Thermowell and its Explanation
According to the American Society for Testing Materials (ASTM), the thermowell is a reentrant tube with closed ends, which can encase the sensor. These devices are equipped with features that ensure tight attachment to a vessel. The thermowell acts as a barrier between the sensing element and the process medium. It protects the sensing element against corrosive process media and fluid pressure and velocity.
Types of Thermowell
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Types depending on stem design:
Depending on the stem design shape three types of thermowell are available:
- Straight Thermowell
- Stepped Thermowell
- Tapered Thermowell
Straight thermowell have the same diameter for the entire insertion length. Stepped thermowell has stepped diameters; Normally at the tip, they have 1/2″ diameter while 3/4″ at the top. On the other hand, tapered thermowell has varying diameters over its full insertion length. Tapered thermowell is suitable for high-velocity applications and possesses a fast response time.
Types depending on end connection:
Depending on End connections, five types of thermowells are available. they are
- Threaded thermowells
- Weld-In thermowells
- Socket Weld thermowells
- Van Stone or Lap Joint flanged thermowell
Depending on material type:
Depending on material types Two types of Thermowells can be found:
- Pipe Fabricated.
- Bar stock thermowells.
Component of a Thermowell
Following are the components of a thermowell:
- Bore Diameter (B):
This is the inside diameter of the Thermowell.
- Bore Depth (S):
The total length of the bore.
- Insertion length (U):
Thermowell immersion lengths are often called the “U” length which is the measurement of the Thermowell from the bottom of the process connection to the top portion of the Thermowell.
- Process Connections:
This is the way a Thermowell is inserted and connected into a process referring to the attachment of the element. The most popular are threaded, socket weld, and flanged connections.
- Shank Construction:
This is the immersion portion (shank) of the construction. The majority fall under three categories: straight, step or tapered.
- Thermowell Shank Construction
The straight shank is the same size all along the immersion length. For general-purpose, where additional strength is not required, the straight shank or stem is used.
The stepped shank has a larger outer diameter at the end of the immersion length in some applications for a quicker response time.
The tapered version decreases gradually along the immersion length. Tapered thermowells are designed for use in high velocities, where extra strength is required. Special attention shall be given to the vibration effects caused by the fluid passing the thermowell.
- Lagging Extension Length (T):
The lagging extension commonly referred to as the “T” length is located on the cold side of the process connection and is usually an extension of the hex length.
- Base diameter Dimension (Q):
This is the outside dimension of the Thermowell shank and is the densest part because the outside surface area is exposed to the hazardous materials of the process.
- Tip Thickness (E):
The thickness of the bottom portion of thermowell.
- Tip Diameter (V):
Thermowell end Diameter.
The following points shall be considered while installing the thermowell (TW):
- Immersion length.
- Type of mounting (elbow, perpendicular, or angle).
- Height from the floor for maintenance.
- Clearance above the thermowell assembly for maintenance.
All thermowells are mounted in three methods:
- Perpendicular to the pipe.
- In a pipe elbow.
- At 45 deg. angle to the pipe.
Of the above three methods, elbow mounting is the most popular because it can accommodate almost total immersion length, if necessary. In elbow mounting, the thermowell protrudes into the exact centerline of the pipe and the thermowell will be in the direction flow area of the pipe.
- The perpendicular mounting will be more restrictive for a given size because the thermowell must not touch the far wall of the pipe.
- If the pipe is not enough to accommodate perpendicular thermowell an angle mounting can solve the problem. An angle mount usually provides additional space for the thermowell.
- If the OD of the pipes is between 3” to 6”, it is slightly more difficult to install the thermowell.
Information required for purchasing thermowell
The following information must be supplied to the thermowell manufacturer/vendor while placing an order.
- Process Connection Size
- Thermowell Insertion Length
- Lagging extension
- Shank configuration
- Process Connection type
- Nominal Bore
- Thermowell Material
- Process Design temperature and pressure
Material of Thermowells
Material selection of thermowell will depend on the chemical, temperature, and flow rate of the process fluid. With the increase in temperature and fluid concentration, the corrosive effects normally increase. At the same time, suspended particles of the fluid will cause erosion. So, all these parameters need to be addressed while selecting thermowell material. Some of the most frequently used thermowell materials are listed below:
- Carbon Steel
- SS 316 / SS 304
- SS316 with Teflon / Zirconium Coated or Tantalum / Titanium sheathing.
- Chromium/molybdenum steels
- Haynes Alloy
However, the most widely used thermowell material is stainless steel as it is cost-effective and highly resistant to heat and corrosion. For pressurized vessels, Chromium/molybdenum steel is used. Cobalt, nickel, chromium, and tungsten constitute the Haynes alloy that is widely used for subsidizing, carburizing, and chlorine-containing environments. The use of carbon steel thermowell is only limited to low-temperature pressure applications due to its very low resistance to corrosion.
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Thermowell Design Code
Piping Designers follow ASME PTC 19.3 for guidelines regarding thermowells.
An improperly specified thermowells will result in:
- Failure due to poor welding practices.
- Poor compatibility with the temperature and media.
- Inadequate temperature transfer.
- Incompatibility with the process velocity leads to failure due to vibration.
- The gap between the OD of the thermocouple sheath and the ID of thermowell must be very close.
- The bore of the thermowell must be uniform and linear.
As thermowells are immersed in the process flow, they can be subjected to vibration due to fluid velocity. The thermowell must have enough stiffness and rigidity to absorb those vibrations. In such a scenario, tapered thermowells are preferred as they have more stiffness.
Importance of thermowells:
Thermowells play a crucial role in the successful measurement of temperature in industrial processes.
- They Protect the sensor.
- Thermowells ensure that process temperature is passed to the sensor (proper heat transfer).
- Improved heat transfer, results in better accuracy.
- Allow the removal of the sensing element while maintaining a closed system.
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