The control valve positioner is a critical component in the control valve loop. During the process control, the positioner helps to accurately position the valve.
Due to failure in the calibration of the positioner and control valves, the positioner may direct the wrong valve positioning which in turn leads to a significant impact on process control where an uncalibrated control valve is being used.
Let us consider an example of supplying a customer natural gas at a pressure of 10bar through a control valve which is being controlled by a controller.
If pressure suddenly rises to 12bar then the controller sends the signal to the control valve to get closed to bring the pressure to 10bar.
In case the control valve is wrongly positioned and failed to close, then the customer gains more quantity than required. This may lead to undesirable consequences.
For accurate positioning of the control valve plug, the positioner needs to be calibrated accurately.
A brief introduction to Fisher 3582 positioner:
- Fisher 3582 is a single-acting pneumatic positioner.
- It is an integral part of Fisher control valves and flow transmitters.
- If the working of the pneumatic positioner is not precise it can impact the overall performance of the control valve or a flow transmitter.
- Hence the calibration of the valve positioner is inevitable.
- It has a diaphragm actuated sliding-stem control valve mechanism.
- This Fisher 3582 pneumatic positioner device is responsible for the accurate positioning of the valve stem in a control valve.
- The control device sends an input signal to the pneumatic positioned, the positioner modulates the input signal to maintain the valve stem at the desired position. And these modulated signals are sent to the control valve actuator to adjust the stem position and alignment.
How does a pneumatic valve positioner works?
The pneumatic signal of 3-15 psi from the control device is sent to the pneumatic valve positioner. The positioner sends the control signal to the control valve to make the valve plug travel properly on the valve seat.
The control valve positioner may be either a direct-acting valve positioner or a reverse-acting valve positioner.
A direct-acting valve positioner transmits an increased output signal to the control valve as the input signal to it increases.
A reverse-acting valve positioner transmits a decreased output signal to the control valve as the input signal to it increases.
To know more about direct acting and Reverse acting valve positioner CLICK HERE
Specifications of fisher 3582 pneumatic positioner:
- Input signal: 0.2 to 1.0 bar (3 to 15 psig).
- Output Signal: Pneumatic pressure as required by actuator up to 95 percent of maximum supply
- Supply Pressure: 0.3 bar (5 psi) above actuator requirement. Valve travel changes less than 1.67 percent per bar (0.25 percent per 2 psi) change in supply pressure
- Supply Medium: Air or Natural Gas
- Pressure Rating: 2.4 bar (35 psig)
- Maximum Steady-State Air Consumption: 1.4 bar(20 psig) to 2.4 bar(35 psig)
- Maximum Supply Air Demand: 1.4 bar(20 psig) to 2.4 bar(35 psig)
- Linearity:1 percent of output signal span
- Hysteresis:0.5 percent of span.
- Pressure Gauges: 40 mm diameter with brass connection triple scale (PSI, MPa, and bar) or dual scale (PSI and kg/cm2).
- Pressure Connections: 1/4 NPT internal.
- Maximum Valve Stem Travel: 105 mm.
Procedure for calibrating a pneumatic valve positioner:
- Turn off the supply pressure to the valve positioner.
- Connect or reconnect the necessary tubing from the valve positioner output to the actuator supply connection.
- Connect the input supply to the valve positioner and set the input signal value at mid-range.
- The flapper assembly may be direct acting or reverse acting type.
- The flapper assembly must be adjusted to the 6th position in the beams operating quadrant.
- Now the valve positioner must be energized with supply pressure.
- The zero-degree index mark on the rotary shaft arm should align with the case index mark.
- The actuator travel position should be in its middle.
- If the actuator is not in the middle position, then check for loose linkage or the cam may not be installed properly.
- To achieve the desired input signal value corresponding to the initial travel point a minor nozzle height adjustment must be done.
- Apply a lower value input signal of the input signal range.
- If your fisher pneumatic positioner input range is 0.2bar (3psig) to 1.2 bar (15psig). Set the input to 3psig.
- Adjust the actuator to the right travel end by loosening the nozzle lock nut.
- For zero trim adjustment, the position of the nozzle needs to be changed.
- When the nozzle position is changed, the zero reference point is changed.
- Apply a higher value input signal of the input signal range.
- If your fisher pneumatic positioner input range is 0.2bar (3psig) to 1.2 bar (15psig). Set the input to 15psig.
- Now observe the actuator stem travel. If the actuator stem travel is less than the expected range, the travel must be increased by moving the flapper assembly to a higher level on the beam.
- If the actuator stem travel is more than the expected range, the travel must be decreased by moving the flapper assembly to a lower level on the beam.
- Repeat the procedure until the correct travel is achieved.
- The position of the flapper assembly must be changed every time to provide a proper zero.
- Moving the flapper assembly toward zero on the beam scale decreases stem travel.