Principle Of Operation - Fisher 3660 Instruction Manual

Instruction Manual
D101402X012
upset the system. Use bypass operation only when the instrument signal range is the same as the positioner output range
required for normal actuator operation.
Labels on the bypass body assembly (key 41, figure 23), and a pointer on the bypass lever (key 42 in figure 23) indicate
if the input signal from the instrument goes to the positioner or directly to the control valve actuator.
With the pointer of the bypass lever over the word POSITIONER, the instrument pressure goes to the positioner and
the output pressure of the positioner goes to the actuator.
With the pointer of the bypass lever over the word BYPASS, the instrument pressure goes directly to the actuator.
Note
A difference between the input signal pressure and the positioner output pressure could cause a transient bump in the controlled
system when the bypass lever is moved to BYPASS.
With a reverse‐acting or split‐range positioner, the bypass lever may be secured in the POSITIONER position so that
bypass cannot be used. To lock the bypass lever in the POSITIONER position, shut off the instrument and supply
pressure to the positioner. Then, move the bypass lever (key 42 in figure 23) so the pointer is over the word
POSITIONER. Align the hole in the pointer with the hole in the body assembly and thread the plastic wire tie (key 79 in
figure 23) through both holes to secure the bypass lever.

Principle of Operation

Refer to figure 21 for the operational schematic.
The instrument pressure acts on the input module, which controls the flapper‐nozzle system of the relay. Supply
pressure is applied to the relay, and the output pressure of the relay is supplied to the control valve actuator.
For a direct‐acting positioner, increases in instrument pressure cause the input module to pivot the beam. The beam
pivots the flapper and restricts the nozzle. The nozzle pressure increases and causes the relay assembly to increase
output pressure to the actuator. With a direct‐acting actuator, this increased pressure moves the actuator stem
downward. Stem movement is fed back to the beam by means of a feedback lever and range spring, which causes the
flapper to pivot slightly away from the nozzle to prevent any further increases in relay output pressure. The positioner
is once again in equilibrium but at a higher instrument pressure, a slightly different flapper position, and a new
actuator stem position.
A decrease in instrument pressure decreases nozzle pressure, which allows the relay to bleed off actuator loading
pressure.
Operation of a reverse‐acting positioner is similar except that the position of the flapper is reversed from that shown in
figure 21. The reversed position uses the alternate flapper pivot point so that increases in instrument pressure rotate
the flapper away from the nozzle to reduce nozzle pressure.
With a 3661 electro‐pneumatic positioner, the electro‐pneumatic (I/P) converter provides a 0.2 to 1.0 bar (3 to 15
psig) output pressure proportional to the 4-20 milliampere input signal. The 0.2 to 1.0 bar (3 to 15 psig) output
pressure becomes the input signal pressure to the input module.
3660 and 3661 Positioners
September 2015
27