How to specify a thermowell?

Temperature measurement and control is an important factor in all process industries and in power plants environments of higher temperature and higher pressure.

          The thermowell allows the temperature sensor Thermocouple and RTD to remove easily from the process environment for the purpose of calibration in case of measurement error, or replacement in case of damage.

For the sensor and operator safety, the thermowell must be permanently mounted in process equipment.

The applications of thermowell are often found in process industries like refineries, petrochemical plants, and in power plants.

To meet service industry needs the thermowell is selected on the basis of pressure, temperature, flow, vibration, and corrosion parameters.

What is a Thermowell?

The thermowell is a hollow metal tube with one side sealed to protect a temperature sensor from corrosive process media under high pressure and high-temperature environment.

The response time of the thermowell is high because the heat to the sensor must be transferred through the thermowell.

The temperature sensor is inserted inside the thermowell such that the sensing part of the sensor is contacted physically with the sealed end.

What are the functions of thermowell?

The functions of thermowell are:

1. To protect a temperature sensor from being exposed to extremely high temperatures
2. The primary function of the thermowell is to provide housing for temperature sensors like thermocouples and RTD.
3. A thermowell serves as a protective barrier between a process medium and the temperature measuring sensor
4. The thermowell allows installing the sensitive part of the temperature sensor tip in the process media.
5. Thermowell protects the sensor against corrosive process media, as well as media contained under pressure or flowing at a high velocity.
6. The reduction in the air space between the sensor and thermowell results in minimum thermal lag.
7. The temperature sensors in the process station must be mounted using the thermowell for mechanical protection.

What are the types of thermowell?

Based on the stem design the thermowell is categorized as:

1.      Straight thermowell:

A straight thermowell has the same diameter throughout the entire insertion length and offers better sensor protection against corrosion and erosion.

2.       Stepped thermowell:

The Stepped thermowell has a diameter of 0.75 inches at the top and is stepped to 0.5 inches near the tip. This reduced surface area allows smoother velocity and has a high-temperature response for the sensing devices.

3.       Tapered thermowell:

The Tapered thermowell has a diameter that decreases gradually over the insertion length from top to bottom. The tapered thermowell has a high strength-to-weight ratio providing greater strength and high sensitivity. And also provides greater resistance to high-frequency vibrations than the straight thermowell.

The installation of a thermowell and temperature sensor in a process line is shown in the figure below:

How to select a Thermowell?

The thermowell is selected based on the following format:

• Process connection size: The size of the thermowell that connects with the vessel must be in the form of thread size, flange size, pipe size, tri-clamp, etc. Sizes are   0.5’’, 0.75’’, 1’’, 1.25”, 1.5 ’’, 2’’, 3’’, and 4’’.

• Insertion length: It is the total length from the top of the mounting threads to the stem end. It is the U-shaped portion of the shank from the underside of the threads to the shank tip which is inserted into the process area to obtain an accurate reading with a thermowell and the entire sensitive portion of the bulb is immersed in the process media where temperature measurement is of the desired value. The lengths are 15/8’’, 2.5’’, 4.5’’, 7.5’’, 10.5’’, 13.5’’, 16.5’’, 19.5’, 22.5’’.

• Lagging extension: It is specified when the vessel or pipe into which the inserted thermowell is insulated. It is determined by the insulation thickness which is the extra length between the process connection and the instrument connection of a thermowell. The lagging extension is up to 3 inches.

• Shank style: It is part of a thermowell that is inserted into the process. A thermowell shank can be tapered, stepped, or straight.

1. Stepped shank – This type of shank has reduced tip diameter to improve heat transfer to the sensing element. And makes the sensor more responsive to the changes in the process temperature.
2. Straight shank – This type of shank is the original design for tube protection this style would be considered in low pressure and low-velocity application.
3. Tapered shank – This type of shank has superior strength and vibration resistance. It is recommended as opposed to a stepped or straight shank.

• Process connection type: It defines the type of connection between the thermowell and sensor probe. Connection types are NPT, Welded, Flanged, and Socket-welded.

• Bore diameter: During the selection of a thermowell, the bore diameter is an important parameter that must be considered.  For the tip sensing thermocouple, the probe must be fully seated in contact with the tip of the thermowell.  For Pt-100 sensors which are of stem sensing the difference between the probe and the bore must be kept to an absolute minimum. The bore diameter must be 0.260 inch or 0.385 inches.

• Material of construction: The selection of materials should be compatible with process parameters like pressure, temperature, and velocity. The materials are selected considering their strength and corrosion resistance. Materials used are Carbon steel, 304, or 316 Stainless steel

• Flange configuration: Flanged thermowell are classified into three distinct configurations namely Flat face, Raised face, and Ring joint.

1. Flat face – These are typically used when the mating flange is made from a casting. And must not be mated with a raised face flange.
2. Raised face – This type of flange is common and found in all process applications. These raised face flanges have a texture to grip the gasket that is placed between the two flanges.
3. Ring joint – These flanges are provided with a groove that holds the gasket and seals metal to metal when tightened through bolts. These are used in a high-pressure and high-temperature environment.

• Thread size: The thread size for threaded thermowell is 0.5-inch NPSM or 0.5-inch NPT.

• Pressure rating:  Class 150, 300, 600, 1500 or 2500.

Thermowell Fittings:

For mounting the temperature sensor assembly into the thermowell some fittings are required.

1. The fitting is made up of brass or stainless steel.
2. The fittings include various threaded unions, bayonet caps, and flanges.
3. To ensure the proper seating of sensor probes in the thermowell the adjustable compression fittings are to be used directly to achieve the required insertion length in the process.
4. Bayonet caps are used for quick fitting into suitable adapters.
5. Commonly used installation formats of thermowell are tapered and straight thermowell.

Thermowell Process Connections:

During mounting of a thermowell, the various processes connection configurations are to be considered. Some of them are discussed below:

a. Threaded connections:

• The parallel or tapered threads make for convenient installation into a welded-in fitting directly into the process.
• These types of connections are suitable for smaller diameter wells which are not likely to be changed frequently where the corrosion rate is less.
• An extended hexagon length can be used to allow for insulation thickness.
• Typical thread sizes are 1⁄8” BSP (T), 1⁄2 ” BSP (T), or 20mm.

b. Flanged Connections:

• Flanged connections are preferred in high corrosion areas where the replacement of thermowell is frequent.
• These connections are used for large pipe diameters in high-pressure areas.
• The diameter of the flanges is usually 2 to 3 inches.

c. Welded Connections:

• These connections are suitable for high temperature and high-pressure applications such as steam lines in boiler main steam.
• These connections are preferred when the process is not corrosive and doesn’t require any routine removal.
• High integrity is achieved and removal of a welded-in well usually involves considerable effort and time.

Application of thermowell:

1. Used in hostile environments and at temperature and high-pressure environments.
2. Widely used in industries like

• Power plants
• Petrochemical
• Refining
• Food  processing 
• Pharmaceutical

Advantages of thermowell:

1. Provides easier serviceability.
2. Reduces operating costs.
3. Protects the sensor from corrosion.
4. Allows removing the sensor without plant shut down.

Disadvantages of thermowell:

1. Increases installation cost.
2. Response time is slow.