What is a Thermowell?
Process control environments can be pretty inhospitable at times; fluids and gases can be corrosive and variables like temperature and pressure can reach extreme valuations. Even a moderately harsh environment can be detrimental to equipment if it is exposed over a long period of time. In these tough working conditions temperature sensors like thermocouples and RTDs face constant wear and need continual replacement to ensure accurate readings. Enter the thermowell, protective armour for a temperature sensor. But what is a thermowell and what does it do? There are many benefits to consider when deciding if a thermowell is the right option for a certain process. We’ll go over them within this blog. But first things first, what is a thermowell?
Figure 1: A basic thermowell
What is a Thermowell?
A protective sleeve, sensor armour, a “pocket in a pipe”; there are many ways to describe a thermowell. In practical terms, a thermowell is simply a metal tube in which you insert a temperature sensor. Then, the whole assembly is placed into the spot where it will measure a process instead of placing the temperature sensor there alone. Just like when you put on a heavy jacket to withstand winter’s cold; or wear rubber boots to keep your feet from getting wet in puddles; a thermowell protects a temperature sensor from a harsh environment.
Figure 2: A thermocouple threads into a thermowell that then threads into a process
As mentioned in our blogs on thermocouples, RTDs, and thermistors, not all temperature sensors are alike. There are different types and designs you can use in a myriad of ways. Thermowells too are infinitely customizable; you can specifically design them to best fit the processes in which you will use them. You can alter, options like dimensions, material type, and style to match with temperature and size requirements. There are many characteristics that we address in a whole standalone blog “What types of Thermowells are there?” Be sure to check out that discussion for more on this incredibly versatile tool.
Figure 3: A diverse range of thermowells
Why use a Thermowell?
Thermowells, as with other optional additions to a process, have a monetary cost. There are many benefits to using one, but is this protective device worth the price? Four reasons why to consider a thermowell:
Although each thermocouple and RTD are built differently, these sensors are often housed in ¼” diameter 304 stainless steel probes. This sheathing alone will give much protection to the sensor and, if your process is relatively inert, nothing more is needed. However, some processes out there are downright inhospitable. Temperatures above 1000ºC will cause stress on the structure of stainless steel and warp a sensor probe. A process medium itself can be strenuous on a probe too. These eroding solutions can be anything from physical particles in a gas to a corrosive liquid like saltwater or acid. High pressures in a process are a force multiplier; a factor that makes the effects from temperature and caustic environments that much more damaging. Thermowells will take the brunt of these types of degrading effects and make the temperature sensor last much longer than it would without.
Depending on the type of temperature sensor an RTD, thermocouple, or thermistor might be relatively cheap or it could be expensive; it varies between processes. If the sensors are expensive, or the process they are used in is harsh, then the protection offered by a thermowell would be more useful. Yes, they will have an item cost themselves, but at some point, that additional line will be worth the expense of continuously purchasing replacement temperature sensors. Just like when facing any other manufacturing decision, a cost-benefit analysis can help identify the range when a thermowell goes from being a luxury to an absolute necessity.
Even if the cost of your temperature sensor is negligible, a thermowell might still be a viable addition. Simply, the act of changing out temperature sensors all the time will have a cost of labour. Be sure to note that the sensors will need replacement eventually. When thermocouples and RTDs are threaded into a thermowell, then sensor replacement is simple. To replace the old sensor: unscrew it, take it out, and screw the new one into place. Also, since the thermowell seals the process, when you swap out a temperature sensor, no outside contamination can enter the process and a worker doesn’t require excessive protection measures or equipment.
4. No Shut-downs
An easily overlooked benefit to thermowells is the operational advantage of the process being completely separated from the temperature sensor. Eventually, you will have to remove the temperature measurement instrument for replacement. When this happens, the thermowell will keep the process contained. Therefore removing the sensor has no bearing on the operation apart from a process temperature not being measured during the swap. This means that a single temperature sensor failure does not result in the shutting down of an entire system. Whereas without a thermowell there is the possibility of a complete or isolated disturbance to the process while a temperature sensor is being replaced. Such a break can be costly or intrusive and should be a considering factor when deciding when to make use of a thermowell.
Figure 4: The view inside a thermowell
Few value-added options for process management are as simple and practical as a thermowell. However, this protective device has an additional up-front cost. But the benefits go far beyond simply saving on the purchase of replacement temperature sensors, thermowells can relieve the many other headaches caused by having to swap out a thermistor, RTD, or thermocouple. Be sure to visit “What types of Thermowells are there?” if you are curious to see the different options available when choosing a thermowell. You can also check out the thermowells that we offer here at Enercorp. We stock some of the standard types, and remember, everything is customizable, even thermowells.