How to control a control Valve using Pneumatic control system

instrumentation

#1

pneumatic control valve

  • A Bellow-lever system controls the pneumatic valve shown in the above system. Here the pneumatic valve is pressure to open type control valve, that is applying pressure opens the valve.

  • The pressure to control the valve is given exerted through the flapper nozzle system

  • The lever that controls the nozzle opening and thus controls the pressure through the nozzle is controlled by four bellows at either end.

  • At the right side top is the process variable (PV) and setpoint bellow is connected to the right side bottom.

  • Left end of the lever is given with the two feedbacks, on the top feedback from stem position is given and at the bottom the lever feedback from the input pressure to the valve. Instead of receiving a pneumatic signal directly from the output line of the controller, the reset bellows receives its signal through another pneumatic line, connected to a location in the control system where the final effect of the output signal (m) is seen.

  • Setpoint and PV is given through two different pneumatic lines at the right side of the lever. As PV increases the nozzle open and the output pressure to the valve reduces and the valve starts closing the flow line.

  • The Setpoint is given opposite to the PV which is directly given from the air supply through a separate pneumatic line.

  • The controller output is given to the left end of the lever counter to the stem position of the valve which is the external reset signal. The external reset signal comes from a pneumatic position transmitter (ZT) mounted to the sliding stem of the control valve, sending back a 3-15 PSI signal representing valve stem position.

  • If something happens to the control valve causing it to freeze position when the controller commands it to move – suppose the stem encounters a mechanical “stop” limiting travel, or a piece of solid material jams the valve trim so it cannot close further – the pneumatic pressure signal sent from the position transmitter to the controller’s reset bellows will similarly freeze.

  • After the pneumatic lag caused by the reset restrictor valve and bellows passes, the reset bellows force will remain fixed. This halts the controller’s integral action, which was formerly based on a “race” between the output feedback bellows and the reset bellows, causing the feedback bellows to “lead” the reset bellows pressure by an amount proportional to the error between PV and SP

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