What is a thermowell?

A thermowell is a hollow tube that is closed at one end and ribbed at the other end. It is permanently installed in pipes or vessels to accommodate temperature probes to measure the temperature of the contents inside the tube. It is also a housing used to protect a temperature sensor probe (RTD or thermocouple) from exposure to hazardous materials or harsh environments.

The ASTM defines a thermowell as a closed-ended, insertable tube designed to house a temperature-sensitive element with a means of securing it to a vessel.

How a thermowell works

A thermowell acts as a barrier between the process medium and the sensor of a temperature measuring device. This barrier protects the sensor from corrosive process media as well as pressurized or high-velocity flow. A thermowell also allows the temperature sensor to be easily removed from the system for maintenance by maintaining a closed system.

Why use a thermowell?

If the measurement is not critical to plant safety or production efficiency, and if the process fluid is not aggressive or hazardous, a 304 or 316 stainless steel, sheathed, industrial thermometer may be inserted into the plant pipeline or conduit without the use of a thermowell through suitable pressure fittings. Obviously, in this case, the thermometer cannot be removed while the plant is operating, and if a measurement problem occurs, maintenance is delayed until the process is shut down and the line is evacuated and secured.

What types of thermowells are available?

Thermowells are available for use with all temperature instrumentation and control products including: thermometers, thermocouples, RTDs, temperature recorders, and temperature controllers.

Connections: Thermowells are available in a variety of process connection styles. Threaded connections in 1/2, 3/4, and 1 NPT are most commonly used. Socket weld, weld, and RF flange connection types are also available. Bimetal thermowells are available with a 1/2 NPSM connection to reduce pressure inside the thermowell.

U-LENGTH: The U-length (penetration length) of a thermowell is the depth of its insertion into a process vessel or piping system and is measured from the tip of the thermowell to the underside of the ribs. The U-length must be equal to or greater than the length of the sensitive portion of the stem or bulb of the temperature instrument.

SHANK: Thermowells are available in stepped, tapered, and straight configurations. Stepped thermowells are typically used in standard applications. Tapered thermowells are designed for use in heavy-duty applications. Straight-stem thermowells are designed for use with instruments that have a wide stem diameter or short stem length.

LAGGING EXTENSION: Thermowells with a lagging tail are used where insulation covers the vessel or piping system. The T-length is the measurement between the instrument connection and the process connection of the thermowell.

What type of material can be used to make a thermowell?

A thermowell protects the device from pressure, flow forces, and chemical effects of the process fluid. Typically, a thermowell is made of a metal rod that is drilled to accommodate a temperature sensor and is mounted using bolts or flanges. The inner end of the thermowell may have a reduced diameter to improve response speed.

The choice of thermowell material depends on the process conditions, such as the corrosiveness of the measured fluid. For low pressures and temperatures, Teflon may be used to make a thermowell. Thermowells can be made from a variety of materials including brass, carbon steel (A105), stainless steel (SS304, SS304L, SS310, SS316, SS316L, SS446, SS410, SS317, SS317L, SS321), Monel and Hastelloy and Carpenter 20, Inconel, Incoloy, Nickel and Titanium.

In cases where the temperature is high and the pressure difference is low, a protective tube may be used with a bare thermocouple sensor. These materials are made of alumina to prevent chemical attack on the platinum or other thermocouple elements and may in turn be encased in an external silicon carbide protective tube if further protection is required.

Since the thermowell mass must be heated to the process temperature and since the thermowell walls remove heat from the process, the accuracy and speed of the sensor response are affected.

Additional Considerations for Thermowell Design and Construction

Thermowells are typically installed in piping systems and are subject to both hydrostatic and aerodynamic forces. Eddy current is the most significant concern for thermowells in counter-flow applications and can force the thermowell into resonance, potentially causing failure of not only the thermowell but also the temperature sensor. The conditions for flow-induced resonance typically govern the design of the thermowell, apart from its pressure rating and material of construction.

Probe Length for Thermowell

The sensor tip should touch the bottom of the thermowell. A temperature sensor such as a thermometer, thermocouple, or RTD is placed at the open end of the thermowell and is typically spring-loaded to ensure that the outer tip of the temperature sensor is in metal-to-metal contact with the inner tip of the thermowell. The sensor fit should be as tight as possible to reduce the annular gap because air acts as an insulator.

There is no standard method for selecting the probe length according to the thermocouple. Thermocouples are available in different standards with different lengths, sensor probes such as thermometers, RTDs, thermocouples are also available in different dimensions. The appropriate probe length should be selected according to the thermocouple.

For example, there are certain cases where the probe length is shorter than the thermocouple, high boiling point oil can be placed in the thermowell to transfer heat to the probe, which is of low priority in measurements and depends entirely on the measurement application.

On the other hand, if the probe is too long, the stem outside the thermowell will bend and there is a possibility of physical damage.

Thermowell manufacturing

Different types of thermowells with different materials can be manufactured and delivered upon request by Petro Parda.