Hundreds, if not thousands, of control valves may be present in any one industrial facility. Whether it is a medical molding manufacturer or a top auto part dealer, no industry can survive without valves.
To keep flow, level, pressure, and temperature, among other process variables, within the set parameters, these valves serve as the last line of defense.
Inadequate attention is paid to control valves, unfortunately. There are two possible outcomes. Engineers, managers, and others who are tasked with maximizing production output frequently fail to recognize the significance of control valves. They tend to ignore the core component of the control loop in favor of focusing on the periphery.
The lack of regular control valve maintenance is a common cause of subpar performance. That could help explain why their value is often overlooked.
There are many types of neglect, and we’ll discuss all here. This section will begin by discussing the function of control valves. Then we’ll examine the rationale and methodology behind control valve servicing and repairs.
So lets get started!
What Factors Lead To Valves Functioning Below The Line?
The most frequent issues with control valves and the nature of these problems and their root causes are here:
- Hysteresis
Hysteresis is a situation in which the upstroke and downstroke of the identical input signal provide different outcomes.
For the sake of argument, let’s say that you have a pressure control valve. The pressure increases from 200 psig to 300 psig at a boost in input signal strength from 20% to 30%. When the signal is reduced to 20%, the pressure does not return to its original value of 0 bar but remains constant at 225 psig. You can imagine this mayhem could cause the process’s inherent flexibility.
The hysteresis of a valve is often the result of a breakdown in the mechanical connection between two or more parts of the valve. (Note: installing a positioner on a valve that doesn’t have one already is a simple approach to lessen its hysteresis.)
- Flashing
A vapor bubble will form if the liquid pressure is less than the vapor pressure. Consequently, the fluid and vapor mixture will be the medium of flow. As flashing cannot be avoided due to process restrictions, the damage caused by flashing must be mitigated through strategic angle body design.
- Deadband
You’ve heard of dead time; now prepare to be horrified by deadband. There is a deadband when the controller delivers a signal, but the valve doesn’t open or close. As a result, the controller is forced to send an even stronger signal, leading the valve to move too far, resulting in either an open or closed position. Obviously, this is not beneficial in the effort to minimize process variation.
- Stiction
Stick and friction come together to form stiction. Valves that become stuck in one position are what this word refers to. The controller then has to send out a stronger signal, similar to what happens with Deadband, causing the valve to overshoot its set point. However, the valve stem becomes fixed in its new location due to its resistance to further movement.
Tips For Ensuring The Continued Functionality Of Control Valves
Selecting the suitable valve for your application, ensuring the valve is placed correctly, and following a regular maintenance and repair plan will help you avoid dead time and its repercussions.
Here, we’ll take a closer look at each of those procedures individually:
- Cavitation
Cavitation results from vapor bubbles forming when the liquid pressure falls below the vapor pressure; cavities are formed when the bubbles collapse as the pressure rises above the vapor pressure. Cavitation can be considered the simultaneous evaporation and condensation of a liquid.
You can see the cavitation on the valve. Increasing the flow area and creating a winding flow route can prevent cavitation. Cavitation can also be avoided by using a tougher trim material.
- Setting Up A Control Valve
Ensure proper installation of your valve for optimal performance. That means following the guidelines provided by the maker.
Because of space constraints, we can’t go into great depth on proper installation procedures; however, we can recommend the following:
- Ensure your valve hasn’t been damaged in transit by inspecting it before using it
- Purge the pipeline and valve
- Be sure you put the valve in the right way up. Depending on the flow direction, many control valves can be mounted either vertically or horizontally
- The piping must be well-supported and aligned with the valve
- Assure that the valve has sufficient clearance for normal operation and maintenance
- Regular Checkups For Control Valves
A brand-new control valve will function perfectly (or at least it ought to). However, a scheduled maintenance routine is essential to ensure its continued operation yearly and decade after decade. In-service diagnostics and preventative upkeep are the greatest approaches for control valves.
Conclusion
Control valves are likely not the single most crucial part of your plant’s control loops, but they shouldn’t be underestimated either. However, they come quite close to being accurate. Make sure you follow the above information in the right manner to avoid any mishaps.
