Chemical Processing - January 2008 - (Page 39) < > 1. t’sautomatic. Testing and model identification are I done automatically rather than manually by engineering personnel. 2. It’smultivariable. Multiple manipulated variables (MV) are tested simultaneously, making test time much shorter than for single variable tests. 3. It’s losedloop.Tests can be performed closed loop c (MPC or PID), resulting in fewer disturbances and operator interventions. In short, this new method of MPC implementation allows the modeling, step testing, integration and commissioning phases to overlap — at the procedure’s peak all four phases, in fact, are underway simultaneously — while requiring fewer personnel resources, at a greatly reduced risk of process upsets. Step-by-step, the methodology ran as follows: Closed-loop operating data were obtained from the plant — approximately one month’s worth of data from when there was a lot of movement in the plant. The data were put through Control Performance Monitor Tai Ji to obtain initial models. These then were incorporated into the MPC and the controller was brought online. The plant was commissioned for two days to ensure that everything was running correctly and that the MV and controlled variables (CV) were functioning as they should. Once the initial commissioning was complete, a Tai Ji step test was performed for three to four days. This test involved moving all the MV simultaneously by perturbing CV target values every minute. “Using Tai-Ji to step test the plant in a closed loop mode was great,” says Mike Golinsky, ROC engineer for Linde. “It reduced testing time, improved model quality and, most importantly, reduced the risk of a loss-producing event occurring during the testing process.” While the step test was underway, we conducted modeling passes every 12 to 16 hours to give the implementation team direct feedback on how the test was going. This feedback led to step size changes to the CV targets (higher signal-to-noise ratio) to better develop the models. Toward the end of the step test a lot of key models had converged — so, the decision was made to put them online with the step test still going on. With the new models online and the step test running, commissioning of the new models began. This is the new methodology at its peak, with all four phases — integration, step testing, modeling and commissioning — occurring at the same time. Once the step test was over, commissioning and fine-tuning of the plant continued. The end result of using this methodology was the commissioning of a reliable grassroots MPC in 12 work weeks rather than the industry standard 3 to 4 months (Figure 1). The greatest challenge faced in this implementation was neither technical nor operational — it was achieving conwww.chemicalprocessing.com fidence in the results of the new method. After all, it’s so much easier to see results when step testing is done one MV at a time versus moving all MV together. Linde had a tough time in the beginning believing the results from Tai Ji but eventually confidence grew and it accepted the results. Getting operations staff, conditioned by five years working with an under-performing MPC, to trust the new controller still was a challenge. The operators were so accustomed to seeing issues with the old MPC that during the step testing and commissioning phases they always were quick to take a loop out of MPC and put it into automatic. Asked why a certain action was taken, an operator replied, “Well, the old MPC would tend to bury this purity, >> Better control Figure 2. With old MPC (top), waste purity suffered severe spikes, now eliminated by new MPC (bottom). January 2008 • 39 http://www.chemicalprocessing.com
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