S-type (platinum-rhodium) thermocouple belongs to the class of noble metal thermocouples. The metals include platinumand...
Type J thermocouple is also a subtype ofbase metal thermocouple with iron-constantan as its constituents, is also acheap...
A thermowell or thermocouple pocket is a cylindrical device used for the protection of temperature sensors. These temperature sensors include thermometer, thermocouple and other temperature detectors. It is used widely in different industries where protection and safety of temperature measuring equipment are required.
The temperature sensing equipment is very sensitive to certain conditions including high pressure and flammable medium. A thermowell is used to avoid any such damage to these sensors. Also, there is an increased risk of spillage and the non-feasibility of pipes in some industrial conditions. Thermowell is the ideal protection to prevent these undesirable damages.
Thermowell finds its application in the oil and gas industry, petrochemical industry, refining industries, food processing plants, pharmaceuticals and cosmetic industries and other commercial industries concerned with mechanical and thermal stress. These industries are related to extremes of temperature and pressure and a lot of toxic substances. All these factors are hazardous for the temperature sensors i.e thermometer and thermocouple thus thermowell is used in such industries.
Based on their connection with the pipeline, thermowell are classified into three main types.
1- Threaded
This thermowell is threaded into the smaller pipelines. They are used to measure the temperature of noncorrosive liquids in a narrower pipeline.
2- Flanges welded
The top portion of this thermowell is flanged which is connected to pipes with bolts and nuts. They have greater application for high-temperature measurement when frequent replacement is needed.
3- Socket welded
The socket welded thermowell is welded into the pipelines, therefore, they have been applied for a permanent basis. They also find greater use for high temperature and high-pressure conditions.
The manufacturing material, insertion length of the sensor, bore of thermowell, shank configuration, lagging extension and keeping account of service conditions like pressure, temperature, velocity, etc. are important parameters to consider while selecting a proper thermowell. The different types of thermowell can also be selected on the basis of time of requirement e.g. socket welded thermowell is used for permanent basis whereas when a temporary connection is needed, threaded thermowell is used.
Diameter and length
The diameter and length of the well should be such that it favors easy immersion of sensor and ability to withstand pressure and vibrations. The average diameter of the bore ranges from 7 to 10mm but other diameters are also available. The length range is between 63 to 572mm and selected according to the length of the sensor.
Tapered or straight
Thernowells are available in two formulations that are tapered and straight. Tapered wells possess higher strength/ weight ratio and thus are less susceptible to higher vibrations whereas straight wells are more prone to these vibrations. Another difference between them is that straight wells have a constant diameter along the entire length whereas tapered thermowell vary in their diameter along its length. These differences help in selecting the thermowell according to the sensors used.
Material
The materials used for manufacturing thermowell also vary for different types of industrial applications. Some industrial boilers use moly steel wells whereas food processing plants used wells made of stainless steel. Similarly, titanium wells are used for offshore application and Monel 400 wells are used for ceramic industries. Other materials include Hastelloy B3, C22,316, and 304 stainless steel and Haynes alloy 556.
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