Principles of Operation
Close on Increasing Signal
The positioner is a force-balanced instrument that provides a control valve posi-
tion proportional to the pneumatic input signal. The control valve will
open
on loss
of input signal. The energy to operate the control valve is obtained from the differen-
tial between the supply and discharge pressures. In steady state, the forces im-
posed on the balance beam by the input signal diaphragm and the range spring are
equal; therefore, the top and bottom balance valves in the positioner are at or near
their closed positions. The cylinder top and bottom pressures are both equal to
power gas pressure, and the control valve is stationary. An increase in the input
signal pressure results in the opening of the lower balanced valve due to an imbal-
ance in the beam forces. This decreases the pressure in the cylinder top., while
the pressure in cylinder bottom remains at power gas. The control valve begins to
close. The actuator rod stretches the range spring, increasing its tension. This
force, which opposes the force on the balanced beam caused by the increasing in-
put signal, continues to increase until the balance beam forces are in equilibrium.
At this point the valve is in the correct position for the applied input signal. The po-
sitioner has a sensitivity adjustment which permits a balance between greater ac-
curacy (from a smaller deadband) and minimal bleed gas.
HPP-5
5
September 1999
HPP-5 Positioner
“How-It-Works”
Power
Gas
Instrument-
Signal
Cylinder
Top
Exhaust
