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Improve Your Temperature Measurement

 In the April 2018 issue of Chemical Processing, Ryan Leino and his colleague Timchan Bonkat published an article discussing three approaches to temperature measurement. (Read the EE365 blog post here.) Ryan was invited back to create a follow-up story for the December 2018 issue where he takes a deeper dive into the most common temperature setup: a conventional sensor placed inside a thermowell sending its signal via a transmitter.

The new article in the December 2018 issue of Chemical Processing, Improve Your Temperature Measurements, sounds pretty basic at first glance, but there are countless variations and combinations possible in the real world when working with this arrangement of devices.

Most process temperature measurements in chemical plants rely upon three critical elements: a thermowell that provides the process containment; a sensor that takes the measurement and translates it into an electrical signal; and a transmitter that converts the sensor signal to a robust communication protocol such as 4–20-mA, HART or Foundation Fieldbus. These elements must work together to provide safe, accurate and repeatable readings.

He starts with the most deceptively simple part of the group, the thermowell. While many may think of it as little more than a hunk of metal, there’s a lot more going on.

The challenge for a thermowell — or, indeed, for any other piece of process equipment for that matter — is when it’s inserted directly into the path of a moving stream. In most situations, the thermowell is perpendicular to the flow, so vortices form on both sides and create high- and low-pressure areas capable of inducing vibration in the thermowell. Sometimes, the vibrations are tolerable but, in certain cases, they can cause serious harm that leads to fatigue failure.

There are ways to predict how well a thermowell is going to work based on a calculated stress value reflecting the operating conditions. This can be very helpful, but the calculations are complex and changes to the conditions can require reworking results. Many users simply respond with a massive thermowell, but there is a new technology that cuts to the heart of the issue.

Suppressing vibration at the source often is possible. This generally involves using a thermowell profile designed to avoid the normal wake-shedding problems. Cylindrical, stepped and tapered thermowells all basically are round, allowing vortices to form along the entire length. Emerson’s patented Twisted Square design disrupts formation of the long vortices and allows them to form on both sides, so they tend to balance and cancel each other. The result is far less vibration — up to a 90% reduction in some cases.

Ryan also explores the Rosemount X-well technology which does away with the thermowell and process penetration altogether. X-well calculates an inferred process temperature based on heat transmitted through the pipe wall, based on understanding the heat transfer characteristics of the pipe, along with compensation for ambient conditions.

The article also deals sensor selection and wraps up with one element that is often overlooked: the transmitter. It boosts the signal and makes it easier to send it to the control system, but it adds a very important extra feature: intelligence.

While all these capabilities are helpful, the most important probably is the transmitter’s intelligence, with the sensor/transmitter becoming a smart instrument able to send diagnostic information to the process automation system. Temperature sensors exhibit telltale signs when they are suffering from mechanical deterioration or wiring and termination problems. The transmitter can spot these and call attention to incipient problems before they escalate to a failure.

Yes, a simple RTD or thermocouple can become a smart device when working with a transmitter like the Rosemount 644, which can make a big difference.

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.

Posted by Deanna Johnson, Director Integrated Marketing Communications for Machine Automation Solutions