By Lysette Hunt
Proper control selection is based on several criteria, including technical capability, plant environment and commercial aspects. When integration, maintenance and lifecycle costs are evaluated, the cheapest proposed control system may end up being the most expensive solution.
A control system must be able to meet the needs of its application. By evaluating the technical needs, some control systems may be eliminated. The larger vendors have several platforms to provide solutions with options from a few to several thousand I/O. The following list will help to determine minimum system requirements:
- I/O count broken down by analog in, analog out, discrete in, discrete out.
- Functional count by controllers, motors, batch processes, sequences, motion control.
- Remote or local I/O, I/O via radio, Ethernet, Control Net, fieldbus, Profibus, other.
- Acceptable processing limits in milliseconds.
- Higher level control applications such as fuzzy logic or model-predictive control.
- Specific control application requiring vendor experience in implementation.
- Safety instrumented system.
- Dependency on information from other systems.
- Communication to other systems.
- Number/types of graphics.
- Operator interaction.
- Alarm management.
- Trending.
- Historical database.
- Scalability.
- Documentation.
Plant Environment
Regardless of the scope of the project, an understanding of the current state of controls at the plant and the plant vision is essential in making a good decision. System architecture drawings and a plant process overview provide a good basis for understanding the current state of controls. An understanding of the plant personnel can help decide whether it is even possible to look at a control system other than what is currently installed. And finally, a master control plan can be essential to the evaluation.
- System Architecture Drawing. A system architecture drawing includes the control hardware, HMIs, engineering workstations, communications, control networks, instrument networks, radios, Ethernet, supervisory servers, including SCADA, data historian, MES, plant network and mill network. There may be several drawings for one plant. If the system architecture drawings do not exist, they should be developed. They are a good maintenance tool, and systems rarely run autonomously anymore.
- Plant Process Overview. The plant process overview is a flow diagram of the plant indicating current control of each process, whether manual control, panel control, PLC, DCS or hybrid, type of HMIs, hardware type and software revisions. Which control room each process is controlled from should be noted. If this document does not exist, a sketch developed in a meeting with the customer may suffice.
- Plant Personnel. An understanding of the number and the level of control support by plant mechanics, technicians, engineers and outside firms responsible for maintaining and modifying the controls is helpful. If the control system chosen can’t be maintained by the plant personnel, this may be an issue. Some applications may be better served by a DCS, but if the maintenance crew is lean and PLC-savvy, then installing a PLC may be a preferable.
- Master Control Plan. With continuous advances in control automation, from the field device to controls to communications to enterprise software, it is important to understand the plant vision when selecting a control system. A master control plan may take months to develop, and can be crucial in allowing a plant to take advantage of the latest technologies and higher level enterprise software.
- A master control plan is a five-to-10-year automation plan or vision for a plant. It should include detailed steps forward from the existing control to overall plant automation. Projects are prioritized for implementation. Projects that are planned for implementation today have all the hooks and memory to be viable and compatible with future automation plans. Rework and reinvestments are kept at a minimum, reducing the total cost of ownership.
If the plant does not have a master control plan, and the project at hand does not allow for such a study, it is still important to have an understanding of the plant vision for controls. You may learn some very interesting facts that will make a difference in your selection of controls.
Information that will help to make a better selection includes plans to upgrade existing control software, replace existing control hardware, consolidate the plant to single control room, add MES, to increase security, reduce workforce, automate a second similar line, or add a higher level of control in the future, such as asset management.
Commercial Costs
There are many factors that affect the real cost of a control system beyond the cost of the vendor proposal.
- Project Cost. Vendor proposals include control hardware, software, engineering, programming and checkout.
Costs that are not always included are start-up and commissioning, wiring, electrical contractor, mechanical contractor, tuning, spare parts and training. These must be accounted for and evaluated as part of the total project cost.
- Lifecycle Costs. The lifecycle costs are more difficult to estimate and can differ greatly from system to system. Hard costs include spare parts, firmware upgrades, Ethernet hubs, additional levels of integration, wiring, licensing, maintenance contracts, training costs and real estate for engineering workstations, operator stations, training manuals and documentation. If the control system being considered is different from the controls currently installed, there may be costs associated with losses due to lack of familiarity by plant personnel to support the system at the highest level, and losses caused by operator action due to differences between operator stations.
Selecting the existing control system may not be the most cost-effective decision.
- Maintainability. Typically, a DCS requires more skilled maintenance than a PLC. If the application is in a remote area rarely visited or maintained primarily by electricians, then a traditional PLC may be better suited due to its limited accessibility.
Other maintenance questions to consider are whether the system is self-documenting, how backups are made, what changes can be made online, how often is firmware upgraded, what is the cost to stay current, whether upgrades be made online, and which modifications require downtime.
If a system is justified by high-level controls, then there must be a maintenance method to ensure the plant continues to benefit from the control. Too often, high-level applications that could be saving a plant tens to hundreds of thousands of dollars per year with proper maintenance are running at less than desirable levels or turned off.
Selection
With all the knowledge at hand, making a final decision can still be very difficult. The mere process of evaluating the systems for technical merit, for suitability to the plant and for commercial costs will help determine the best recommendation. A structured decision analysis (SDA) allows factors to be listed, weighted and compared, and is a great tool to aid in the evaluation.
On a recent project, plant personnel prepared to automate a water delivery system by placing a DCS controller in each of four areas to allow easy integration to the main DCS at the boiler. However, further evaluation showed the cost would have been very high, the maintenance crew was lean and had little experience with the DCS, and, although the existing DCS and the new DCS were from the same vendor, the integration between the two platforms was no more seamless than the integration between the PLC and DCS. The plant opted to install PLCs consistent with the other PLCs in the plant.
In another plant, the choice was between two DCS vendors. The majority of the plant was controlled by Vendor A. Vendor A appeared to be the most sensible choice, but Vendor B was chosen in the end. Vendor A had several similar, but no identical applications for the particular process, and its pricing only allowed it to do one unit within the plant’s budget. Vendor B had several identical applications and was priced, so that a second unit could be automated. The lifecycle costs, including additional spares, maintenance, training and increased real estate, were deemed insignificant compared to the assurance that the unit would run as desired, and that a second unit could be automated.
Lysette Hunt is a senior controls engineer with the Harris Group (www.harrisgroup.com).
A chart detailing how this process works is available online at www.controlglobal.com/1012_DCSChart.html.
