Calculating wake frequency for thermowell we must need, fluid velocity with other process data. But when a thermowell is to be installed at the bottom of a vessel where some fluid will enter? it will be stored there for process reason, and then transfer some other place via gravity or pump. Now in such case i don't have the flow rate/velocity of the fluid into the vessel. Note please vessel will be agitated. Now in that case how can i calculate wake frequency for the thermowell installed into the vessel? should I consider the in late /out late/flow rate of the vessel or the rpm of the agitator? Any suggestion guys?
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Depending on the location of the thermowell it can be exposed to inlet, outlet or fluid velocity inside the vessel. Pl. confirm which will be more applicable in this case. For example, if the thermowell is installed in opposite wall of the vessel, away from the inlet or outlet, it will be mainly exposed to fluid velocity inside the vessel due to the agitator rotation. Or with the help of process engineer you can calculate the 3-velocities and take the maximum of them. Most of the time there is no need to do any wake frequency calculation for the thermowell inside the vessel. ASME PTC 19.3 TW-2010, gives the following conditions when wake frequency calculation is not required.
For slow flowing process fluids, there is not enough energy transferred from the process fluid to the
thermowell to cause fatigue failure. If the following conditions are met, there is no need to conduct
frequency limit calculations as the risk of thermowell failure is negligible.
1. Process Fluid Velocity, V < 0.64 m/s (2.1 ft./sec)
2. Wall Thickness, (A - d) ≥ 9.55 mm (0.376 in)
3. Unsupported Length, L ≤ 0.61 m (24 in)
4. Root and Tip Diameter (A and B) ≥ 12.7 mm (0.5 in)
5. Maximum Allowable Stress, S ≥ 69 Mpa (10 ksi)
6. Fatigue Endurance Limit, Sf ≥ 21 Mpa (3 ksi)