Are you among the process automation professionals who consider temperature control loops among the most difficult to control in your plant? If so, then Modeling and Control blog’s Greg McMillan‘s recent article in Quality Digest may be for you. The article, Don’t Sweat That Temperature Loop, looks at the various types of temperature sensing technologies, their characteristics with respect to sensitivity, repeatability, and drift, and best practices for installation.
In the article, Greg notes the critical nature of temperature control across a wide range of industries and applications:
Temperature is a critical condition for reaction, fermentation, combustion, drying, calcination, crystallization, extrusion, or degradation rate, and is also an inference of a column tray concentration in the process industries.
The issue most process automation engineers have with temperature is the process dynamics. Greg writes:
Curiously, the slowness of the response of the temperature process is the biggest source of problems and opportunities for tight temperature control. The slowness makes it difficult to tune the controller because the persistence and patience required to obtain a good open- or closed-loop test exceed the capability of most humans. At the same time, this slowness, in terms of a major process time constant, enables gain settings larger than those permissible in other types of loops except for level.
RTDs and thermocouples are the primary temperature sensing elements used in the process industries. Greg provides a quick comparative summary:
There are many stated advantages for thermocouples, but if you examine them more closely, you realize they are not as important for industrial processes. Thermocouples are more rugged than RTDs. However, using good thermowell or protection tube design and installation methods make an RTD sturdy enough for even high-velocity stream and nuclear applications. Thermocouples appear to be less expensive until you include the cost of extension lead wire and the cost of additional process variability from less sensor sensitivity and repeatability.
He describes the various types of RTDs (wire-wound, thin-film) and thermocouples (Type E, Type J, Type K, Types R and S, Type T). The description includes their construction, theory of operation, and installation considerations.
The part of the article you may want to jump to is the installation section at the end. Greg provides guidance on minimizing length of sensor wiring, mounting temperature transmitter as close to process as possible, minimizing conduction error, thermowell piping locations, and distance from process equipment such as heat exchangers, static mixers, and desuperheaters.
Give the article a read for a few tips if you’re one of the plant engineers currently sweating your temperature loops.
