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mbac123 (Mechanical) (OP)
11 Jan 10 20:17
Hello,

I am trying to compute the pressure drop across a 20 inch butterfly valve at various valve positions for throttling a supercritical carbon dioxide stream in a pipe. I found an equation for calculating the pressure drop for compressible fluids across valves in an Emerson manual (the equation is on page 120):
http://www.documentation.emersonprocess.com/groups/public/documents/book/cvh99.pdf

However, I am unsure what value I should use for the expansion factor. I see in the manual it also talks about the critical pressure drop, but I do not know how to determine this point.

Scotsinst (Petroleum)
12 Jan 10 3:29
I recommend you download a vendor program (Emerson, Valtek, Metso, Masoneilan).  Use the FL and Xt factors for your particular make of butterfly valve (noting how the FL factor changes with opening) and you can calculate the valve openings (you will need to iterate the FL factors for different openings to get improve accuracy) for different flowrates.
It sounds like your upstream pressure is constant and you wish to know the pressure drop across the valve at different openings. Note that depending on the system hydraulics the downstream pressure will change based on flowrate which will affect valve DP.  You need to understand the system hydraulics to know what process conditions the valve will encounter.
mbac123 (Mechanical) (OP)
12 Jan 10 12:27
My problem is that my valve requires very high performance (Temperature of 750 deg. C and Pressure of 220 bar), and Emerson cannot manufacture such a valve. However, I still want to use their compressible flow equation since I have not found any other equation for modelling the pressure drop across the valve. Is it common practice to have these FL and Xt factors from various manufacturers?
To clarify, my upstream pressure and flowrate will not be constant if the valve is being used to throttle. If I cannot use these set of Emerson equations is anyone aware of any other equations that I could apply?
Helpful Member!  PaoloPemi (Mechanical)
12 Jan 10 13:03
possibly Neles software has built-in custom equations, don't know other vendors but I presume most do use the ISA correlations, these require to specify parameters as XT, FL CV/aperture etc. these are not easy to define as they are related to the specific internal geometry of the valve, as suggested by Scotsinst you can use software from Masoneilan, Fisher etc. which include these parameters for their models, when the vendor doesn't provide specific software I use Prode Valves (see http://www.prode.com ) and I adopt the values suggestd in ISA standards.  

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