Critical flow calculations -- proper fluid properties

[denvercheg]

Hello, I have done numerous critical flow calculations for both restriction orifice sizing and tube failure scenarios. I have typically based my calculations on those in Crane's handbook, using upstream properties (density, pressure) to calculate maximum flow. I was recently given an equation to use that appears based on sonic conditions (at P2 as calculated by critical pressure ratio) instead of upstream conditions; I have never seen the equation or used it, and as it obviously will generate a lower flow at the lower pressure and density, am reluctant to consider it an appropriate basis for my calculations. However, I don't want my calculations to be needlessly conservative.

Has ANYONE ever heard of or used conditions at critical flow for calculating flow through an orifice?

 

[zdas04]

Nope, and I don't beleive that it makes sense as long as the downstream pressure is low enough to allow critical flow. You get the same flow rate for P2 from the maximum value (which can be as high as 60% of upstream pressure) down to zero so using P2 conditions to determine flow rate or fluid properties would give you ambigious results.

David