Maintenance - Emerson Fisher 3660 Instruction Manual

3660 and 3661 Positioners
FEEDBACK
PLATE
31B3960-C
B2152-4 / IL
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.
24
OUTPUT
RANGE
SPRING
A
PILOT
PIVOT
SHAFT
FEEDBACK
LEVER
INPUT
MODULE
TYPE 3660
Figure 21. Operational Schematic
RELAY
GAIN ADJ
A
SUPPLY
BEAM
BEAM
INPUT
MODULE
I/P CONVERTER
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 Type 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 to 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.

Maintenance

Positioner parts are subject to normal wear and must
be inspected and replaced as necessary. The
frequency of inspection and replacement depends
upon the severity of service conditions. The following
procedure describes disassembly and reassembly of
the positioner. When inspection or repairs are
required, disassemble only those parts necessary to
accomplish the job. When reassembly is complete,
Instruction Manual
March 2006
FLAPPER
DIRECT
ACTING
PIVOT
NOZZLE
REVERSE
ACTING
PIVOT
SECTION A-A
RANGE
RELAY
SPRING
GAIN ADJ
A
A
PIVOT
INPUT SIGNAL
TYPE 3661
Form 5265
SUPPLY