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Instruction Manual
D103626X012
Table 2. Available Constructions
PISTON
ACTUATOR SIZE
DIAMETER, INCH
12
12
14
14
16
16
18
18
20
20
22
22
24
24
26
26
Table 3. Thrust
ACTUATOR SIZE
12
14
16
18
20
22
24
26
1. Consult your Emerson Process Management sales office for supply pressures below 40 psig.
2. Maximum available thrust.
Principle of Operation
685 piston actuators utilize a pneumatically controlled piston that moves inside of a cylinder to generate thrust. A seal
contained on the circumference of the piston provides a seal between the piston and the cylinder, preventing supply
pressure leakage.
From an equilibrium state, the actuator operates by reacting to a force unbalance that is created by increasing supply
pressure on one side of the piston, and decreasing it on the other. This moves the piston up or down, and results in a
repositioning of the attached control valve. Travel can be adjusted using travel stops or cutoffs within a valve
positioner, which both limit the travel range of the actuator.
An optional handwheel manual override is capable of extending or retracting the actuator manually and can be
engaged at any position from full open to full close. This override utilizes a worm gear assembly that is attached to the
stem connector and not attached to the cylinder or piston rod. This enables the manual override to reposition the
control valve even if the actuator cylinder or piston is removed for maintenance.
Specifications
Specifications for 685 piston actuators are given in table 1. Refer to the nameplate affixed to the actuator yoke for
specifications specific to individual constructions.
PISTON AREA,
CONNECTOR SIZE,
2
INCH
113.10
153.94
201.06
254.47
314.16
380.13
452.39
530.93
4.1 barg (60 psig)
5.5 barg (80 psig)
1526 (6786)
2076 (9236)
2712 (12064)
3432 (15268)
4238 (18850)
5127 (22808)
6102 (27143)
7161 (31856)
VALVE STEM
YOKE BOSS
DIAMETER, INCH
INCH
1 1/4 or 2
5H or 7
1 1/4 or 2
5H or 7
1 1/4 or 2
5H or 7
1 1/4 or 2
5H or 7
1 1/4 or 2
5H or 7
1 1/4 or 2
5H or 7
1 1/4 or 2
5H or 7
1 1/4 or 2
5H or 7
THRUST AT SUPPLY PRESSURE, N (LBF)
6.9 barg (100 psig)
2034 (9048)
2543 (11310)
2769 (12315)
3461 (15394)
3616 (16085)
4520 (20106)
4577 (20358)
5721 (25447)
5650 (25133)
7063 (31416)
6837 (30411)
8546 (38013)
8136 (36191)
10170 (45239)
9549 (42474)
11936 (53093)
685 Piston Actuator
VALVE TRAVEL, INCH
Minimum
Maximum
8-7/8
23-7/8
8-7/8
23-7/8
8-7/8
23-7/8
8-7/8
23-7/8
2
23-7/8
2
23-7/8
2
23-7/8
2
23-7/8
(1)
10.3 barg (150 psig)
3814 (16965)
5191 (23091)
6780 (30159)
8581 (38170)
10594 (47124)
12819 (57020)
15255 (67858)
17904 (79639)
May 2013
(2)
3
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