MKS 640B - Using Different Gases

How to Verif
y
the Orifice Selection Appendix B: Valve Orifice Selection
58
Using Different Gases
The valve orifice selection data is based on nitrogen gas. If you will be using a gas other than
nitrogen, you need to compensate for the density difference between nitrogen and your process
gas before you can select the appropriate valve orifice.
1.
To calculate an orifice sizing factor, use the equation:
N Density
Gas Density
Orifice Sizing Factor
2

Appendix C: Gas Density Table, page 59, lists the standard density for common gases.
2.
To calculate the valve orifice index number for the new gas:
(valve orifice index number for nitrogen) x (orifice sizing factor) = new valve index number
3.
Proceed with step 3 of How to Verify the Orifice Selection, page 56, to determine the
correct orifice size.
Example 2: Using Helium (He)
1.
You want to control a vacuum process at a pressure of 0.5 psia, with a flow rate of 2000
sccm of helium. Your inlet pressure is 15 psia, giving a differential pressure (delta P) of
15 psi, resulting in an uncorrected Index Number value of 90.
The standard density of He is 0.179 g/liter and the standard density of N2 is 1.250 g/liter,
so the equation is:
ට
ଵ.ଶହ଴
଴.ଵ଻ଽ
ൌ 2.6 Orifice Sizing Factor
2.
Calculate the valve index number, using the equation above.
(90) x (2.6) = 234 corrected valve index number for He
3.
Use the index number and the maximum flow rate to determine the orifice number from
Figure 24, page 57.
Using the maximum flow rate of 2000 sccm (in our example) and an index number of
234, the correct orifice number would be 3.