6
Instruction Manual
SECTION 1 - PNEUMATIC CAPACITY CONTROL
In operation, an increase in instrument air pressure
(Pi) upsets the balanced forces and moves the
diaphragm assembly to the left. This lifts the pilot
valve above its seat to reduce the air pressure drop
across the pilot valve and increases the metered
supply pressure, (Psm) on the downstream side
of the pilot valve, which is imposed on the large
side of the air piston. The metered supply pressure
will continue to increase until sufficient force is
developed to move the piston to the right which
increases the pump capacity.
As the piston moves, the range spring feeds back
a force proportional to the piston location so that
when the range spring pressure plus the diaphragm
force from the instrument air signal are again in
balance with the suppression spring force, the pilot
valve closes and maintains a balanced pressure
condition.
A decrease in instrument air pressure moves the
diaphragm assembly to the right which seats the
head of the pilot valve and lifts the pilot valve stem
off the exhaust seat. This vents the pressure (Psm)
on the large side of the piston at atmosphere (Pa),
then the supply pressure on the small side of the
piston will move the piston to the left to decrease
the pump capacity. Again the range spring feeds
back to the positioner the location of the air piston
until the forces are again in the balanced condition.
