Which Relative Density to use in ANSI/ISA-75.01.01 Equations for Control Valve sizing??

[dean427]

Hi all.

Hoping someone can help me with the following query:

ANSI/ISA-75.01.01 equations for incompressible fluids make use of the fluid relative density. The symbol used in the equations is "rho1/rho0" and in section 5 it defines "rho1" as the density of the fluid at P1 and T1. Therefore, is it correct to assume the calculation of relative density used in the standard is simply...

relative density = [fluid density at flowing conditions (i.e. @ P1 & T1)] / [1000]?

This may seem like a stupid question, however I am used to dealing with relative density of heavy hydrocarbon liquid streams where I would quote a relative density at the same temperature & pressure as water i.e. @ 4degC and 1 atm or 15 degC & 1 atm...i.e. I would work with a specific gravity [4C/4C] or specific gravity 15C/15C].

However...I am currently dealing with a cold NGL liquid stream which would be vapour at standard conditions therefore to quote a relative density on an equivalent basis would not work.

That being said, my process simulator (Honeywell UniSim Design) contains a property calculation (in the Refinery stream Correlations set) for specific gravity at 15/15C or at 60/60F, which seems to use the "Std. Ideal Liq. Mass Density [kg/m3]" property to calculate the specific gravity at 15/15C or 60/60F.

Depending on which method I use the relative density will vary only slightly - i.e. between a relative density of 0.5 or 0.47 - so it does not make any meaningful difference to the rating calculations (soaked up in the noise of all uncertainties on compositions, fluid props, non-ideality etc etc) however I feel that I should know this basic stuff and select the correct SG in the calculation just for the sake of professionalism.

Hopefully someone with more experience can clarify but any help would be much appreciated.

Thanks!

 

[PaoloPemi]

ISA 75.01 adopts the concept of CV as defined by Masoneilan as the number of U.S. gpm of 60°F water which will flow through a valve at a specified opening with a pressure drop of 1 psi across the valve... accordingly my copy of ISA 75.01 gives for "rho1/rho0" the value of 1.0 for water at 15 C (~60 F) ,
for these calc's I adopt a thermodynamic library (Prode) but the same with a simulator, you can simply calculate the relative density to water at 15 C (see the definition of CV)

 

[dean427]

Thanks for your post PaoloPemi. Your mention of Masoneilan prompted me to check their sizing handbook and they are explicit in their note that the "rho1" is at flowing fluid temperature. Therefore the SG is simply the flowing fluid density at upstream conditions divided by water at 15 degC

Thanks