3660 and 3661 Positioners
Instruction Manual
Form 5265
March 2006
24
FLAPPER
DIRECT
ACTING
PIVOT
REVERSE
ACTING
PIVOT
NOZZLE
OUTPUT
RANGE
SPRING
RELAY
GAIN ADJ
A
A
FEEDBACK
PLATE
FEEDBACK
LEVER
PILOT
SHAFT
INPUT
MODULE
BEAM
PIVOT
SUPPLY
RANGE
SPRING
RELAY
GAIN ADJ
SUPPLY
A
A
INPUT SIGNAL
I/P CONVERTER
INPUT
MODULE
BEAM
PIVOT
SECTION A-A
TYPE 3660
TYPE 3661
31B3960-C
B2152-4 / IL
Figure 21. Operational Schematic
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 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,
