Maximizing Your Control Valve Performance: A Guide to Control Valve Selection, Maintenance, and Repair
In any given process plant, you may find hundreds, or even thousands, of control valves. These valves are the final control elements, the equipment that is ultimately responsible for keeping process variables like flow, level, pressure, and temperature within the desired operating range.
Unfortunately, control valves are often neglected. This happens in two ways:
- The importance of control valves is often overlooked by engineers, managers, and other personnel responsible for improving process efficiency. They tend to focus on other sections of the control loop, meanwhile ignoring the component at the center.
- Control valves are often not properly maintained, which means they can’t work at the desired performance level. This may contribute to their importance being ignored.
In this article, we’ll tackle both forms of neglect. First, we’ll explore the important role control valves play. Then, we’ll take an in-depth look at the whys and hows of control valve maintenance and repair.
What your control valves do for you
It’s tempting to think of control valves as just components that regulate the flow, temperature, and so on of the media running through your pipelines. That is, of course, their main function. But the value they bring to your operations goes far beyond that.
Better product quality
In manufacturing, product quality is intricately related to consistency. No matter what you make, the product that comes off your line should be the same every time. If it isn’t, you risk having to throw product away or disappointing customers, who have certain expectations.
Consistency in your product relies on consistency in your process. You can’t afford high levels of process variability — it needs to work the same way every time.
This is exactly the job of your control valves. By having the right valve size and type installed on your lines and by keeping those valves in good working order, you reduce your process variability and, in turn, maintain consistently high product quality.
Higher levels of efficiency
Control valves make several contributions to the overall efficiency of your plant — in terms of both production and energy.
- First, by reducing process variability, they minimize waste. This means you have less to throw away across the entire line, from raw materials to finished products.
- Second, by precisely controlling process variables, control valves improve the energy efficiency of your plant. For example, by maintaining temperature control, they can improve your heating efficiency.
These are just two examples. In reality, every step of your process provides an opportunity for greater efficiency. And the control valve is a key factor in achieving it.
Improved bottom line
By increasing your product quality and raising your efficiency, the humble control valve can have a huge impact on the profitability of your plant as a whole.
Control valves that work properly also boost your environmental sustainability and decrease excess materials costs. What’s not to love?
What factors affect control valve performance?
The trick is that you won’t realize the many benefits of control valves unless those valves do their job. Here, we take a quick look at three main factors that affect control valve performance.
Valve sizing
Many of the control valves in use today are oversized. This means that, under normal operating conditions, they operate at a low percentage of their rated flow capacity.
Valve oversizing often happens for one of two reasons.
- The engineers don’t have enough information or have the right information when designing the process to select the ideal valve, OR
- The valve was oversized deliberately to leave room for process adjustments in the future.
Regardless of the reason, if your valve is too big, you lose your ability to control the process variable very precisely. This means you give up on the quality, efficiency, and profitability improvements control valves can bring.
Valve response time
A valve’s response time is how quickly it reaches its new position after it receives a signal to move. The faster a valve reaches position, particularly in response to small signal changes, the more precisely the process variable can be controlled. And, as we saw earlier, precise control is what counts most.
When a valve doesn’t move quickly, the result is known as “dead time.” This is the time between when the signal changes and when the valve gets to where it’s supposed to go. Dead time is essentially lost time on your production line — it destabilizes your control, allowing process variability to sneak in.
Other components of the valve assembly
Control valves don’t work on their own. Instead, each control valve is part of a larger valve assembly — including an actuator, perhaps a positioner, and various optional accessories. All of these components, as well as the design of the assembly as a whole, contribute to the overall performance of the valve.
What causes valves to operate below their peak performance?
Deadband
If you think dead time is bad, wait until you hear about deadband!
Deadband occurs when the controller sends a signal, but the valve doesn’t move. The consequence is that the controller must send an even bigger signal, which then causes the valve to travel too far, which means it either opens or closes more than desired. Obviously, this doesn’t help in the fight to reduce process variability!
Deadband can be caused by several factors:
- A wrong-sized actuator
- Loose mechanical linkages (aka, backlash)
- Excess friction in the valve body or actuator
- Faulty accessories, such as the positioner
Hysteresis
Hysteresis refers to the condition in which the same input signal produces different results on the upstroke from the downstroke.
For example, suppose you have a control valve regulating pressure. When you increase the input signal from 20% to 30%, the pressure goes from 200 psig to 300 psig. But, if you then put the signal back down to 20%, the pressure doesn’t return to its starting position — it stops at 225 psig. You can see how this can wreak havoc on your process variability.
Hysteresis is frequently caused by loose mechanical linkages between different components of the valve assembly. (Note: If a valve doesn’t have a positioner already, adding one is a quick way to reduce hysteresis.)
Stiction
Stiction is a combination of stick and friction. The term describes a situation in which a valve gets stuck in a particular position. Like with deadband, the controller must then put out a larger signal, which causes the valve to overshoot its desired position. Then, however, because the valve stem is resistant to motion, it gets stuck in the new position.
Several underlying causes can lead to stiction, including but not limited to:
- Sticky valve internals
- Undersized actuators
- Tight shutoff
- Media viscosity
For most processes, problems arise when stiction creeps above 0.5% per cycle.
Nonlinearity
Finally, control valves work best when they respond to input signals in a linear fashion. As you can imagine, deadband, hysteresis, and stiction all cause valves to become nonlinear.
How to keep your control valves working properly
The biggest consequence of all of the control valve problems above is instability in the control loop. This most often takes the form of dead time. The result is that during the delay between the controller sending the signal and the valve traveling to its desired position, the process variable level will be either too high or too low.
The trick to avoiding dead time — and its consequences — is to select the right valve for your application, make sure the valve is installed correctly, and follow a plan of regular maintenance and repair. In this section, we’ll look at each of these steps in turn.
Control valve selection
Selecting a control valve isn’t like going to the hardware store and picking a faucet. You need to have an arsenal of information at the ready to ensure you get the right type, size, and characteristics for your application.
The Emerson Control Valve Handbook, 4th Edition, identifies 30 pieces of information you need when selecting a control valve:
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Control valve installation
For your valve to work correctly, it must be installed correctly. That means according to the manufacturer’s instructions.
While we don’t have the room here to detail specific installation guidelines, here are some best practices:
- Always inspect your valve to make sure no damage occurred during shipping.
- Clean both the valve and the pipeline.
- Install the valve in the appropriate orientation. Many control valves can be installed either vertically or horizontally depending on the flow direction.
- Ensure piping has ample support and is properly aligned with the valve.
- Provide the valve with enough headroom to accommodate normal operation as well as be accessible for maintenance.
This is not an exhaustive list, but it gives you an idea of the types of things you need to pay attention to.
Control valve maintenance
A brand new control valve will operate at peak performance (at least it should!). But to keep it running year after year, and decade after decade, requires a regular maintenance plan.
For control valves, the best plan is based on in-service diagnostics and predictive maintenance.
Control valve diagnostics
Using advanced diagnostics, control valve technicians can detect problems like deadband and stiction, as well as perform a root cause analysis on your complete control system. In other words, the technicians can find any issues and what’s causing them without having to shut down the system. If it is necessary to remove the valve from service for repair, the technician will already know exactly what’s wrong and how to fix it, which means that valve will be back in action in no time at all.
Predictive maintenance
Beyond diagnosing current problems, advanced diagnostics allows technicians to predict where future problems might occur and take measures to guard against them. This is called predictive maintenance, and it can greatly reduce downtime as well as costs associated with maintenance work.
The tool also helps technicians develop more accurate maintenance schedules.
For example, a technician may test a valve several times to establish a performance baseline. Then, when the annual maintenance time comes along, the technician will test the valve again. If the valve is still operating within range of the baseline, no service may be necessary. By doing this, you may be able to stretch your maintenance schedule out so that certain valves receive service only every two or four years, as long as they’re performing well.
This is more efficient than the traditional maintenance model. It also means you won’t waste any time or money on maintenance tasks for valves that are still as good as new.
Control valve repair
Of course, there are times when repair becomes necessary. In particular, our technicians are called out to job sites for four main reasons.
Internal leakage
If you notice a control valve leaking internally, first check the maximum allowable leakage (MAL) specified by ANSI/FCI Standard 70-2. Not all control valves are designed to be bubble-tight.
If a valve is leaking above the MAL, it could be that the internal components are worn out or that there’s a problem with the actuator, the positioner, or the controller.
External leakage
This is arguably the biggest problem with control valves, at least environmentally speaking.
Control valves are by far the worst offenders for fugitive emissions. Roughly 50% to 60% of a plant’s fugitive emissions come from valves. And 70% of that amount can be attributed directly to control valves.
More often than not, the problem has to do with the valve packing, with causes ranging from improper maintenance to material incompatibilities.
Stickiness
Earlier we identified a few situations that can cause valves to stick in place. Here’s another common one: overtorquing the valve packing in an attempt to curtail leakage.
This may seem like a no-win situation — if the packing is too loose, the valve will leak; if it’s too tight, the valve won’t work. But it doesn’t have to be — you don’t need to choose between the environment and your production line. You can prevent both of these problems from occurring by selecting the right valve for your specific application to begin with.
Giving up the ghost
Occasionally control valves just stop working entirely. This can happen for many reasons, but the underlying cause for most of them is that the valve wasn’t designed for the service conditions in which it’s being used. When you pair the right valve with the right application, you can get decades of use out of your equipment.
Control valve maintenance and repair techniques
It’s difficult to make too many blanket statements about servicing and repairing control valves because every situation our technicians walk into is unique. But we’ll try! Here’s are a few of the more common maintenance and repair procedures our technicians perform.
Hydrostatic/Shell testing
Control valve hydrostatic testing is governed by ANSI/ISA-75.19.01-2013.
Comprehensive assessment of the process and service conditions
We’ve noted several times the importance of having the right valve and valve components for the service conditions. Especially if it’s the first time a technician visits your site, the first thing he’ll do is look at the complete process, including the valve assembly, the control loop, and what’s in the pipe.
Packing replacement
Both leakage and stickiness can result from packing problems. Specifically, problems frequently arise at the packing seal, that is, the plug stem where it goes through the bonnet.
To work properly, the surface finish of the plug stem needs to be at a particular RMS, which is a measure of surface roughness. If the RMS isn’t right to provide a tight seal, then the packing either needs to be replaced or machined back to its original specifications.
Actuator adjustments and positioner calibration
On seven out of 10 service calls, the problem is the valve. The other three times, it’s usually the actuator, the positioner, or another component of the valve assembly.
For actuators, it’s crucial that all adjustments conform to OEM instructions. For positioners, the problem usually lies in the calibration.
The right positioner can also extend the life of your valve. For example, you can configure the Masoneilan SVI II AP Digital Valve Positioner to disallow conditions that are hard on the valve, such as when there is large inlet pressure but small outlet pressure. This can prevent you from having to take the valve apart for repair every time there’s an outage.
Conclusion
It may be an exaggeration to say that control valves are the most important elements in your plant’s control loops. But they’re pretty darn close!
At Allied, we’re dedicated to making sure your control valves perform at their best. Our technicians are certified to service and repair all control valves from all manufacturers. Send us a note or give us a call to let us know how we can serve you.