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Your temperature sensors are an important part of your process. You need to keep accurate measurements throughout their lifespan. A vital part of this is a thermometer well, and you need to select just the right one for maximum reliability and efficiency.
Thermometer wells also save money in the long term by extending the lifespan of your sensors.
The Basics of a Thermometer Well
A thermometer well, also known as a thermowell, is the protective barrier placed around thermocouples, thermosensors, or RTDs. It guards against damage from excessive pressure and material velocity, as well as corrosion. In certain applications, they also allow you to replace sensors without draining the system. The thermowell remains connected to the system even if the sensor itself has been removed. This means operations don’t need to be interrupted while sensors are switched out.
This helps ensure accurate readings and extends the life of the sensor. Thermometer wells are mainly used in chemical applications, but are useful in any situation where a temperature sensor might be exposed to harsh environments, including liquids and gases. They make processes more efficient and reduce costs.
Key Features
The key to thermometer wells is durability and resilience. This means that materials such as stainless steel and Iconel are commonly used. Stainless steel is the most common material as it is cost effective and resistant to heat and corrosion. In some cases, chromium/molybdenum steel might be used, typically for extree pressure situation. Other materials might be used for specific chemical environments, such as ones containing chlorine. Thermometer wells are typically designed to withstand harsh environments including high pressure, extreme temperatures, and exposure to corrosive materials.
Thermowells for high-pressure environments are typically machined from bar stock for maximum integrity. Some smaller thermowells are constructed from tubing, with one end welded closed, but these are typically reserved for lower pressure environments. For situations with flow-induced vibrations, thermometer wells are tapered, as straight wells tend not to last as long.
How Does a Thermometer Well Work?
A thermometer well is a pretty simple device. It is a closed tube that is inserted into the medium being measured. Temperature is transferred to the sensor without the sensor being exposed directly.
This isolates the sensor from the medium, making temperature readings more accurate and, most of all, reducing damage from heat, corrosion, pressure, fluid flow, and physical interaction.
The thermometer well is attached to the outer wall of the vessel, and the sensor can be accessed from outside. This means that sensors can be inspected or changed without any need to access the interior. Operations can typically continue and there is no downtime caused by draining vessels or pipes to access sensors.
While not complicated, the thermowell has to be the right shape and made of the right materials for your specific operation. You may need expert help choosing the right thermowell.
Why Isolation Matters
Thermocouples and other sensors are extremely sensitive. Flow vibration, pressure changes, etc, can make their readings go off very quickly. This can affect your entire process and potentially impact safety. For example, if a fluid goes over temperature, this can cause an explosion or a fire. Thermometer wells provide protection from this. Only the well comes into contact with the potentially corrosive or abrasive material, and only the well is exposed to vibration and pressure changes.
Premature failure of sensors is dramatically reduced, lowering costs and maintenance times. When a sensor does need to be checked, repaired, or replaced, it can be done without disrupting the system.
Meanwhile, this isolation has no negative impact on the accuracy of sensors. Temperature data can “pass through” the well even if nothing else can. Thermowells are made of metals that conduct temperature well.
Common Applications of Thermometer Wells in Industry
Thermowells are essential across many industries, but are most often used for chemical processes and when chemicals have to be transferred over long distances, such as through pipelines.
Industries and Uses
Oil and Gas
Thermometer wells are used in oil and gas pipelines to monitor temperature and reduce the risk of a blowout along the pipeline’s route. Temperatures are also measured in refineries, and temperature measurement of wells is equally important.
Monitoring temperature reduces the risk of accidents significantly, protecting workers and the environment. Pipeline accidents can have a major impact on the local environment, residents, and wildlife.
Power Plants
Boiler and turbine systems work at very high temperatures, but this still has to be monitored. Typically, this is done with resistance temperature detectors (RTDs) or thermocouples. Accurate measurements ensure operational excellence, improve safety, and lower maintenance costs. Thermowells are particularly valuable at these high temperatures or in nuclear plants.
Power plants also need to avoid downtime as much as possible to ensure that users don’t experience blackouts or brownouts.
Chemical Processing
Many chemical processing operations involve working with and containing substances that are corrosive, abrasive, flammable, toxic, etc. Sensors exposed to corrosive materials will fail quickly, resulting in expense and downtime.
Thermowells are standard in the vast majority of chemical processing operations, reducing downtime, extending sensor life, and reducing the risk of worker exposure to chemicals.
Food and Beverage
Maintaining accurate temperature ranges is vital when transporting and handling food-grade substances such as milk and alcoholic beverages, whether for safety or to ensure that the product remains palatable.
In this case, thermometer wells not only protect sensors from flow, pressure, and mechanical damage, but ensure that sensors do not come into contact with food, reducing the risk of contamination. The thermowell itself can be kept clean and made of food-safe materials to protect consumers.
Advantages of Using a Thermometer Well
In quick terms, here are the advantages of using a thermometer well over directly exposing sensors to the medium.
- Reduces wear and tear, prolonging the life of sensors and reducing maintenance and replacement costs.
- Enhances safety by containing leaks in case of sensor failure. This protects workers, nearby residents, and the environment.
- Makes readings more accurate.
- Reducing downtime for maintenance by allowing sensor removal without shutting down and draining the system.
Key Factors To Consider When Choosing a Thermometer Well
Thermometer wells have to be carefully chosen to fit your specific application. This includes choosing the right material and the right shape. Tapered wells are vital for applications with a lot of flow vibration, such as oil pipelines.
In food and beverage applications, materials should be chosen with hygiene in mind. Materials and well shapes also need to be compatible with the sensors you are using; check with the manufacturer for any specific requirements.
Material Selection
Materials need to be chosen to ensure longevity and performance in industrial settings. Consider the following key factors:
- Chemical resistance. Make sure to choose a material that can tolerate the chemicals you are using. For example, in sulphidising environments, you should choose Haynes alloy, which contains cobalt, nickel, chromium, and tungsten.
- Temperature tolerance. Pick a material that aligns with your temperature conditions. Extremely high or extremely low temperatures may require different materials. For example, Inconel is good for high-temperature applications and can tolerate higher temperatures than aluminum or steel.
- Pressure capability. Measure your system’s operational pressure and ensure that the material chosen can restore it.
- Erosion resistance. If you are dealing with high flow rates or abrasive particles, you need materials with superior mechanical strength to resist the slow wear over time.
You need to balance these needs with costs. There is no need to “overbuy” and use expensive materials your use case doesn’t need, but at the same time the consequences of buying the wrong thermowell can be expensive and potentially dangerous to your employees and even your neighbors and the environment if leakage is caused.
Common Thermowell Materials
Thermowell materials need to conduct heat well and be durable in the specific situation you need. Some of the most common materials are:
- Stainless steel. This is the most common material used as it is versatile and cost-effective. It can handle mild corrosive conditions and a decent temperature range, and is used when there is no need to go to added expense of a more resistant substance.
- Inconel. This is a proprietary alloy that is excellent for high-temperature operations. It can also handle more aggressive chemicals without issues.
- Titanium. Used for systems with high chloride content, as it is exceptionally resistant to corrosion.
There are many other materials used which might be right for your specific application.