Instumentation • Computing • Control
same model quality analysis can pinpoint a specific part of a controller and compare it with the original model, thereby determining the underlying cause of the degradation in performance. First generation APC tools delivered a lot of benefits, but they did not solve the biggest cost issues. With aggressive step-testing the cycles are much shorter, but they are more disruptive. Of course, when the controllers are off-line this results in lost capacity and benefit related to quality that normally accompanies an APC implementation. Tis type of testing requires constant supervision on the part of plant operators and engineers. Te fundamental problem in the past was that the controller needed to be turned off to collect open- loop data suitable for the model identification technologies. Revamping a controller often required up to 80% of the original effort and cost. Tere is also the latent cost of the loss of capacity and quality. Te lack of precision is also an issue in identifying problem areas of the models. Co- linearity detection and repair were not integrated with the other modelling workflows, preparing for model identification was manually intensive and generating candidate models required a lot of activity by the control engineer. Another core issue that companies face is that APC maintenance was performed infrequently and the methodology used mirrored that of the initial
Fig. 2. Many companies have recently reported that advanced control delivers benefits ranging from 2 per cent to 6 per cent of increased profit by reducing process variability and allowing plants to be operated closer to their true constraints.
project. Tat would produce some undesirable side- effects. For example, maintenance was commonly deferred until turnarounds and the interim controller performance was allowed to degrade over time. Sometimes, operators would lose faith in
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