When examining distributed control system ( DCS) migration projects, most discussions focus on the early planning phase or potential vendor selection. These are all important, but if the project is nearing the end of its lifecycle, and the new automation system arrives at the factory awaiting installation and startup, significant risks exist throughout the installation process, particularly the switch from the old system to the new one, which can have unpredictable impacts on production.
A well-planned project should have a detailed implementation roadmap. If your company has already partnered with a system integrator or automation system supplier, you should plan a detailed switchover process. If they haven't, or if you are still planning, let's take a look at what must be included in the switchover process.
Switching refers to transferring components of a larger automation system that need to be retained to a new platform. This typically includes field instruments, valves, motor controllers, and the supporting networks and cabling. These components interface with the system via input/output (I/O) cards, and every connection cable must be moved from the old platform to the new platform.
Before the new DCS arrives on site, it is crucial to verify the location, operation, valid terminals, and wire markings of the equipment when preparing the field equipment.
Steps before DCS delivery
Switching is the final step in the project, so the following should be completed when the new DCS system is delivered to the site:
●Review and update the documentation for facilities or installations;
●When necessary, field equipment, wiring, and all labels should be inspected, assessed, and repaired;
● Perform and pass factory acceptance testing of the new automation system;
● The infrastructure supporting the new automated system should be functioning properly.
These four points cover many aspects; let's consider their impact one by one.
1. Documentation can make or break a migration project. Successful implementation depends on a detailed, complete, and accurate description of the existing facility. When replacing a DCS, the facility inevitably undergoes numerous changes and updates since the automation system was installed. If the documentation doesn't reflect all these changes, various problems can arise at different stages of the migration process. If discrepancies persist after the initial project phase, they will certainly surface during the switchover process when each connection needs to be identified and seamlessly integrated.
During project execution, documents should be reviewed in detail. This includes:
● Piping and Instrumentation Diagram (P&ID)
● Loop Diagram
●Pantai Chart
●Rack Room Diagram
● Cable and conduit progress
When it comes to switching, if a company brings in a systems integrator or other external help, it is crucial that the team leader review the documentation at some level. The level of detail varies depending on the situation, but it depends more on the documentation, so its accuracy should not be taken for granted.
2. The field equipment and its wiring should be verified:
● All equipment should be located where shown on the P&ID diagram.
●Equipment is working normally
●They should be connected as shown in the diagram.
● Cables should be connected to junction boxes and I/O cards and should be correctly labeled.
3. Factory acceptance should be completed before the system arrives on site, and there is much to be done during this step. Essentially, when the cabinets and racks are delivered to the factory, the control software and hardware should undergo a thorough inspection by the supplier and project team. If the testing is successful, there should be no unexpected issues such as faulty I/O cards or hardware malfunctions.
4. Upon arrival of the automated system, any infrastructure required to support the new system should be installed and operational. Any improvements to power distribution, HVAC and lighting systems, wiring and conduit openings, and even wall painting should be completed. These components may seem insignificant, but they can become major problems if left to the last minute. Adding extra holes in cinder block walls will generate significant amounts of grit and dust and potentially damage the cabinets, so this work should be completed before the new equipment is in place.
After DCS delivery
After the new racks and cabinets are installed and powered on, on-site acceptance testing can begin. During on-site acceptance testing, some of the tests performed during factory acceptance testing need to be repeated to ensure no damage occurred during transportation; however, spot testing rather than comprehensive testing is permitted. This primarily includes basic verification of both hardware and software.
Those involved in the switchover should ensure that the I/O card installation location, network connection, etc., are correct. Before the switchover begins, technicians should verify the terminals and communications.
Cold switching and hot switching
There are two basic switching methods: cold switching and hot switching. Cold switching is performed when nothing is running. The old system is disconnected, and all field devices and other components are then connected to the new system and tested during the downtime. When the process restarts, it will be fully operational on the new platform.
Hot switching refers to the introduction of a new platform while the old system continues to operate. Both systems must run simultaneously and be coordinated under control so that they perform the same control functions. This process continues as technicians methodically switch each piece of equipment and loop to the new platform, one after another, thus requiring coordinated parallel operation. If implemented correctly, this will not cause any production losses.
It sounds like a clear choice, but it's not actually that easy. Why would a company choose one method over another? In most cases, a cold switch is a better option, provided there are specific conditions for doing so.
First, the company must be able to shut down and schedule the shutdown at an appropriate time, or schedule the switchover within the shutdown period. This means all components must be integrated within a specified timeframe. With good planning, this is certainly achievable, but few companies are willing to offer an indefinite time window. Just as a shutdown has a start date, it also needs a finish date, otherwise problems arise. During a shutdown, production ceases, and the subsequent startup will be included in larger financial calculations.
Because cold shutdowns must be completed within a given time window, they require significant resources. Most companies rely on external resources, making cost calculations particularly interesting. How do you compare the profit gained from a day's production to the increased costs incurred by hiring external resources to complete a project a day or several days ahead of schedule? Every penny of these hard costs must be evaluated.
Some people like hot-swapping
Hot switching is attractive when processes cannot be easily stopped, or when downtime would incur prohibitively high costs, because production doesn't stop. The new DCS system is already in place and up and running. It connects to the old system so both can operate in parallel, executing the same commands simultaneously. Technicians then move equipment and support loops from one system to the other, with both systems sharing control responsibilities. While the equipment is disconnected for the switchover, the control room cannot view the instruments, so any functions that depend on it must be handled manually. This sounds daunting, but there's a reason for it.
First, technicians must understand exactly what happened. They should proactively identify the connections of specific devices, disconnect the wiring, and transfer it to the terminals of the new I/O card. If a wiring cabinet exists, the terminals on the DCS side can be located, and multi-strand wires can be connected to the I/O cards in the new DCS.
This is a very important part of the documentation. Technicians holding a disconnected cable don't want to find a terminal they didn't intend to connect it to. Depending on the installation, there are several ways to perform such a switch. In simple cases, it can be done quickly in the junction box. In the opposite extreme, it may be necessary to pull the cable out of the cabinet so that it can be connected to a new cable.
Secondly, technicians need to plan the sequence of the switchover. This requires a thorough understanding of the different equipment and loops, and how they interact in the process. Documentation also plays a crucial role. Detailed International Society of Automation (ISA) loop diagrams can help you make decisions more easily, but the selection requires the expertise of the relevant planners to make the final decision. Don't rush into implementation.
Third, when switching cables to field instruments, the DCS and control room will lose contact and will be unable to receive this data. A few minutes of lost data from level instruments on the tank won't be a major problem because process variables don't change that quickly. Many field instruments behave similarly in monitoring applications.
The situation becomes more complex if the final control device performs real-time functions. If the process is generally stable and tends to operate in a steady state, the company might be willing to attempt a hot switch. If the DCS requires constant adjustments, operators might have to sit next to the disconnected equipment and relay information to the control room via walkie-talkie: “Open the valve a little wider…too much, open it halfway…”
Fourth, while the process may not be able to be completely shut down, manual control work performed by operators and technicians may result in products that do not meet specifications. Of course, the likelihood of this happening and its impact will vary depending on the circumstances, but users should not assume that production will not be affected.
These are all factors to consider, but besides maintaining operation, hot switching has other important and positive impacts. Scheduling is much easier because it avoids switching during downtime. There is also less pressure regarding switchover time, requiring fewer resources than cold switching. Technicians can reconnect terminals in a logical sequence, and because errors during hot switching can have serious consequences, these technicians must be highly skilled.
Hybrid switching: Balancing risk factors
Typically, the final decision on whether to choose a cold or hot switch will manifest itself in various forms of risk, including:
●The time required for cold switching exceeds expectations, and downtime must be extended, which may lead to the inability to meet production targets.
●The risk of a safety accident occurring in hazardous devices when the control is switched to manual mode during a hot switch.
●If the process control function is partially lost, there is a risk of producing defective products during hot switching.
Fortunately, the choice between cold and hot switchovers is not an either-or one. Many factories opt for cold switchovers for the reasons discussed above. This is often assumed when some system integrators work with customers, unless the migration team requests a hot switchover. Some customers do require a true hot switchover, but this is relatively rare.
Many companies opt for a hybrid switchover: the most critical loops and functions are migrated during short downtime, while less critical loops and monitoring functions can be hot-swapped at the appropriate time. This approach still requires downtime, making it impractical for some companies. Nevertheless, most process plants of similar size can implement planned downtime.
Of course, this requires precise planning and flexible resources. All critical functions must be checked, identified, and assembled in the optimal order. After downtime, relevant personnel must be prepared to ensure that all switchovers can be completed within the specified timeframe.
In most cases, hybrid switchover combines the advantages of both approaches, reducing the risks of hot switchovers while lowering the costs and resources required for cold switchovers. Partnering with a good project manager helps ensure a smooth hybrid switchover operation and guarantees the benefits of both.
The intense activity during downtime requires a sufficient number of skilled technicians, while pre-operation preparation and follow-up work require fewer personnel; a balance must be struck. Few companies possess the flexibility of internal resources to minimize downtime and maintain adequate manpower for subsequent operations. Introducing an effective automation partner can change this situation.
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