4 replies to this topic

[mrbabu]

when calculating a control valve for gas flow (ISA S75 formulation) with PRODE PROPERTIES

in EXCEL should I use the macros

=EStrGCp(1,t,p)

=EStrGCv(1,t,p)

which return the REAL Cp, Cv for gas mixtures calculated with Soave or Peng Robinson EOS

or the methods which return the ideal gas cp and cv ?

[PaoloPemi]

there are ISA papers discussing the errors introduced adopting isentropic exponents calculated with a EOS such as Soave Redlich Kwong or Peng Robinson,

for specific applications you may adopts a method such as

Enhanced Valve Sizing Methods for Fluids Exhibiting Real Gas Behavior, ISA 1992, Riveland

or the rigorous formulations allowable with Prode Properties

[mrbabu]

thanks,

in control valves formulation is cp/cv related to evaluation of critical flow condition ?

[PaoloPemi]

it is, but also to expansion factor Y

in my copy of ISA 75.01 Expansion factor Y is defined as

expansion factor accounts for the change in density as the fluid passes from the valve inlet

to the vena contracta, it also accounts for the change in vena contracta area

as the pressure differential is varied,

the method requires several parameters which depend from valve's geometry.

[mrbabu]

thanks Paolo,

I'll review the ISA formulation and report doubts, if any.