Chemical and other industrial plants have all sorts of applications where the temperature is a critical factor. In chemical plants, temperature measurements are used to control and monitor reaction rates, product viscosity, distillation fractions and many other processes. How temperature gets measured is also critical since the method can affect the value of the data in a variety of ways.
Ryan Leino and Tim Bonkat look at three main techniques in their article in the April 2018 issue of Chemical Processing: Instrumentation: Measure Temperature Right. They compare and contrast common techniques often used in chemical plants.
A direct measurement requires immersing a temperature sensor in the medium, allowing heat to flow through a thermowell or other component to the sensing element. Typically, these temperature sensors are protected by a stainless steel sheath and mounted permanently in a thermowell to get a specific measurement from the process. An alternative approach involves contact but not directly with the fluid itself. Instead, it measures the surface temperature of a pipe and infers how hot the fluid within is. Most non-contact measurement methods determine temperature by analyzing the infrared (IR) radiation thrown off by a surface using a combination of optics and detectors to zero-in on the area of interest.
That’s a lot of ground to cover. Fortunately, Ryan and Tim unpack those concepts and dig into each more deeply. They start with an approach most of us see every day: direct measurement of fluids using a sensor in a thermowell. Everybody knows how the concept works, but the mechanical side effects can be more daunting.
This method probably is the most common at chemical plants. It uses a sensor to measure the temperature of a liquid or gas — getting the sensor to the desired point in the process is essential but not necessarily easy. The method is intrusive; it requires inserting the sensor into the product stream, typically using a thermowell. When designing a thermowell installation, mechanical issues become the most problematic elements for a variety of reasons.
Mechanical issues in this context include the ability to withstand the pressures of the process, the corrosiveness of the fluids, and so on. While these issues are usually pretty easy to grasp, end users often struggle with ways to mitigate thermowell failures due to vortex-induced vibration caused by flowing fluids.
The thermowell faces stresses caused by fluid movement. Its protrusion into a flowing stream creates alternating pressure cells called vortices. The thermowell experiences vortex-induced vibration. If the frequency of the vortices matches the natural frequency of the thermowell, the resulting side-to-side movement eventually can lead to thermowell fatigue failure. Understanding the extent of the problems and tackling these challenges requires careful thought when designing an installation. Using known process parameters, product designs, best practices and thermowell calculations can ensure a specific thermowell is strong enough to endure the conditions it will face without failure.
Typically, these calculations end up recommending a massively thick thermowell because the vibrations can be extremely powerful and destructive when the frequencies are within a certain range. But there is another answer:
Another method is to suppress the formation of damaging vibrations within the process by modifying the thermowell shape. Replacing the traditional round profile with a hard-cornered square, twisted like rifling, disrupts long vortex formation. Shorter vortices tend to balance each other reducing vibration.
These special thermowells are now available as Rosemount Twisted Square Thermowells, and they are part of the standard product line. This design eliminates more than 90% of dynamic stress, reducing the need for thick wall thermowells. Of course, Rosemount X-well Technology can solve many application problems by getting rid of the thermowell entirely. For many difficult situations, it may be the solution. The article talks about where and how to apply these as well. Give it a read because there’s lots of useful information.
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.