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Temperature Measurements: Going with the Heat Flow

Anybody who has had to deal with the challenges of adding a temperature sensor to a process knows it’s easier said than done. Drilling into the pipe or vessel and adding a thermowell means shutting down the process, and maintaining the thermowell going forward.

 

Natalie StrehlkeEmerson’s Natalie Strehlke offers a simpler approach with this nuts-and-bolts article in Process Heating magazine titled Temperature Measurements: Going with the Heat Flow. She makes the point that temperature readings invariably depend on heat flowing through metal:

 

For most industrial temperature measurements, the sensing element is never directly exposed to the process. The electronic element such as a thermocouple (TC), resistance temperature detector (RTD) or thermistor is encased inside a metallic enclosure, typically a section of stainless steel tubing closed at one end. This approach works because heat is conducted through the metal sheath and eventually reaches the actual sensing element.

 

Nobody will argue with this fact, but it seems many users still believe temperature readings must be invasive, with a thermowell inserted right into the heart of the process. But as Natalie points out, there’s a better way:

 

A pipe or vessel wall transmits heat if the interior temperature is higher than the exterior ambient. Users may try to exploit this characteristic by seeing if it is practical to infer the interior temperature by measuring the external surface, clamping a sensor to the outside of the pipe.

 

A great idea, but it only goes so far:

 

Heat dissipation from the pipe’s surface keeps the external temperature from ever fully reaching the internal value. Adding insulation can help, but the two readings may never fully match, and changing external conditions will cause fluctuations in the difference.

 

So, that externally mounted sensor, by itself, will have a hard time giving a useful reading. But if it can be used in conjunction with an ambient temperature reading, the heat transfer characteristics of the pipe and some sophisticated signal processing—magic can happen:

 

Some instruments use a different methodology. The user enters factors for the pipe material and thickness, and the instrument does the rest. When clamped on a pipe, an RTD is held in contact with the pipe surface to ensure predictable and consistent heat transfer. Once insulated, the reading from the surface RTD is compared to a second RTD in the transmitter housing, which monitors ambient conditions. These two readings are continuously run through the algorithm to infer the temperature inside the pipe even when ambient and other conditions change significantly. The result is an accurate temperature reading without the need for a process penetration.

 

What Natalie is describing isn’t just a good idea, it is something you can have right now because Emerson's Rosemount X-well technology does all these things. Mount it on a pipe with no penetration, add some insulation and you’re in business. X-well will send temperature data via your plant WirelessHART network.

 

It’s clever and creative engineering, but to someone who’s had to deal with thermocouple problems, it really is magic. The ability to add a temperature measurement point without any drilling or welding is huge. Natalie adds a few typical examples of situations where X-well is particularly effective including:

 

  • Small Line Sizes: not enough room for thermowell
  • Difficult Locations: hard to access for cutting, welding, etc.
  • Uninterruptable Production: no process penetration allowed
  • Problematic Piping and Fluid Flows: difficult or impossible to install and/or maintain thermowell

 

Do those sound like the kinds of things you have to deal with? Here’s the solution:

 

The ability to take an accurate temperature reading through a pipe or vessel wall without a process penetration using a self-contained instrument solves many temperature measurement problems. Adding wireless transmission of temperature and other data via a wireless network provides additional attributes able to address many process challenges.

 

There are lots of users fighting the same problems, and you can probably find many right here in the Emerson Exchange365 community. Look for the Temperature Group to interact with your peers and our experts.

 

Tags: Natalie Strehlke, temperature, X-well, Rosemount, thermowells, pipe temperature, WirelessHART

 

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