Brooks Instrument Mfi Series - Manual (Page 52)

4-14
Brooks ® Mf Series
Section 4 Maintenance &
Troubleshooting
Installation and Operation Manual
X-TMF-Mfi-Mfx-MFC-eng
Part Number: 541B074AAG
August, 2009
The preload determines the initial force that is required to raise the valve
seat off the orifice and start gas flow. If the preload is insufficient, the valve
will not fully close and gas will leak through. If the preload is excessive, the
magnetic force generated between the plunger and stem will be insufficient
to raise the plunger and the valve will not open.
The air gap is the space between the top of the plunger and stem. The air
gap determines the force between the plunger and stem at a given voltage
and the total travel of the valve. If the air gap is too small, the plunger travel
may be insufficient to fully open the valve. Also, the magnetic force may be
too high for a given valve coil voltage. If the air gap is too large, the
magnetic force will be insufficient to raise the plunger and the valve will not
open.
Note: Prior to starting the valve adjustment procedure, check to insure that
the orifice is properly seated and that the valve parts are not bent or
damaged.
Adjustment Procedure (Normally Closed Valve)
(Refer to Section 5, Spare Parts for Spacer/Shim Kit)
a. Remove the electronics housing (6) from the controller. Insure that the
connector from the coil assembly (15) is properly reconnected to the
PC board after the electronics cover is removed.
b. Perform the electrical and gas connections to the controller following
the instructions in Section 2 of this manual. Use a clean dry inert gas,
such as nitrogen, for this procedure. Do not apply gas pressure to the
controller at this time.
c. Disassemble the control valve following the procedure given in Section
4-4A above. Note the number, locations and thicknesses of all spacers
(28A and 28B).
d. Decrease the preload of the valve by 0.005 inches by either removing a
0.005-inch small preload spacer or by adding a 0.005-inch large
preload spacer. Refer to Figure 4-3a.
e. Reassemble the valve following the assembly procedure in Section
4-4B.
f. Adjust setpoint for zero percent flow, apply normal operating pressure
and check for valve leak through by observing the output signal.
g. If the valve leaks through, increase the preload by 0.005" and go to
Step h. If the valve does not leak through, repeat Steps d, e, f and g.
h. Apply normal operating gas pressure and adjust setpoint for 100% flow.
Note: Due to possible heat capacity and density differences between
the test gas and actual process gas for which the MFC was sized, it
may be necessary to increase the inlet pressure to obtain proper
control at 100% flow.