The project
Site management is responsible to the company head office corporate management for the economic operation and performance of the site. The site was required to submit monthly, quarterly and annual performance reports. Previous practice had been to base forecasts solely on factors known at the time of the forecasts. These included such influences as planned maintenance and shutdowns and to a limited extent plant mechanical condition, known material supply arrangements, staffing levels and other site conditions. Subsequent reports included details of unexpected events (weather, supply interruptions, unexpected plant failures, industrial action, etc) by way of explanation for discrepancies between actuals and projections.
Plant performance will deviate from its projected targets as a result of any change to any assumption upon which the forecasts were based. These might include,
- interruptions to raw material supplies
- mechanical plant problems leading to reductions in capacity or equipment breakdowns (unscheduled maintenance)
- weather-induced difficulties (excluding force majeure)
- industrial action causing plant operating limitations or shutdowns
- deteriorating performance of plant but not of such magnitude as to cause a plant outage
- limitations imposed by environmental release restrictions.
The site management came to the view that its management practices could be improved by the use of plant modelling to predict and quantify the consequences of abnormal but not unprecedented events which influence the site's ability to meet its performance targets.
The use of a dynamic process model of the plant allowed the systematic analysis of the plant's hour-to-hour operations over a defined period. The operating periods and throughput of the individual work centres were identified for a wide range of actual and anticipated operating circumstances. Operations were considered over a long period, ranging from hours to months. The analysis considered material flows within the plant, transformations of the working materials in work centres, provision and consumption of resources, and energy flows. The analysis treated the statistically variable nature of the individual processes and performed several “runs” through the designated time period (one week) to generate average operational statistics. These were then extrapolated to a 3 month period, reflecting a typical reporting interval, and yearly to provide a long-term planning basis.
Some results
A significant result was the reduction in throughput as a consequence of what might be called “normal” plant availability. In comparison with an assumed 100% availability, an average 90% availability of major plant brings about a 6% reduction in average throughput and a significant increase in on-site inventories.
Of the disturbances, the 3 day weather-induced problems of the ore preparation plant (crusher/dryer) had the greatest impact (8% reduction in throughput for the week), as might be expected as this plant is in almost continuous operation and the ore preparation plant is on the critical production flow path of the plant. A secondary consequence was the rapid build-up of raw ore on the site, assuming that normal deliveries continue without interruption.
The loss of one product finishing stream was of less consequence for the overall plant production rate but resulted in a substantial build-up of intermediate product storage waiting for processing by the finishing plant. This result can be of value in defining the required capacity of this storage.
Case Study 1. A Dynamic Simulation of the Energy Plant at a Large Steelworks
Case Study 2. A Cogeneration Plant Financial Assessment
Case Study 3. A Process Plant Operations Analysis
Case Study 4. A Dynamic Analysis of Steam Utilities in an Oil Refinery