Thursday   28-Nov-2024

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As legacy control systems based on PLC's and first-generation DCS's approach the end of their useful lives, plant owners are increasingly faced with the uncertainties of migrating to a more modern platform including unknown costs and the business risks associated with extended downtime. In many cases the original requirements specifications are unavailable or are simply irrelevant due to changes made to the system over the years, often by personnel who are no longer with the organisation. Furthermore, such changes were often poorly documented so even a snapshot of the code with comments (if any!) may not accurately reflect the control requirements. This leaves the client facing the daunting task of trying to analyse the system and attempting to extract the control requirements from it.

Because Intuitive has experience of a wide variety of control system platforms, we are able to "reverse engineer" obsolete systems with confidence. We can translate the control functionality into a format appropriate to the technology - for example, we might use the concept of open PLC programming using the standard IEC 61131-3. In the case of batch applications we would make use of flow charts and physical and procedural model diagrams in line with standard IEC 61512 (the ANSI/IEC S88 Batch Standard).

Documenting the control system in this way and translating the control functionality into a readable, high-level format means that the client's process engineers and management can be involved in updating and enhancing the requirements, without knowledge of specialist programming languages.

Translating the requirements into such a format also has the advantage of being vendor independent; we can use the documents as the basis for implementing the control system on the replacement platform, or they can be passed directly to a systems integrator in the knowledge that they are in an industry-standard format.

The client wanted to migrate a control system supervising an existing chemical plant onto a new platform. The legacy system consisted of Fisher-Rosemount Regulatory Controllers and Gould Modicon PLC's, whilst the new system was based on Emerson Process Management's DeltaV system.

Intuitive was asked to translate the control functionality from the old platform to the new. This was achieved by analysing the existing system (which was still in operation), reviewing the control strategy with the client's engineers and incorporating numerous enhancements. The existing control system included a number of conventional hard-wired control loops and Intuitive's brief included a detailed review of the loop diagrams and the proposal of replacement software-based control strategies.

The output of this exercise was a comprehensive set of documentation which was used directly as the basis for specifying and coding the new DeltaV system.

Outcome: control of the plant was successfully transferred to the new system during a shutdown. The project was brought in within the original budget and timescale.

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