-4-
screw(s) (37) and moving the travel indicator until it is
properly aligned with the indicator washer (12).
Note A: a control valve which has been adjusted to
provide 3 psig starting pressure plus valve closure
force (with pressure in body) will have a considerably
higher start-pressure than 3 psig, when tested at 0
body pressure.
Note B: air pressures quoted are relative. Actual pres-
sures required in operation may vary with pressure
drop conditions existing and/or actuator springs used.
Actuator Maintenance
Disassembly and Inspection of Actuator
1. Close inlet and outlet stop valves. Operate system by-
pass, if necessary. Remove the actuator as described
in “Valve Disassembly”, Step 3.
2. Remove the nuts (27) and bolts (7) which hold the up-
per actuator case (26) to the lower actuator case (16)
and remove the upper actuator case.
3a.
Direct Acting Actuator (ATC):
remove the actuator
stem bolt (34) and lift spacer (40)( 85M only), fastener
seal (21), seal washer (22), diaphragm (23), diaphragm
plate (24), support plate (39) (85M only), and spacer
from the actuator stem. Remove the actuator stem
(17). To check the stem o-ring (31), unscrew the gland
nut (20) from the gland (19) and remove the bushing
(36) and stem o-ring from the actuator case (16).
3b.
Reverse Acting Actuator (ATO):
remove springs (25)
from diaphragm plate (24). Remove the actuator stem
bolt (34) and lift spacer (38), backing plate (39) (85m
actuators only), diaphragm plate (24), diaphragm (23),
seal washer 22), seal ring (21), and spacer (40) (85M
actuator only) from the actuator stem (17). Remove
the actuator stem (17). To check the stem o-ring (31),
unscrew the gland nut (20) from the gland (19) and
remove the bushing (36), gasket (18), and stem o-ring
(31) from the end of the gland.
4. Clean all parts with an approved, non-residue-forming
solvent. Remove encrusted material with crocus or
very mild aluminum oxide cloth. Inspect all parts for
excessive wear and/or damage. Replace any worn or
damaged parts.
Reassembly of Actuator
After all parts have been thoroughly cleaned and inspect-
ed, reassemble the actuator.
1.
Direct Acting Actuator (ATC):
insert the actuator stem
(17) through the gland (19), greasing stem o-ring (31),
if available, before inserting the actuator stem and
bushing (36) and gland nut (20). Place the springs
(25) in the lower actuator case (16). Place the backing
plate (39)(85M actuator), diaphragm plate (24) on the
springs so that the stamped bosses are nested in the
springs. Install the diaphragm (23) so the bolt holes
line up with the bolt holes in the lower actuator case.
Place the seal washer (22) on the diaphragm and bolt
together with the seal ring (19), and spacer (40) (85M
actuator) and actuator stem bolt (34) and tighten.
2.
Reverse Acting Actuator (ATO):
lightly grease the
stem o-ring (31) and install in the c’bore in the gland
(19), bushing (36), gasket (18) and gland nut (20). In-
sert actuator stem (17) through the gland. Assemble
the spacer (40) [85M actuators only],fastener seal
(21), seal washer (22), diaphragm (23), diaphragm
plate (24), backing plate (39)(85M actuator), and
spacer (38) to the actuator stem bolt (34), and thread
into the actuator stem (17). Line up the bolt holes in
the diaphragm with the holes in the lower diaphragm
case. Place the springs (25) on the diaphragm plate
nested over the punched bosses.
3. Place the upper actuator case (26) on the diaphragm,
lining up the bolt holes.
4. Replace four bolts (7) and nuts (27) 90° apart and
tighten finger-tight. Replace the remainder of the bolts
and nuts and tighten evenly and alternate across the
actuator case.
5. Thread the valve stem (9) into the actuator stem(17)
and adjust the spring preload as described previously
(“Actuator Spring Preload Adjustment”).
Reversing Action
Changing the valve action from direct to reverse acting or
from reverse to direct acting. * Different springs would be
needed only if the valve has a positioner.
Different actuator springs are required to meet the pub-
lished pressure drop ratings when changing action. Fail-
ure to do so will change the allowable pressure drop ca-
pability of the valve. Consult factory for details.
The action of the Mark 78 control valve is reversed by
reversing the action of the actuator.
1. Disassemble the actuator as described in “Actuator
Maintenance”.
2. Reassemble the actuator with the desired action. Re-
fer to “Actuator Maintenance”.
Troubleshooting
If you Experience Erratic Control:
Over sizing causes cycling or hunting – recalculate
size required.
Under sizing causes control setting to drop under
peak loads.
Steam traps downstream may need reconditioning.
Safety relief valve may be jammed open.
Excessive foreign matter may be lodged in seats.
