Critical pressure ratio
Correct calculation of emissions is an important basis to reasonable select safety valve and judge its reliability. GB 150-89 "Steel Pressure Vessels", according to different valve flow condition, it proposes two kinds of displacement formula, so to determine safety valve is in a critical flow state or a sub-critical flow state, is premise of the correct selection of displacement formula.
Recently There are two kinds of view of safety value critical pressure ratio: ① many norms are the view that the critical pressure ratio is the same with nozzle critical pressure ratio, its value is 0.528 [1,2]. ② Many experts and researchers believe that the safety valve critical pressure ratio is less than the nozzle critical pressure ratio, its value is about 0.2 ~ 0.3 [3].
critical pressure ratio rcr is defined as the out inlet pressure ratio when air flow rate gets to the local sound speed in the minimum flow channel cross-section. The nozzle critical pressure ratio can, in theory, determined by a formula. When the nozzle out inlet pressure ratio is less than or equal to the critical nozzle pressure ratio, due to the outlet cross section is the sound speed flow, a out inlet pressure ratio disturbance can not exceed the speed of sound surface, so disturbances can not affect the flow inside the nozzle. The outlet cross-section air pressure with the maintenance of p2/p1 = rcr unchanged, the outlet cross-section air flow is still the sound speed flow, the relative displacement remain unchanged, that is, W / Wmax = 1, so, the nozzle is in a critical or super critical flow state [4]. In addition to nozzle, other structure critical pressure ratio often requires testing to determine while the critical pressure ratio determined by test is also said as second critical pressure ratio, which is provided for difference.
As the complex valve structure, it is difficult to determine the minimum flow channel cross-sectional area air flow rate, and thus can not be accurate to determine the safety valve critical pressure ratio under the minimum flow channel closure area with the sound speed. At present, the methods of determining safety valve whether achieves the critical flow state is to mensurate safety valve discharge coefficient, considering that the discharge coefficient is not changed with pressure ratio, the safety valve reaches a critical flow state [3]. The result is always measured with the pressure relief valve displacement and change the ratio only when the safety valve pressure ratio is less than 0.2 ~ 0.3, the safety valve displacement changes smaller with pressure ratio, and people think that such small change is caused by measurement error, and thus to judge the fall lift safety valve critical pressure ratio is about 0.2 ~ 0.3. This test measured the critical pressure ratio approach, its theory basis is under the critical and super critical flow states, pressure ratio can not exceed the speed of sound surface disturbance, leaving the relative displacement of nozzle remains unchanged, that is, W / Wmax = 1. However, in critical or super critical flow state, the nozzle outlet cross-section flow is sonic flow and make the relative displacement flow W / Wmax remain unchanged, on the other hand, if the relative displacement remains unchanged, we determine the outlet cross-section flow has been is the sonic flow, safety valve in a critical flow condition with a lack of theoretical basis and experimental evidence.
