Leakage during shutdown of high-temperature and high-pressure flanges in residue hydrogenation
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During the shutdown of the residue hydrogenation unit, leakage occurred at the high-temperature and high-pressure flanges as the temperature and pressure were reduced. Experts, please analyze the reasons. Material 321, operating temperature 375°C, pressure 17 MPa. Recently, when the operation was stopped and the temperature was reduced to 220°C at 7 MPa and 45°C at 4 MPa, leaks occurred in each of the two flanges respectively. There have been a few leaks before as well.
Sealing leaks occur in high-temperature and high-pressure static equipment, mainly at large-sized gaskets. In addition to the regular inspection of the sealing system (gaskets, bolts, materials), special attention should be paid to the rate of pressure reduction in the process system; too rapid a pressure reduction can lead to gasket leakage or cracks in the sealed joints. We’ve made observations in this regard: there is no leakage when driving or under normal use.
1. The flange, bolts, and gaskets, as part of the sealing assembly, represent variables among themselves; numerous such variables reach a dynamic equilibrium under certain operating conditions. This equilibrium point is above the minimum required sealing pressure, preventing leakage; otherwise, leakage occurs. 2. Under high temperatures and prolonged operation, the sealing assembly experiences \"creep relaxation,\" resulting in a gradual decrease in the sealing pressure, although this decline is slow under stable operating conditions. 3. When operation is halted, the temperature of the medium inside the pipe drops faster than the temperature changes in the surrounding environment; as a result, the flanges cool down more rapidly than the bolts, and the sealing pressure drops sharply. This makes it easy for leaks to occur in seals where the sealing pressure is balanced at lower points. 4. By understanding the relationship between sealing components and temperature, it becomes easier to comprehend why leaks occur when operation is stopped. 5. To mitigate or prevent leaks that may occur during shutdowns, certain management measures can be taken, such as: 5.1. Reducing pressure first before gradually lowering the temperature; 5.2. Removing the insulation from some flanges before shutting down, so that the bolts are exposed to the atmosphere; 5.3. Checking the tightness of the bolts after pressure is reduced but before the temperature is lowered, and tightening any loose bolts as necessary. 6. Austenitic stainless steel should be used as the material for sealing components, as it is more sensitive to temperature changes
1. The cooling rate should not be too fast. 2. The rate of blood pressure reduction should not be too fast. 3. Is the material selection for the sealing gasket correct?
With similar devices, the pressure is high during the shutdown process
Flanges, gaskets, and bolts have different cooling rates during the shutdown and cooling process; if the cooling rate is slower, remove the necessary insulation