|This article needs additional citations for verification. (March 2012)|
Thermowells are tubular fittings used to protect temperature sensors installed in industrial processes. A thermowell consists of a tube closed at one end and mounted in the process stream. A temperature sensor such as a thermometer, thermocouple or resistance temperature detector is inserted in the open end of the tube, which is usually in the open air outside the process piping or vessel and any thermal insulation. The process fluid transfers heat to the thermowell wall, which in turn transfers heat to the sensor. Since more mass is present, the sensor's response to process temperature changes is delayed. If the sensor fails, it can be easily replaced without draining the vessel or piping. To be representative of the average temperature of fluid, the thermowell must extend a few per cent of the inside diameter of the process pipe or vessel.
A thermowell is typically machined from solid bar stock and gun-drilled to ensure a long straight bore that closely approximates the diameter of the installed sensor (ex: a .260" bore matching a .250" sensor). A thermowell is typically mounted into the process stream by way of a threaded, welded, sanitary cap or flanged process connection. The temperature sensor such as a thermometer, thermocouple or resistance temperature detector is inserted in the open end of the thermowell and typically spring loaded to ensure that the outside tip of the temperature sensor is in metal to metal contact with the inside tip of the thermowell.
Thermodynamically, the process fluid transfers heat to the thermowell wall, which in turn transfer heat to the sensor. Since more mass is present with a sensor-well assembly than with a probe directly immersed into the process, the sensor's response to process temperature change is slowed by the addition of the well. Since the mass of the thermowell must be heated to the process temperature, and since the walls of the thermowell conduct heat out of the process, sensor accuracy and responsiveness is negatively impacted by the addition of a thermowell.
To be representative of the average temperature of fluid, the thermowell must extend a few percent of the inside diameter of the process pipe or vessel. A rule of thumb that is sufficient for most industrial applications (apx. 1% accuracy) is to ensure that the thermowell projects 5 times its own diameter into the process plus the length of its sensing element. So, for a grounded thermocouple spring loaded into a thermowell with a 1-inch diameter shank and a tip thickness of .25" a typical immersion length would equal 5.5 inches (1" OD x 5 + .25" well tip thickness +.25" sensor wall thickness = 5.5").
Materials and construction
The thermowell protects the instrument from the pressure, flow-induced forces, and chemical effects of the process fluid. Typically a thermowell is made from metal bar stock. The end of the thermowell may be of reduced diameter (as is the case with a tapered or stepped shank thermowell) to improve the speed of response.
Where temperatures are high and the pressure differential is small, a protection tube may be used with a bare thermocouple element. These are often made of alumina or other ceramic material to prevent chemical attack of the platinum or other thermocouple elements. The ceramic protection tube may be inserted into a heavy outer protection tube manufactured from silicon carbide or other material where increased protection is required.
Thermowells are typically installed in piping systems and subject to both hydrostatic and aerodynamic forces. Vortex shedding is the dominant concern for thermowells in cross-flow applications and is capable of forcing the thermowell into resonance with the possibility of fatigue failure not only of the thermowell but also of the temperature sensor. The conditions for flow-induced resonance generally govern the design of the thermowell apart from its pressure rating and materials of construction. Flow-induced motion of the thermowell occurs both in-line with and transverse to the direction of flow with the fluid forces acting to bend the thermowell. In many applications the transverse component of the fluid forces resulting from vortex shedding tends to govern the onset of flow-induced resonance, with a forcing frequency equal to the vortex shedding rate. In liquids and in high pressure compressible fluids, a smaller but nonetheless significant component of motion in the flow-direction is also present and occurs at nearly twice the vortex shedding rate. The in-line resonance condition may govern thermowell design at high fluid velocities although its amplitude is a function of the mass-damping parameter or Scruton number describing the thermowell-fluid interaction.
For drilled bar-stock thermowells, the most common form of failure is bending fatigue at its base where the bending stresses are greatest. In extreme flow conditions (high velocity liquids or high velocity, high pressure gases and vapors) catastrophic failure may occur with bending stresses exceeding the ultimate strength of the material. For extremely long thermowells, the static component of the bending stresses may govern design. In less demanding services, fatigue failure is more gradual and often preceded by a series sensor failures. The latter are due to the acceleration of the thermowell tip as it vibrates, this motion causes the element to lift off the bottom of the thermowell and batter itself to pieces. In cases where the acceleration stresses have been measured, sensor accelerations at resonant conditions often exceed 250 Gs and have destroyed the accelerometer.
The natural frequencies of thermowell bending modes are dependent upon the dimensions of the thermowell, the compliance (or flexibility) of its support, and to a lesser extent dependent upon the mass of the sensor and the added mass of the fluid surrounding the thermowell.
The ASME Performance Test Code PTC 19.3TW-2010 ("19.3TW") defines criteria for the design and application of thermowells. However, these thermowells must be manufactured from bar stock or forged material where certain dimensional requirements and manufacturing tolerances are met. Coatings, sleeves, velocity collars, special machined surfaces such as spirals or fins are expressly outside the scope of the 19.3TW standard.
- H. M. Hashemian (ed), Sensor Performance and Reliability, ISA 2005 ISBN 1-55617-897-2, page 16
- Thomas W. Kerlin & Mitchell P. Johnson (2012). Practical Thermocouple Thermometry (2nd Ed.). Research Triangle Park: ISA. pp. 79–85. ISBN 978-1-937560-27-0.
- J.V. Nichols D.R. White(2nd ed) "Traceable Temperatures", John Wiley & Sons, Ltd. 2001 ISBN0-471-49291-4, page 136
- Johnson, Mitchell P. & Gilson, Allan G. (August 2012). "Do Your Thermowells Meet the ASME Standard?". Flow Control. XVIII (8).