Stainless Steel Thermowells: Functions and Types Explained

A quick overview of thermowell characteristics and their impact on implementation

Thermowells are an excellent option for improving the service life of sensors used to monitor piping processes. However, like most piping components, choosing the proper design and fit for your intended application is essential to ensuring proper operation and maximizing the benefits on offer.

In this guide, we’ll cover everything you need to know to purchase the perfect thermowell for your needs.

What Are Thermowells?

Used to guard temperature sensors, thermowells are cylindrical fittings designed to integrate directly into the piping system. Typically categorized by the design of their stem, thermowells offer excellent protection against excessive pressure, material velocity, and corrosion for thermocouples, bimetal thermometers, thermistors, and other common sensors.

Thermowells also allow for easier sensor replacement. Since thermowells are closed to the surrounding flows, thermowells allow you to avoid the need to reroute or stop processing or drain system sections prior to replacement.

Thermowell Design Considerations

Most thermowell designs are available in three different body styles -- also known as shank types:

  • Straight thermowells: Maintain a consistent diameter throughout the entire insertion length. Due to the extra diameter and metal, straight thermowells offer high mechanical strength at the cost of response time. While corrosion and abrasion resistance is high, straight shanks are not intended for use in high-velocity flow systems.

  •  Stepped thermowells: Include a diameter step down between the stem and the tip. This allows for faster sensor response however it weakens the mechanical strength of the thermowell making it less than ideal in high-velocity flow systems.

  • Tapered thermowells: The diameter of the thermowell decreases smoothly from the stem to the tip. Combining benefits of both straight and stepped thermowells, tapered shanks provide an excellent balance of sensor response time and mechanical strength. The tapered design also provides good vibration resistance making them ideal for high flow rate and high-pressure use.

You also need to consider immersion length. In most cases, larger insertion lengths are ideal as it further extends the temperature-sensitive zone of the sensor into the medium you wish to measure. However, you’ll also need to consider vibration and wear on the thermowell as well. Finding an optimal balance of length and sensitivity is essential. 

With the shank choice and immersion length out of the way, we can look at the thermowell stem designs available.

Threaded Thermowells

Typically used in conjunction with Thredolets®, threaded thermowells provide easy installations and removal when necessary. However, they are prone to leakage. This makes them less effective for processes using corrosive, explosive, or toxic chemicals.

Welded Thermowells

Typically used in conjunction with Sockolets®, welded thermowells are typically used in permanent installations requiring the ability to handle high-velocity flows, high temperatures, or extremely high pressures. While they provide a leak-proof seal, removal often requires cutting the thermowell out of the piping system.

Flanged Thermowells

Flanged thermowells provide a stronger seal than threaded designs without the removal concerns of welded thermowells. Using a mating flange welded to the pipe, these thermowells are bolted in place, providing easier installation, high pressure ratings, and simple replacement. Due to these characteristics, these thermowells are ideal for processes involving corrosive environments, high pressure, high temperature, or high velocity.

Vanstone or Lap Joint Thermowells

Offering the ability to use different materials for the thermowell contacting the process and the overlaying flange, lap joint thermowells mount between a mating flange and lap joint flange. By eliminating welds in the installation process, the design is ideal for corrosive applications. Greater material flexibility also makes lap joint thermowells an effective solution in cost-sensitive scenarios.

Thermowell Materials

Like most piping components, thermowells are available in a variety of materials. This allows you to better tailor the thermowell to its intended use environment to ensure long-lasting performance.

Popular materials include:

  • Stainless steel
  • Carbon steel
  • Molybdenum/Chromium steels
  • Titanium
  • Specialized alloys

Due to its high corrosion resistance, stainless steel is an excellent choice in many situations.

With the information in this guide, you have everything you need to know to match thermowells to your intended usage. However, if you need additional help, the experts at Unified Alloys have helped industries across North America find the stainless solutions to their piping needs for more than four decades. Contact us today to find out how our selection of olets and thermowells can work for you.