I am recient college graduate (University of North Dakota 2009).  I am working in the nitrogen fertilizer business and one of the tasks given to me is to perform a 120 # steam balance over our major users.  I am attempting to figure out how to calculate the flow through a pressure control vale that regulates the 120 # head pressure.  The valve vents directly to the atmosphere.  I am confused when I read about "critical pressure drop" does this mean the discharge at the point of exit of the valve is not actually at atmospheric pressure?  Also, I cant find anything on how to calculate this "critical pressure drop" just that its about 42 % of the inlet pressure.  So, what I am looking to do is to calculate the mass flow rate of 120 # steam that is being vented based on valve position and I cant find any equation to aid in this calculation.  This may sound like a lot but I could really use some guidence on this.  Thank you so much to those who respond.

I have use the "ISA handbook of control valves" for my sizing of control valves. They have a good discussion of choked or critical flow, and also 2-phase flow and cavitating and flashing flow.

Other than Emerson, there are also texts by the valve vendors , such as Fisher, that use their proprietary coeficients, but to be non-discrimantory, I use the ISA method.

For compressible choked flow, such as dry steam above 15 psig inlet pressure, the max flow will be a function of the valve Cv ( at current % open) and the valve throat's "Xt". The Xt is a representation of the degree of oblique shock waves formed at pressure ratios less than critical, and will vary according to valve internal geometry.

 Some valves , such as CCI Self drag valve, have no acoustic choking, so Xt=1.0, but a typical globe valve has an Xt= 0.85, and a streamlined ball valve may have a Xt=0.15 . So, using the ISA equations, the critical presure ratio is not simply a function of ratio of heat capacities but is also a function of internal geometry. 

critical pressure drop is the condition where you reach the maximum flow at specified input conditions, for gas flow it means that in some section of the valve fluid speed is near sound speed (the same condition as the critical flow in piping), the difficult is that each valve has a different internal geometry so you need (as in safety valves) to ask the manufacturer for some parameter (in ISA it's XT or something similar) to calculate critical pressure drop. Usually manufacturers distribute also software which include these parameters, for a good discussion see the IEC 60534.

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