Thermowells are used to protect temperature sensing elements such as Thermocouples, RTD’s, Bimetallic Thermometers, and Filled System Bulbs from breaking if they are installed in a tank, vessel, or pipe. Thermowells also allow a temperature measuring element such as a thermocouple, RTD, or bulb to be removed for servicing without the requirement that the plant be shutdown. Sometimes “Protection Tubes” instead of thermowells are used to protect thermocouples. These protection tubes are usually fabricated of ceramic materials and they are installed in high temperature flue gas and duct applications. Protection tubes are also available made of Inconel and with their ends open in order to improve speed of response and heat transfer.
Thermowells are used for almost all applications except for some specific applications (that is, for very-low-pressure air systems, for ventilating systems, bearing temperature measurement, lube oil drain in compressor and so on) where faster response required or space limitation such as casing drain in compressor. Temperature elements can be installed without Thermowells in following applications or condition exist,
- The process fluid is not corrosive or otherwise hazardous.
- The process is not under significant pressure.
- Air-in leakage is permissible.
- The primary element has the necessary static and dynamic mechanical strength for the application.
- Either a failure of the element may be tolerated until the process can conveniently be shut down, or operation can continue without the use of the element.
There is no personnel hazard if the thermocouple is inadvertently removed
In case if test Thermowells are required, then devices with suitable plugs shall be provided. Design of all the Thermowells should be same for all elements (i.e. for bimetallic gauges, RTDs or thermocouples) so that interchangeability can be obtained. Factors to be reviewed while choosing Thermowells are resistance to corrosion, flow induced vibrations, erosion, ease of installation, preferred installation point of Thermowell, insertion length, material, velocity considerations, lagging extension etc.
The Thermowells are available with following types of connections:
- Weld-in Type (W) Thermowell
- Socket Weld Type (S) Thermowell
- Threaded Type (T) Thermowell
- Flanged Type (F) Thermowell
Flanged type Thermowells are most commonly used in all Oil & Gas industries. The drawing for this type of Thermowell is shown in the figure below.
Standard Thermowells should be stamped with the ASTM material designation, “U” (insertion length) dimension, Rating and the tag number.
Thermowells installation shall be per Piping & Instrumentation Interface Diagram or as specified in the piping specification. The location for a Thermowell should also be checked with respect to accessibility. In particular, there should be sufficient space to allow removal of the Thermowell assembly.
In-case if Thermowells are installed in pipe lines which are less than 4”, then a section of this pipe line shall be expanded to 4”. This expansion (3” or 4”) shall be decided based on specific project requirement.
Thermowells can be installed in Tee at a 90-degree pipe elbow in case of smaller pipe sizes.
The above typical flanged Thermowell drawing is only for reference. Construction of he device shall be as per specific project requirement. Care shall be taken while specifying teak weld or full penetration weld for flanged Thermowells based on flange ratings/pipe ratings.
Different variation of construction is available. In case if there is no specific and clear requirement, API construction can be followed.
Thermowell Insertion Length
Thermowell insertion length should be sufficient to measure the bulk temperature of the fluid without conduction effect. A Thermowell installed perpendicular or at a 45-degree angle to the pipe wall should have a minimum immersion length of 2” and a maximum distance of 5” from the wall of the pipe.
If the Thermowell is installed at an angle or in an elbow, the tip should point toward the flow in the process line.
Thermowells installed in lines through which fluid flows at a high velocity may be subject to vibration, which can rupture the well below the mounting. Tapered stems and U lengths established by means of stress analysis are recommended for high velocity.
On Equipment or Vessels minimum immersion length should be between 300 to 500mm based on requirement.
While deciding insertion length, nozzle stand out, insulation thickness, pipe thickness etc shall be taken in account. This activity should be coordinated with mechanical/ Piping to avoid any mismatch during selection and installation.
Special applications such as agitated vessels, separation columns, etc., with internals, may require shorter Thermowell lengths and should be checked for interference.
The Thermowell material is selected as per piping class and specification. For better accuracy in temperature measurement, Thermowells must have a smooth, shiny surface with no pitting and corrosion.
Pipe insulation or lagging extension is the extra length provided between the Thermowell piping connection and the temperature element connection. This extra length is provided to ensure the temperature element connection is located outside the insulation and for ease of temperature element installation or removal. This extra lagging extension length shall be taken into account while deciding Thermowell insertion length.
- Thermowell Flange Material Different than Pipe Material: In case, if material of construction (MOC) of Thermowell is different than the pipe class material, the flange rating of Thermowell shall be suitable and withstand pipe line design condition that is, pressure temperature. For example, if pipe class is Carbon Steel (CS) and MOC for Thermowell is SS316, then Instrument Engineer should check the rating required for Thermowell based on line design pressure & temperature.
- Incase if the flange rating of Thermowell is different than the pipe rating, it should be communicated to Piping and Process to take necessary action at their end (provide nozzle with same rating as Thermowell rating).
- Requirement of Insulating Gasket shall be communicated to Mechanical & Process in case material of construction for Thermowell is different than piping material.
- In case if Thermowells are with collar to protect them from failure due to high vibration, the nozzle ID shall be checked with respective to collar diameter. This information should be communicated to Mechanical to avoid any mismatch during installation.
- Care shall be taken when Thermowells are selected with collars for lined vessels & cladded vessels. In these cases nozzle ID shall be checked with equipment drawings to avoid mismatch of Thermowell during installation.
- Requirement of Thermowell connections (Flanged, Threaded or Welded) shall be checked as per specific project requirement.
- NDE requirement for the device shall be checked and specify as per piping requirement.
- When the Thermowells are installed on the equipment & vessel, care shall be taken while deciding insertion length of these Thermowells to avoid fouling with equipment/vessel internals. Equipment/vessel internal arrangement drawing & nozzle location shall be reviewed by Instrument engineer to avoid any mismatch.
Multipoint Temperature Element with Thermowell
Multipoint temperature elements (RTDs or Thermocouples) are used for tank gauging system & process reactors. The purpose of these elements is to measure the temperature at different location or different levels.
In the tank gauging system, the requirement is to measure fluid temperature at different level, which can be obtained by installing multipoint temperature elements assembly, which consist of number of elements of different length in one Thermowell. This device is prefabricated and installed in the tank by tank supplier. The numbers of temperature elements (i.e. 4, 8,10,12,16 etc) shall depend upon tank height & process requirement. The figure below shows a typical drawing for this type temperature element.
In case of process reactors, the device made from pipe is used to installed 3 or 4 temperature element of different length (to measure temperature at different location on reactor diameter) to measure temperature of chemical bed at specific location.
In some reactors requirement is to measure temperature of chemical bed at different location and different levels. In this case thermocouple/coupling assembly of required elements (8, 10,12,14,16 etc) inserted/installed on reactor by clamping the elements inside reactor. Care shall be taken while deciding the insertion length of element. The length also depends upon routing of these elements in reactor.