What are the usual problems of control valves?

Field Instrumentation Valves
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What are the usual problems of control valves?

The problems of control valves usually manifest themselves as stiction, deadband and hysteresis. image

Deadband

Dead band is also called neutral zone or dead zone. Deadband is a general phenomenon where a range or band of controller output values fails to produce a change in the measured process variable. This is bad for process control.

Process control systems these days execute at a rate of about 3 times per second. On top of that, each time it executes, the output changes in the magnitude of usually less than 1%. But most relevant in the case of deadband, the changes can occur in either direction.

If a control valve is suffering from a deadband problem, when the controller output reverses direction, the control valve does not respond. Therefore the process variable also does not respond to the command of the controller. The controller does not know it, it thinks that its previous command is not good enough and so issues another (sometimes more drastic) command. When the control valve finally comes out of its deadband, the controller command has caused it to overshoot.

The controller then tries to go back the other direction only to be faced with the same situation. And the process will be driven to overshoot in either directions and cycles continuously forming what is called a limit cycle.

image Hence deadband is a region of input values in the domain of a transfer function in a control system or signal processing system where the output is dead or no action has occurred.

Stiction

The word stiction is made from the words stick and friction. This is somewhat similar to deadband except that it does not only happen when the controller changes direction. Again stiction (also known as ‘sticky valve’) can be due to a variety of reasons, a common one which is packing friction.

As far as process control is concerned, the effect of stiction is also like deadband whereby the valve fails to respond when required and when it does respond, overshoots the setpoint. The controller then tries to bring it back the other way.

Stiction is caused when the static (starting) friction exceeds the dynamic (moving) friction inside the valve. Stiction describes the valve’s stem (or shaft) sticking when small changes are attempted. Friction of a moving object is less than when it is stationary.

Stiction can keep the stem from moving for small control input changes, and then the stem moves when the force is enough to free it. The result of stiction is that the force required to get the stem to move is more than is required to go to the desired stem position. In presence of stiction, the movement is jumpy.

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Hysteresis

The maximum difference in output value for any single input value during acalibration cycle, excluding errors due to dead band. For example, when the controller output is 20%, the process variable is 30°C. when the controller output increases to 25%, the temperature increases to 35°C. However, when the controller goes back down to 20%, the temperature only goes down to 33°C.

This results in different process gains in both directions and will confuse the controller, which has been tuned for only one process gain. We have to remember that industrial controllers are linear. The following figure illustrates a case of valve hysteresis.

Hysteresis is a path dependent characteristic that can be attributed to materials not being able to return to their original shape and size after being stretched or deformed. Dead band represents the amount of signal change required to reverse direction and is characterized by a zero slope region where the input signal reverses.

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Also…

  1. Leakage from packing (must replace the packing).
  2. Leakage from gaskets and/or fasteners (tighten the screws or replace gaskets).
  3. Stem mechanically stuck (can use WD-40 or any other rust remover).
  4. Passing (is known by process, and usually we need to readjust the zero position mechanically from the positoner feedback or remove the valve and replace the plug)
  5. External body cracked. ( should replace the valve if the crack is big)
  6. Internal body cavitation and/or erosion (must isolate the valve from process and remove the valve to check and make sure that it will need to go to the workshop for maintenance or even replaced)…

I hope this can add to the topic

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I know this thread is a bit old but i would like to point out some more disadvantages :

  • Control valves are relatively complex mechanical devices, which means they can be more challenging to design, install, and maintain than simpler types of valves.

  • Control valves can also be relatively expensive compared to other types of valves, especially if they are designed for high-performance applications or have specialized features.

  • Control valves require significant energy, especially for high-pressure or high-flow applications. This can lead to higher operating costs and increased energy consumption.

  • Control valves can be more prone to failure than other types of valves, especially if they are subjected to high pressures, high temperatures, or corrosive environments.

  • Because control valves are designed to modulate flow, they can be more sensitive to changes in flow rates, pressure drops, and other factors that can affect valve performance.

  • Control valves may require specialized training and expertise to install, operate, and maintain, which can be a barrier for some organizations or individuals.