Control valve diagnostics

Aug. 8, 2007
Health monitoring leaves fewer reasons to live with stiction, wrong-sizing and excessive wear.

The routine of monitoring and prioritizing maintenance targets is becoming increasingly automated and criteria-driven. Technicians needn’t assess each valve’s current operating condition. Rather, control systems using open communications standards allow individual valves to monitor their own health and call home for help when they start exhibiting some threshold level of sub-optimal performance. This increases throughput and uptime, and it decreases valve-related maintenance, technician travel time and energy costs.

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Underlying technologies

 A major enabler of control valve optimization is standards-based, interoperable fieldbuses. “Fieldbus is taking off like crazy. Some consider it a mature market, but billions of dollars are still being spent every year on the technology,” says Robert Dunlap, founder of Core Global Consulting. “Its greatest benefit is the information it can collect in support of maintenance. If programmed correctly, the system can warn you of process weaknesses or impending valve failure.”

Because manufacturers can chose from multiple open protocols, health monitoring and automation solution providers such as Emerson, Honeywell and Invensys speak to multiple languages. Emerson, for one, offers HART, Foundation fieldbus, AS-i, Profibus DP and DeviceNet cards.

George Buckbee, director of marketing at ExperTune, agrees that open communications is the key to automating valve maintenance decisions. Most, if not all, of the major control system manufacturers support the OPC interface in their field devices. Once available on OPC, the data can be shared virtually anywhere. ExperTune’s PlantTriage, a performance supervision system, sits on top of the control system and uses OPC to pull real-time information into control loops.

Automated performance monitoring

Systems like PlantTriage make it easier to spot which valves or control loops are the bad actors. This system can perform 70 assessments on every control loop and apply systematic economic weighting to each loop to determine problem criticality. For instance, the stock flow valve on a paper machine is fundamental to a paper plant’s operation, whereas a valve used elsewhere may have comparatively less significance. Weighting shows the plants where to focus maintenance effort.

Triggers and filters that consider valve type, economic impact and baseline or threshold conditions make it easy to establish and maintain best practices. For example, if a valve tells you it’s exhibiting less than one-half percent of stiction, you decide to react or not.

Targeting correctable conditions

“An often overlooked problem is that companies try to stabilize and solve process variability by putting in advanced controls,” says Buckbee. “However, they can’t achieve stability if they don’t correct the underlying problems in control valves, instrumentation and control loops.”

Stiction affects process efficiency and produces continual upsets in the plant. Processes also can be constrained when a valve is too small. A component might have been designed for one purpose but, over time, the materials, production or process conditions have changed. Correcting these problems can be an inexpensive way to increase production levels and plant performance.

Poor controller tuning and improperly filtered instruments can cause control valves to move more than necessary. This condition reduces the valve’s useful life and induces process upsets, when the whole point is to reduce process variability. When you can see that loop tuning is damaging a valve or fluid handling system, you can correct it.

Optimization outlook

Control valve optimization will remain a priority for several reasons. Knowledge is walking out the door as the workforce retires. Manufacturing equipment is aging, which makes extending the asset life cycle more important. Competition from foreign manufacturers, who share our open communications standards, is increasing the emphasis on cost control and production and maintenance efficiency. Predictive maintenance technologies and systematic decision-making provide answers to each of these concerns.

Dunlap acknowledges that some changes are politically driven. “When a pro-environment administration is elected to office, being better stewards of the environment becomes a higher priority. Oil and chemical companies in particular might find themselves under greater scrutiny, and will pay more attention to packing flanges, accident and leak avoidance, efficient production methods, and how they clean up the ground field.”

Buckbee anticipates greater emphasis on bringing in key information from a variety of systems and putting it into context – not just the valve or control loop, but how the component affects the overall process. Having this contextual perspective will make it easier to focus on achieving business value and financial benefits.

E-mail Contributing Editor Sheila Kennedy, managing director of Additive Communications, at [email protected].

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