Valve Sizing Equation

Hi,

I ran into the following valve sizing equation for gases:

Q(SCFM) = 16.05 Cv ((P1^2 - P2^2)/ SGg T)^0.5 [Eq. 1]

Where P1, P2 and T are absolute pressures (inlet/outlet) and temperature, in PSIA and °R.
SGg is the gas specific gravity, that is, the ratio between the density of the gas and the density of air at the same conditions, effectively the ratio of molecular weights.

Some of the sources say that the equation is from standard FCI 62.1 but I cannot find it, even on FCI's website.

This equation does not appear on ISA 75.1.

Now, I know how to start from the basic equation for turbulent liquid flow,

Q(USGPM) = Cv ((P1 - P2)/SG) [Eq. 2]

Where SG is the specific gravity of the liquid, referred to water at 4°C, and derive this equation for gases, cited as "traditional" by Emerson:

Q(SCFH) = 59.64 Cv P1 ((P1 - P2)/P1)^0.5 (520/(SGg T))^0.5 [Eq. 3]

But, as much as I try, I cannot derive Eq. 1 from Eq. 2.

Does anybody know its origin?

Thank you

Sorry, I missed the square root on Eq. 2:

Q(USGPM) = Cv ((P1 - P2)/SG)^0.5 [Eq. 2]

This is the equation I use.

Thank you, Tom.

The equation you use is the same as Eq. 3 in my posting, after multiplying
59.64 × 520^0.5 and dividing by 60 to get SCFM instead of SCFH. We are on the same page.

I will have to follow this...good info.

What is the equation for calculating CV?

Hi NetNathan,

The most basic one (for non-choked turbulent liquids) is (same as Eq. 2 in my posting, rearranged):

Cv = Q(USGPM) / ((P1 - P2)/SG)^0.5

For an extensive explanation of many other cases, please refer to ISA 75.1

Saulo35 said:

Hi NetNathan,

The most basic one (for non-choked turbulent liquids) is (same as Eq. 2 in my posting, rearranged):

Cv = Q(USGPM) / ((P1 - P2)/SG)^0.5

For an extensive explanation of many other cases, please refer to ISA 75.1

Click to expand...

Does this cover gases also?

Yes it does. But first, you have to convert usual liquid volumetric units (USGPM) to usual gas volumetric units (SCGH, SCFM), work with the specific gravity considering your gas an ideal gas, etc. and finally you get Eq. 3 (where the temperature appears, for instance).

You can see Eq. 3 in Emerson's Valve Sizing Calculations (Trditional Method), Equation (A), Page 632.

And, other forms, also for gases, again, in ISA 75.1