What are the main causes of control valve hunting?

In process industries, valve hunting is a prevalent problem. This occurs when a control valve opens and closes quickly instead of staying in one place. This oscillation makes process variables unstable, wears down the packing, and makes the loop work poorly. This guide tells you what valve hunting is, what causes it, and how to find and stop it.

What is Valve Hunting in Control Valves?

• 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 valve hunting in process control systems?

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.

Defective hardware

• 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 fix valve hunting in control valves?

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.

Can improper valve sizing lead to hunting?

• Yes. One of the most prevalent reasons for valve hunting is that the valves are not the right size.
• If the control valve is too big, it will open and close quickly around the setpoint when the controller output changes by a small amount.
• This frequent movement hurts the packing and makes it less accurate.
• On the other hand, a valve that is too small can create severe pressure drops, cavitation, trim wear, and unstable flow conditions, all of which can lead to hunting.
• To get steady process control, you need to use the right Cv calculation to size the valve correctly.

How can valve positioner calibration reduce hunting?

• A control valve positioner makes sure that the valve stem goes to the exact place where the controller needs it to go.
• Hunting can happen if the positioner is not set up correctly, which means the valve may go too far, not far enough, or keep changing its position.
• When the actuator response is in sync with the controller signal, there are no deadband or feedback problems, and the valve movement is stable.
• Modern smart positioners also give you diagnostic data that helps you find stiction, leakage, or misalignment early on, which cuts down on hunting even more.

Quick Checklist to Diagnose Control Valve Hunting

Step	What to Check	Possible Issue	How to Interpret / Action
1	Put controller in manual mode	Loop tuning issue	If oscillations stop when in manual, the root cause is poor controller tuning. Re-adjust PID parameters to stabilize the loop.
2	Observe valve stem travel	Stiction or trim wear	If the valve stem hesitates, sticks, or suddenly jumps, it indicates stiction or internal wear. This leads to overshoot and unstable control. Inspect gland packing and trim for damage.
3	Check Cv sizing vs process demand	Oversized / undersized valve	An oversized valve causes rapid open/close movement (hunting), while an undersized valve forces high pressure drop and cavitation. Compare valve Cv with actual process requirements.
4	Inspect the valve positioner	Spool valve wear or feedback issues	A worn or stuck spool valve, faulty feedback linkage, or air leakage in tubing can cause constant re-adjustments and hunting. Calibrate or replace the positioner if deviations are detected.
5	Review maintenance and operating history	Packing deterioration or actuator leakage	Repeated cycling damages gland packing and actuator seals, leading to leaks and instability. If hunting is consistent across control ranges, schedule preventive maintenance and replace defective hardware.

Control valve maintenance procedures

If the control valve became stuck while it was being operated, correct the problem by following the instructions in the following link.

How to do maintenance on struck control valve?

Check out the link below for more information regarding the control valve positioner’s calibration technique.

Calibration of control valve positioner

The bench setting and calibration technique of the control valve can be found by following the link provided below.

How is control Valve calibrated?

Check out the link below for further information on how to check the stroke of control valves.

Stroke checking procedure of control valves

If you would like to acquire the preventative maintenance process for the control valve as well as a printable PM checklist in excel format, please click on the link that has been provided for you below.

How to do Control Valve Preventive Maintenance?

Control valve – PM Maintenance checklist – Download

FAQ on Control valve Hunting

What is valve hunting in control valves?

When a control valve is valve hunting, it keeps opening and closing instead of staying in the right place. This oscillation happens when the valve can’t stay at the setpoint because of things like bad controller tuning, stiction, valves that are too big, or positioners that don’t work. Hunting makes processes less stable, wears out gland packing faster, and causes maintenance problems.

What causes valve hunting in process control systems?

There are a number of things that might trigger valve hunting:

• Bad PID loop tuning or controller setup
• Control valve sizing that is too big or too little (Cv mismatch)
• Packing friction causes stiction in the valve stroke.
• Positioner problems or mistakes in calibration
• Leakage from the actuator or damage to the trim 

Each of these problems makes it hard for the valve to follow the controller output smoothly, which causes it to oscillate around the setpoint.

How do you fix valve hunting in control valves?

To fix valve hunting, you need to figure out if the problem is with the control loop or the valve hardware. The steps are:

1. If the oscillations stop when you switch the controller to manual mode, the problem is loop tuning.
2. Check the valve stem travel for signs of trim wear or stiction.
3. Check the size of the valve Cv to make sure it meets the needs of the process.
4. If you find any differences, calibrate or replace the valve positioner.
5. Do preventative maintenance to change out worn seals or packing.

You may stop hunting and get steady process control back by diagnosing in a systematic way.

Can improper valve sizing lead to hunting?

Yes. An oversized valve reacts excessively strongly to slight changes in signal, which makes it cycle open and closed quickly. A valve that is too small makes the pressure decrease, raises the risk of cavitation, and makes the operation less stable. Hunting happens in both cases. Correct Cv-based sizing makes sure that the valve responds smoothly and steadily.

How can valve positioner calibration reduce hunting?

When the positioner is calibrated correctly, the actuator moves the valve stem exactly as the controller says it should. If the positioner isn’t calibrated correctly, the valve can go too far or keep changing, which might cause hunting. Calibration gets rid of deadband, makes feedback more accurate, and keeps control stable. Modern smart positioners can also find stiction, leaks, or misalignment, which makes hunting even less likely.

How do you identify valve hunting?

When the process should be stable, valve hunting can be seen by watching the valve stem or actuator pressure keep changing. Some signs are:

• The process variable goes up and down around the setpoint.
• The valve stem doesn’t stay in one place.
• Fluctuations in the movement of the actuator or valve that can be heard
• Packing or seals wear out faster

Smart positioners and valve travel graphs are examples of diagnostic instruments that can confirm hunting.

What is the difference between valve stiction and valve hunting?

• Static friction in packing or trim can cause the valve stem to stick, which is called stiction. The valve doesn’t move until enough force builds up, and then it jumps.
• Hunting happens when the valve keeps moving about the setpoint because of tuning, sizing, or hardware problems.

Stiction often causes overshoot, which is one of the key things that makes hunting happen.