Accurate Piping Surface Temperature Measurement

by | Oct 3, 2016 | Measurement Instrumentation, Temperature | 0 comments

We see the impact of the Internet of Things in our homes with devices like thermostats that we can control from our mobile devices. And we see it in industrial processes with the ever expanding variety of sensing devices to improve the safety, efficiency and reliability of these processes.

One important area of innovation is being able to measure temperature without having to penetrate the piping or vessels with thermowells. Temperature can be measured at the outside surface and transmitted wirelessly to control and asset management systems to improve performance.

Emerson’s Natalie Strehlke

Emerson’s Natalie Strehlke shared a new whitepaper with me, Achieve Accurate Process Temperature Measurement with Surface Sensor Innovation. The whitepaper opens noting some challenges with traditional temperature measurement:

A thermowell and temperature sensor assembly is the most frequently used method of measuring a temperature internal to a process. Thermowells allow for direct sensor immersion into a process which helps provide an accurate measurement, but introduces many complex design challenges and risks associated with creating a possible leak point.

rosemount-xwellThe innovation is in the Rosemount 0085 Pipe Clamp Temperature Sensor, which direct mounts to the Rosemount 648 Wireless temperature transmitter with X-well technology to externally measure the fluid inside the pipe. It works by using:

…a heat flux algorithm that applies known thermal conductivity properties of both the process pipe or vessel and the surface temperature measurement assembly to calculate an accurate and repeatable internal process temperature value.

There are many complexities in the design and use of thermowells:

The process fluid type, density and state as well as properties such as pressure, temperature, flow-rate and viscosity all play roles in the design of a thermowell to ensure a proper selection and safe installation. Material compatibility is also a concern for corrosive or abrasive process fluids. Wake frequency calculations (based on ASME PTC 19.3TW) are performed to ensure proper design for thermowells, however, this calculation is based on a single set of process data points. If the process parameters change from those used for wake frequency calculations, the thermowell may no longer be appropriately designed for the application.

Surface measurements eliminates these challenges. But they have challenges as well:

Ambient factors can severely impact the surface measurement reading, producing unpredictable results and complicating any attempt to relate surface temperature to process temperature. The relationship between surface temperature and process temperature is heavily dependent on the difference between ambient temperature and internal process temperature. Even a uniformly applied correction to the surface temperature measurement as an attempt to equate it to an expected temperature drop through a pipe or vessel wall loses validity if either process or ambient temperature changes.

The Rosemount X-well technology avoids these challenges by:

…implementing an algorithm with an understanding of the thermal conductive properties of the temperature measurement assembly and corresponding piping or vessel, a surface temperature sensor solution can be utilized to accurately calculate internal process temperature.

Assuming steady state conduction and negligible impact of ambient heat convection which is eliminated by proper use of insulation, the process piping and temperature measurement assembly can be viewed as a series of planes with different thermal conductivities…

Download the whitepaper for detailed graphs on the performance of the surface measurements versus inline measurements, overall performance considerations, suitable applications, and industries where this measurement technology may be best applied.

If you’ll be attending the October 24-28 Emerson Exchange conference here in Austin, make sure to catch the Rosemount Temperature X-well Technology Overview workshop and other temperature measurement-related sessions. You can also connect and interact with other temperature measurement experts in the Temperature group of the Emerson Exchange 365 community.

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The opinions expressed here are the personal opinions of the authors. Content published here is not read or approved by Emerson before it is posted and does not necessarily represent the views and opinions of Emerson.

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