The "trick" becomes figuring out when to become concerned that your control system will require replacement. So, ask yourself some crucial questions:
• Is it increasingly hard to secure spare parts and are the ones available too costly?
• Does the inflexibility of your platform make integration with manufacturing execution systems, enterprise resource planning, etc., either impossible or extremely expensive?
• Is knowledge available about your legacy system (not just internally but from your DCS vendor) decreasing year-on-year?
• Do you worry the existing engineering functionality of the legacy system, optimized over many years, will be lost?
If you answered "yes" to several of these questions and don't already have an automation plan or roadmap, it's past time to create one. This plan must specify your corporate policy on automation and should form part of the annual corporate planning process. An automation roadmap generally follows one of two paths: continuous/incremental change, where pieces of the control system are upgraded on a regular basis as the technology evolves; or replacement of the entire system at fixed periods or based on a predetermined set of criteria perhaps similar to the ones listed above. Every organization must choose the path that works best for it.
Once you've decided it's time to replace your control system, the first step is to complete a front-end-loading study to determine the complete project scope, timing and implementation strategy. The study should identify any potential difficulties with a migration project and provide plans to mitigate them.
This study must consider the lifecycle of a system based on different tiers of longevity (Figure 2). Wiring, for example, typically is good for 30 years or more. I/O and termination panels can last up to 20 years, even though their core chip sets may no longer be available. Controllers usually have a lifecycle of around 15 years, while workstations and consoles generally need replacement after around five years.
Some other major factors to consider include:
• Minimizing downtime during the transition. You must make sure the plant still is safe while shut down and choose a migration strategy. Typical migration options include:
Phased migration. Modernization occurs in gradual steps, replacing the human/machine interface (HMI) or a particular unit first. Installing the full new DCS may take several years.
Complete replacement. The entire system is ripped out all at once during a planned outage. In some cases, hot cutover can minimize downtime and ensure seamless integration of current control assets .
System upgrade. The site retains the legacy platform but modernizes elements of it. This may make sense if the DCS supplier continues to offer parts and support.
• Migrating applications, including basic regulatory control. You'll want to take advantage of best practices on how to operate the facility that are incorporated into your legacy system's control algorithms. Bear in mind, however, that the capabilities of the existing platform may limit "best practices" and the new environment likely can enable improving many of these. In addition, in some cases it will make more sense to rewrite the control algorithms rather than to migrate the legacy applications with all their patches and revisions. What's important is replicating the functionality of the algorithms.