ASME PTC 19.3 TW:2016 pdf free download – Thermowells
Figure 4-1-1 shows a schematic diagram of a thermowell, along with its characteristic dimensions. Typical thermowell attachment configurations include threaded, socket weld, weld-in, lap-joint (Van Stone), and integral- flanged as shown in Figs. 4-1-2, 4-1-3, and 4-1-4 (see also Table 4-1-1). These figures are representative of common practice but do not display all allowable attachment configurations. The selection of a specific 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- structure surface that passes through the shank of the thermowell. 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 flanges or welded into weld adaptors, the support plane will be a flat plane. However, for thermowells mounted by direct welding into a pipe wall, the support 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 distance from the thermowell tip equal to the largest actual distance from the tip to any point on the true curved support surface. For thermowells welded to a flange 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 defining L and A, the support plane shall also be defined (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 flange, socket weld collar, 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 thermowell shank, in addition to the dimensions defined 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 and −0% with no upper limit for t. If tolerances for A, B, or d are not met, calculations shall be made according to subsection 6-2, using as the corrosion allowance the linear sum of the actual tolerance and any corrosion allowance. If tolerances for L or L S are not met, calculations shall be made assuming 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 finish of 0.81 ?m (32 ?in.) Ra or better. Stress limits given in subsection 6-12 are not valid for thermowells manufactured with rougher surfaces.