Industrial Plant Design - How it is affected by an ...

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... eines modernen Zementwerks als Grund/age fur die Entwicklung von Funk- tionsangaben fur ein ... trucks, railroad cards, and by ship (barge). ...... Grand Total.
Industrial Plant Design - How it is affected by an integrated On-Line Computer System K.M.Wiig The operational and fimctional requirements for a modern cement plant are reviewed as a basis for the development of functional specifications for an integrated on-line computer system. The performance and design specifications of the computer control and information system are then presented. Given the computed specifications of the plant, its requirements, and the computer system, the aspects of the plants which may be modified and improved to give better cost effectiveness are discussed and analyzed. Es werden die Betriebs- und Funktionserfordernisse eines modernen Zementwerks als Grund/age fur die Entwicklung von Funktionsangaben fur ein integriertes On-Line Rechnersystem besprochen. Danach werden die Lei'stungs- und Konstruktionsangaben fur das Steuerungs- und Informationssystem dargelegt. Auf der Grund/age der vorgegebenen berechneten Spezifikationen des Werks, seiner Erfordernisse und des Rechnersystems, werden die Aspekte des Werks, die zur Erzielung h6chster Kostenrentabilitiit modifiziert und verbessert werden ·konnen, diskutiert und analysiert. Le rapport discute les exigences d'exploitation et de fonction d' une cimenterie moderne, comme base du developpement de donnees de fonction pour un systeme integre a traitement direct des donnes. La-dessus, on explique les donnees de rendement et de construction pour un systeme de commande et d'information. Sur la base des specifications calculees qui ont ete donnees prealablement, des exigences posees a cette usine et du systeme calculateur, on discute et analyse !es aspects de l'usine susceptibles d'etre modifies et perfectionnes afin d'obtenir la rentabilite la plus elevee des frais.

1.

Introduction

This paper discusses the effects the introduction of an extensive, integrated on-line computer system (IOLCS) has on plant design and subsequently on plant operation. The plant under consideration is a large scale cement plant comparable to plants which have been placed in operation in Europe and the United States during the last five years. The plant considered here will utilize conventional cement making processes, and will be controlled by one operator from a central control room with the aid of a computer system in a manner described by McMorris et al [1] and by Wiig [2]. The computer system which is proposed in this paper consists of hardware components which currently are readily available. It is a distributed system utilizing medium sized minicomputers which are organized as a three levels hierarchy where the functions are distributed between the individual levels. This philosophy is similar to systems now under implementation and its lowest level functions are much as the programmable controllers now becoming available and being installed for motor control center operation [3]. The control approaches to be used are multivariable control algorithms of the type described by Wong et al [4] and which have shown superior performance in complex environments such as the cement plant when compared to approaches such as direct digital control. Overall plant optimizing and scheduling will be performed in a manner similar to that described by Skull [5]. The computer system is economically justified on a straight cost replacement basis but has impacts on the

plant beyond this by providing improved regulation, operation, scheduling and control. In this analysis, considerable emphasis has been placed on the functional and technical descriptions of the computer system. This has been done to indicate and highlight the relationships which lead to new objectives for the design of the plant, and since the particular effects on plant design are very dependent on the details of the approaches adopted for the computer system and its functions. 2.

General Description of the Plant

The plant under consideration in this paper is a cement plant which produces 3600 metric tons daily of several types Portland Cement, primarily for bulk shipments in trucks, railroad cards, and by ship (barge). It is located adjacent to mineral deposits and receives the main raw materials by trucks and belt conveyors directly from the quarry operations. The raw materials, limestone, and shale supply calcium carbonate and silicates, respectively. Siliac sand and other additives are used to change the composition of the raw mix. The layout of the plant accentuates facilitating transportation between the major plant machinery, stockpiles, and silos, raw material receiving, and shipping, A schematic diagram of the plant is shown in Figure 1. 2.1

Materials Handling System, Stockpiles, Silos

Raw materials enter the plant from the quarry through primary crushing facilities for size reduction (upper

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2. Rechnergestiitzte Anlagenplanung

left of Figure 1). The materials are then carried by the belt transport system either to intermediate stockpiles, or directly to intermediate storage silos adjacent to the slurry mill after secondary crushing. There are three stockpiles, one each for coal (fuel), limestone, and shale. All are served by the same feed belt and the same reclaiming belt. Only one type of material can be transported to the stockpiles at any one time, and only one type of material can be transported to the intermediate silos at any one time, although these transports are

2.2

Slurry Mill

The plant is equipped with one 8000 horsepower rotary ball mill for grinding limestone, shale, and silica sand with water to a slurry. This grinding is performed by ,,closed circuit" screening out coarse material leaving the mill and feeding it back into the inlet. The slurry is pumped to a large slurry basin which is constantly agitated and which again feeds into a similar basin giving two basins in series for better mixing of the raw slurry

Clinker Stock Pile Coal

Stone Shale

Intermediate Storoge & Treatment of Dust

Receiving and



u

0 0

u

E 'iii ::> o' u >-

a.

() (i)

Legend: Motor control center computer

~

Area control computer

D

Central supervisory computer

Cement Silos

Fig. l. Cement Plant Schematics

independent and hence two materials can be in this part of the transport system intermediate simultaneously. The ten intermediate silos are used for crushed limestone and shale, silica sand, coal, cement clinker, gypsum, and for additives for the cement burning process. As indicated in Figure 1, these silos are fed with a single transport system (belts, elevators) which is time shared similarly to the system serving the stockpiles. There are however, several transport systems for reclaiming materials from these silos, all equipped with elaborate weighing devices to feed the kiln and mills with accurately metered quantities of materials. Each silos is equipped with weigh scales and most silos may selectively feed either of the three mills over separate belt systems. Coal is retrieved from the coal silo through a specially designed weighing system to a pulverizing mill which is in the primary air circuit for the kiln combustion process.

to lessen inhomogenuities in composition, etc. The slurry is pumped to the cement kiln where it is metered and fed into the high end of the kiln. 2.3

Kiln

The rotary cement kiln is over 200 meters long, and over 7 meters in diameter. It is inclined towards the discharge end thus making the solid material flow as the kiln rotates. Gases flow counter current to the material and are pulled out of the system by an induced draft fan and discharged through a stack. The gases are heated by a coal flame at the material discharge end. Immediately after the exit from the kiln, the flue gases pass through electrostatic precipitators which remove the majority of suspended solids as dust which after treatment is insufflated into the kiln.

Industrial Plant Design

As the slurry passes through the kiln it is dried, the limestone is calcined (C0 2 is removed by heating), and finally reacts chemically and forms clinker (by sintering). The reaction kinetics of this process is largely unknown and is heavily dependent on chemical composition, particle size, and the crystallographic properties of the individual raw materials. The overall chemical balances are, however, well known. On leaving the kiln, the clinkers are cooled by air in a moving grate cooler equipped by several fans to provide air under pressure and may be transported by a dedicated belt system to the clinker stockpiles, or directly to the intermediate silos. 2.4

Cement Mills

Two rotary ball mills driven by 8000 horsepower motors grind the cement clinker with appropriate amounts of gypsum and other additives to fine cement powder. The fineness of the produced cement strongly affects its properties, thus requiring a classification and recycling system with high control requirements. The finished cement is pumped to one of eight storage silos from with it can be dispensed and weighed into trucks, railroad cars, and barges over conveyor belts. The plant is equipped with an elaborate dust control system throughout which collects dust at conveyor belts, weigh scales, and allows dispensing of dust at each floor from mechanical and manual dust collection to reduce wear of plant machinery. This dust is pumped to the dust treatment plant where waste dust from the kiln in collected and chemically treated to remove alkalis. Treat• ed dust is then pumped toward to the kiln discharge end to be insufilated into the kiln with the primary air for resintering. 2.5

Motor Control Centers

Figure 1 indicates that the plant will be equipped with ten (I to X) motor control center computers, one in each major motor control center. This plant is equipped with several hundred electric motors which mostly are clustered in specific areas of the plant. Each motor control center contains the power relays and branching of the power cabling to each motor, and the required control logic to provide interlock and sequencing controls, etc., for all motors and related pieces of equipment. The plant operator communicates with the motor control center to start and stop selected motors and to check the status of overload, etc. The motor control centers are fed 240 and 440 volts from the plant substations and are located so as to both minimize cabling costs and provide good dust control and ventilation with minimal building costs. Figure 1 also shows the relative location of four area control computers and the central supervisory computer. The four area control computers communicate directly with the supervisory computer which is located adjacent to the central control room from which the plant is controlled by one operator. 2.6

General Operating Conditions

In a cement plant of this type, the bottleneck of opera-

tion is the cement kiln which is dimensioned to operate

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continuously for extended periods with perhaps only a one month scheduled shutdown once a year outside the cement shipping season. Uninterrupted runs may last as long as 30 days or more. The total capacity of the plant is essentially set by the annual clinker capacity of the kiln. The slurry mill is dimensioned to operate less than two 12 hours for each 24 hours of kiln operation. cement mills have a total capacity comparable to the slurry mill. Cement clinker can be stored for several months, hence allowing the build up of a sizeable inventory during the low demand winter months. The daily operating cycle allows running the slurry mill at night when the cost of electric power is less. Likewise ,the cement mills are operated during low power cost times of the day during the low shipping demands periods to reduce the power costs. During the high shipping season (summer), the cement mills may partially operate on an around-the-clock schedule to keep up with the demand and hence they become the limiting factor of the plant capacity during this period since large quantities of finished cement cannot be manufactured in advance and held in stock.

The

3.

Description of Computer Control System

A comprehensive control computer system is planned for this plant. It will allow one operator to control the operation of the total plant through a double CRT display and by the use of computer controlled alarm detection, full automation, and elaborate multivariable algorithms for the automatic control of the plant. There will be no backup for the computer in the control room except for two analog controllers for control of cooler hood-draft air pressure and primary air temperature. All other backup will be manual at the individual motor control centers. In addition, no analog instruments will be located in the central control room for compute~ backup. The computer system is expected to be highly reliable due to its design with dual processors with extended autonomy of the supervisory computer, and with each level in the hierarchy being independent on the next higher level for short term operation. Also, since individual processors are exchangeable and pre-programmable, it is expected that repair times will be very short i.e. maximum less than one hour and normally less than 15 Minutes.

3.1

Functional Description

3.11

General

The Integrated On-Line Computer System (IOLCS) automatically performs all normal and expected abnormal control functions not requiring human intervention or intelligence including start-up and shutdown of all major equipment. It allows the plant operator to override or modify any control functions on a partial or temporary basis, or semi-permanently and completely. It allows him to individually exercise control of any part of the plant. It keeps a history on all plant operation for a 36-hour period and stores permanent history on selected variables.

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2. Rechnergestiitzte Anlagenplanung

The IOLCS generates periodic reports to: a) operator, b) supervisor, c) plant laboratory, d) plant management. It allows the laboratory to enter information on product and raw material qualities for use in control of the plant and permits supervisors to request specific hardcopy reports to be produced on I/O devices in their offices.

3.1 2 Normal Operation IOLCS communicates with the plant operator through CRT displays with light pen and keyboard for him to monitor and control operation of the total plant. It displays proper graphical representations of the plant and indicates the status of each unit. It also produces permanent records of the plant operation for product quality assurance, operating history, and for plant accounting purposes. It establishes and indicates possible maintenance problems.

3.1 3 Abnormal Operation The system alerts and alarms the plant operator of any abnormal conditions throughout the plant and their causes or associated events whenever these can be determined. It takes cautionary moves to prevent mal-operation or damage to equipment as a result of abnormal conditions to the extent that this can be determined in advance.

3.1 4 Motor Control The IOLCS performs all electric motor control functions. To achieve this, it oporates all motor power relays to start and stop motors. It also senses and controls: a) b) c) d) e) f) g)

motor overload, current leakage, coolant failure, mechanical drive failures, restart timing safety devices, sequencing or dependence on run conditions of other motors and devices.

without human guidance. It operates the belts to also prevent contamination of the individual materials or general material spillage.

3.1 7 Intermediate Silo Control The IOLCS controls the material levels in the individual silos and operates the transport system according to a schedule which allows the replenishing of the silos from stockpiles without interference, contamination, or exhaustion of the intermediate inventories in the silos. By manual intervention, the transport schedule may be changed to accommodate arrival and unloading of specific raw materials through the receiving system. On command the IOLCS produces expected schedules and silo levels for the immediate future and up to 24 hours based on milling scheduled and current status.

3.1 8 Slurry Mill Control The IOLCS controls the operation of the slurry mill system to fulfill the following objectives: a) b) c) d)

produce sufficient slurry for kiln operation, produce slurry of sufficient quality, minimize production costs (electricity), minimize raw material costs.

To fulfill objectives a) and c) the system produces a mill operating schedule which takes into account the kiln operating level, slurry basin levels, and power contract terms. It generates this schedule to work with the milling schedule generated for the cement mills. The system allows the operator to modify this schedule based on changes in ·equipment availability, 'kiln operating schedule or in the cement mill schedules. Associated with this schedule is a preventive maintenance and emergeny maintenance schedule. The IOLCS further calculates and maintains the costoptional raw material mix d) which satisfies the raw mix criteria for the cement type to be produced. It starts up, shuts down, regulates, and controls the total mill system and its feeders to consistently produce slurry with the desired chemical and physical properties, even when this requires automatic mill shutdown during periods of feed starvation.

3.1 9 Slurry Basin Control

It provides audible alarms before starting major equipments. It performs these functions through analog inputs, digital inputs and outputs, and interrupts as required.

The system agitates and recycles the two slurry basins so as to maximize the mixing ability of this system and minimizing the risk of letting quality disturbances from the mill system propagate to the kiln.

3.1 5 Actuator Control

3.110 Kiln and Cooler Control

The IOLCS controls and positions actuators for devices such as valves, fan dampers, weigh scales, speed regulators, etc., using appropriate control algorithms. It moves these actuators to desired positions determinded by functions which control the operation of specific plant areas.

The IOLCS controls and regulates the total operation of the cement kiln, its cooler, feed and drive mechanisms, and all its auxiliary devices. It maintains optimal temperature profiles by adjusting heat flows, gas flows, and material flows. It uses a multivariable optimal control approach for this purpose and automatically determines desired temperature profiles and other operating conditions and allows operator intervention in this process. It estimates the values of variables which cannot be measured directly and generally controls the system to the following objectives:

3.16 Transport System The IOLCS provides sequencing control of the individual belt systems. It routes individual materials correctly through the transport system from stockpiles to silos

Industrial Plant Design

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a) maintain a production level commensurate with desired operating capacity, b) maximize clinker quality, c) minimize undesired system effluents, d) maximize equipment life.

shown in Figure 2. The minicomputers selected for this plant are medium priced and generally as offered by many manufacturers throughout the world. The basic motivation for this arrangement is to: a) distribute the computers physically to:

The kiln operations control generates a kiln operating schedule which also reflects the need for preventive and emergency maintenance, and which may be subject to supervisory intervention. It controls the production level of the kiln to reflect the capacity and availability of the major systems and of the individual auxiliary subsystems.

minimize cost of instrument cabling, minimize information transfer requirements, increase system reliability. b) share the computational work load on several individual processors.

3.1 11 Precipitator and Dust Treatment Control

IOLCS controls the operation of the various electrostatic precipitator chambers to optimal voltage and current conditions. It monitors and controls the dust removal system and routes the gas through optimal subsets of the precipitator system as a function of equipment availability. It controls the dust treatment process to maximize recycled dust quality and minimiz waste and pollution. 3.112 Cement Mill Control

The IOLCS controls the operation of the cement mills to fulfill the following objectives: a) b) c) d)

produce sufficient cement to satisfy demand, maintain desired cement specifications, minimize production costs (electricity), maximize throughput of the cement mills.

Based on the cement shipment schedules, the cement silo inventories, and the power contract terms, weekly cement milling schedules are established to fulfill objectives a) and c) above. IOLCS regulates and controls flow rates and separator settings in the total mill systems to satisfy the above objectives. It regulates the mill to provide adequate mill loadings by varying feed rates, and it regulates cement fineness by modifying the blade settings of the separators. It monitors the cement transport system from mills to silos and ascertains that proper cement types are stored in the correct silos. The cement mill control utilizes a multivariable control algorithm. 3.113 Shipping and receiving Control

IOLCS controls the shipping of cement by communicating with the shipping operator. It weighs the correct cement type into trucks, railroad cards, and barges and posts these transactions to accounting files and produces permanent hard copy for immediate verification and for papers of lading. ·

3.2

Technical Discription

3.2 1 General

The computer system selected to control the cement plant is a three-level hierarchical distributed computer system consisting of 15 separate minicomputers, one of which, the supervisory computer, utilizes a dual processor. A diagram of the integrated computer system is

All computers in the system consist of identical central processors, equipped with computer/computer interphases for 4800 baud asynchronous transmission under program control over two conductor dedicated lines. All computers have interrupt capabilities, hardware multiply/divide, automatic restart, and internal clocks. Core memories, I/O drivers and all special circuity are plug-in module which mafoes it possible for each cpu to serve in any location. The foreground processing in all computers is organized similarly in the three levels of the hierarchy to simplify programming and operating systems. It works this way: distinct input tables are periodically read into the computer from the process as analog or digital signals or from other computers in the system. Output tables are likewise read out to the process or to other computers on periodic bases. Control programs, alarm programs, filter programs, etc. are execut on period bases and synchronized with, but often at lower frequencies than the inputs and outputs. The application programs work through pointer vectors for the respective input and output vectors to each program, hence allowing very simple mechanisms for changing between variables in the programs. 3.2 2 Supervisory Computer

The highest level of hierarchy is the supervisory computer which basically is a data processor, and its functions are: a) Communicate with the operator through typers, keyboard and CRT's with light pen. b) Receive information on plant conditions from the four Area Control Computers (ACC) at 4800 baud. c) Transmit information on control goals to the ACC's at 4800 baud. cl) Produce hard-copy reports on typers located in shipping area, plant chemical laboratory, supervisor's office, and control room. e) Store operating information on disc and tape. f) Load programs into spare minicomputer from tape. g) Generate on-line operating schedules for mills, materials handling system, kiln operation, etc. h) Generate off-line schedules for maintenance, shipping, etc. i) Allow program development and modification in the background for any computer system. j) Perform off-line plant data processing in the background. k) Produce hard-copy periodic plant reports on line printer. This computer system consists of two identical powerful 16 bit minicomputers with separate 32k word memories. One computer is a slave to the other and

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2. Rechnergestiltzte Anlagenplanung Motor Control Center Computers (4 k, process 1/0 only) II

Front of kiln and cooler

III

1. Cooler control Front of kiln control Cooler dust collection control

IV Slurry tanks, pumps, compressors

Kiln drives, mid kiln

End

Area Control Computers

2. Controls: Kiln drives End of kiln

(8 - 12 k core, process 1/0 only)

Precipitator Slurry Dust treatment

VI

Silo bottoms, weighscales

VIIICrushing, stock piling, mat'I handling

x

Shipping, Receiving

V Dust treat-

4. Controls: Crusher Material distribution Shipping

3. Controls: Slurry mill Cement mills Weighscales

OPERATOR Spare minicomputer

r----, I

L - - -

I ......i:-------------.i

Supervisory Computer (32 k)

CRT, Keyboard

Card Reader

PROGRAMMER

line printer

\~ ~

Chemical laboratory Supervisor

Fig. 2. Integrated Computer System

operates in the background. The foreground computer performs all on-line data communication, and operator communication (functions a-f). The background computer performs function g-k. Both computers are equipped with a separate disc drive capacity of 1.2 million words each, and tape drives. The system also has a 300 !pm printer and a 200 cpm card reader. The supervisory computer system is arranged with a large degree of redundancy, permitting continued, but reduced operation with one computer, one memory, one disc drive, or one tape drive out of service. The system is equipped with two CRT's, one with full graphic capability and with its own graphic processor, the other is a character display scope for communication of written messages and quantized information. 3.2 3 Area Control Computers (ACC)

The four ACC's perform the following functions for each areas: a) Digitally filter all variables to remove effects of process disturbances. b) Multivariable adaptive control for each control system in its area.

c) Alarm identification and transmission of alarm conditions to supervisory computer on express basis. d) Communication to and from supervisory computer and from Motor Control Center Computers. The four computers have only computer/computer interfaces and have 8k to 12k of core each. Their mainframes are of the same type as for the supervisory computers. The programs ar.e organized with separate data tables for common areas and for input/output. These data tables are operated on by the control program modules. The four computers monitor and control the operation of the following subsections of the plant: 1) Cooler with all its pressures and fans, grate speeds, and temperature. Front of kiln with primary air, dust, and insufflated and additive control. Cooler dust collection and control. Primary air temperature controls. 2) Kiln drive systems (kiln speed). Total kiln control (new data communicated to ACC-1 through Supervisory Computer). Precipitator Slurry basin Dust treatment

Industrial Plant Design

3) Slurry mill Cement mills Weigh scales Intermediate silos discharge system. 4) Material handling and distribution system (raw material and clinker reclaiming). Shipping and receiving Crushing System

3.2 4 Motor Control Center Computers (MCCC) The ten MCCC's perform the following functions m each control center: a) control electric motor operations (as described earlier), b) actuator operation and control, c) reading status of all rotating machinery and of on/off devices, d) reading analog inputs from the plant, e) receiving plant interrupts from critical events, f) communicate with ACC. The programs in the MCCC are organized similarly to the ACC programs and the cpu's are equipped similarly except for the process I/O equipment which is required in the MCCC. The ten motor control centers control the following functions: 1) Front (discharge) end of kiln and cooler with all

2)

3) 4)

5) 6)

7)

8)

9) 10)

4.

fan motors, control mechanisms, dust collection machinery, etc. This is the largest motor control center and also supplies the central control room and plant office utilities. This MCCC also computes the Hood Draft Pressure multivariable control algorithm due to timing problems if done by the ACC. Kiln drives, and mid kiln instrumentation controls. Kiln drives are a SCR driven pair of DC motors and with electric emergency drives. End of kiln area with electrostatic precipitators and kiln feed controls, kiln induced draft fan. Slurry tank area with agitators, slurry pumps, and air compressors for agitation. Dust treatment area with intermediate storage on dust. Intermediate silos (area below silos) weigh scales, elevators, transport belts, coal mill, etc. Mill motors and operation of the three ball mills. Compressors for cement transport, bottom area cement silos. Crushers, transfer house, stockpile discharge and reclaiming system, and part of material handling system. Material handling system on top the intermediate silos, and cooler exhaust fan. Shipping and receiving materials handling and weighing systems.

Effects on Plant Design and Operation

The computer system described in this paper has definite effects on the design of portions of the plant. It is designed to allow control and monitoring of the plant from one central control room with no conventional instrumentation, indicators, or displays. All information regarding

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the state of operation of the plant is measured and converted to digital information locally in each motor control center, thus eliminating the need for the traditional extensive instrument wiring and termination system. Due to the high degree of automatic operation which is required to support the IOLCS system, additional sensors and actuators will be installed. The plant layout will not be affected significantly by the introduction of the IOLCS, except that it may become desirable to locate the central control room closer to the plant offices than in conventional plants. The cement making equipment (kiln, mills) is not directly affected by the introduction of the IOLCS. However, due to the higher degree of operating efficiency realized with such systems as a result of better scheduling and fewer interruptions of operation, larger, and hence more cost effective, ball mills and kilns are employed instead of more and smaller units which traditionally are used partially to ensure continuity of operation. Particular effects on design and operation caused by IOLCS are indicated below.

4.1

Control Room

Jn the control room the majority of conventional equip-

ment is replaced by the IOLCS. Information from the plant is now multiplexed and transmitted as digital data between computers and as a result the complex cabling system with termination panels, etc. is eliminated. The costs of the replaced termination equipment within the control room and termination room is estimated to be $ 40,000. The traditional instrument panels are replaced by the CRT displays [l], and the value of these with internal wiring is approximately $ 60,000. The IOLCS replaces all but two analog controllers, recorders, indicators, sequencing indicators, alarm circuits and displays, and specialized control circuitry for mills, weigh belts, and precipitator. The total cost of the replaced instrumentation and control equipment approaches $ 300,000. In addition, the control room can be physically smaller and no space is required for terminating instrument wiring. Additional space is required for the supervisory computer system with its data processing input/output equipment, although this will be comparable to space required by conventional process computers.

4.2

Instrumentation and Instrument Cabling

A number of instruments must be added to the plant. On-off sensors for sensing operating conditions of equipment, and for sensing gate and valve positions are added. Operating conditions are sensed on the links furthest away from the actuators and are required to ensure extended information on operating conditions. A majority of equipment is dependent on proper operation of other equipment. In addition, all silos are weighed using strain gages built into the support columns to measure silo levels. This requires that each silos is constructed to be free standing. Weigh scales are added to the '8ilos supply belts to weigh in material to the '8ilos and prevent overflow. Material indicators are included on all belts. The cost of this added instrumentation may be estimated to be over $ 100,000.

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2. Rechnergestiltzte Anlagenplanung

Instrument cabling from instruments and actuators to the nearest motor control center is performed similar to current practices. However, due to shorter transmission distances cabling requirements are less severe. Cabling from motor control centers is completely modified, requiring one single four-conductor cable with overall shield replacing hundreds of pairs of instrument cable making cable trays and extensive routing of cables obsolete. Power cabling, however, remains the same. These changes are expected to reduce the cost of cabling by $ 120,000. Simon [6] discusses this in further detail. tail.

4.3

Motor Control Centers

In the ten motor control centers the MCCC will replace

motor control relays and circuitry (except sensors) to perform the functions specified in section 3.1.4. Power relays and start/stop control relays will remain, though since these are needed for operation both by the computer and manual operation locally. For variable speed motors necessary power controls (such as SCR) will remain. An estimate of the value of equipment and installation work which are replaced and not duplicated by the MCCC indicates that their total costs exceed $ 120,000. Secondary effects such as smaller total physical space requirements for the motor control centers are not considered. Particular advantages of motor control by computer, such as digital smoothing of voltage and current signals and more sophisticated overload control philosophies, are expected to be of significant importance. Stopping of motors due to abnormal conditions will be more positive than with a conventional system and will be accompanied by specific computer generated information supplied to the operator to indicate the source of the problem and to aid immediate correction.

4.4

Material Handling System

The physical construction of the materials handling system remains similar to any modern cement plant. However, due to the higher efficiency of the scheduling function, its overcapacity will be somewhat reduced by reducing belt widths and elevator capacities. This will reduce costs somewhat, but it is expected that some of these savings will be offset by a more extensive dust control system which will be required to prevent physical brealcdowns and hence, aid in assurance of the continuous operation which will be required to meet the system operation objectives.

4.5

System Backup

As has been indicated previously, no extensive system backup features will be included in the design of the plant. The IOLCS is designed in such a fashion that failure of one supervisory computer will allow continued operation on pre-stored, fixed schedules and on a reduced level of responce for long periods. Total failure of the supervisory computer will allow operation by the ACC and MCCC for shorter periods (hours), but without

computer supplied information plant status. Failure of ACC will allow very short term operation, but mills would be shut down by MCCC. Kiln would be allowed to operate until operator forced a shutdown. Failure of MCCC would shut down any system under its control leaving necessary motors running as a fail-safe condition until the MCCC would be replaced or repaired. Failure of one MCCC or ACC would automatically cause preparation for loading of the appropriate programs into the standby spare computer by the supervisory system. If the failing computer is the MCCC I (front of kiln), the operator will have the option to manually set and control the fuel rate and use analog controllers for hood draft pressure and primary air temperature control. All other auxiliary equipment will be left in a running condition when that MCCC fails.

4.6

Maintenance

The introduction of IOLCS results in elimination of the majority of the electromechanical switching devices in the plant. The complexity of instrument wiring and number termination points are reduced to less than one third. The service life of the systems components that have replaced this equipment is also generally much greater. As a result, the error sources in the system are greatly reduced, and the cost and time of maintaining it is expected to be significantly smaller than for a conventional system.

4. 7

Plant Staffing

Plant staffing is affected in a number of ways, mostly resulting in a general upgrading of the technical requirements of the personnel. Some of the effcts are: a) One plant operator per shift, with a backup of two field operators and one shipping operator. These persons must be able to operate plant sections manually for short periods if vital computer equipment fails. b) One maintenance person per shift must be able to troubleshoot and repair computer systems (by changing modules or whole computer). c) Electricians' and instrument technician's task become very similar and may require only one craft. d) A small staff of two systems analysts/programmers for system maintenance and modifications must by added to the plant organization.

5.

Conclusions

The costs of equipment which are replaced by the IOLCS have been estimated above. These are average estimates for several plants and include labor costs for installations and may be compared to the costs of the computer equipment. Engineering and drafting costs are not included, nor are systems analysis and programming costs, since these vary with the actual implementation programs. It is expected, however, that these will be comparable when comparing IOLCS to a conventional plant. The computer costs shown below are based on a medium-sized minicomputer and reflect all specified and required equipment:

Industrial Plant Design

a) Control Room Supervisory computer CRT displays 4 Area Control Computers Sub Total

$ 175,000 40,000 75,000 290,000

b) Motor Control Centers 10 Motor Control Center Computers Sub Total

250,000 250,000

c) Extra Equipment Added Instrumentation and wiring Spare computer Sub Total

120,000 25,000 145.000

Grand Total a) Control Room Control Room Instrumentation Termination, and wiring costs Instrument Panels Sub Total b) Motor Control Centers Motor Control Center equipment Plant Instrument Cabling Sub Total

$ 685,000

$ 300,000 40,000 60,000 400,000 120,000 120,000 240,000

$ 640,000 From these estimates it is seen that IOLCS compares closely in total capital equipment cost to that of a con-

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ventional system. The MCCC's are comparable in cost to the conventional equipment they replace. Modern programmable motor controllers would be less expensive, but do not provide the extended ability to handle analog information, on-site control functions, and computer/ computer communication. These features are required to support the centralized computers and to provide the extensive stand alone fail safe capabilities of the MCCC. The central computer system with its equipment for operator supervision and programmer communication, is less expensive than the equipment it replaces. The added plant instrumentation, and its wiring and the required spare computer system makes the overall cost of the two approaches comparable. Plants equipped with process computers traditionally have a larger amount of backup compared to the system discussed in this paper. This generally results in a higher total cost than the conventional approach used for cost comparisons here. 6.

References

[l] McMorris, A.H., Kelleway, J. L., Tapadia, B., Dohmann, E. L.: Are Process Control Rooms Obsolete? Control Engineering 18 (1971) No. 7 p. 42-47 [2] Wiig, K. M.: Centralized Control Room for a Cement Plant. IEEE Detroit Chapter Lecture Series on Modern Control Systems, December 1967 [31 Bentzen-Bilkvist, [.:Private Communication [4] Wong, K. Y., Wiig, K. M., Allbritton, E. A.: Computer Control of the Clarksville Cement Plant by State Space Design Method. IEEE Cement Industry Technical Conference, St. Louis, May 1968 [5] Skull, A.: The Process Control/Operational Research Interface. Operational Research Quarterly 21 (1970) No. 3 p. 21-36 [6] Simon, H.: Multiplex Systems Save Multibucks in Refinery and Chemical Plants. Instrument Society of America paper No. 70564

Diskussionsbemerkungen zu Rechnergestiitzte Anlagenplanung

Frage an Luttmer

Frage an K.M. Wiig

Herdick, Troisdorf

Maddy, USA

1) 2) 3) 4) 5) 6)

Your system is not only extensive hard ware wise, but I think it would also be extensive software wise. I noted no reference to software cost in your presentation., Was this a problem?

Dauer der Entwicklung des Systems? Aufwand in Mannjahren? Welche Programmiersprache? Auf welchem Rechner? Speicherplatzbedarf? Prograrnmlaufzeit?

Antwort Antwort

Zu 1 und 2: An der Entwicklung der einzelnen Programme arbeiteten drei Mitarbeiter von 1968 bis 1971. Es waren also zwolf Mannjahre erforderlich. Ein erheblicher Teil der Prograrnmierleistungen fiel dabei im Rahmen der Programmverbesserungen an. Zu 3: Die Programme wurden in FORTRAN IV geschrieben Zu 4: Fiir die Bearbeitung der Programme wurde ein Fabrikat unseres Hauses, nfunlich der Rechner TR 4, herangezogen. Zu 5: Fiir die Bearbeitung der einzelnen Programme ist eine Speicherkapazitat 19 k Wortem von 48 Bit Wortlange erforderlich. Um die technischen Daten der anzuwendenden Geriite abzuspeichem, sind abermals 5 Magnet-Speichereinheiten notwendig. Zu 6: Die Laufzeit der Programme ist abhangig vom Umfang der zu bearbeitenden Anlage. Je nach GroBe der Anlage ist eine Laufzeit von 5-45 Minuten erforderlich.

It is true that this system is extensive software wise as well as hardware wise. Appreciable efforts will have to be expended in designing and implementing the software. However, the software design is based on a very modularized approach with individual modules utilizing pointer vectors to communicate with input/output arrays. The modules are also all written in a higher level language such as Fortran IV, hence allowing minimization of efforts to implement the software.

Our experiences to date indicate that the cost of systems analysis, programming, documentation, and implementation compares favorably with the cost traditionally encountered with designing and documenting for example the circuits for motor control center operation and for plant instrumentation. The modular approach of the higher levels of the computer control system makes the software costs comparable, if not/less than for traditional computer control systems with perhaps a more limited scope than the system proposed in this paper.

D