Open On Increasing - Becker HPP-5 Instruction Manual

Power
Gas
Open 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 close on loss of
input signal. The energy to operate the control valve is obtained from the differential
between the supply and discharge pressures. In steady state, the forces imposed
on the balance beam by the input signal diaphragm and the range spring are equal;
therefore, the top and bottom balance valves of the positioner are at or near their
closed positions. The cylinder top and bottom pressures are both equal to power
gas, and the control valve is stationary. An increase in the input signal pressure re-
sults in the the opening of the upper balance valve due to an imbalance in the beam
forces. This decreases the pressure in the cylinder bottom, while the pressure in
the cylinder top remains at power gas pressure. The control valve begins to open.
The actuator rod stretches the range spring, increasing its tension. This force, which
opposes the force on the balanced beam caused by the increasing input signal, con-
tinues 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 positioner has a sen-
sitivity adjustment which permits a balance between greater accuracy (from a
smaller deadband) and minimal bleed gas.
HPP-5
Instrument-
Signal
6
HPP-5 Positioner
"How-It-Works"
Cylinder
Top
Exhaust
September 1999