Relay Operation - Fisher 3570 Instruction Manual

3570 Positioners
September 2015
relay action, the pressure to the underside of the piston decreases. The force exerted by the actuator spring
overcomes the force of the pressure below the piston, and the piston moves downward. This changes the valve plug
position.
In the 3573 positioner, piston movement is fed back to the beam by means of a range spring, which is connected to
the piston rod extension. The downward movement of the piston rod extension extends the range spring until the
torque of the beam balances the torque exerted by the instrument bellows.
In the 3577 positioner, feedback is provided to the range spring by a wire that is connected to the actuator‐valve stem
connector.
As the input signal decreases, the reverse action takes place. The bellows contracts, and as the beam pivots, it covers
the relay nozzle. Through relay action, the pressure on the underside of the piston increases to overcome the force
exerted by the actuator spring, and the piston moves upward.

Relay Operation

Refer to figure 9, which shows a sectional view of a typical relay.
Supply pressure reaches the relay(s) through passages in the positioner base and is channeled to fixed restriction R and
to point A between the supply valve B and the balancing O‐ring of the relay valve. The fixed restriction is an integral
part of the relay restriction plug and wire assembly G. The orifice in nozzle F is larger than the fixed restriction. This
allows the supply pressure to bleed to atmosphere faster than it enters the unit through the fixed restriction when the
beam flapper is away from the nozzle.
Assume that a change in the input signal causes the beam flapper to cover the nozzle of a relay. The supply pressure
flows through fixed restriction R into the chamber between the two relay diaphragms. Due to the restricting effect of
the flapper over the nozzle, pressure builds up in the chamber between the diaphragms, forcing the diaphragm head
assembly E downward to open supply valve B, allowing output pressure to increase.
The supply pressure flows past supply valve B to increase the output pressure to the actuator cylinder. The cylinder
pressure (relay output pressure) also acts on the area D. This provides an air feedback that returns the diaphragm head
assembly E and the movable nozzle F to their original positions, thus preventing any further increase in output
pressure. The feedback arrangement and the movable nozzle ensure accurate and stable positioning of the actuator
piston without introducing cycling or over‐correction. After any change in the output pressure, supply valve B and
exhaust valve C always return to the closed position to put the nozzle back in its original, or equilibrium, position. The
spring behind supply valve B aids in closing the valve as the diaphragm head assembly is forced upward.
When the beam flapper moves away from the nozzle F, the supply pressure bleeds out at a greater rate than it enters
through the fixed restriction R. The pressure then decreases in the chamber between diaphragms. The force of the
cylinder pressure acting on area D pushes diaphragm head assembly E upward, opening exhaust valve C. Cylinder
pressure bleeds through the exhaust port to atmosphere. As the cylinder pressure decreases and the force on area D
decreases, the force of the nozzle pressure in the chamber between the diaphragms returns the assembly to its
original position. The unit is again in equilibrium, but at a lower nozzle pressure and a lower output pressure.
Each relay has a 4:1 ratio between the nozzle pressure and the output pressure. For example, a 0.7 bar (10 psig) nozzle
pressure change, produces a 2.7 bar (40 psig) output pressure change; a 1.4 bar (20 psig) nozzle pressure change
produces an 5.5 bar (80 psig) output pressure change. With a constant input signal pressure, the internal parts of the
relay are at equilibrium with the supply and exhaust valves closed.
20
Instruction Manual
D200137X012