The correct calculation of the safety valve discharge is a reasonable choice of the safety valve, and an important basis to judge the reliability of the. GB150-89 "steel pressure vessel", according to the state of different flow valve, puts forward the calculation formula, 2 kinds of discharge therefore, judge the safety valve is in a critical state of flow or subcritical flow state, is the prerequisite of the correct selection of capacity calculation formula.
At present, the safety valve critical pressure ratio value of 2 Views: ① codes are of the view that the safety valve critical pressure ratio and nozzle critical pressure ratio is the same, the value of which is 0.528 (1,2). ② many experts and researchers considered, the critical pressure ratio safety valve critical pressure ratio is less than the nozzle, the value of about 0.2 ~ 0.3 (3).
Critical pressure ratio RCR is the minimum flow section, the air flow to the local speed of sound when the inlet pressure ratio. The critical nozzle pressure can be determined by the ratio formula in theory. When the nozzle inlet pressure of less than or equal to the critical nozzle pressure ratio, due to the exit section is the sonic flow, the inlet pressure disturbance can not exceed the speed of sound than the surface, so the disturbance does not affect the nozzle flow. Exit section of the air pressure to maintain p2/p1=
RCR unchanged, the export section of the stream flow is still sound, relative displacement remain unchanged, that is w/wmax=1, at this time, the nozzle is critical or supercritical flow state (4). In addition to the nozzle, other structure of the critical pressure ratio is often required by the test, and to test to determine the critical pressure ratio of second critical pressure ratio, to distinguish.
Because of the complexity of structure safety valve, it is difficult to determine the minimum flow valve cross-sectional area at the air flow, thus unable to closure according to the minimum flow area at the speed of sound and accurately determine whether the safety valve critical pressure ratio. At present, the safety valve to determine whether the critical flow state is to determine safety valve discharge coefficient, as long as the discharge coefficient does not change with the pressure ratio
, the safety valve to achieve the critical flow state (3). The result is always measured with the pressure ratio safety valve discharge changes
, but when the safety valve pressure ratio less than 0.2 ~ 0.3, little change of safety valve discharge with pressure ratio, and people think that such small the change is caused due to measurement error, so as to determine the safety valve critical pressure ratio is about 0.2 ~ 0.3. Method for the determination of the safety valve critical pressure ratio of the test, the theory is based on the critical and supercritical flow condition, pressure ratio can not exceed the speed of sound surface disturbance, and the nozzle relative displacement remain unchanged, that is w/wmax=1. However, in a critical or supercritical flow state, the nozzle exit section of the flow is supersonic flow and the relative displacement w/wmax unchanged, on the contrary, if the relative displacement remains unchanged, will determine the export section of the flow is subsonic flow, the critical safety valve in the flow state is lack of theoretical basis and experimental evidence.
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