There are instances where higher pressure steam may enter the line.  In this case, someone manually opens this valve to reduce the pressure.
I requested process data from the client and was told the flowrate on the particular line of interest is "however much 80# saturated steam can flow through a 4" globe valve that is 25% open."
I found the Cv for the existing model valve and intended to use it and Cv = Q / sqrt(dP) to determine the flowrate.
A 4" globe control valve should have a Cv of somewhere around 200. This varies more than you would think. The Fl will be around 0.9.  If God smiles upon you the Cv will be on the nameplate of the valve or you can identify it well enough to look it up.   If it has equal-percent trim and it is at 25% of travel, then the Cv at that point is only around 15. Equal-percent curves vary, too.
I ran the numbers on my Valtek sizing program for a 4" Mark1 globe valve with =% trim, and I get 3400#/H with 80 psi in, atmospheric out, with 326 degree steam, at 24% of valve travel. it has a full-open Cv of 179, but the Cv at theses conditions is 22.385.

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