4660 Pressure Pilot
Instruction Manual
Form 5200
September 2004
12
3. Release the upper end of the flapper such that
the flapper will snap back and contact the nozzle
beam.
4. Replace the case cover, and attach it with the
case cover screw.
Startup
Verify that the desired high and/or low set point
pressures have been set. Refer to the Mounting
section for the desired pilot installation procedures.
Reduce the external process pressure source to
zero and disconnect the source from the process
pressure connection. Reconnect the process
pressure line to the pilot and open the process
pressure line from the process to the pilot.
Reconnect the pilot output pressure line. When
adjustments and startup have been completed,
replace the cover (key 4), and the cover screws
(key 6).
Performance
The performance characteristics shown in figure 6
illustrate several important functional parameters.
S
L
and S
H
represent the low and high set points,
respectively. The set range is 3 to 97 percent of the
Bourdon tube rating. However, with a single high-low
unit or a high-only/low-only pair, there is a limit on
how close to each other the set points can be
adjusted. This limit is defined as set point
D
P
MIN
and
is shown as S
L’
and S
H’
.
Trip-to-reset is the combined effect of pilot dead
band and hysteresis. After the pilot has tripped, it will
automatically reset when the process pressure
returns to the set range. However, full output is not
instantaneous. The difference between the set point
and reset to full output is the trip-to-reset zone. This
parameter is also a function of the Bourdon tube
rating as shown in figure 6.
Finally, repeatability is the switch point deviation
around the set point as a percentage of the Bourdon
tube rating.
Figure 7. Principle of Operation Schematic
LOW
SETPOINT
PIVOT
HIGH
SET POINT
PIVOT
FLAPPER
NOZZLE
BEAM
BOURDON TUBE
PROCESS PRESSURE
LOW SETPOINT
ADJUSTMENT
HIGH SETPOINT
ADJUSTMENT
BLOCK AND BLEED
RELAY ASSEMBLY
SUPPLY PRESSURE
OUTPUT PRESSURE
SPRING
38A3803-A
A2898-2 / IL
Principle of Operation
Refer to the schematic in figure 7. The following
explanation describes the principle of operation for a
high-low pilot.
Process pressure is connected to the pilot Bourdon
tube sensing element. As the process pressure
decreases, the Bourdon tube contracts; as the
process pressure increases, the Bourdon tube
expands. While the process pressure remains above
the low set point and below the high set point, the
flapper does not contact either set point pivot, but
contacts the nozzle beam. This keeps the relay
nozzle capped off, maintaining full output pressure.
A decrease in process pressure below the low set
point, or an increase in process pressure above the
high set point causes the flapper to contact the
respective low or high set point pivot and uncap the
relay nozzle. This trips the relay assembly, which
blocks supply pressure and vents (bleeds) the output
pressure to zero.
