ASME PTC 19.3 TW-2010 pdf download.Thermowells.
4-1 CONFIGURATIONS Figure 4-1-1 shows a schematic diagram of a ther- mowell, along with its characteristic dimensions. Typical thermowell attachment confgurations include threaded, socket weld, weld-in, lap-joint (Van Stone), and integral-fanged as shown in Figs. 4-1-2, 4-1-3, and 4-1-4 (see also Table 4-1-1). These fgures are representa- tive of common practice but do not display all allowable attachment confgurations. The selection of a specifc attachment method is subject to the governing piping or pressure vessel code. Use of ball joints, spherical unions, or packing gland installations are not permissible in Performance Test Code applications. The dashed line in Fig. 4-1-1 indicates the support plane, which is an imaginary extension of the supporting-struc- ture surface that passes through the shank of the ther- mowell. The unsupported length, L, is calculated as the distance from the tip of the thermowell to the intersection of the thermowell axis with this surface. For thermowells mounted on fanges or welded into weld adaptors, the support plane will be a fat plane. However, for thermo- wells mounted by direct welding into a pipe wall, the sup- port plane will actually be a curved surface with the same curvature as the inner pipe wall. For this case, the support plane should be approximated as a plane located at a dis- tance from the thermowell tip equal to the largest actual distance from the tip to any point on the true curved sup- port surface. For thermowells welded to a fange or pipe wall at an angle, the support plane will not be normal to the thermowell axis. For nonstandard attachments, this Standard covers the design requirements of the thermowell only. The designer shall account for the support compliance of the attachment (refer to subsection 6-6), and the attachment method shall meet all the requirements of the governing piping or pressure vessel code.
For the purpose of defning L and A, the support plane shall also be defned (see subsection 6-7). The root of the thermowell is located where the thermowell shank makes a transition to (a) a machined transition to a fange, socket weld col- lar, or threaded section of the thermowell (b) a weld-joint transition to other piping components The Standard also applies to step-shank thermowells within the dimensional limits given in Table 4-2-1, where L S is the length of the reduced-diameter section of ther- mowell shank, in addition to the dimensions defned for Table 4-1-1. Refer to Fig. 4-1-1. Calculations should be made using the nominal dimensions provided that a corrosion allowance is not used (see subsection 6-2) and that the thermowell is fabricated with manufacturing tolerances of ±1% for lengths L and L S and ±3% for diameters A, B, and d. If tolerances for A, B, or d are not met, calculations shall be made according to subsection 6-2, using as the cor- rosion allowance the linear sum of the actual toler- ance and any corrosion allowance. If tolerances for L or L S are not met, calculations shall be made assum- ing that the lengths L and L S each equal the nominal length plus the respective manufacturing tolerance. External pressure calculations shall be made based on the minimum material condition, as discussed in subsection 6-13. This Standard applies to thermowells with an as-new surface fnish of 0.81 µm (32 µin.) Ra or better. Stress lim- its given in subsection 6-12 are not valid for thermowells manufactured with rougher surfaces.
6-1 GENERAL CONSIDERATIONS 6-1.1 Overview of Design Criteria Thermowells shall be designed to withstand static pressure, steady-state fuid impingement, turbulence, and dynamic excitation due to von Karman vortices. Excitation by structure-born vibration is a possibility and should also be considered, but is not addressed by this Standard, since this type of excitation is determined by the design and support of the entire piping system. Consideration of these loads on a mechanical model of the thermowell results in pressure and velocity lim- its due to the combination of steady-state and oscilla- tory forces acting on the thermowell. In evaluating an existing design or in designing a thermowell for given applications, the complete range of operating conditions for the thermowell, from start-up to emergency condi- tions, shall be considered. Factors that reduce the ther- mal mass of the thermowell and measurement errors are those that tend to reduce strength. Thermowell design consists of achieving accurate and reliable temperature measurement without compromising mechanical integ- rity or fuid containment. In all cases, the mechanical strength requirements shall control. 6-1.2 Optimization of Thermowell Design Proper design of a thermowell requires that the sensor mounted inside the thermowell attain thermal equilib- rium with the process fuid. Thermal modeling of the sensor response is outside the scope of this Standard (refer to the latest version of PTC 19.3 for guidance). This Section briefy summarizes general design rules that will optimize the sensor performance within the constraints of the mechanical strength requirements.