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Tuesday, 7 July 2015

Valve Positioner

In some process applications, it is often necessary to provide additional control accuracy to the final control element.  This is usually accomplished by using valve positioners.  The valve positioner adjusts the valve plug position so that it is in exact proportion to the incoming control signal commanded by the controller.  However, there is some disagreement to arbitrary use of positioners in certain control situations.  Positioners are not normally required for ON-OFF services or in a fast moving process such as in flow loops.  In fast moving process control loops, positioners may even cause stability problems like overshooting.
Overshooting is a continuous fluctuation of the process caused by over reaction of the control valve to a slight input change
The following are the application of positioners on which there is general agreement that they should be used;
  When a maximum loading pressure to the actuator is greater than 20 psi is required.  Some control valve actuator needs 6-30 psi to close and open the valve.
   When reversing action is necessary (this may be accomplished with a reversing relay also).
   One valve sizes 6 inches and larger.
   Where long transmission lines exist.
   Where the pressure drop across the valve is high (100 psi or more).
   Where excessive friction may occur in packing glands and guides.
   In sludge services, which may cause sticky stems and guides.
   In split range operation which requires full valve stroke from some fraction of the controller signal (for example, a fully open and fully close action of one valve if the output of the controller is 3-9 psi and 9-15 psi to close or open the second valve.

The Split Range Operation
The use of two or more control valves from one controller signal is called split range operation.  This operation is usually accomplished by using valve positioners calibrated to give a full valve travel at a signal range less than the full range of the controller.  The most commonly used split range operation consists of two control valves, one control valve is operating at 3-9 psi and the other is 9-15 psi.
Figure 7 illustrates the application of split range valves to control gas pressure from the separator and knock out drum.

Pressure control valves, PCV-101A AND PCV-101B are controlled by one controller, (PIC-101).  Each positioner moved its valve for only a part of the controller output range.  For example, PCV-101A is fully close when the input signal is 3 psi and fully open at 9 psi.
While PCV-101B is still fully close at 9 psi and fully open at 15 psi.  These two control valves connection is called split range.
During normal plant operation only PCV-101A is controlling the pressure of the knockout drum.  When the pressure increases the control valve opens to regulate the pressure according to the set point.
However, when something goes wrong that PCV-101A cannot provide the needed range ability to control the pressure build up in the knockout drum, PCV-101B will start to open to release the pressure up to the flare stack.  By opening the second valve in this split range type control will definitely reduce the pressure down back to the set point.

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