What is control valve hunting?
Table of Contents
- Control valve hunting is a condition when the valve rapidly closes and opens.
- It means that the control valve is unable to settle on the proper position to keep the predetermined process conditions.
- This makes the process variable in a closed loop oscillate about its set point.
- Hunting is a phenomenon that has several causes.
- The issue might stem from the controller, from faults in specific components of a control valve, or even from the way the process is being executed.
- Continuous valve hunting will result in gland packing deterioration and significant departures from the desired set point.
- It is essential to identify the true cause of the low performance in order to take the required corrective action and solve the issue and improve control loop performance.
- The many causes of hunting are covered in this article, along with methods of diagnosis.
What causes a valve to hunt?
Control loop issues
- The controller mode is switched from automatic to manual and the response is evaluated as the conventional approach to determining whether the issue is within the loop.
- If the oscillation stops, then the loop is at fault. These issues are typically present in non-linear processes.
- Hunting may also occur as a result of the hysteresis effect. The result is a sluggish behaviour of the process loop.
- The configuration of the controller will not be able to fix this mechanical issue. Control valve hunting caused by loop issues can be resolved by properly tuning the controller.
- If it is not hunting in manual, other reasons, such as actual variances in process variables, valve sizing, etc.,
Sizing of control valves
- The controllability of the valve is significantly influenced by the control valve’s size.
- The flow coefficient, abbreviated as Cv, which is the volume of US gallons of 600°F water that can pass through a completely open valve with a 1 psi pressure drop, is used to calculate valve sizing.
- The Cv, which is established by the valve’s design, remains constant.
- If the valve body style or valve trim differs, control valves of the same size may have varied Cv.
- Problems with control valve sizes arise when the total process gain is either low or extremely high.
- Control valves are frequently sized to handle increased flow rates in the future. This results in the purchase of a valve that is marginally larger for the application, which can result in inaccurate control.
- Due to its excessive opening and closing, an oversized valve will result in hunting, packing damage, and imprecise control.
- A valve that is too small will need a significant pressure drop across it in order to maintain the proper flow and may not have the necessary capacity.
- This increases the stress on the pump and increases the risk of cavitations.
- Cavitation and flashing are the two main issues that cause damage to the trim on the control valves, which results in fluctuations in process control.
Control valve positioner
- In order to get the control valve actuator into the balanced position needed for controlling the process variable, a valve positioner is a device that raises or lowers the air pressure applied to it.
- It has a spool valve to control the air flow, but it eventually wears down from constant use or from airborne dust particles.
- As a result, the spool valve may become stuck at a specific location, which may lead to an increase in air pressure.
- Overshoot results from the spool piece being released from its position as a result of the increase in air pressure.
- As a result, the location of the valve fluctuates and control over it is lost, which causes hunting.
- The positioner’s exposure to high temperatures owing to radiation from surrounding process tanks is another factor in the control valve hunting caused by the positioner.
- Damage to positioner seals and tubing will result from this. A positioner may regulate its output by using a feedback link to determine the actual position of the valve.
- The valve may not operate in the proper position if the feedback link is broken or severed due to fluid forces, friction, etc.
- Modern smart positioners are equipped with unique characteristics to find such deviations.
Stiction in valve stroke
- When a valve experiences static friction, also known as stiction, it stops moving at a specific location and requires extra forces to move again.
- Hardened gland packing or viscous flow in the trim could be the culprits here. The valve then shifts to an overshoot position after the increased forces are sufficient to pull it away from the stiction point.
- The process variable exceeds its set point as a result of this overshoot. Monitoring the correlation between the controller output and the process variable allows for the observation of stiction.
- The valve actuator needs to have the right size, and the torque operating on the gland packing needs to be within the acceptable range to prevent stiction.
- Additionally, internal wear in trims might induce valve hunting. This prevents the valve from being shut off completely.
- The control valve will become uncontrollable at higher operating ranges due to a damaged trim.
- In a control valve, gland packing is utilized to guarantee that the process medium remains contained within the valve body.
- If it is destroyed, leakage will occur through the bonnet, threatening the working environment.
- Leakage in the valve actuator is another element that contributes to valve hunting.
- The stem is initially positioned by the positioner, but because of leakage, the stem is permitted to move continuously, forcing the positioner to re-adjust its output and creating an infinite search for the stem’s position.
- Control valves frequently hunt with steady state control signals, and this is one of the most frequent causes of this.
How do you stop valve hunting?
Analysis and diagnosis of hunting
- Problems with the loop or other influences can result in oscillations or hunting.
- Put the controller in manual mode and watch to see if the hunting stops to find the issue.
- If it stops, the loop itself is the issue, which may be fixed with the appropriate adjustment.
- Internal oscillations might be brought on by tuning issues or malfunctioning machinery.
- The issue may be caused by damaged valve components or changes in process parameters if the valve is still hunting in manual mode.
- Stiction and positioner overshoot are the most frequent causes of control valve hunting. Figure depicts the output response of a valve with stiction.
- To determine whether the control valve is hunting due to improper controller tuning or mechanical problems in the control valve, it is suggested to output a constant pressure to the control valve actuator by temporarily bypassing the controller’s output and then observing the output response.
- A linear potentiometer (position transmitter) is used to detect the valve stem’s movement, and a pressure sensor(smart positioner) is used to measure the output pressure to the positioner.
- By attaching the sensors to a data capture card and visualizing the data in the monitor (Labview), it is possible to generate the valve stem travel vs controller output.
- The microcontroller receives the input from the controller’s set point and pressure sensor.
- It is noted that the pressure deviates from its comparable set point.
- This is considered as hunting behaviour if it occurs more than five times.
- In that instance, the controller’s output is isolated, and current to pressure converter (i to p converter) automatically generates the pressure corresponding to the set point and provides it as input to the control valve positioner.
- A few seconds later, the deviation is once more tracked. It is safe to conclude that there is no issue with the control valve and its accessories if the deviation is reduced.
- Because of this, the controller needs to have a special tuning applied to it for the loop.
- But if the valve is still hunting, there may be trim damage or stiction from gland packing causing the issue.
- It is simple to locate the stiction location with the assistance of the gathered graph where there is a dead band.
- In situations where there is continual hunting, there is no evidence of a dead band.
- Thus, trim damage is mostly responsible for the possibility of hunting.
- This technique also enables determining whether hunting is present across all control ranges or only in regularly operated areas.