LowFlow Mark 708HP Series 8000 psi Control Valve - Manual (Page 3)

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Valve Reassembly
Lubricate the seat (2) threads with a suitable lu- 1.
bricant such as NO-LOK and thread the seat into
the body (1) (Note: Mk708HPA body is an angle
pattern with the inlet at the bottom, however all
instructions still apply). Tighten to 20 ft-lbs (240
in-lbs).
Install the metal seat ring (34) into its groove in the 2.
body.
Coat the guide portion of the plug/stem (5) with 3.
DOW CORNING G-n Metal Assembly Spray (750°
F) according to manufacturer’s instructions. Insert
the plug/stem into the bonnet (3).
Coat the coarse exterior bonnet threads on the 4.
body (1) with a suitable anti-seize compound in-
tended for use on stainless steel. Install the bon-
net (3) onto the body, and then thread the bonnet
ring nut (4) over the bonnet and onto the threads
on the body. Carefully push down on the stem to
be sure that the plug has fully engaged the seat.
Tighten the bonnet lock nut until a positive stop is
felt.
Bring the yoke (6) into full contact with the bonnet 5.
(3) centering it on the hub. Install the yoke lock nut
(7) and hand tighten.
Lubricate the packing set (33) with Dow Corn- 6.
ing #6 and install the packing set into the bonnet
bore. Thread the Packing nut (8) onto the bonnet
(3) and tighten loosely. Assemble the two stem
nuts (32) and indicator (19) all the way on to the
stem threads.
Actuator Preload Adjustment
Direct Acting Valves (Air To Close)
Insert the valve stem (5) into the actuator stem 1.
(11). Hold the actuator stem with a wrench to pre-
vent it from turning. Spring preload is adjusted by
threading the valve stem (5) into the actuator stem
(11). Hold the actuator stem from turning with a
wrench and using the stem nuts (32) turn the valve
stem. Threading the valve stem into the actuator
stem increases preload and threading out de-
creases preload.
Load an air supply into the upper case. Slowly 2.
increase the air pressure up to 18.5 psig while
observing the stem for movement. When the plug
hits the seat, tighten the yoke lock nut (7) and also
tighten the packing nut until it stops.
Correct spring preload is when the valve plug 3.
contacts the seat at the time the actuator pressure
reaches 18.5 psig.
If seating occurs before 18.5 psig then increase 4.
the preload. If seating occurs at a pressure higher
than 18.5 psig then decrease the preload.
After the preload is set, lock the stem nuts (32) 5.
against the actuator stem (11).
Reduce pressure to 11 psig and the valve should 6.
stroke approximately full open (0.500”).
Reverse Acting Valves (Air To Open)
Insert the valve stem (5) into the actuator stem 1.
(11). Hold the actuator stem with a wrench to pre-
vent it from turning. Spring preload is adjusted by
threading the valve stem (5) into the actuator stem
(11). Hold the actuator stem from turning with a
wrench and using the stem nuts (32) turn the valve
stem. Threading the valve stem into the actuator
stem decreases preload and threading out in-
creases preload.
Slowly decrease the air pressure down to 11 psig 2.
while observing the stem for movement. When the
plug hits the seat tighten the yoke lock nut (7) and
the packing nut until it stops.
Correct spring preload is when the valve stem 3.
starts to move when the actuator pressure reaches
11 psig.
If movement occurs before 11 psig then increase 4.
the preload. If movement occurs at a pressure
higher than 11 psig then decrease the preload.
After the preload is set, lock the stem nuts (32) 5.
against the actuator stem (11).
Increase pressure to 20 psig and the valve should 6.
stroke approximately full open (0.500”).
Actuator Maintenance
Remove the actuator as described in “Removing 1.
Actuator.”
Remove the nuts (24) and bolts (25) which hold 2.
the upper actuator case (23) to the lower actuator
case (9) and remove the upper actuator case.
Direct Acting Actuator (ATC):
Remove the actuator stem bolt (22) and lift the 1.
seal washer (17), diaphragm (16), diaphragm plate
(15), and spacer (27). Remove the actuator stem
(24).
To check the stem o-ring (35), unscrew the gland 2.
nut (13) from the gland (12) and remove the stem
o-ring from the end of the gland.
Reverse Acting Actuator:
Remove springs (38)& (39) from diaphragm plate 1.
(15). Remove the actuator stem bolt (22) and lift
spacer (27), diaphragm plate (15), diaphragm (16),
seal washer (17) and seal ring (18) from the actua-
tor stem (11).
Remove the actuator stem (11). To check the stem 2.
o-ring (35), unscrew the gland nut (13) from the
gland (12) and remove the stem o-ring from the
end of the gland.
Clean all parts with an approved, non-residue- 3.
forming solvent. Remove encrusted material with
crocus clother or very mild aluminum oxide cloth.
Inspect all parts for excessive wear and/or dam-
age. Replace any worn or damaged parts.