VRG Controls LLC. 9 of 35 JANUARY 2019
VPC “SN” SERIES VALVE PILOT CONTROLLERS
INSTRUCTION MANUAL
HOW IT WORKS DESCRIPTIONS:
DOUBLE ACTING VPC-DA-SN
When the SENSING pressure is equal to the VPC-DA-SN
setpoint, the net force on the VPC-DA-SN power module is
zero. This is the equilibrium or “balanced” condion where
the sensing pressure that pushes down on the sensing
diaphragm and the control spring force that pulls up on the
sensing diaphragm are equal. When the VPC-DA-SN achieves
equilibrium, the OPEN seat & nozzle assembly and CLOSE
seat & nozzle assembly will be posioned at equal openings
maintaining a constant OUTPUT pressure to the top and
boom chambers of the control valve actuator. The VPC-DA-
SN will exhibit constant emissions at this state as referenced in
Table 1.0. From this posion two possible scenarios can occur,
the sensing pressure can rise above or below the set point.
If the sensing pressure rises above the VPC-DA-SN setpoint
the net force on the VPC-DA-SN power module is downward.
The CLOSE seat & nozzle assembly will move toward closed
posion and divert pressure to the CLOSE chamber of the
double acng actuator. The OPEN seat & nozzle assembly will
open more and pressure shall be reduced on the OPEN side of
the double acng actuator. The combinaon of these acons
creates a dierenal pressure to be applied to the double
acng actuator that will move the valve toward the closed
posion.
If the sensing pressure falls below the VPC-DA-SN setpoint
the net force on the VPC-DA-SN power module is upward.
The OPEN seat & nozzle assembly will move toward closed
posion and divert pressure to the OPEN chamber of the
double acng actuator. The CLOSE seat & nozzle assembly
will open more and pressure shall be reduced on the CLOSE
side of the double acng actuator. The combinaon of these
acons creates a dierenal pressure to be applied to the
double acng actuator that will move the valve toward the
open posion.
An adjustment for sensivity is achieved via a rotang drum
at the center of the VPC-DA-SN. Rotaon of the drum to the
LEFT (Increasing Numbers) will increase the xed distance of
the internal assembly, requiring greater travel of VPC-DA-SN
internals to aect a change in CLOSE PRESSURE and OPEN
PRESSURE. Conversely, rotaon of the drum to the RIGHT
(Decreasing Numbers) will decrease the xed distance of
the internal assembly, requiring lesser travel of VPC-DA-SN
internals to aect a change in CLOSE PRESSURE and OPEN
PRESSURE.
Adjustable orices are installed upstream of the SUPPLY
PRESSURE that aect the maximum achievable ow rate to
CLOSE PRESSURE and OPEN PRESSURE independently. These
Adjustable Orices may be ulized to adjust the CLOSING and
OPENING speed of travel of the control valve actuator with
both Adjustable Orices being set equally. Alternavely, the
Adjustable Orices may be set at dierent levels to achieve a
dierence between CLOSING and OPENING speed necessary
to opmize control performance for certain applicaons.
Note that the VPC-DA-SN atmospheric emissions may be
completely eliminated by discharging exhaust to a nearby
or downstream pressure system. Addionally, addion of
an NVD No-Vent Device will eliminate emissions when the
control valve remains in the full-open or full-closed posions
such as a standby, overpressure monitor or relief type
applicaon.
