Performance; Principle Of Operation; Maintenance - Fisher 4660 Instruction Manual

Performance

The performance characteristics shown in figure 6 il-
lustrate several important functional parameters.
S and S represent the low and high set points, re-
L H
spectively. 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 P
as S and S .
L' H'
Trip-to-reset is the combined effect of pilot dead band
and hysteresis. After the pilot has tripped, it will auto-
matically reset when the process pressure returns to
the set range. However, full output is not instanta-
neous. 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.

Principle of Operation

Refer to the schematic in figure 7. The following ex-
planation 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 de-
creases, 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 con-
tact either set point pivot, but contacts the nozzle
beam. This keeps the relay nozzle capped off, main-
taining 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 re-
spective 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.
When the process pressure returns to a value be-
tween the low and high set points, the flapper no long-
er contacts one of the set point pivots, but contacts
the nozzle beam, again capping off the relay nozzle.
This resets the relay assembly, restoring full output
pressure.
and is shown
MIN
LOW
SETPOINT
PIVOT
HIGH
SET POINT
PIVOT
FLAPPER
NOZZLE
BEAM
BOURDON TUBE
38A3803-A
A2898-2 / IL
Figure 7. Principle of Operation Schematic

Maintenance

Select the appropriate maintenance procedure, and
perform the numbered steps. Each procedure requires
that supply pressure and process pressure be shut off
before beginning maintenance. All key numbers are
shown in figure 8.
WARNING
Avoid personal injury or property dam-
age from an uncontrolled process or
sudden release of process pressure. Be-
fore starting disassembly:
Provide some temporary means of
process control before taking the pilot
out of service,
Shut off the process pressure line
and the supply pressure line, and bleed
all the supply pressure.
Bleed all process pressure from the
pilot so that no pressure is contained in
the Bourdon tube.
Type 4660
PROCESS PRESSURE
LOW SETPOINT
ADJUSTMENT
HIGH SETPOINT
ADJUSTMENT
SPRING
BLOCK AND BLEED
RELAY ASSEMBLY
SUPPLY PRESSURE
OUTPUT PRESSURE
11