48-2
Flowserve Corporation, Valtek Control Products, Tel. USA 801 489 8611
INTEGRAL
ALIGNMENT
PIN
SECTION A-A
BRACKET
A
A
Figure 2: Positioner Mounting with
NAMUR shaft
Figure 3: Positioner mounting
Figure 1: XL90 Positioner Schematic for Air-to-Open
When the opposing forces balance exactly, the system
will be in equilibrium and the stem will be in the exact
position called for by the instrument signal. If the
opposing forces are not in balance, the input capsule
will move up or down and, by means of the pilot-valves,
will change the output pressures, moving the stem until
the tension on the feedback spring exactly opposes the
instrument signal pressure.
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 supply-
ing air to, nor exhausting air from, their respective sides
of the piston.
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
Balance Adjustment
Zero Adjustment
Range
Adjust Screws
Cam
Feedback
Spring
Input Capsule
Feedback
(shaft)
Flapper
Detecting
Nozzle
Instrument Signal
Upper Diaphragm
Lower Diaphragm
Piston
Cylinder
Cylinder
