With the passing of a number of very popular control platforms into automation history, and inevitable support end dates looming for others, there is an increasing need to replace obsolete control system hardware either by updating it or by migrating it.
“Migration” is loosely defined as the movement of some entity from one place to another. In the case of a programmable logic controller (PLC) migration, it could be described as “moving control from one system or platform to another”. The problem with this definition, and so many actual PLC “migrations”, is the idea that it is a simple movement from one place to another. The reality is, it’s not always that simple.
Over the past few years many popular PLC processor models have reached the point where spare parts are no longer available. However, aging unsupported platforms are not necessarily obsolete; many of these systems still meet the technical requirements of the specific application. It is also important to note that expansion equipment such as Input/Output (I/O), communication, and specialty modules may not reach the end of support stage at the same time as the PLC platform. Expansion systems may be adapted to new PLC platforms or newer systems may be adapted to work with the old platform. This extends the lifecycle of expansion systems and potentially allows for retrofit of a processor.
Replacing an end of life PLC system with a more advanced system is never just a move from one system to another. When dealing with smaller systems that are lacking advanced functions, remote I/O, and integration with other systems it is tempting to view the retrofit as a simple migration during the early stages but there are always other factors that need to be addressed when considering such projects.
Functional Safety Considerations
A key factor in determining the status of a retrofit project is whether or not the PLC that will be replaced is Safety Integrity Level (SIL) rated and/or part of a SIL certified system. The most common scenario where SIL rated equipment may be required involves the potential requirement for SIL. It is often seen that client standards and/or regulations may have changed resulting in a requirement for SIL rated hardware where general hardware had been acceptable before.
New client standards and/or regulations may also drive a requirement to replace a single PLC with two functionally separate systems. For example, an older compressor station PLC may need to be split into separate Basic Process Control System (BPCS) and Emergency Shutdown (ESD) PLCs.
If a functional safety component is identified; a number of other safety assessment requirements and SIL verifications may be needed depending on the application, owner and industry standards.
When evaluating a potential PLC retrofit project it is important to also consider operational factors. The question you should ask is “If I replace this PLC will there be unintended operational impacts?”. There are a number of potential impacts to be considered;
- Safety – Is there a potential for increased safety risk?
- Reliability – Is there potential for long term system reliability impacts related to the system retrofit?
- Maintainability – Is there potential for system maintenance impacts? These impacts may be related to increased costs, reduced accessibility and spare parts availability.
- Constructability – Is there potential for operational impacts resulting from construction activities related to the system retrofit?
Every retrofit project is unique. As part of the preliminary study it’s important to assess the retrofit options and methodology in order to decide on the optimum retrofit solution.
A few things to consider include:
- Will the footprint of the existing system will suit the space requirements of the new system? If the answer is no, plan for additional space for the new hardware?
- Will the IO point migration be performed offline or online? Based on the answer the approach, preparation, schedule and cost will vary.
- Were there existing problems in the current system that need to be fixed after retrofitting? If so, that should be considered in the next step (Analysis and Evaluation).
Analysis and Evaluation
Most PLC retrofits have inherent complexities that may not be apparent at first glance. This makes a documented early-stage analysis and evaluation critical to the success of any retrofit project. A fully completed system analysis/evaluation will provide a detailed list of potential issues involved with a given retrofit project. If possible, the retrofit analysis/evaluation should be at least partially completed during the proposal stage. This will allow for the development of a more detailed scope of work, cost estimate, and schedule.