Gas flow through an installed valve - Average Specific Volume Approximation Method ASVAM

4 Gas flow through an installed valve - Average Specific Volume Approximation Method (ASVAM) 

This final method is based on the approximate methods for calculating compressible flow in a pipe, as described in Sections 6.8, 6.9 and Appendix 2, and in a valve, as described in Section 9.11.2. The major approximation made is that the specific volume of the gas in large stretches of pipework can be represented adequately by a notional average specific volume. The benefit of ASVAM is that it can be programmed to avoid implicit loops, and hence the associated problems of convergence in the main equations, while maintaining a very reasonable accuracy. 

Having found Kt, we may estimate the critical pressure ratio, Pic/P, that will lead to sonic flow at the exit from the downstream section of pipe from polynomial equation (6.66)  

Having derived the ratio of supply vessel to pipe outlet pressures, p /p), we may estimate the notional average specific volume throughout pipe and valve from equation (6.62), namely  

We shall use the long-pipe approximation to the compressible flow equation (cf. equation (6.65)) in order to calculate the flow before sonic conditions are reached in the valve  

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