Understanding Thermowells: Selection, Design and Applications

Introduction

Temperature measurement is an important part of many industrial, commercial and mechanical systems. Depending on the application, temperature sensors and thermometers may be exposed to pressure, fluid flow, corrosive media, vibration, or other conditions that can damage the measuring instrument or make replacement difficult.

A thermowell provides a protective barrier between the temperature-measuring instrument and the process.

Thermowells are used with a variety of temperature-measuring devices, including bimetal thermometers, liquid-in-glass thermometers, remote-reading thermometers, thermocouples, resistance temperature detectors (RTDs), and other temperature sensors.

Properly selected and installed, a thermowell can protect the instrument, allow the temperature device to be removed without opening the process, and provide a practical means of maintaining a temperature measurement point.

However, a thermowell is more than simply a protective tube. Its material, dimensions, configuration, process connection and installation must be appropriate for the operating conditions.

What Is a Thermowell?

A thermowell is a closed-end protective component installed into a pipe, vessel, tank or other process equipment. The temperature sensor or thermometer is inserted into the internal bore of the thermowell rather than being directly exposed to the process fluid.

The thermowell therefore performs two important functions:

• Protects the temperature instrument from the process environment.

• Separates the instrument from the process, allowing the instrument to be removed or replaced while the process remains contained.

  
  

This second function can be particularly important in pressurized or continuously operating systems. When a suitable thermowell is permanently installed, the temperature instrument can be removed for inspection, calibration, repair or replacement while the thermowell remains installed in the piping.

The thermowell itself becomes part of the pressure boundary or process containment system, so its design and installation should be treated accordingly.

Why Use a Thermowell?

There are several reasons a thermowell may be appropriate for a temperature measurement application:

Protection from the Process

A thermowell can protect a temperature instrument from:

• High process pressure

• Corrosive fluids

• Contaminated or abrasive media

• Fluid movement and turbulence

• Mechanical damage

• Certain environmental conditions

  
  

The thermowell material can also be selected independently of the temperature instrument to provide greater compatibility with the process.

Easier Instrument Replacement

One of the most practical advantages of a thermowell is the ability to remove the temperature instrument without necessarily disturbing the process connection.

For example, a thermometer installed in a pressurized water system can be removed from the thermowell while the thermowell remains installed in the piping.

Greater Flexibility

A permanently installed thermowell can accommodate different temperature instruments over the life of an installation, provided that the replacement instrument is compatible with the thermowell's bore, insertion depth and connection.

This can be useful when an application changes from one type of temperature instrument to another.

Thermowell Designs

Thermowells are available in several configurations. The appropriate design depends on the installation, process conditions and mechanical requirements.

Straight Thermowells

A straight thermowell has a relatively uniform outside diameter along its insertion length. Straight designs are commonly used where process conditions and mechanical requirements permit their use. Their relatively simple geometry can make them economical and practical for many applications. However, the appropriate design should not be determined solely by cost or convenience. In applications involving significant fluid velocity, the thermowell must be evaluated for the mechanical forces generated by the flowing process fluid.

Tapered Thermowells

A tapered thermowell decreases in diameter toward the tip. The larger diameter near the mounting point provides greater stiffness, while the reduced diameter toward the tip can reduce the thermal mass surrounding the sensor.

This combination can provide useful mechanical and thermal characteristics, particularly in applications involving fluid flow.

Stepped Thermowells

A stepped thermowell incorporates one or more changes in diameter along its length.

Like tapered designs, stepped configurations can be used to balance mechanical strength with thermal response requirements.

The appropriate geometry depends on the particular installation rather than there being one universally superior thermowell design.

Process Connection Types

Thermowells can be supplied with different methods of attachment to the process:

Threaded Thermowells

Threaded thermowells are commonly used in smaller piping systems and applications where a threaded connection is appropriate. The process connection must match the requirements of the piping or equipment, including thread type, size and pressure requirements.

Socket-Weld Thermowells

Socket-weld thermowells are installed into a suitable socket-weld connection.

They can provide a robust permanent connection and are commonly considered for applications involving higher pressure or temperature.

Weld-In Thermowells

Weld-in thermowells are installed directly into the process piping or vessel. This configuration may be appropriate where a threaded or flanged connection is unsuitable or where the process design requires a permanent welded installation.

Flanged Thermowells

Flanged thermowells use a flange connection to the process. They are often selected for larger process connections or applications where the thermowell needs to be removed without disturbing a threaded connection.

Sanitary Thermowells

Sanitary thermowells are designed for applications where hygienic considerations are important, including certain food, beverage and pharmaceutical processes. Their design, surface finish and connection must be appropriate for the applicable sanitary requirements.

Thermowell Materials

Material selection is an important part of thermowell specification. The thermowell material must be compatible with the process environment and capable of withstanding the mechanical and thermal conditions to which it will be exposed.

Factors to consider include:

• Process fluid or media

• Operating temperature

• Operating pressure

• Fluid velocity

• Corrosive characteristics

• Potential erosion

• Mechanical strength

• Installation method

• Welding requirements, where applicable

  
  

Common thermowell materials include brass, low-lead brass, carbon steel and stainless steels such as 304 and 316 stainless steel. More specialized alloys may be appropriate for particularly corrosive or demanding environments.

There is no single thermowell material that is appropriate for every application. Material selection should therefore consider the actual process media and operating conditions rather than simply selecting the most commonly used material.

Thermowell Length and Insertion Depth

The thermowell must extend sufficiently into the process to provide meaningful temperature measurement. However, longer is not necessarily better.Insertion length affects both the temperature measurement and the mechanical characteristics of the thermowell. Increasing the unsupported length can reduce mechanical stiffness and natural frequency, while the location of the temperature-sensitive portion of the instrument also affects measurement performance.

The required insertion length depends on factors such as:

• Pipe or vessel dimensions

• Location of the measurement point

• Flow characteristics

• Sensor dimensions

• Required measurement accuracy

• Thermowell construction

• Mechanical requirements

  
  

The thermowell should therefore be selected as part of the complete measurement system rather than simply specified according to the length of the thermometer stem.

Thermowell Response Time

A thermowell provides important protection, but that protection introduces additional thermal resistance between the process and the temperature sensor. As a result, a temperature measurement using a thermowell may respond more slowly to a rapidly changing process than a sensor directly exposed to the process.

The response time depends on numerous factors, including:

• Thermowell material

• Wall thickness

• Bore diameter

• Stem geometry

• Sensor construction

• Sensor fit within the thermowell

• Process fluid

• Fluid velocity

• Temperature difference between the process and sensor

  
  

A smaller practical bore and appropriate thermowell geometry can help optimize thermal response, but mechanical requirements must always be considered. In other words, the fastest possible thermowell is not necessarily the correct thermowell.

The thermowell must provide an appropriate balance between temperature response, mechanical strength and process containment.

Improving Thermal Transfer

The fit between the temperature sensor and the internal bore of the thermowell can also influence thermal response. An excessive air gap between the sensor and thermowell can increase thermal resistance and slow the transfer of heat from the thermowell to the sensing element.

Where appropriate for the temperature range, sensor construction and application, a suitable thermally conductive material may be used within the thermowell to improve thermal coupling between the thermowell and the sensor. Depending on the application, these materials may include thermal-transfer gels, pastes or other conductive compounds designed for temperature measurement.

Any such material should be selected with consideration for:

• Operating temperature

• Chemical compatibility

• Sensor materials

• Thermowell material

• Required response time

• Ease of sensor removal

• Cleanliness or sanitary requirements

  
  

The use of a thermal-transfer compound should not be viewed as a substitute for proper thermowell design. It is one possible method of improving the thermal interface between the sensor and the thermowell.

Fluid Flow and Thermowell Mechanical Design

One of the most important considerations in thermowell selection is the effect of flowing fluid. When fluid flows around a thermowell, alternating vortices can form downstream of the thermowell. This phenomenon, commonly associated with a Kármán vortex street, produces alternating forces on the thermowell. The frequency associated with these alternating forces is commonly referred to as the wake frequency or vortex-shedding frequency.

Every thermowell also has a natural frequency at which it tends to vibrate.

If the flow-induced excitation approaches a critical relationship with the natural frequency of the thermowell, significant vibration can occur. Under unfavorable conditions, repeated vibration can produce fatigue stresses and eventually lead to thermowell failure.

For this reason, thermowells used in applications involving significant fluid velocity should be evaluated for mechanical suitability rather than selected solely on the basis of temperature, pressure and material compatibility.

Wake Frequency and Thermowell Calculations

A wake-frequency calculation evaluates the relationship between the fluid-induced excitation of the thermowell and its mechanical characteristics.

The calculation can take into account factors such as:

• Fluid velocity or flow conditions

• Fluid density

• Operating temperature

• Operating pressure

• Thermowell material

• Thermowell geometry

• Unsupported length

• Process connection and mounting conditions

  
  

ASME PTC 19.3 TW provides an engineering framework for evaluating thermowell suitability under applicable conditions. Its evaluation includes criteria addressing frequency, dynamic stress, static stress and hydrostatic pressure. This evaluation illustrates an important design tradeoff. For example, reducing unsupported length and changing the geometry of the thermowell can affect its natural frequency and mechanical strength, while changes intended to improve thermal response can have different mechanical consequences. Therefore, a thermowell cannot necessarily be optimized for thermal response without considering its mechanical requirements.

Because wake-frequency and mechanical calculations are application-specific engineering evaluations, they should be performed by a qualified professional using the actual thermowell design, installation and process conditions. For demanding applications, the thermowell manufacturer or qualified engineering supplier will typically perform or support this evaluation as part of the thermowell selection process.

The result applies to the specific thermowell design and operating conditions evaluated; changing the thermowell geometry or process conditions can change the result.

Heat Numbers and Material Traceability

For applications where material traceability is important, the thermowell should be supplied with documentation identifying the material from which it was manufactured.

A heat number identifies a particular heat or melt of metal produced during the material manufacturing process. It provides a means of linking the physical material to its associated material certification.

For a thermowell manufactured from solid bar stock, the heat number can provide traceability between the finished thermowell and the material documentation supporting its specified material grade. Depending on project requirements, the thermowell may be permanently marked with the heat number, while the corresponding Material Test Report (MTR) or material certificate provides information about the material's chemical composition and applicable mechanical properties.

For applications requiring material traceability, documentation may include:

• Specified material grade

• Heat number

• Material Test Report or mill certificate

• Traceability between the material and finished thermowell

• Permanent marking of the heat number where required

• Additional inspection or certification requirements specified by the project

  
  

Heat-number traceability is particularly valuable in applications involving pressure containment, corrosive or demanding service, regulated industries, or customer-specific quality requirements.

It is important to note that heat-number marking and MTR documentation are not automatically required for every thermowell. The requirement depends upon the applicable specification, project requirements, material standard, customer requirements and service conditions.

Where Thermowells Are Commonly Used

Thermowells are used across a broad range of applications, including:

HVAC and Mechanical Systems

Thermowells are commonly used with temperature instruments in heating and cooling systems, hydronic piping, boilers and other mechanical systems.

Industrial Processes

Industrial applications may require thermowells for temperature measurement in:

• Process piping

• Tanks and vessels

• Heat exchangers

• Boilers

• Chemical processes

• Manufacturing equipment

• Compressed-air and gas systems

  
  

Oil and Gas

Temperature measurement in pipelines, process equipment and other energy applications may require thermowells capable of handling elevated pressure, temperature and fluid velocity.

Food, Beverage and Pharmaceutical Applications

Where hygienic requirements apply, specialized sanitary thermowell designs may be required.

When a Thermowell May Not Be Appropriate

Although thermowells are extremely useful, they are not automatically required for every temperature measurement.

A direct-mounted temperature sensor may be appropriate where:

• The process does not require a permanent pressure barrier around the sensor.

• The sensor can safely withstand the process conditions.

• Rapid response is particularly important.

• The installation permits safe sensor replacement.

• Mechanical and process requirements do not justify a thermowell.

  
  

The decision should be based on the complete application rather than simply following a rule that every temperature instrument requires a thermowell.

Selecting the Right Thermowell

When specifying a thermowell, it is useful to consider the entire application rather than selecting the thermowell independently of the temperature instrument.

Process Conditions

• Media

• Pressure

• Temperature

• Flow velocity

• Corrosive or abrasive characteristics

  
  

Thermowell Construction

• Material

• Outside diameter

• Bore diameter

• Stem profile

• Insertion length

• Tip configuration

  
  

Installation

• Threaded, welded or flanged connection

• Process connection size

• Pipe or vessel configuration

• Available installation space

• Mounting and support conditions

  
  

Temperature Instrument

• Thermometer, RTD, thermocouple or other sensor

• Stem or probe diameter

• Insertion length

• Required response time

• Sensor-to-bore fit

  
  

Mechanical Considerations

• Flow-induced vibration

• Natural and wake frequencies

• Thermowell geometry

• Static and dynamic stresses

• Pressure containment

• Applicable design standards

  
  

Material and Documentation

• Material compatibility

• Required material grade

• Heat-number traceability, where required

• Material Test Reports

• Customer or project-specific certification

  
  

For demanding applications, sufficient process information should be provided to allow the thermowell design to be evaluated before manufacture and installation.

Conclusion

A thermowell may appear to be a relatively simple component, but it plays an important role in both temperature measurement and process protection. The correct thermowell protects the temperature instrument while providing an appropriate path for heat transfer from the process. At the same time, it must withstand the pressure, temperature, chemical environment and mechanical forces present at the measurement point.

For straightforward applications, selecting a suitable standard thermowell may be relatively simple. More demanding installations may require consideration of material compatibility, thermal response, heat-number traceability, insertion length, stem geometry, fluid velocity and wake-frequency calculations.

The most important principle is therefore simple:

A thermowell should be selected for the application—not simply for the thermometer.

About Merit Instruments

Merit Instruments provides pressure and temperature instrumentation and related products for industrial, mechanical-trades and OEM applications. Our technical team can assist with product selection and application requirements, including standard and custom thermowell configurations.