Innovative thermowell design improves reliability in oil and gas pipelines

A manufacturer of temperature measurement products has launched a unique design of thermowell that can easily and safely be installed in process pipelines if a standard thermowell has failed the ASME Performance Test Code (PTC 19.3, 2004).

In petrochemicals, thermowells are used widely in temperature measurement applications. There are multiple variations of two basic types: low pressure and high-pressure versions. Thermowells are used to provide isolation between a temperature sensor and the environment, in this case oil or gas. A thermowell enables the temperature sensor to be removed and replaced without compromising either the ambient region or the process. The service life of a thermowell is therefore critical and the mechanical design needs to ensure that the unit can operate under ever increasing flow rates within process pipelines.

Fluid (oil or gas) flowing past a thermowell creates a turbulent wake as vortices form at both sides of the well. These vortices detach, first from one side and then from the other, which causes alternating lateral forces on the well perpendicular to the direction of flow. This is known as the Von Karmann effect. This wake frequency is proportional to the fluid velocity and well dimensions. Generally, if the wake frequency coincides with or comes within 20% of the natural frequency of the well, the resultant vibration could cause mechanical failure of the well. Thermowells are usually safe if the natural frequency is well below the wake frequency or if the fluid velocity continually fluctuates through the critical velocity point.