You’ve seen these signs around plants: “If it ain’t broke, don’t fix it.” Sometimes that may be useful advice, but it’s also dangerous because it suggests we shouldn’t try alternate approaches to ongoing activities.
Case in point: implementing a typical process temperature measurement. There’s the “it ain’t broke” approach, and there is the better approach which uses current technologies capable of improving performance and reducing lifecycle costs. That discussion is the topic of my article in the November 2019 issue of Flow Control, Moving Beyond Basic Temperature Measurement Instrumentation to Improve Performance and Reduce Costs. Let’s look at how this can work by considering a typical example, inserting a sensor in a thermowell into a pipe.
In a traditional approach, a hypothetical engineer would select a stock thermowell with a compatible mating connection and made from a suitable material. For selection of a sensor, there are several possibilities suitable for the temperature range. In this example, the engineer would select a type K thermocouple (TC), since it is capable of handling anything the process is likely to experience. It can be connected to a home-run cable via the terminal block at the thermowell head. A reel of type K extension wire is sufficient to cover the 200 feet from the thermowell to the I/O rack of the host system via one marshalling cabinet.
OK, that’s the basic approach, pick a nice hefty thermowell and connect the TC via its appropriate extension cable. There’s nothing wrong with using a TC, but the larger implementation is not ideal. The thermowell is probably too thick, which slows reading changes, and all that cable is a great way to pick up EMI to corrupt the reading. So, what is the better approach? Rethink component selection from one end to the other, beginning with the thermowell.
Thermowell selection requires careful analysis to avoid maintenance problems related to vortex-induced vibration (VIV) caused by wake shedding effects. Design analysis using software based on formulas outlined in the ASME PTC 19.3 TW-2016 standard can determine the viability for a given thermowell profile based on its physical strength and vibration characteristics at specific operating conditions. With effective software, it is a simple matter to work through what-if scenarios to ensure the thermowell selected will avoid VIV problems at a range of operating conditions including startup, product grade change, likely upsets and others, in addition to “normal” operation.
So don’t just pick a thermowell because it fits and may be available, give it some additional thought and choose the best profile possible to ensure long service life and highly responsive readings. Emerson can help by providing Rosemount Thermowell selection guidance and software. Now, what about the other part, sending data to the host system?
Adding a temperature transmitter at the thermowell terminal enclosure converts the weak voltage of a TC or small resistance of an RTD to a more robust 4-20 mA current signal. A current signal can be carried by standard instrument cabling and added to the automation host system via a standard 4-20 mA input card. If a different type of sensor is required, it is a simple matter to reconfigure the transmitter. Sophisticated HART and fieldbus transmitters support remote configuration and diagnostic functions, which can call attention to developing problems and assist in troubleshooting.
Those diagnostic functions, available with Emerson’s Rosemount 644 Temperature Transmitter, include diagnostic capabilities, such as Hot Backup, sensor drift alert and thermocouple degradation.
You can find more information like this and meet with other people looking at the same kinds of situations 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 Group and other specialty areas for suggestions and answers.