.valve positioner

A positioner is a motion-control device which actively compare stem position against the control signal, adjusting pressure to the actuator diaphragm or piston until the correct stem position is reached.

Relationship between the v/v steam movement and actuating pressure will be linear only if there are two forces acting against each other, one is downward air pressure to actuator and other one is upward spring force.

Unfortunately, there exist many other forces acting on a valve stem besides the actuator force and the spring’s reaction force. Friction from the stem packing is one such force, and reaction force at the valve plug caused by differential pressure across the plug’s area is another. These forces conspire to re-position the valve stem so stem travel does not precisely correlate to actuating fluid pressure.

Due to these other forces relationship is not linear. A common solution to this dilemma is to add a positioner to the control valve assembly.

Positioners essentially act as control systems within themselves: the valve’s stem position is the process variable (PV), the command signal to the positioner is the setpoint (SP), and the positioner’s signal to the valve actuator is the manipulated variable (MV) or output. Thus, when a process controller sends a command signal to a valve equipped with a positioner, the positioner receives that command signal and applies as much or as little air pressure to the actuator as needed in order to achieve that desired stem position. Thus, the positioner will “fight” against any other forces acting on the valve stem to achieve crisp and accurate stem positioning according to the command signal. A properly functioning positioner ensures the control valve will be “well-behaved” and obedient to the command signal.

Boiler level controller (2 element):

Initially the 2-element control system is operating under normal load condition and MV = SP, PV = CSP and LCV= 50% open.

  If large & sudden↑ in steam load arises, the 2-element control system will respond as follows:  

• FT senses rise in steam flow
  • PV ↑
  • O/P (output) of R-A slave controller↓
  • ATC LCV opens more increasing feed water supply 
  • In the meantime, steam pr ↓ and ACC increase fuel to burner and water level ‘Swell’ in steam drum
  • MV↑
  • O/P of D-A Master controller↑
  • CSP of Slave controller↑
  • PV ↓ w.r.t. CSP↑
  • O/P of R-A Slave controller↑
  • ATC LCV closes towards 50%

The two-element control system provides better control than single-element system because the LCV was opened more than 50% and then closes back towards 50% resulting in a net increase in feed water supply.  Thus a dangerous drop in water level is avoided, as shown in ‘green’ making it suitable to deal with large and sudden increase in load.  

Two element boiler level controller:

Explanation → start from slave controller

Boiler demand increase→ flow reduce → goes to slave controller (start like this)

PV and P bellows same side means→ negative feedback

PV and P opposite side means → positive feedback