Would it be possible to use a control valve without a positioner?

Would it be possible to use a control valve without a positioner ?

Yes, it is possible to use a control valve without a positioner, but whether it’s advisable depends on the specific application and requirements. Here are some key points to consider:

Basics of Control Valves and Positioners

  • Control Valve: A device used to regulate the flow of a fluid by varying the size of the flow passage.
  • Positioner: A device that ensures the valve reaches the desired position by comparing the control signal (usually from a controller) to the actual position of the valve and adjusting it accordingly.

Using a Control Valve without a Positioner

  1. Direct Control: In the simplest systems, the control valve can be operated directly by the control signal (pneumatic, electrical, or hydraulic). The controller sends a signal to the actuator of the valve, which moves the valve to a position that corresponds to the signal.
  2. Advantages:
  • Simplicity: Fewer components mean a simpler system with potentially lower costs and reduced maintenance requirements.
  • Lower Initial Cost: Positioners add to the initial cost of the control system.
  1. Disadvantages:
  • Accuracy and Precision: Without a positioner, the accuracy of the valve position may be reduced, especially if the control valve is subject to varying loads or friction.
  • Hysteresis and Deadband: Direct control may suffer from issues like hysteresis (lag between input signal and movement) and deadband (range in which no movement occurs), leading to less precise control.
  • Non-linearity: Many control valves have a non-linear relationship between the input signal and the valve position. Positioners can linearize this relationship, improving control performance.
  • Response Time: Positioners can improve the response time of the valve, which is critical in fast-acting control loops.

Applications Where Positioners Are Not Essential

  • Simple On/Off Control: In applications where the valve is either fully open or fully closed, a positioner is often unnecessary.
  • Slow Processes: In processes where the fluid dynamics are slow and do not require highly precise control, direct control without a positioner might be sufficient.
  • Constant Conditions: If the process conditions (pressure, temperature, etc.) are relatively constant and do not impose significant variations on the valve, a positioner may not be needed.

Applications Where Positioners Are Beneficial or Necessary

  • High Precision Requirements: Processes requiring precise control of fluid flow, pressure, or temperature will benefit from the enhanced accuracy provided by a positioner.
  • Variable Conditions: When process conditions change frequently or significantly, a positioner helps maintain accurate valve positioning.
  • Complex Control Strategies: Advanced control strategies, such as PID control, typically require the enhanced performance that a positioner provides.

While it is technically feasible to use a control valve without a positioner, the decision should be based on the specific requirements of the control system. For simple, low-precision applications, omitting the positioner can be a cost-effective choice. However, for applications demanding high precision and responsiveness, a positioner significantly enhances control performance and reliability.