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.
The nature of the process determines the degree of complexity for each of the three. So, let’s look at them individually.
This device (Figure 1) generally seems the least complex technically. However, more often than not, it actually poses the most problems. These stem from mechanical issues. Therefore, let’s examine how a thermowell works.
Most applications require a thermowell because inserting the temperature sensor directly into the process usually isn’t practical. The sensor normally is encased in a stainless steel sheath, which ideally is relatively thin to ensure fast response. In addition, maintenance technicians may need to remove the sensor from its mount; if it’s a continuous process, they want the ability to do this without losing product containment.
These situations call for permanently mounting the thermowell through the pipe or vessel wall; sometimes it’s threaded into place but usually it’s welded. The thermowell thus becomes part of the product containment but still allows removal of the sensor without interrupting production.
For processes with frequent shutdowns such as batch operations, the uninterrupted containment aspect isn’t as critical. In these situations, the thermowell mount may be attached to the vessel permanently or, alternatively, the thermowell, sensor and transmitter may be a single removable assembly.
There are multiple ways to mount thermowells (Figure 2) depending on the degree of permanence desired and the severity of the process. High temperatures, elevated pressures and product aggressiveness often demand exotic materials and more robust construction, just as with any process equipment.
Many applications don’t subject a thermowell to taxing conditions. For instance, a thermowell extended into a reactor where the chemistry is benign and fluid movement is minimal usually can operate without problems for as long as any other part of the equipment.
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.
Understanding the extent of the problem and properly addressing challenges requires following three critical steps when designing a thermowell installation:
1. Prevent problems by utilizing known process parameters, product designs, best practices and calculations. These working together ensure the thermowell is strong enough to stand up to the conditions it will face without failure.
2. Suppress the formation of vibrations within the process by modifying the equipment or the process itself.
3. Eliminate the need for a thermowell altogether if possible, reducing the risk of process leaks and thermowell failure to zero.
Let’s look at how you can use these three steps when designing an application.
Begin by choosing the right shape, construction material and connection method for the particular process. Specify the correct thickness, length and profile (Figure 3) to withstand the stresses and achieve a tolerable response time. You can get thermowells in all shapes and sizes, and with the wall thickness necessary to withstand a variety of stresses.
Most thermowells are straight cylindrical shapes with parallel sides. This design is strong but has the most material at the tip, slowing response time. A stepped design is thickest at the base for strength and thinner at the tip for better response. It sacrifices some strength overall but reduces drag force in the pipe. A tapered profile provides a compromise — retaining strength and improving response but increasing drag. Making the best choice can be a challenge.
Fortunately, the American Society of Mechanical Engineers has compiled formulas — outlined in its thermowell standard (PTC 19.3 TW-2016) — that you can use to calculate the extent of the stresses anticipated, thus eliminating much of the guesswork. This complex series of calculations requires knowing many details about the specific process conditions but can provide a good prediction of success. The standard covers four test criteria: