Instruction Manual
D200163X012
4660 Pressure Pilot
June 2017
18
Principle of Operation
Refer to the schematic in figure 7. The following explanation describes the principle of operation for a high‐low pilot.
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
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.
When the process pressure returns to a value between the low and high set points, the flapper no longer 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.
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.
