In previous unit, you have learned about valves and actuators.
Figure 1 below, shows how a controller is connected to an actuator to drive a valve to close and open a piping system.
As in any control situation, instability is one of many problems in process system. Instability is caused when time lags, causes the control loops to react too late to any disturbance. Time lags also occurred in control loops where large actuator is being used. A large actuator is like a capacitance in the process. Valves do not change position when airflow starts to enter in the diaphragm chamber of the actuator. Valves will move only when enough air accumulates to build up pressure inside the diaphragm chamber and cause motion to actuator stem.
To overcome time lags due to actuator capacitance a device called positioner is installed in between controller and control valve connections. The positioner provides a substantial improvement in valve and control loop performance because it produces large airflow amplification. The effect of large airflow amplification reduces the effective actuator capacitance in a control loop.
In this unit you will learn to:
S Identify the parts and function of a positioner
S Identify some other control situations where application of positioners is necessary
S Install and calibrate a Fisher 3582 pneumatic valve positioner.
The principle of operation
Figure 5 Positioner Response to an Increasing Instrument Signal
When the instrument pressure increases, the bellow expands and moves the beam. The beam pivots the flapper and restricts the nozzle. The nozzle pressure increases and moves the relay diaphragm assembly to open the supply valve.
Output pressure to the diaphragm actuator increases moving the actuator stem downward. Stem movement is fed back to the beam by means of a cam, which causes the flapper to pivot slightly away from the nozzle. Nozzle pressure decreases and the relay supply valve closes slightly to prevent any further increases in output pressure. The positioner will now be in equilibrium but in a higher instrument pressure, a slightly different flapper position and a new actuator stem position.
When the instrument signal decreases, the bellows contracts (aided by an internal range spring) to move the beam and to pivot the flapper slightly further from the nozzle. Nozzle pressure decreases and through relay operation, the exhaust valve in the relay opens to release diaphragm actuator pressure to atmosphere, permitting the actuator stem to move upward. Stem movement is fed back to the beam by the cam to reposition the beam and flapper. When equilibrium conditions are obtained the exhaust valve closes to prevent any further decrease in diaphragm case pressure.
The principle of operation for reverse-acting positioner is similar except that as the instrument pressure increases, the diaphragm case pressure is decreased. Conversely, a decreasing instrument air signal causes an increase in the pressure to the diaphragm actuator.
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