What are the common issues with control valves?

Today we will discuss the practical common problems we mostly face with control valves and step-by-step troubleshooting tips.

Common Control Valve Problems

Follow these tips to troubleshoot any problem you are facing with a control valve.

1. The first step is to visually check the control valve and all its accessories. Check for any air leakage or physical damage. If found any issues then correct them.
2. Check the air supply from the air filter regulator (AFR) to the valve positioner or solenoid valve whichever is applicable. If no air supply is present, then check the isolation valve status in the air supply header. Open the isolation valve if found closed.
3. Now verify the AFR set pressure and it must be as per the value mentioned in the control valve datasheet.

4. If the air supply pressure is below the recommended pressure then valve movement will be sluggish. If the air supply pressure is more than the recommended pressure, then this may cause damage to the actuator. So, set the air pressure as per recommended settings. 
5. Give the command to open the control valve or close the control valve as per field conditions from the PLC or DCS system. If the valve doesn’t make any movement, then check the supply voltage. If voltage is not present, then check the cable connections thoroughly by back tracing method i.e. from the field to the marshalling cabinet.

6. Also, check the fuse healthiness in the marshalling cabinet. If found blown, then replace the fuse with the same rating fuse. If still problem persists, then check the cable healthiness.
7. Checking the resistance of the cable will help you to identify the problem with the cable. If found an issue in the cable pair, then replace the existing cable pair with a healthy spare cable pair. This step will help to get the supply voltage to the control valve.
8. Give 0 to 100% command from the control system to operate the control valve and do the valve stroke checking test. Also, verify the control valve feedback on the graphics during the stroke test.
9. If the valve stroke is ok as per system command but feedback is not ok, then feedback setting/configuration is required.

10. Here two types of valve feedback designs are there. One is open feedback and close feedback or the other is full 0% to 100% full analog feedback (depends on your control valve design). If the issue is there in analog feedback, then in most cases control valve calibration will solve this issue.
11. If the control valve doesn’t make any stem movement, then check for interlocks and permissive in the PLC or DCS logic i.e the control system may not send the command to the valve if the logic is not healthy.
12. If there is an issue in the control valve stroke, then carrying out the calibration will solve the issue.
13. After calibration, check the stroke again. If still there is an issue in the stroke, then check the mA to the input of the valve positioner. Give different commands from the control system and check the mA in the field. There might be an issue with the PLC/DCS analog output channel. But, this is a very rare case.

14. If doubt is there about the valve positioner’s functioning, then replace the positioner and do the calibration. After calibration, again check the stroke of the control valve.
15. Check all the control valve accessories like the quick exhaust valve, air volume booster, trip valve, and vent port. Verify that there should be no issue of blockage in them. Also, replace them if found any serious issues.
16. Later you can carry out the repair in your workshop (for replaced items). If you don’t have spares then first repair them and install the same.
17. If still there is an issue in the operation of the control valve, then remove all tubing and check the control valve movement by applying external air directly to the actuator.

18. Check whether the actuator is passing or not. If the actuator is having passing issues then do an overhauling of the actuator.
19. Now apply the air pressure as per the bench set mentioned on the nameplate or the datasheet of the control valve. The control valve’s movement should be smooth.
20. Verify the fail action of the control valve.
21. If the valve movement is sluggish or getting stuck, then decouple the actuator from the valve body (if the control valve is of the rotary type).
22. Check the actuator response again. If found ok, then there will be an issue on the valve body side. Drop the valve body for inspection and overhauling.
23. If the control valve is of linear type movement, then a dropping valve is necessary. Before dropping the control valve, make proper markings.
24. After overhauling, reinstall the control valve and do stroke checking again.

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Control valves that aren’t working correctly can cause problems for your entire system. While some issues are easy to spot, others require a powerful diagnostic tool, like GE’s ValScope. This article provides a brief introduction to five common control valve problems you should watch out for.

Deadband

Deadband is a range of input that doesn’t result in any output. In this range, the system is essentially “dead.”

For a control valve, deadband refers to a range of controller signal that fails to trigger any activity of the valve. Deadband happens when the valve needs to change direction. To compensate for deadband, the controller must send additional output, which can cause the valve to overshoot its target position. This in turn causes the process to overshoot its setpoint.

There are several consequences of high deadband:

• Increased dead time

• More errors from load disturbances

• Higher possibility of rupturing pressure relief discs or vessels

• More oscillations in the control loop

• Increased control variance

Hysteresis

Process Industry Forum provides a good, simple definition of hysteresis: “the difference between the valve position on the upstroke and its position on the downstroke at any given input signal.”

For example, imagine your controller output is 30% and the process variable, pressure, is 125 psig. When the controller output increases to 40%, the pressure increases to 150 psig. So far, so good. But then, when the controller output decreases back to 30%, the pressure only decreases to 130 psig.

About 1% to 3% hysteresis is normal, depending on if the valve has a positioner, but more than that can cause problems including:

• Process cycling around the setpoint

• Slower controller responses

• Increased control variance

Hysteresis can be caused by a faulty mechanical linkage, such as between the valve and the actuator. When this happens, it’s called backlash.

Stiction

Stiction is a combination of the words stick and friction. Like deadband, stiction causes a valve not to move, but instead “stick” in a particular position. Moving the valve requires additional force, which, also like deadband, can lead to the valve overshooting its position and the process overshooting its setpoint. Then the valve gets stuck in the new position.

Stiction can be caused by several factors: sticky valve internals, undersized actuators, tight shutoff, media viscosity, and more. The consequence is that the controller will never reach the desired setpoint.

Nonlinearity

Control valves are designed to be linear. This means that the flow increases linearly with valve travel. Deadband, hysteresis, and stiction can all cause nonlinearity, which means the relationship between the flow and valve travel isn’t stable.

Nonlinearity affects the tuning of the controller and make it so that the controller responds optimally at only one operating point. The further the valve position from this operating point, the more variable the flow.

Over- or undersized valves

Improper sizing can make all of these problems worse, especially for smaller valves.

When control valves are oversized, a small change in valve position can have a big effect on flow. When control valves are undersized, process bottlenecks occur. ISA-75.01 provides control valve sizing equations.

Are your control valves causing you problems? We’re here to help. Learn more about GE’s ValScope-PRO, a diagnostic tool for testing and monitoring control valve performance.