As temperature sensors are typically not designed to be exposed directly to process conditions or media, how do you install them so they can do their job? The answer is a thermowell. Thermowells are a piping component that allows process piping integrity to be maintained while providing a protective sheath around and connection for a temperature sensor. Thermowells also allows for the maintenance and replacement of thermometers and temperature sensors without the need to shut down a process. Want to learn more? Read on.
What Is a Thermowell?
Most people consider a thermowell as part of a temperature measuring system, but in actuality, a thermowell is first a piping component regulated by industry-standard ASME 19.3 TW. Why a standard? Because a thermowell is a fixed bluff projecting into a process and is extremely susceptible to the effects the process fluid will impose upon it. These effects are much like a radio antenna on a car moving down the highway. The antenna tends to vibrate and rotate in a circular pattern. If these imposed effects are allowed to reach excessive proportions the metal comprising the thermowell will weaken and the thermowell will fail, oftentimes with very undesirable effects.
At its basic purpose, the thermowell is a protection tube for a thermometer or temperature sensor which allows that sensor to interface with a process through existing piping. Thermowells must be matched to the specific dimensions of the temperature sensor with which it is to be used to ensure the 2 devices join together in a manner allowing for accurate temperature measurement.
A sensor too short will not contact the thermowell, leaving the sensor hanging in an air gap which will certainly be cooler than an actual process temperature. Air is a great insulator. Too Long and a sensor could be damaged when being installed causing premature failure. Finally, the sensor must “connect” or attach to the thermowell, typically through a threaded connection. The threads must be of the same type and pitch to ensure proper connection.
Thermowells come in a variety of piping and sensor connections and are typically constructed from a single piece of bar stock. Process connections are typically threaded or flanged to meet existing piping standards but may also be welded in high pressure or dangerous applications. Sensor connections are most often threaded but can also be bayonet (twist lock) style, or compression (slide in and tighten to hold in place). The shank of the thermowell (the part exposed to the process) can be altered to meet specific needs.
Tapered shanks are common because they offer strength at the process connection but less material at the tip for better temperature conduction. Stepped shanks try to improve on these attributes by cutting some more material away towards the tip creating a “step”. Finally, straight shanks are typically used in applications where strength due to high velocities or pressure is mandated.
How to Correctly Protect Your Temperature Sensor
Whether you are installing a bi-metal thermometer, RTD, or thermocouple in your thermowell, there are some basic considerations to keep in mind.
- U-Length – better known as insertion depth, U-Length is the depth of the thermowell below its process connection, and the length of the sensor below its connection to the thermowell. The U-Length of the thermowell must be matched with the U-Length of the temperature device.
- Bore – the hole through the middle of the thermowell in which the temperature device resides. Standard industry bore sizes are 0.250” and 0.375”. A 0.250” diameter temperature device will fit in a 0.375” bore, but the air surrounding the temperature
device will insulate it and you will never see the actual process temperature. And if it’s not obvious, a 0.375” temperature device won’t fit in a 0.250” bore. For these reasons, always match device diameter with thermowell bore.
- Resonance Frequency Calculations (or Wave Frequency Calculation) – Per ASME 19.3TW each customer must use the formula provided in the standard and perform a frequency calculation to ensure the metallurgical integrity of the thermowell is maintained in every process condition. Typically, worst-case process conditions are used for this calculation as a minimum. A safety factor is recommended here but consult your piping engineers or call Sagacity for further information. Frequency calculations are required for every installation and can be very complicated to perform.
- Hydrostatic Testing – is the process of putting a thermowell in a test apparatus and pressuring the thermowell up to at least 1.5x working pressure. This is done to prove the integrity of the thermowell material and to test the integrity of any welds used during the manufacturing process of the thermowell. Your piping engineers will let you know if this is a plant standard and is a requirement.
When You Need Precision Parts or Fabrication, Count on Us
Sagacity Allstream Fabrication Engineering can help you refit aging industrial equipment including temperature devices and thermowells. Our applications team can help with MRO replacements, or with new installs, and make recommendations based on our decades of applications experience. Please don’t hesitate to contact Sagacity Allstream Fabrication Engineering today at (844) 514-9170.
Alternatively, connect with us online via chat, drop us an email, or send us a text. We look forward to helping you with your new or existing applications.