LowFlow Mark 708SH Series Short Pattern 1/4 - Manual (Page 4)

If there is valve movement at less than 3 psig:
thread the stem connector out of the actuator stem,
wrenching on the locked stem nuts and keeping
the actuator stem from turning to increase spring
preload. If there is no valve movement at 3 psig:
thread the stem connector into the actuator stem,
wrenching on the locked stem nuts and holding on
to the actuator stem to keep it from moving, to de-
crease spring preload.
Actuator Maintenance
Disassembly and Inspection of Actuator
Note: The actuator stem has two retaining rings. The pur-
pose of these rings is to maintain compression on the
springs (to keep them in place) should the actuator be
removed from the valve bonnet. The upper ring functions
with reverse action and the lower ring functions with di-
rect acting actuators.
Follow instructions under Maintenance section to 1.
remove valve from line.
Remove the actuator as described in “Removing 2.
Valve Actuator”.
Direct Acting Actuator (ATC) 3. : separate the cases
by removing the bolts. Use a wrench on the flats
of the actuator stem to prevent it from turning and
remove the bolt and lockwasher. Lift off the dia-
phragm washer, diaphragm, diaphragm plate, and
ThredSeal. Remove the springs. If the actuator
stem is to be removed, remove one retaining ring
and push the actuator stem out. Remove and dis-
card the o-ring and two backup rings.
Reverse Acting Actuator (ATO 4. ): separate the cases
by removing the bolts. Remove the springs. Use a
wrench on the flats of the actuator stem to prevent
it from turning and remove the bolt and lockwasher.
Lift off the diaphragm plate, diaphragm, diaphragm
washer, and,ThredSeal. If the actuator stem is to be
removed, remove one retaining ring and push the
actuator stem out. Remove and discard the o-ring
and two backup rings.
If there is a need to clean the parts, use a solvent 5.
that will not harm Buna-N elastomers, such as Stod-
dard solvent.
Reassembly of Actuator — 14M
Replacement of the actuator stem seal is recom- 1.
mended. The o-ring and back–up rings are stan-
dard Buna-N, size 112. Lubricate and install the two
back-up rings and the o-ring into the cavity in the
lower case. The back-up rings are installed on ei-
ther side of the o-ring.
The retaining ring closest to the top end (bolted) of 2.
the actuator stem must be in place. The retaining
ring nearest the other end ((with wrench flats) must
be removed.
3a. Direct Acting Actuator (ATC): attach the following
parts to the top of the actuator stem in the order
indicated: ThredSeal, diaphragm plate, diaphragm,
diaphragm washer, and lockwasher. Secure them
with the bolt. Hold the actuator stem on the wrench
flats and tighten the bolts.
3b. Reverse Acting Actuator (ATO): attach the fol-
lowing parts to the top of the actuator stem in
the order indicated: ThredSeal, diaphragm plate,
diaphragm, diaphragm washer, and lockwasher. Se-
cure them with the bolt. Hold the actuator stem on
the wrench flats and tighten the bolt.
4a. Direct Acting Actuator (ATC): hold this assembly
by the bolt head in a vise, stem pointing upward.
Install the springs over the dimples in the dia-
phragm plate. Take the lower case and carefully
install it onto the actuator stem by firmly pressing
the stem through the stem o-ring seal, keeping
the springs in place. Install the retaining ring into
the groove in the actuator stem. This will keep the
springs compressed and prevent them from com-
ing out of place.
4b. Reverse Acting Actuator (ATO): install this assem-
bly into the lower actuator case by pressing the
actuator stem through the stem o-ring seal. Then
install the lower retaining ring onto the actuator
plate. Next, install the springs over the dimples in
the diaphragm plate.
Position the flange of the diaphragm on the flange 5.
of the lower actuator case and align the holes. At-
tach the upper actuator case with the bolts.
Reassemble the actuator to the valve as described 6.
in “Valve Reassembly” steps 8 & 9.
Reversing Action
Changing the valve action from direct to reverse acting
or from reverse to direct acting. Reversing the action of
the actuator reverses the action of the Mark 708 Control
Valve. The same parts are used for either action.
Disassemble the actuator as described in “Actuator 1.
Maintenance”.
Reassemble the actuator with the desired action. 2.
Refer to “Actuator Maintenance”. To change the ac-
tion of a motor valve, refer to the instructions for the
motor actuator, which are provided with all Mark
708MV motor valves.
Motor Valve Seat Spring Preload
Position the valve to be full open. 1.
The two stem nuts should be loosened and thread- 2.
ed down the stem and then locked together.
Slowly stroke the motor to closed position, while 3.
rotating valve stem and stem nuts back and forth
(approximately ¼ turn). Caution: valve stem should
rotate freely. When it no longer rotates freely, the
plug is in contact with the seat, and the motor must
be stopped or damage may occur. If required, the
valve stem may be threaded into the actuator stem
to allow motor to complete stroke.
With motor in full closed position and stopped by 4.
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