Brooks Instrument 5851EM - Manual (Page 51)

4-13
Brooks ® Models 5965, 5851EM
Section 4 Maintenance
& Troubleshooting
Installation and Operation Manual
X-TMF-5965-5851EM-MFC-eng
Part Number: 541B122AAG
September, 2009
3. Determine Critical Pressure Drop
Critical pressure drop occurs when the outlet pressure (psia) is less
than half the inlet pressure (psia) or
If these conditions exist, the pressure drop (Dp) should be calculated
as follows:
Dp = Pressure drop (psi)
P
in
= Inlet pressure (psia)
If these conditions do not exist, pressure drop equals the inlet pressure
minus the outlet pressure.
Then Dp = 64.7 - 44.7 = 20 psi
4. Using the nomograph, locate the pressure drop (psi) on the vertical line
marked "Dp" (Point A).
5. Locate the Nitrogen equivalent flow rate (sccm Nitrogen) on the vertical
line marked "Q
Nitrogen
" (Point B).
6. Draw a line connecting Dp and Q
Nitrogen
and extend it to the baseline.
Mark this point (Point C).
7. Locate inlet pressure (psia) on the vertical line marked "P
in
" (Point D).
8. Draw a line connecting P
in
(Point D) and baseline (Point C) and then
extend this line to the vertical line marked D
o
(orifice diameter, inches)
(Point E).
9. This point on the line is the minimum orifice size for the given
conditions. If this point is between two orifice sizes, select the next
largest size orifice to ensure adequate flow. If the orifice selected falls
below .0013, choose .0013 size orifice.
For the example in Figure 4-5, the .007 size orifice would be selected.