I am looking for the equation for delta P across a control valve.  I am looking for non-choked, compressible, turbulent flow with or without fittings attached.  I have the equation to solve for C, but I was hoping someone has this equation re-worked for DP. 

I'm sure I know what you are asking. For turbulent non-compressible flow it would be Flow squared, times Specific gravity over Cv squared equals delta P.

For compressible flow you first have to determine the delta P to P1 ratio. Then Weight, Mass or Vapor basis and what kind of valve globe, ball or butterfly.

What I was trying to say before is that if you have an arbitrary inlet pressure and the xT, then you have xT=(P1-P2)/P1abs (since the xT is just a pressure drop ratio).  You should then have your pressure drop where the valve becomes choked.  

As far as the equation goes, I wouldn't break up Y at first, use Y as 0.667 to begin with, or leave it as Y.  When I went through the equation, I ended up with pressure drop on one side and W^2/(rho*C^2*N6^2*Y^2) on the other side, with the P1 term dropping off because it appears in the numerator and the denominator.  

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