A positioner should be considered in the following circumstances:
- To increase the pressure that a particular actuator and valve can close against.
- When exact valve positioning is required.
- To linearize a non-linear actuator.
- To speed up the valve response. The positioner uses higher pressure and greater air flow to adjust the valve position.
- When controlling with wide throttling range.
- When valves are handling sludge or solids in suspension.
- As?valve positioners?know the valve’s exact position, they provide more precise control than can be achieved by an actuator on its own.
- Positioners make control valves respond more rapidly to changes in the process variable, which as a result, minimizes the amount of time operating by the system above or below the set point.
- Positioners can cope with large variations in forces acting on plug.
- Varying differential pressure across the valve shows instability in the control loop. A positioner helps stabilizing valve position.
- A positioner allows you to put distance between the controller and the control valve; therefore, more flexible configurations and functions will be provided.
- They remove the effects of large distances between valve and positioner.
- They minimize the effects of friction.
- They eliminate hysteresis which reduces productivity.
- It allows for split ranging. For instance, one controller for two valves.
- It allows use of piston actuators with high instrument air supply pressures.
- It allows a wide range of flow variation. For instance, operate at less than 10% travel under normal conditions.
- It allows the use of characteristic cams in rotary valves.
