Determining flowrate through an existing valve 1

[CharlieH]

We are replacing a vent valve that keeps a steam header at 80#. 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.

Does this sound like a valid approach to estimate the flowrate through the existing valve or is the error using this equation too great? I suspect the formula is for liquid flow.

Any advice / suggestions would be appreciated.

Thanks in advance,
Charlie

 

[JimCasey]

Cv = Q / sqrt(dP) is the wrong equation for steam,it's for liquids, and if you are dropping 80 psig ( 94.7 psia) steam to atmosphere it is choked flow and you can't even use the "easy" steam equations.

Here is an online interactive steam valve calculator.

http://www.engineeringtoolbox.com/steam-control-valves-41_502qframed.html

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.