Instruction Manual
D100291X012
480 Series Actuators
August 2012
7
the piston to the other. For most actuators in the 480 Series, this device is a 3570 positioner. However, a separate
loading device must be provided for actuators without positioners.
For actuators with positioners (figure 4), the pneumatic output signal from a controller or instrument is piped to the
positioner bellows. As long as the bellows receives a constant input signal pressure, the beam remains motionless and
allows supply pressure to bleed through both relay nozzles so that a constant pressure is maintained between the
nozzle and the fixed orifice. The relays are in equilibrium with their inlet and exhaust valves closed.
Assume that a downward piston motion is required and the bellows receives a corresponding change in input signal
pressure. This causes the beam to pivot so that it covers the nozzle on relay A. (Beam movement is accomplished
either by increasing the input signal pressure on a direct-acting positioner to expand the bellows, or by decreasing the
input signal pressure on a reverse-acting positioner to contract the bellows.)
The nozzle pressure in relay A increases due to the restriction created by the beam over the nozzle. Through relay
action, the air pressure to the top of the piston is increased. At the same time, relay B reacts to the change in beam
position to decrease the pressure to the underside of the piston. Due to the resulting unbalanced forces acting on the
piston, it moves down, changing the valve plug position.
Piston movement is fed back to the beam by means of a range spring which is connected to the beam and to the
piston rod extension, applying a force to the beam opposite to that caused by the expanding or contracting bellows.
This feedback arrangement prevents overcorrection and ensures a definite position of the piston and valve plug for a
given instrument signal.
If upward piston motion is required, the beam pivots over the nozzle on relay B. The result is relay, piston, and
feedback action opposite that for downward piston motion.
Reversal of positioner action is accomplished simply by removing four screws, inverting the bellows, and installing two
bellows posts for support it the change is from direct to reverse action. Bellows posts are stored in the positioner case
and are not used if the change is from reverse to direct.
Actuator with Snubber
As the actuator piston strokes, the snubber piston moves inside an oil-filled cylinder, forcing oil from one side of the
piston to the other through two check valves (see figure 6). The resistance to flow created by the settings of the check
valves and the shock absorbing quality of the oil combine to damp out any tendency of the valve plug to jump. The
plug of each check valve is held off its seat by the positioning of the adjusting screws. Thus, with the adjusting screws
backed off all the way, maximum damping will be obtained.
Pressure Connections
480
The 3570 positioner, an integral part of 480 actuator units, is factory-assembled to the actuator. Two connections,
“Supply” and “Instrument”, remain to be made after valve installation. Both are 1/4 NPT holes in the positioner and are
labelled. Minimum supply pressure is 2.4 bar (35 psig), but for optimum performance, supply pressure should be held
as near as possible to the 10.3 bar (150 psig) maximum. Supply pressure air or gas should be clean and dry, as well as
non-corrosive. Use of a Fisher 252 filter is recommended.
The positioner case should always be vented to prevent pressure buildup. If the valve is installed with the actuator
below the pipeline, provide a new vent location in the lowest part of the case by removing the cadmium-plated pipe
plug from the case and screwing it into the standard vent location. This also serves as a drain hole to prevent
accumulation of condensate.
