This article was printed in CONTROL's March 2009 edition.
By Jim Montague, Executive Editor
Old dogs may be uncooperative, but you can teach old valves to sit up and roll over. In fact, for technologies that are 80 years old and sometimes older, there’s a lot going on with valves and the actuators and positioners that move and control them.
Sure, everyone knows how valves evolved from manual and motorized actuation, then to relay-based and integral controls, and onward to today’s microprocessor-based, non-intrusive, controls that can perform remote data processing and communicate via fieldbus, Ethernet and even wireless. But that’s not the end of the story. Traditional valves, actuator and positioner technologies still are being updated and improved, and many versions are more affordable now, so they can be used in smaller applications. And, as fieldbus-enabled and microprocessor-based valve technologies have gained traction, they too have evolved new capabilities.
Perhaps because many valve, actuator and positioner technologies have been used for decades, they’re often accompanied by some equally persistent problems and technical limits. For example, the fact that air and other gases are inherently compressible means it’s been difficult to control valves using traditional pneumatics. This is mainly because the data signals from these systems also fluctuate, and so it’s hard for their controls and operators to respond as quickly or efficiently as desired or required. Sometimes added pressure is needed to overcome valve stiction, but that pressure may also cause some valves to open too far or close too tightly. Also, compressed air is costly to produce, and its components are subject to leaks. These factors and normal wear can lead directly to inaccurate valve performance, which indirectly reduces end-product quality in many applications.
Electric actuators and positioners have promised more precise control, but these newer methods are limited by the simple fact that they can’t operate without power. So developers have been faced with how to make electrically driven valve components fail-safe.
For instance, to expand its operating schedule, replace worn valves and run more efficiently, the Las Vegas Public Works Department’s Environmental Division recently updated its phosphorus filtration facility by replacing close to 200 valves and actuators on its 30 filters. These filters handle influent and effluent water, backwash water and air, and effluent isolation to remove phosphorous as required by state permit.
Figure 1. The 90-mgd filtration plant at Las Vegas Public Works’ Environmental Division added K-Tork’s Vane actuators (right) to expand its operating schedule and run its 30 filters more efficiently.
Engineers at the wastewater facility conducted a complete field survey before starting the $6.1 million construction project in early 2008 and finished this past December. More than 75% of the new pneumatic Vane actuators from K-Tork Actuators and Controls had to be retrofitted into the filters, which meant adapting, cutting and inserting them into existing 12-in. to 36-in. lines (Figure 1).
“We moved from clarifiers to filters when we upgraded the filtration plant in 1999-2000, but now a lot of our existing actuators were reaching the end of their lives, and were beginning to have maintenance problems and just wear out,” says Jay Karvonen, the Environmental Division’s information systems coordinator. “We also needed to put in isolation systems, so we could take down a section of the facility for maintenance without stopping the whole plant.” As a result, the filtration plant split its air system in half, and divided the water system into five sections.
“With the old actuator, we also had a lot of air leaks and over-use of air, and so we were faced with upgrading our whole compression system,” adds Karvonen. “With our new actuators, we no longer have any air bleeds, and so we now have excess air capacity and didn’t have to upgrade our air system.”
Dick Scholtz, K-Tork’s business development vice president, adds, “Traditional spool-valve positioners will give a response in seconds, but smart positioners can respond in milliseconds, which greatly improves performance. Our vane actuator is 0.25% accurate for all applications, and this is two to four times better than the ±1.0% accuracy that actuators used to have, which means less deadband or slop. This is important because distributed control systems (DCSs) and other control systems are more sophisticated, and so end users must keep installed devices up to the DCS’ new capabilities.”
Better Data = Better Performance
Similarly, Rohm and Haas reported that a reliability study of its acrylates unit in Deer Park, Texas, identified more than 800 hours of production losses in 2006, mostly due to reactor trips, leaks, sticking, bad positioners, aging components and spare parts. To make its operations safer, improve asset use, make operations more consistent and productive, and increase control valve mean time between repairs (MTBR), Rohm and Haas worked with Emerson Process Management to develop a strategy to define its critical control valves, evaluate existing equipment reliability, document control valve repairs, implement Emerson’s AMS Suite Intelligent Device Manager and Fieldvue/Valve Link software, conduct control valve training, solve spare parts issues, standardize its valves, address solenoid valve and air filter reliability, improve its valve repair process, add its new control valve strategy to the plant’s turnaround process, and establish metrics for all these new procedures.
Antonio Alves, Rohm and Haas’ instrument reliability engineer, reports that he and his colleagues also set up failure reports to find bad control valve actors, conducted root-cause analyses, checked for installation and design issues, sought process issues affecting valve performance, defined spare parts needed, and implemented action plans to identify and solve control valve problems before failures occurred. “We also installed multiplexers in two units that can monitor more than 32 valves per unit by trending alerts and performing performance diagnostics (PD),” he explains. “And we located AMS Device Manager’s server to monitor multiple devices, and allow it to access PC data to review alert logs for predictive maintenance and perform PD on critical tags.”
Alves adds that Rohm and Haas’ asset managers at Deer Park also attend daily production meetings and monthly reviews, and that the acrylate unit’s engineers, I/E specialist, technical steward, reliability engineers and operators all cooperate. Together, they generate daily valve behavior reports, add ValveLink and FlowScanner software reports to their preductive maintenance plan, and create standard control valve documentation to develop plantwide repair principles.
As a result, Rohm and Haas reports that production losses due to control valves at its acrylate unit were cut by 75% to a total of 79 hours during the first quarter of 2008. The firm adds that its proactive maintenance program has reduced unplanned outages, its equipment standardization has reduced inventory and repair downtime, and other reliability and profitability metrics have improved. Woof!
To Design a Valve Project
Before starting a valve actuator or positioner project, there are several primary questions that must be answered, according to Bruce Grumstrup, director of Fieldvue and positioner instruments at Emerson Process Management. They include:
- What and where are your critical control loops?
- Which are the critical control valves in those loops?
- Where do you need alerts and alarms and how should you set them up?
- Have you prioritized your loops and their alerts?
- Where are you communicating alerts to?
- Once an alert is received, such as performance is off or a failure is approaching, what’s the next step in your work process and practice?