Orifice area critical flow

Critical and Subcritical Flow - The maximum vapor flow through a restriction, such as the nozzle or orifice of a pressure relief valve, will occur when conditions are such that the velocity through the smallest cross-sectional flow area equals the speed of sound in that vapor. This condition is referred to as "critical flow" or "choked flow . 

Calculate individually the orifice area required to pass the flashed vapor component, using Equation (5a), (3b), (4), (5), or (6), as appropriate, according to service, type of valve and whether the back pressure is greater or less than the critical flow pressure. 

Calculate individually the orifice area required to pass the unflashed hquid component, using Equation (8). The pressure drop term Pj should be made equal to the set pressure minus the total back pressure developed by the vapor portion at critical flow pressure, except when the critical flow pressure is less than the calculated total back pressure (superimposed plus built-up), considering the combined liquid and vapor flow. In the latter case, P should be made equal to set pressure minus the calculated total back pressure. 

Calculations of Orifice Flow Area using Pressure Relieving Balanced Bellows Valves, with Variable or Constant Back Pressure. Must be used when backpressure variation exceeds 10% of the set pressure of the valve. Flow may be critical or non-critical for balanced valves. All orifice areas. A, in sq in. [68]. The sizing procedure is the same as for conventional valves listed above (Equations 7-10 ff), but uses equations given below incorporating the correction factors K, and K,, . With variable backpressure, use maximum value for P9 [33a, 68]. 

Calculate relieving area by applicable equation for critical or non-critical flow, using tlie flow rate determined in (3) above. (See Equation 7-10 and following). The area actually selected for orifice of safety type valve must have orifice equal to or greater than calculated requirements. For a rupture disk application, the full free open cross-sectional area of pipe connections in inlet and exit sides must be equal to or be greater than the calculated area. 

Perforated plates are defined by a critical flow rate above which the orifice operation is asynchronous and the liquid flow in the sparger region is relatively unstable. As the hole spacing decreases, the critical flow rate decreases as well. At the same time, perforated plates require a minimum pressure drop in order to achieve uniform orifice activity. In other words, a critical flow rate is also created at the lower end such that a lower flow rate would lead to instability as well (Kang et al., 1999 Ruzicka et al., 2003 Su and Heindel, 2005a). This effect would produce additional complications in making comparative analysis between research works using different open area ratio adjustment methods. 

The general equation to estimate the orifice area for critical flow is... 

Calculations of Orifice Flow Area for Conventional Pressure Relieving Valves, and Flow is Critical (sonic) Through Part of Relieving System, i.e., backpressure is less than 55% of the absolute relieving pressure (including set pressure plus accumulation). See Figure 7-7A, use... 

Obtain an expression for the maximum flow for a given upstream pressure for isentropic flow through a horizontal nozzle. Show that for air (ratio of specific heats y = 1.4) the critical pressure ratio is 0,53 and calculate the maximum flow through an orifice of area 30 mm2 and coefficient of discharge 0.65 when the upstream pressure is 1.5 MN/m2 and the upstream temperature 293 K,... 

Arching is caused by the outlet size being less than the critical arching size for the strength of the bulk material in the particular hopper neck construction. Air-retarded flow is the consequence of the limited rate of failure of the unconfined surface of the material. However, the solution required both a high flow rate and a bulk flow condition that is not excessively dilated. The surface area of failure demand for this latter function would require an excessively large orifice cross-section. 

As we found for separation performance described above, i e-similarity is not critical for the pressure drop, either. In Chap. 4 we found that many of the empirical models for cyclone pressure drop only contain the ratio of inlet to outlet areas, implying that Eu will be the same between geometrically similar cyclones, irrespective of f e-similarity. Obviously, as was the case for separation efficiency, this is only valid when Re is high enough that the friction factor is essentially independent of Re. This should come as no real surprise since the same situation holds true for most flow devices (such as pipes, elbows, orifices, contractions and expansions, etc.) that operate in fully developed turbulent flow. In such cases, pressure loss can be characterized by the formula ... 

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