July/August 2012
Automation Basics

Enhanced maintenance efficiency with third-generation control valve diagnostics

By Niklas Lindfors and Juha Kivelä

For more than two decades, maintenance managers and engineers at plants and mills have had a chance to use control valve diagnostics as help when planning shutdown activities. The first diagnostics tools were developed during the 1980s, and since then the technology has taken giant leaps, further providing a wide range of new possibilities. For a rather long time, real-time diagnostic information has been available, including when the process is online, making it possible to predict-and prevent-possible process disturbances. Users are now taking advantage of the additional information available and adopting predictive maintenance strategies to gain more value in the process industry every year. The latest development in the field of diagnostics, the so-called "third generation of diagnostics," is also playing a role in this transition by further smoothening the shift from traditional corrective and schedule-based to predictive maintenance.

Traditional control valve diagnostics

The separate diagnostics tools from the 1980s, the first digital positioners from the 1990s, and the intelligent valve controllers that have been available since the early 2000s differ greatly when compared with each other from the perspective of the diagnostics information they provide. The first diagnostics tools were based on stand-alone devices that needed to be separately connected to the control valves to be able to collect the diagnostics. Typically with these devices, separate tests needed to be conducted on the valve to collect the diagnostics. The tests could only be performed when the process was offline, meaning that the valves were not in their normal operation environment during the tests and that the users did not have any online diagnostics information available. The digital positioners of the 1990s were able to perform this same task, removing the need for the separate stand-alone diagnostics tools.

Enh1  

After the change of the millennium, the first intelligent valve controllers brought the possibility to collect diagnostics from the control valve performance when the process was online. Besides collecting the information, these devices are also capable of analyzing this information and providing online alarms and warnings of possible control valve performance degradation. These kinds of devices are often described as providing "second generation diagnostics" because of the possibility to conduct the basic analysis on the data collected offline and online.

Enh2

Third-generation diagnostics: improved user experience

It has been roughly ten years since the launch of the first intelligent valve controllers, and the technology has been further developed and refined. Nowadays, state-of-the-art devices are capable of processing the collected diagnostics information to visualize the condition of the valve in question. This makes the available information more user-friendly and easier to understand. Whereas, with the first- and second-generation diagnostics, a valve specialist has usually been required to be able to analyze the available data and make conclusions based on it; with the third-generation diagnostics this is no longer the case. Improved valve management software is now available, and it works with intelligent valve controllers to process the existing data and represent it in an easy-to-understand display, which includes five different indices: control performance index, valve condition index, actuator condition index, positioner condition index, and environmental conditions index.

The valve condition indices are typically displayed using software. Illustrated in Figure 1 are the control performance index, valve condition index, actuator condition index, positioner condition index and environmental conditions index, shown with Metso's Valve Manager software for the ND9000 series intelligent valve controllers.

The screen also displays information available from a new generation of intelligent valve controllers that provide more information using several built-in sensors that measure various internal parameters. Since the indices are generated using experience-based mathematical algorithms, users with less valve know-how can evaluate the condition of the control valve in question and make informed maintenance decisions.

Enhanced maintenance efficiency and process performance

What is, then, the end-user benefit to having third-generation diagnostics? As mentioned previously, with processed and visualized diagnostics information, the maintenance managers and engineers and process operators are capable of making educated decisions concerning the control valve maintenance without deep valve know-how. Unexpected shutdowns can be avoided, and the control valve performance can be maintained at optimum level, by continuously monitoring the condition of the installed base at the plant or mill and by taking the necessary preventive actions based on the available diagnostics information. With the help of the diagnostics, the available maintenance resources can also be more efficiently allocated during shutdowns.

When so-called preventive or schedule-based maintenance strategy is followed, many valves are often needlessly maintained during shutdowns. This new level of information provides significant cost savings through reduced spare part purchases and reduced need for maintenance resources.

Future development areas

Whereas the third-generation diagnostics are already available for control valves, the same level of information cannot yet be received from on-off-or from emergency shutdown (ESD)-valve performance. It should be only a matter of time before device manufacturers start to make these features available for the on-off and ESD valves as well.

Currently, the industry standard for on-off valve control is more or less using solenoid valves to control the on-off movement. One of the downsides is the solenoids cannot provide any diagnostics data on the on-off valve performance; thus, it can already be seen with ESD valves, which are the most critical type of on-off valves, that, in new green field projects, smart partial stroke testing (PST) devices are being implemented. The state-of-the-art PST devices can be used to conduct different tests for the ESD valves, both online and offline, providing diagnostics information on the ESD valve condition. The next step would be to have the smart PST devices analyze the data further and provide similar indices that are presently already available for control valves.

For other on-off applications, besides the ESD, several device manufacturers have brought integrated solenoid and limit switch combinations to the market. An example of such a device is Metso's Axiom on-off valve controller, which can also provide basic diagnostics by giving fault alerts of possible changes in the on-off valve condition. Some end users have already identified the need for diagnostics on the process-critical on-off valves, and the integrated devices can be seen as a first step in this direction.

Intelligent on-off valve controller devices on the on-off side already include diagnostics features that are especially designed for on-off applications. The future will tell how well this new technology will be accepted and adopted among the process industry end users.

In summary

Control valve diagnostics tools have taken a giant leap since the first devices were introduced in the 1980s. The third-generation diagnostics are taking predictive control valve maintenance to a new level, making it possible to enhance maintenance efficiency, improve process performance, and achieve significant maintenance cost savings. In the years to come, it remains to be seen whether users will take full advantage of these new features. These new possibilities should help improve the value of predictive maintenance practices, leading them to be more widely adopted among the industry and to become the industry standard.

ABOUT THE AUTHORS

Niklas Lindfors (niklas.lindfors@metso.com) joined Metso's product management almost four years ago. After graduating as an M.Sc (Mechanical engineering) and later on as a Lic.Sc., he has been working as project manager and researcher in product and production development projects. Currently, Lindfors is holding a position of Product Manager for Neles Smart Products. Juha Kivelä, M.Sc., currently works as a product manager for the valve controls product line at Metso Automation Inc. Kivelä has more than five years of experience working in the valve industry, with a special focus on intelligent valve controllers and valve diagnostics.

Index guide

Control performance index is the most important index out of the five. It shows how well the whole control valve assembly is performing its task to control the flow in the pipeline.

Valve condition index presents the status of the valve based on its use history and current measured values. Based on the index value, the user can also estimate how soon the valve will require maintenance.

Actuator condition index presents the status of the actuator based on its use history and current measured values. Based on the index value, the user can also estimate how soon the valve will require maintenance.

Positioner condition index presents the status of the positioner based on its use history and current measured values and also on several self-diagnostics parameters. Based on the index value, the user can also estimate how soon the positioner will require maintenance.

Environment conditions index takes into account the conditions of the assembly (e.g., supply pressure value and its steadiness) and also selected aspects of valve assembly configuration.