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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:
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.
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.
There are many factors that affect the real cost of a control system beyond the cost of the vendor proposal.
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.
Selecting the existing control system may not be the most cost-effective decision.
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.
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.