Flowserve StarPac 3 - Manual (Page 13)

13
StarPac Intelligent Control System FCD VLENIM0066-05 04/14
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7.3 Positioner Operation
Figure 1: StarPac 3 Positioner Schematic (air-to-open configuration)
The StarPac 3 Positioner is an electric feedback instrument. Figure 1 shows a StarPac 3 installed on a double-acting
actuator for air-to-open action. Positioning is based on a balance of two signals: one proportional to the modulator input
signal and the other proportional to the stem position.
The supply pressure for the positioner modulator is tapped off the main supply and is filtered as it passes through a field-
replaceable, coalescing filter element in the module. Next it passes through an internal pressure regulator that regulates
it to approximately 21.5 psig. The air then goes through an orifice that restricts the flow and air consumption (refer to
Figure 1).
The air is further controlled to 6-12 psig using a spring-diaphragm flapper that is attracted by an electromagnet to a noz-
zle. A temperature compensated Hall Effect sensor mounted on a circuit board senses the spool valve position. The Hall
Effect sensor and circuitry create a feedback loop, which determines how much current to send to the electromagnet for
a desired spool valve position. The electromagnet in the feedback loop varies the nozzle-flapper spacing, which regulates
the output pressure to 6-12 psig proportional to the command input signal.
When these opposing signals are equal, the system will be in equilibrium and the stem will be in the position called for
by the command signal. If these opposing signals are not equal, the spool valve will move up (or down) and, by means
of the modulator, will change the output pressures and flow rate. This will cause the piston to move until the signal of the
feedback sensor equalizes with the command signal.
The detailed sequence of positioner operations are as follows: An increase in the command signal forces the modulator
signal capsule and spool valve upward. This motion of the modulator also pushes the pilot valve spool upward from its
equilibrium position. This opens the pilot valve ports, supplying air to port one and exhausting air from port two. This
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Electromagnetic Coil
Nozzle Flapper Digital Position
Algorithm
Spool Valve
Flame Arrestors
Pressure Sensor
Air Supply
Orifice
Exhaust
OUTPUT 1
OUTPUT 2
Exhaust
Flame
Arrestor
Air-to-Open
Configuration
Flame Arrestor
Filter
Regulator Hall Effect
Sensor
Stem
Position
Sensor