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Accessories
n
Atmospheric Bleed Control (AB control): Maintains minimum
pressure differential across the cylinder� The AB control is
required to provide the necessary output to operate the control
valve under all design conditions�
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NBV No Bleed Valve: Same function as DPS, no adjustments or
tubing required� Works up to 150 psig power gas�
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DPS Series Non-Bleed Sensor: Achieves non-bleeding condi-
tions in either full open or full closed positions� Selection based
upon power gas pressure and discharge gas pressure�
Principles of Operation
Close on Increasing Signal
The positioner is a force-balanced instrument that provides a
control valve position 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 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 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 imbalance in the beam forces� This decreases
the pressure in the cylinder top, while the pressure in the 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 input 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 positioner has a sensitivity adjustment that
permits a balance between greater accuracy (from a smaller
deadband) and minimal bleed gas�
Summary of Contents for Becker HPP-5
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