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Safety valve critical pressure ratio concept
The critical pressure ratio rcr refers to the ratio of the inlet pressure to the inlet velocity at the local velocity of sound at the minimum flow path cross section. The critical pressure ratio of the nozzle can theoretically be completely determined by the formula. When the nozzle outlet pressure ratio is lower than or equal to the critical pressure ratio of the nozzle, because the outlet section is already sonic flow, inlet pressure ratio of the disturbance can not exceed the sonic surface, so the disturbance can not affect the nozzle flow. The pressure in the outlet cross-section is maintained at p2 / p1 = rcr, the flow in the outlet cross-section is still sonic flow and the relative displacement remains unchanged, ie W / Wmax = 1, at this time the nozzle is in critical or supercritical flow status. In addition to the nozzle, the critical pressure ratio of other structures often need to be determined by the test, and the critical pressure ratio determined by the test said the second critical pressure ratio, for the capital difference. Due to the complexity of the safety valve structure, it is difficult to determine the flow velocity of the relief valve at the minimum cross-sectional area of ​​the relief valve. Therefore, it is impossible to accurately determine the critical pressure ratio of the relief valve based on whether or not the sonic velocity is reached at the minimum flow passage cutoff area. At present, the method of judging whether the safety valve reaches the critical flow state is to measure the displacement coefficient of the safety valve, and the safety valve reaches the critical flow state as long as the displacement coefficient does not change with the pressure ratio. The measured result is that the displacement of the safety valve always changes with the change of the pressure ratio, except that when the pressure ratio of the safety valve is less than 0.2~0.3, the displacement of the safety valve changes little with the pressure ratio, and it is considered that this is smaller The change is due to measurement error, thus judging the critical pressure ratio of all-purpose safety valve is about 0.2 ~ 0.3. This test method to determine the critical pressure ratio of the safety valve, the theoretical basis is that in the critical and supercritical flow conditions, the pressure ratio can not exceed the speed of sound than the disturbance surface, leaving the nozzle relative displacement remains unchanged, that W / Wmax = 1. However, in the critical or supercritical flow regime, the flow at the nozzle exit cross-section is sonic and the relative displacement W / Wmax remains constant. Conversely, if the relative displacement remains constant, the flow at the exit cross-section has been judged Is the speed of sound velocity, the safety valve is in a critical state of flow is the lack of theoretical basis and test proved.