Flowserve APEX 8000 High-Performance Positioner - Manual (Page 3)

Figure 1: Apex 8000 Positioner Schematic for Air-to-Open
The sequence of operation is as follows: An increase in instrument signal pressure forces the input capsule downward. Displacement of the
capsule in turn moves the flapper away from the detecting nozzle. This allows a larger flow rate through the nozzle, decreasing the pressure
exerted on the top of the pilot valve capsule.
Supply air biases the pilot-valve in an upward direction. As the capsule moves up, it will close the exhaust seat of the upper pilot poppet and
open the supply seat, which applies increased air pressure to the bottom cylinder port. At the same time, the pilot-valve capsule will open the
exhaust seat for the lower pilot poppet; thus, decreasing pressure to the top cylinder port.
This difference in pressure will drive the piston outward, rotate the pinion and stretch the feedback spring until the spring tension exactly
opposes the force resulting from the instrument signal pressure. At this point, the flapper will be moved toward the detecting nozzle to restore
the pressure above the pilot-valve capsule to its equilibrium value. As a force-balanced condition is approached, the pilot-valve capsule will be
forced back to a neutral position where the pilots are neither supplying air to, nor exhausting air from, their respective sides of the piston.
A decrease in instrument signal pressure reverses the described actions and cause a proportional inward movement of actuator
pistons and a reversal in pinion direction.
Figure 2: Positioner Mounting with NAMUR Shaft
3
Apex 8000 High-Performance Positioner AXENIM0130-06 11/10
Piston Cylinder
Feedback
(Shaft)
Cam
Range Adjustment
Screws
Zero Adjustment Feedback Spring
Balance Adjustment
Supply Seat
Upper Pilot
Poppet
Exhaust Seat
Port No. 1
Restriction
Supply
Pilot Valve
Capsule
Exhaust Seat
Port No. 2
Lower Pilot
Poppet
Supply Seat
Flapper
Detecting
Nozzle
Input Capsule
Lower Diaphragm
Upper Diaphragm
Instrument Signal