Frequency in thermowells
In 1995, an accident caused by a broken thermowell at the Monjo nuclear plant reactor caused severe damage to nearby facilities. Thermovols located in a line carrying sodium coolant were subjected to severe vibration and eventually broke. As a result, 3 tons of sodium coolant came out of the pipes. When in contact with air, liquid sodium reacts spontaneously with oxygen and moisture in the air, filling the room with sodium smoke and producing a temperature of several hundred degrees Celsius.
The intense heat generated damaged several steel structures in the room. Post-accident investigations were conducted due to the damage caused by the thermowell as the main cause, and this accident led to the development of the Performance Test Codes (PTC) 19.3 (2010) to be followed by the manufacturers of the thermola. These codes apply the maximum vibration frequency limit to thermogels.
ASME PTC 19.3 TW-2010 is divided into dynamic and static calculation results. For gas media, the previous limit frequency , r max = 0.8, from PTC 19.3-1974, is still valid. For liquid media, in many applications, the newly introduced new limit frequency r max = 0.4 can be used for in-line amplification. Evaluation of dynamic results is performed using NSC damping coefficient. For gaseous media, a characteristic value of NSC is> 2.5.
Fluids usually have an NSC <2.5. Whether the frequency ratio r <0.8 can also be used as an evaluation limit with the liquid process medium is determined by examining the allowable stresses in the thermocouple with respect to the actual resonance pressures. In addition, the strength of the thermowell material is evaluated due to the flexural fatigue stress in the heat closure zone. The results of static analysis according to ASME PTC 19.3 TW-2010 are produced from the maximum allowable process pressure (depending on the process temperature and geometry of the thermola) and the bending stress in the thermola root area. Bending stresses are caused by the accident current in the thermocouple and depend on the protective length of the flange adapter.
For built-in thermowells, the Dittrich / Klotter calculation method should be used as this construction is not covered by ASME PTC 19.3 TW-2010. These are compared to allowable pressures for thermowells and safety factors.
Changes in thermowell structure for better performance:
At the maximum frequency limit, maximum r, for “in-line” – or main resonance, the following structural changes must be made:
A) Shorten the length
This is the most effective method (and the method proposed by ASME PTC 19.3 TW-2010) to improve the frequency ratio r.
- B) Increase the tip diameter
As the tip diameter increases, the fs natural frequency decreases, and the r frequency ratio also optimizes.
- C) Anchor support
Anchor thermostat support is usually not recommended by ASME PTC 19.3 TW-2010 and is outside the scope of the ASME code. At the request of the customer, the anchor can be used, in accordance with the exact specifications of the customer and the components of the side thermulation process under the anchor connection in accordance with the design and calculation criteria of ASME PTC 19.3 TW -2010, without falling. In the range of ASME PTC 19.3 TW-2010. The operator is responsible for the rigid support of the anchor in the adapter, this may mean the need to reprocess the adapter.
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