Wake Frequency Calculations for Thermowells

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A thermowell is a receptacle used to insert a temperature sensor into a pipe and protect the sensor as gases and liquids pass through the pipe. Thermowell calculations are made to ensure the receptacle will remain in place in the pipe and that the thermowell materials and chemicals passed through the pipe will not interact to damage the pipe or thermowell.


A thermowell is commonly designed as a piece of drilled out metal that protects the temperature sensor within from high pressure, temperatures and the harsh environments commonly contained in a pipeline. Thermowells can be attached to a piping system by screwing in the threaded end of the thermowell or by welding techniques for socket weld and flange welded thermowell designs. Wake frequency calculations are completed to ensure the design of the thermowell is correct compared to the environment of the pipeline into which it will be introduced.


Wake frequency calculations are completed for thermowells using procedures set out in ASME PTC 19.3 TW-2010. ASME (formerly known as the American Society of Mechanical Engineers) is a globally recognized organization responsible for formulating standards and procedures for testing materials. Wake frequency calculations are completed to ensure the stress limits of thermowell materials and designs are not exceeded when the receptacle is positioned within a pipeline, according to Wika.


The information required for the completion of wake frequency calculations include the interior diameter of the thermowell, the height of the thermowell and the thickness of the walls of the thermowell. Calculations are completed for periods of dynamic and static movement within the pipeline. Wake frequency calculations are completed to ensure the harmonic frequency of the materials passing through a pipeline are not similar to the harmonic frequency of the thermowell materials during the transmission of materials in the pipeline, as a similar frequency can result in damage to the thermowell and the pipeline.


The calculations regarding wake frequency ensure the thermowell is composed of the correct materials, construction and design to maintain the pressure created within the thermowell as materials flow through the pipe. Static forces are created by the flow of fluids that impact against the stem of the thermowell. When the fluids flowing through the pipelines reach high flow rates, the stem can begin to emit material through harmonic frequencies. The material shed during high flow rates is required to be no more than 80 percent of the harmonic frequency of the thermowell, ABB reports.