups, and other specialty areas for suggestions and answers.