Control Valve

Working of Pneumatic Valve Positioner

PNEUMATIC VALVE POSITIONER
PNEUMATIC VALVE POSITIONER

What is a pneumatic valve positioner?

The pneumatic valve positioner is an instrument used for positioning the control valve stem in accordance with the pneumatic signal of 3-15 PSI received from a controller or from a manual loading station.

A valve positioner is used to increase or decrease the air pressure to drive the actuator.

The pneumatic positioner ensures a reliable and accurate operation of the Control Valve.

On what principle the pneumatic valve positioner works?

The pneumatic valve positioner works on the principle of force balance principle.

VALVE POSTIONER

In this force balance principle, the chamber is furnished with three ports, one each for the instrument air supply input, the control signal output, and the exhaust.

The pressure on a lever arm is varied or adjusted by a flexible diaphragm through the external sensor.

When the sensor presses the right-hand end of the lever in the downward direction, the left-hand end of the lever is lifted in the upward direction to open the supply air valve.

Due to this the pressure in the chamber and the control signal at the output pipe are increased, causing a movement in the controlled device.

As the air pressure in the chamber increases, the diaphragm is forced in an upward direction against the sensor pressure until the system is again in balance at higher pressure with the supply air valve closed.

What is the purpose of a pneumatic valve positioner?

The main role or purpose of the positioner in a control valve is to interface between the PLC and actuator system to adjust the position of the plug precisely for the opening and closing of a control valve.

The positioner in the control valve for a particular loop acts as a translator. It converts the PLC language to the corresponding actuator language.

It means the positioner converts the given 4 to 20 mA DC current signal to a proportional 3 to 15 PSI pneumatic air pressure signal.

 Explain the working of the pneumatic valve positioner?

The pneumatic positioner contains an I-P transducer inside.

In general, this I-P transducer converts the given 4 mA DC current signal to a 3 PSI pneumatic air signal, and a 20 mA DC current signal to a 15 PSI pneumatic air signal.

But these pneumatic signals are proportional in the middle range so we call 3 to 15 PSI air pressure the pneumatic signal.

Operation of control valve through PLC:

So now we can understand that in order for PLC to open the control valve for 50% of its full range the PLC must send a 12 mA DC current signal to the electro-pneumatic positioner then the positioner will convert the received current signal of 12 mA to its corresponding pneumatic signal to 9 PSI accordingly, and this pneumatic signal is directly sent to the actuator.

The instrument air supply is required as another input to the valve positioner.

This instrument’s air supply is filtered and regulated through an air filter and regulator to provide clean air to the valve positioner.

Using sufficient instrument air pressure the valve positioner is able to convert the required pressure signal for moving the actuator to the right amount.

The positioner also needs to receive feedback to precisely position the valve stem for opening and closing at desired ranges.

This feedback is sent by the control valve to the positioner using a mechanical mechanism.

In this way, the positioner decides the required amount of pressure on the actuator to move the valve stem.

What are valve positioners?

Valve positioners are a device that allows us to throttle the control valve at desired set point.

The input signal and the feedback signal are the two inputs given to the valve positioner.

The input signal is a control signal that provides a set point to the positioner, and the feedback signal indicates the current position of the control valve.

The input signal is provided by the pneumatic control systems bellows.  As the input signal is increased the bellows will expand and acts on a beam. The beam pivots and moves a flapper in relation to a nozzle. 

As the flapper position has changed the nozzle pressure varies accordingly and operates a pneumatic relay.

At this stage, we have a pressure-to-pressure transducer for the input.  To make the positioner we must have feedback. The feedback confirms that the control valve is responding to the input signal and the valve position corresponds to the given input signal. Here the mechanical components provide the feedback signal from the control valve to the positioner.

The cam is attached to an arm that rotates as the valve stroke. The feedback also acts on the beam and is an opposing force to the input signal in the bellows.

 If an increase in the input signal  is made to the device the bellows will again act on the beam and move the flapper close to the nozzle.  As the nozzle and output pressures increase the valve moves and feedback from the cam acts on the opposite side of the beam moving the flapper away from the nozzle.

The beam is referred to as a summing component. The input and feedback are both applied to the summing beam and are constantly compared to each other.

If the forces are equal the nozzle and the flapper relationship remain stable and output pressure from the device is constant.

The valve position is then maintained. If a change to one of the forces should occur the flapper position is again adjusted in a relation to the nozzle and output pressures change until both forces are once again equalized.

 If the positioner is properly calibrated the valve position will correspond to a given control signal

Let us consider an example:

3 to 15 PSI control signal is used for the input.

Control signal Valve opening Valve position
Pneumatic Current
3 PSI4 mAFully  Opened0%
9 PSI12 mAHalf Opened50%
15 PSI20 mAFully Closed100%

Referring to above table

  1. The valve is fully opened at 0% for a control signal of 3 PSI. 
  2. The valve travel will be at 50% for a control signal of 9 PSI. it means the valve is half-opened. And
  3. The valve is fully closed 100% for a control signal of 15 PSI.

The positioner gives the control valve ability to overcome valve friction and process forces that may cause deviation in valve position.

There are many additional benefits to using a control valve positioner other than just a stem position.  Accuracy positioners are often selected to achieve throttling control of piston actuators to accommodate non compatible control signals. To ensure proper shut off of the control valve, to enable split ranging, and to change the gain characteristics of the control valve.

What are the types of valve positioners?

There are 3 primary types of positioners:

  1. Pneumatic Valve Positioner.
  2. Electro-Pneumatic (EP) Valve Positioner.
  3. Digital Valve Positioner.

Where is a valve positioner located?

  • In linear control valves the valve positioners are usually mounted on the yolk or top casing of a pneumatic actuator.
  • In rotary control valves the valve positioners are usually mounted near the end of the shaft

The positioner is connected mechanically to the valve stem or valve shaft.

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