Positioners play a critical role in improving control valve performance by addressing issues such as friction, deadband, and process stability. Below are key guidelines for the use of positioners in various applications:
1. Reducing Deadband
Control valves without positioners—even those with “low-friction” packing—can exhibit deadband up to 5%. Deadband greater than 1% can impair control loop performance. Positioners help reduce deadband to below 1% by overcoming friction in valve movement.
2. Minimizing Stick/Slip Effects
Stick/slip behavior occurs when a valve momentarily sticks, then suddenly slips to a new position. This results in a limit cycle that causes the process variable to overshoot the setpoint. Positioners with stem feedback mitigate stick/slip, helping to keep the process variable closer to the desired value.
3. Enabling Split Range Control
Split-range applications often require two valves to operate over different segments of the control range. Positioners are necessary to ensure accurate valve travel over compressed input signal ranges and to maintain control in the overlap region where both valves may be partially open. This ensures seamless transition and avoids control gaps.
4. Improving Shutoff and Seating Force
Positioners can increase actuator force at valve travel limits. For example, in air-to-close actuators, a positioner can apply full supply pressure to tightly seat the valve. In air-to-open actuators, the air can be fully vented, allowing the spring to apply maximum force to the seat.
5. Required for Double-Acting Actuators
Double-acting piston actuators require positioners to control pressure on both sides of the piston. Precise pressure control can only be achieved using stem position feedback.
6. Process and Valve Linearization
Positioners with feedback cams can linearize valve characteristics to match process requirements. However, this method is generally effective only in slow-reacting loops, such as temperature control applications.
7. Superior Performance of Two-Stage Pneumatic Positioners
Two-stage pneumatic analog positioners (using nozzle-flapper and relay designs) offer better accuracy and faster response than single-stage designs (which use a spool valve). High gain and airflow make them ideal for fast processes like liquid flow and pressure control.
8. Improved Control in Fast Processes
Historically, positioners were discouraged in fast loops due to instability with pneumatic controllers. However, modern digital controllers offer wide tuning flexibility. Loop stability can now be maintained by lowering controller gain while retaining fast integral action.
Guidelines for Digital Positioners
Digital or “intelligent” positioners enhance the functionality of traditional analog models by integrating diagnostics, automation, and advanced communications.
1. Automated Calibration
Digital positioners can perform automatic zero and span calibration in minutes—significantly reducing time compared to manual calibration of analog devices.
2. Output Signal Characterization
Digital positioners can linearize the output signal, improving loop performance by ensuring a consistent process gain. Unlike cam-based linearization, this method operates independently of valve stem feedback.
3. Digital Noise Filtering
Digital filters can reduce process noise. While beneficial, filters increase process response time and may require controller retuning.
4. Alarm Generation
Digital positioners can monitor conditions such as valve deviation, travel limits, or unexpected behavior. These alarms can be relayed to the operator interface for timely response.
5. Maintenance Data Collection
Positioners can log valve activity—such as total stem travel or reversals—to aid in predictive maintenance scheduling and improve reliability during outages.
6. Controlled Stroke Speeds
In applications prone to hydraulic hammering, digital positioners can slow valve movement to prevent system damage.
7. Travel Limit Enforcement
Some processes require that valves never fully close or open. Digital positioners can enforce travel limits to meet these application constraints.
8. Automated Performance Testing
Digital positioners integrate with control valve diagnostic software to perform automated performance tests. Comparing test results over time helps identify valves needing maintenance, optimizing outage planning.
9. Resistance to Vibration
With few moving parts, digital positioners are more resistant to vibration. Solid-state electronics ensure consistent performance in high-vibration environments.