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Optimizing Control System (OCS)Advanced Control for Rotary Calciners

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Advanced Process ControlMetso has developed an advanced or “upper level”process control system called OCS, (an acronym forOptimization Control System). OCS is referred toas an upper level control system because it isbasically installed over top of the more fundamentalPLC-based controls. OCS process control is basedon a proprietary set of operating “rules”, which,when applied to the various process variables,continuously evaluate and update control setpointsto optimize system performance. The addition ofOCS to operating lime kiln systems has been shownto improve product quality, increase productioncapacity, and improve fuel and electric powerconsumption rates.

Future GrowthNow more than ever, as we enter the 21st century,producers are compelled to continuously operate atpeak performance. Demanding product qualityspecifications, lower operating costs, and surges in mar-ket demand requiring over-capacity production rates,have become daily challenges. One of the ways in whicha producer can bring his kiln closer to peak performanceis to apply “advanced”, or “upper level” process controlsystems.

Common Goals of OCSOCS is one type of advanced control system that isconsidered part of this next level in the evolution ofprocess systems control. It was developed by MetsoCISA to optimize the control of complex processes byimitating the decision-making regimen of a system’sbest operator on his/her best day, but doing so on a 24/7 basis.One of the more intuitive reasons for variations in acalcining process, and in turn the product produced, isthe subjective nature of the kiln operator’s decision-making. A kiln is operated by a number of differentindividuals over the course of time. During this timeeach operator brings his/her own personal set ofdecision-making criteria into the control system. Wherethese individual approaches differ significantly is in theprocess conditions that can also vary widely, resultingin a somewhat less than uniform and consistentproduct, and a less than optimal use ofresources.(Combustion).

Typical Control Room - Showing Metso Integrated Computer Control System

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Key Features

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OCS Basics:OCS is a rule-based process control software that runson a PC using Windows NT/XP, and uses a “fuzzy logic”to carry out the decision-making process. It is not onlycontinuously evaluates the values of a set of criticalprocess variables, relative to their respective setpoints,but also evaluates on a continuous basis theappropriateness of these setpoints themselves. It is this“extra effort” that makes it an Optimizing ControlSystem.

Operator Interface:Much of OCS is “invisible” to the kiln operator and takesplace behind a single graphics control screen, the onlymeans of actually interfacing with the upper levelcontrol system. The screen was developed inWonderware (used visualization system), for the mostpart to simply let the operator know what OCS is doing.The one input where the operator is affecting controlover the process– when OCS is enabled – is the enteringof the lab result quality once every 1 to 3 hours. Anyother required input by the operator would impose alimit to the production rate. In the event OCS cannotbe allowed to maximize production, due to low marketdemand for instance, the operator in the control screen

enters an artificial limit. All of the remaining screeninputs are range limits on fuel and air rates, which aremore one-time entries. These limits are set simply toensure operation of the kiln system remain within the“normal” range.

The control screen includes notifications of systemalarm conditions, which make the operator aware of apotential problem with feed, the kiln burner, or thecooler. These alarms should prompt the operator tointensify focus on control of the system and/orphysically inspect certain components of the calciningsystem.

In addition to the graphics control screen, there is oneother tool available to the operator and, as equallyimportant, to plant management… the trendingscreen. All variables used by OCS can be manipulatedin the statistical module, which allows for trending ofall process control input values, their averages, anddeviations. Histograms can also be configured todetermine operating parameters.

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Operator Screen to interface to OCS

OCS Philosophy:In order to optimize control of the calcining process,OCS carries out four basic operations:

(1) Instrument Verification – Just as the kiln operatorwould simply know that a reading from a given fieldinstrument may be inaccurate based on conditionselsewhere in the system, the OCS programming also firstevaluates whether or not a particular reading can becorrect—given the larger picture, before affecting anychange to a process variable.

(2) Temperature Profile Analysis – One of the waysin which the OCS determines whether or not the systemis operating properly is to “look at” the various processtemperatures throughout the system, simultaneously.A properly operating kiln system has a certain processtemperature profile. One specific way in which the OCSmaintains control of the process is to watch fordeviations to this set of temperatures.

(3) System Stabilization – The OCS is constantlywatching the calcinations process and evaluates notonly changes to a process variable, but also the rate atwhich the change is occurring (in real time). In doingso, the program of the OCS is able to respond to anydeviation from the process control scheme before itbecomes critical. In addition, because the OCS doesmaintain a constant vigil over the process, it will “see”momentary excursions of a process variable that wouldmost likely go unnoticed and in turn, undocumentedby a human operator between log entries.

(4) System Optimization – When in control of thecalcinations process, the OCS never becomes “satisfied”with current process conditions. Even at steady-stateoperating conditions, with all of the process variablevalues within the acceptable ranges, the OCS willcontinue to make every effort to affect additionalimprovements. It will constantly be striving to increaseproduction and/or decrease fuel consumption withinthe user-defined settings by slightly “disturbing” thesystem.

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Instrumentation & Controls

The Algorithms: (Building an OCS Application)The critical and challenging part of developing anyupper level control system was “writing the rulebook”.To do so, a number of different information sourceswere used. The instrumentation and controls engineersat Metso Minerals, responsible for developing thecontrol programming conducted an extensive series ofinterviews. These included, among others, talks with kilnoperators, shift foremen and production managers. Thediscussions were aimed at determining which processvariables were thought to be the most critical tomaintain stable system operation and in turn, thecontinuous production of quality lime.

In addition to the information obtained through theinterviewing process, supporting historical data is alsoretrieved from process control computers. With thisinformation in hand, and combined with the experienceof the Metso Minerals commissioning engineers andother field service personnel, the operation of the kilnsystem could be broken down into a set of interwovencause-and-effect relationships. Once these

relationships were established and well defined, theoperational “rules” were written (algorithms). Definingrules can be difficult because some rules are very subtleand may not be even conscious decisions of the human.Some can be overlooked or are difficult to define.

Metso’s goal, as true for the industry in general, is toincrease system throughput, improve product quality,and at the same time, use less fuel and electric powerto do so. This means that variations in the calciningprocess would have to be minimized. Process controlwould have to stabilize the operation of the calciningsystem. Although “steady-state” operation alone, wouldresult in improved system efficiency, stable operationwould reveal other limiting factors of production. Onceidentified, these other limiting factors could beaddressed, solutions applied, and furtherimprovements realized. OCS would prove to be a goodsolution all your rotary kiln applications, especiallypebble lime, lime recovery, and coke kiln applications.

OCS Program - Showing the fuzzy cuves

Subject to alteration without prior notice Brochure No. 1601-04-03-MPR/Danville-English © 2004 Metso Minerals Printed in Sweden

Metso Minerals Industries Inc.350 Railroad StreetDanville, PA 17521-2046USAPhone: +1 (570) 275-3050Fax: +1 (570) 275-6789

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