How To Verify the Orifice Selection A
endix B: Valve Orifice Selection
62
Example 3: Using 30% Hydrogen (H
2
) and 70% Nitrogen (N
2
)
Using a mixture of gases, such as 30% hydrogen and 70% nitrogen, may impact the orifice size.
Following the example in
How To Verify the Orifice Selection,
page 58, with this gas mixture:
1.
Calculate the orifice sizing factor, using the equation page 60.
The standard density of H
2
is 0.0899 and N
2
is 1.250, so the orifice sizing factor is:
1.250
(0.0899)(0.3) + (1.250)(0.7)
1.18 Orifice Sizing Factor
=
Since the gas mixture is largely nitrogen, the orifice sizing factor is close to 1.
2.
Calculate the valve index number, using the equation page 60.
(175) x (1.18) = 207 valve index number for 30%H
2
/70%N
2
3.
Use the index number and the maximum flow rate to determine the orifice number from
Figure 24, page 59.
Using the maximum flow rate in our example of 1000 sccm and an index number of 207,
the correct orifice number would be number 2.
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