Control System Buying Check List

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.