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Control valve performance over the lifetime.

The main drivers for control valve modernization projects have been:

• To improve process stability and increase production efficiency
• Increase process uptime by reducing field device related process disturbances
• Replace old control systems with up to date fieldbus capable DCS systems
• To reap the full benefits of predictive maintenance that is enabled by modern field devices, like intelligent valve controllers.

Successful valve modernisation cases range from the upgrade of a few critical control valves in key locations in the most important process during plant shutdown to complete revamps of the refinery while the process is online.

What does control valve modernization achieve?
The principal benefits of control valve modernisation are:

• Improved control valve accuracy
• Increased control valve reliability and availability, because potential valve failures can be predicted
• Seamless integration to modern DCS systems, which releases ‘hidden’ intelligence in the valve controllers
• Enables process optimisation during plant run time, because online control valve performance monitoring always offers up to date information about control valve performance
• Enhanced maintenance efficiency, because maintenance actions can be targeted only at control valves that actually need maintenance. Investment payback within months.

Several factors, such as valve type and nominal size together with control valve condition, affect valve modernisation investment. However, modernization projects that Metso has carried out together with customers show that payback time for this type of investment is short and has the added benefit that the payback time can be estimated quite accurately in advance (Figures 3 and 4).

Long before control valves fail, they may show early warning signs of premature wear and ‘stickiness’ that lead to poor control performance, which often goes unnoticed, even though it impairs product quality and process efficiency.

In a recent example of a refinery heating application, Metso’s flow control solution gives greatly improved process performance, with smoother control and more accurate step response. The increased control performance represents a payback time for the modernization investment of only three months. In addition, all control valve data are stored automatically in Neles ND9000 intelligent valve controllers to provide the basis for predictive maintenance.

Improved reliability and availability
A 15 or 25 year old control valve may seem to work rather well, but for how long? By replacing the valve positioners with intelligent valve controllers, control valve reliability will increase as instrumentation problems will disappear, but for most it will show the estimated remaining lifetime of the valve.

As with all mechanical devices, valves, actuators and positioners eventually need maintenance or replacement. How often valves need to be repaired or replaced typically depends on the severity of operating conditions (pressures, temperatures, flow media, etc). The lifetime of positioners and actuators is often defined by the amount of operations carried out.

Typically, positioners define the level of control accuracy at which control valves can perform. When evaluating old, used control valves, the replacement of worn parts like trims and gland packing’s can return valve performance to almost new condition. However, replacing an old positioner with a new intelligent valve controller, like Neles ND9000, can also significantly enhance control valve performance.

Once an old positioner has been replaced with ND9000, performance changes by an operating valve are spotted quickly thanks to an alert sent by the valve controller. This allows operators or maintenance personnel to predict with a fairly high degree of accuracy when repair or replacement of the control valve may be necessary. This early warning system also helps to determine the root cause of a problem, leading to quicker, more efficient repair.

Improved maintenance efficiency
Because intelligent valve controllers gather control valve performance data online from the control valve operation, the need for control valve overhaul can be estimated in greater detail than before (Figures 1 and 2). Control valve service reports made during several past maintenance projects show that 47% of the control valves were removed from the pipeline without a good reason.

By utilising the intelligence built into Neles ND9000 intelligent valve controller, the majority of the unnecessary maintenance work could have been avoided.

Analysis shows that maintenance costs cover almost half of the total ownership cost of a control valve. Thus increased maintenance efficiency brings significant savings over the life cycle of a valve. When utilizing the ND9000’s intelligence for predictive maintenance, the maintenance savings can be equal to control valve purchase price.

Introducing fieldbus technology
Many installed control systems dated from the 1980s or 1990s are due for modernisation now or shortly for two main reasons, firstly with time, service of the existing system becomes too difficult and too expensive, because systems are completely outdated and no longer supported by their manufacturers. Secondly, some systems might still run reliably at the controller level, but do not fulfill modern requirements regarding integration to asset management systems.

When replacing old conventional control systems with a modern DCS, the utilisation of fieldbus technology offers a great opportunity to move up to the next level of plant asset management. According to research, the greatest benefits of fieldbus are found in maintenance and operations, long after a plant has been commissioned. In other words, fieldbus itself is often not the cost saver, but an enabler of a new level of asset management effectiveness that can significantly reduce operating costs and help manufacturers to achieve operational excellence.

Financial benefits of control valve modernization
Valve modernisation enables financial savings during the remaining lifetime of the old control valve, in operations and maintenance. The purchase price of the intelligent valve controller is only a small proportion of the remaining lifecycle cost of even an old control valve. Maintenance cost and indirect costs due to a device’s effect on process performance account for a much higher proportion.

Neles ND9000 has low installation and commissioning costs and due to predictive maintenance, control valve maintenance is cost efficient. Predictive maintenance helps to prevent unplanned shutdowns, while increased control valve accuracy enables better overall process performance.

Originally published in Hydrocarbon Engineering and Neles News.

Metso Automation Ltd. Tel: 0870 606 1478

Published in Valve User Magazine Issue 14

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