Principle Of Operation - Fisher EMERSON VSX4 Series Instruction Manual

OVERPRESSURE SHUTOFF (OPSO)
ADJUSTING SCREW
UNDERPRESSURE
SHUTOFF (UPSO) SPRING
OVERPRESSURE
SHUTOFF (OPSO) SPRING
CaM
RESET BUTTON
M1141
INLET PRESSURE, P
u
OUTLET PRESSURE, P
d
ATMOSPHERIC PRESSURE, Pb

Principle of Operation

VSX4 and VSX8 Series Controller
The pressure measuring element of the controller
consists of a diaphragm that senses downstream
pressure. The downstream pressure is controlled
by the regulator. The top side of the diaphragm
encounters the force imposed by the overpressure
shut-off spring and underpressure shut-off spring.
When the downstream pressure increases above the
Overpressure Shutoff (OPSO) setting, the diaphragm
moves up.
When the downstream pressure decreases below the
Underpressure Shutoff (UPSO) setting, the diaphragm
moves down.
RESET PIN
Figure 3. Type VSX8 Operational Schematic
VSX4 and VSX8 Series
UNDERPRESSURE SHUTOFF (UPSO)
ADJUSTING SCREW
DIAPHRAGM
VALVE PLUG SPRING
Both of these actions result in the rotation of the cam
and the release of the reset pin.
The valve plug spring moves the valve plug against
the regulator port, stopping the fl ow of gas. The reset
button is then used to reset (relatch) the controller
(refer to the Resetting Procedure section for detailed
resetting instructions).
Standard downstream pressure registration for the
controller is external; however, internal sensing line is
made available by removing the downstream sense
blocking screw, installing a 1/4 NPT plug in the control
line port and removing the downstream control line.
CONTROL LINE PORT
DOWNSTREAM SENSE
BLOCKING SCREW
VALVE PLUG
5