2004 IEEE International Conference on Industrial Technology (ICIT)
Automation of Sugar Boiling Process in Batch Vacuum Pans using ABB-Freelance PLC (AC 800F) and Conductor NT SCADA Maheshwarnath Behary, Robert T. F. Ah King and Harry C. S. Rughooputh Faculty of Engineering University of Mauritius Reduit, Mauritius
[email protected] Abstract- In this paper, the design and implementation of the sugar boiling process in batch vacuum pans using the ABB-Freelance PLC and the Conductor NT is proposed. With a wei-designed automated system, the need for operators’ supervision is greatly minimised and hence permits the labour cost to be reduced and optimising the repeatability and efficiency of the process done by the system. Thus, the operator may readily and effcientty take real time action on the flow of the process permitting the latter to have an upper hand on the system as the latter may decide his own desirable set points at critical intervals during the sugar boiling process. Using Function Block Diagrams (FBD) and Sequential Function Chart (SFC) as the main programming languages for the PLC and with the appropriate interfacing of the designed system on the SCADA as per the Conductor NT manual, a complete centralised control of the automated system for batch vacuum pans is proposed.
2 Overview of Cane Sugar Manufacture
Keywords: Automation, control, PL C, SCADA, sugar
boiling process.
1 Introduction Nowadays to maximise profit we need to minimise the cost of production. The sugar industry [ 1,2] will definitely need decrease the cost of production, which can be achieved by reducing labour or by displacing labour into another sector. Furthermore, energy consumption will have to be used in an optimum way without any unnecessary wastage. Centralisation will be the next target, that is, small industries might have to close down and their activities taken up by major industries. After considering these suggestions, the best way to sustain these ideas is automation. In fact, through innovation and upgrading the machineries, the costs of production, as well as the harvest will be more promising to back the above ideas. The aim of this project is the automation of type batch vacuum pans. The boiling process that is undergone in the batch vacuum pans is very compkx and lengthy. To develop the control statement the modular technique has been used and this is enhanced by the use of the Sequential Function Chart. The compIete automation design is proposed for the batch vacuum pans.
0-7803-8662~0104/$20.00 02004 fEEE
The different processes involved with the cane stalks are discussed. Prior to extraction, the cane should be converted to strands of thread-like fibre with pith (ff ufTy in form), for an optimum juice extraction. Afier juice is extracted it is screened, heated, defecated, decanted, filtered to remove dirt and clay. After filtration, the juice passes through a series of evaporators in order to remove maximum water. As water is removed, the juice becomes more viscous and the brix (dissolved substance in solution) rises rapidly and this viscous liquid is called syrup. The syrup is hrther heated in a vacuum pan until it reaches the point of saturation, then seedling can be initiated by the addition of slurry (mixture of ethanol and icing sugar). The boiling process continues and the icing sugar crystal grows and the mixture becomes more viscous. The resulting viscous mother liquor is termed as “massecuita”. Finally, the “massecuite” is sent to centrifugal where the sugar crystals and the moIasses are separated. The process continues until sugar of type A, B and C are manufactured. Further processing is done to the crystals and stored in silos for ease of transportation. The processed sugar is now ready for domestic and commercial uses.
2.1 Steps invoked in a sugar boiling cycle The flowchart in Fig. 1 shows the complete sequences involved in a sugar boiling cycle. For ease of programming the sequences have been defined into discreet steps. All the actions that follows within a particular step have been fiather defined in complete details so as to define the states of all the input and output devices of the proposed system in the control statement of the system,
3 Design for the New Automated System of the Pans After considering the various requirements of the new system and considering its economic feasibility, a proposed design has been brought forward. It should be noted that the developed system should be very user fiiendly so as the operators can hlly interact with the system. To accomplish this an easily interpreted GUI (Graphical User interface) should be developed.
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3.1 Functional Decomposition of Proposed Design
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Fig. 2 shows the detailed design of the batch vacuum pans 1 and 2. Tags have been labelled in the design and are defined completely in Table 3.1 and Table 3.2 that follow. As it can be seen in the design, the lines in red show the input signals from the transmitters namely the pressure, resistivity, vacuum and level transmitters. Lines in blue show the processed output signals to actuate the regulation valves to achieve desired conditions. Control is achieved by using PID (Proportional Integral And Derivative) Controller [6], which is a block function in the PLC that is the algorithm of the PID ,exists in software in the PLC application program. In calandria of the pan, a pressure transmitter is placed which will be used to sense pressure and send feedback to a PID block, which will regulate the pressure valve in order to reach the desired setpoint. Likewise, the other closed loops in the system perform respective control actions. -Moreover, the real-time values from the transmitters will be used for monitoring, hence the need for these to be displayed with appropriate units on the SCADA. The proposed system also contains ON/OFF valves, which can operate in only two modes, which is either opened or closed position. The states of these valves also need to be displayed on the SCADA for monitoring purposes.
Fig. 1 showing the various steps involved in a sugar boiling cycle
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Fig. 2: Detailed design of the batch vacuum pans
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3.2 Control Principtes Used to Control the Parameters in the New System
3.3 Identification of Inputs and Outputs for the Batch Vacuum Pans
Regulation of vacuum (0 inch mercury to 29.2 inches of mercury) is achieved by controlling the entry of finely sprayed cold water in the barometric condenser, since vapour evolving kom the juice being heated in the batch vacuum pans enters the barometric condenser, which is a closed chamber. So this can be used to create vacuum by direct contact of cold water with the evolved vapour. Obviously, there will be condensation of the vapour and hence leading to volume contraction in the condenser. This will lead to the creation of vacuum. Regulation of pressure (- 1.0 bar to + 0.6 bar) in the caIandria can be achieved by varying flow of steam into the calandria. To control the flow rate a regulation valve can be used. By increasing the opening of the valve, flow rate of steam can be increased and hence leading to an increase in pressure. Likewise, a decrease in pressure can be achieved by gradually closing the valve. Regulation of the hot water valve (0-100%) and syrup/molasses valve (0-100%) will be used to control the desired slope (variation of concentration with time) set by the operator especially during the feed crystal growth stage. Increasing the opening of the valve will lead to increase in flow rate and vice-versa. The water valves, syrup valves, pressure valves, cold water inlet valve in the barometric condenser are expected to be controlled automatically according to the IeveUpercent opening of water valve, resistivity/percent opening of syrup valve, actual pressure in cahndridpercent opening of pressure valve, actual vacuum in pansipercent opening of cold water inlet valve in the barometric condenser. These control valves are supposed to response according to the instruction given by the controller. The remaining valves are the ON/OFF valves. These valves have only two states, either they are opened or closed. So, a signal from the Programmable Logic Controller is sufficient to either activate or deactivate the solenoid of the valve .which turn opens or closes the valves by positioning the controller of the valves to allow flow of compressed air in the correct direction. As it can be seen in Figure 2, lines in red shows input analogue signals kom the transmitters involved. However, lines in blue shows the controlled output signals from the controller to the corresponding rcgulation valves. It should be noted in the proposed design, an ‘or condition’ is defined for the inputs from the two vacuum transmitters. This is done so that each pan can work independently of each other. Secondly, an averaging function has also been used to average the two incoming signals from the two transmitters, so as to give priority to the pan with the lowest vacuum.
After analysing the proposed design (Figure 2), first a list for all the inputs and outputs were identified. Moreover, all tag5 used in the proposed design have been defined. It should be noted that the states of all ONlOFF valves are being monitored; to accomphsh this feedback fiom the position of the valves will be used. This signal will be obtained from the inbuilt limit switches in the valves.
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Moreover, the state of the discharge valve will not be monitored, since this valve shall normally be closed in the process, hence no need to monitor its state. A visual alarm consisting of a lamp will also be used. The lamp will be placed near the pan, and will be used to signal for the satisfaction of preset parameters (setpoint) set by the operator. For example if the operator sets a setpoint for the brix, the moment the setpoint has been attained the lamp shall light up to catch the attention of the operator. The latter can then take appropriate Iocal action that is opening or closing valves. This will be used more frequently when the system will operate in the manual mode, or is controIled manually, that is, the operator’s intervention to confirm that the next step can be started.
4 Description of Equipment Used A brief overview of the hardware and software that will be used for the realisation of the project is given. Then, the details and technical specification of the hardware used will be elaborated further. After the selection of the materials, selected quotations for the equipment will be analysed in details for the selection of the cost-effective components. 4.1 Understandingthe Working Principles of the ABBPLC (AC SOOF) [10,12] After considering the working principles of this PLC, for instance a proper understanding of the communication protocols that have been used namely the Profibus DP protocol and the Ethernet protocol. The input and.output addressing techniques of this PLC has been mastered, as this will be useful in the control development, 4.2 Ethernet Protocol
Ethemet is quite a rapid medium of communication especially when dealing with bulky softwares like the SCADA, which consists of synoptic views. That is why Ethemet has been used to communicate with the SCADA instead of Profibus-DP. The use of Erhernet also permits several computers to be connected to the network at different locations in the plant for control and monitoring. It should be noted that Ethernet assign a unique address to each hardware connected to the network, that is, it uses the TCP/IP protocol (Internet Protocol).
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maintainability, power supply requirements, ruggedness, availability, cost and so on.
4.3 Profibus DP Protocol
Profibus specifies the functional and technical characteristics of a serial fieldbus. This bus interconnects digital and analogue field devices in the low (sensor/actuator level) up to medium (all level) performance range. The system contains master and slave devices. Masters are called active stations. Slave devices are simple devices such as valves, actuators and sensors.
4.6 Selection of Components
For this project, the sensors were obtained from the suppliers and they also have their inbuilt signal conditioning circuit. That is, standard output like 4-20 mA was readily obtained from the ordered transmitters. The various transmitters that were required for this project are as follows:
For the actual case, the PLC with the profibus module is the master and the data logger modules are the slaves, which supply .data (serial form) to the PLC. One main advantage of using profibus-DP is to minimise the cost of wiring, Instead of using many wires for taking signals from isolated field devices, a single twisted pair cable, which transmits all the data into a serial communication [4] can be used instcad to minimise the cost of Wiring.
Resistivity transmitters (Radio frequency probe)
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4.7 Criteria for the Selection of Valves
4.4 The SCADA (Supervisory Control And Data Acquisition) System
Control valves are called to handle all kinds of fluids at broad range of temperatures. So selection of a control valve body assembly requires particular consideration to provide the best available Combination of valve body style, material and trim construction design for the intended service. In order to select the control valve best suited for the existing service conditions the following application guidelines should be considered.
The SCADA system communicates With specific controllers to retrieve data. SCADA was traditionally used for data collection from PLCs and plant floor controllers. SCADA systems were also used for monitoring and supervisory control of processes. The role of SCADA systems has expanded. SCADA systems [3] are a vital part of many manufacturers’ information systems. The manufacturing data, real-time values of parameters being monitored can also he used to program dynamic attributes (visual alarm like flashing and so on). Furthermore they can also be used to program database for parameters of interest and collected data are clearly visible in the form of graphs, this is normally known as historical trending. As a whole the SCADA is use to develop user-fiiendly graphical intcrface. The main advantage of using SCADA is that graphical user interface (GUI) can be developed in an easiIy programmable way.
Type of fluid to be controlled. Temperature range of fluid. Viscosity range of fluid. Minimum and maximum flow required. Minimum and maximum inlet pressure at the control valve. Minimum and maximum outlet pressure at the control valve. Pressure drop across the valve expected during normal flow conditions. lnlet and outlet pipeline size and schedule of pipe.
4.5 Selection of Sensors 171
After considering the specifications of all the components, these components have been ordered so as to complete the proposed design.
In selecting a sensor for a particular application, there are a number of factors that need to be considered:
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Pressure transmitters [9] Vacuum transmitters Level transmitters
Identify the nature of the measurement required that is factors like the variable to be measured, its nominal value, the range of values for measurements, the required speed of measurement, the reliability required, the environmental conditions under which the measurement is to be made. Identify the nature of the output required from the sensor; this determines the signal conditioning requirements in order to give suitable output signals fkom the measurement. Possible seiisors should be identified taking into account factors like range of operation, accuracy, linearity, speed of response, reliability,
5 Understanding the Software for the Control Statement I
The selected languages used are namely (1) The Function Block Diagram (FBD) and (2) The Sequential Function Chart (SFC). Some basic function blocks used for the software development with the FBD are discussed in details. The rules of thumb of the SFC are also explained as per the AC-800F manual. As per the requirements of the control statements, approximately 20 function blocks were used to develop the control statement. So, a deep understanding of these fbnction blocks is needed.
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is, the speed and capacity at which the data is gathered from the system being monitored (Fig. 3).
5.1 Programming the PLC
The control statement was developed into different folders with specific task that is using the modular technique. This simplifies our task in the testing stage. During the programming stage, some compulsory limits of the function blocks were not defined and these have to be entered by the operator. Hence, parameters for pan one was deveIoped so as to simplify the task of operator so that the latter can easily enter the setpoint from the GUI. This characteristic of the program enable the operator to have full control on the boiling process and makes the system flexible and adaptive in the cases of poor yielding fiom the sugar cane that is low purity syrup obtained. The programming stage includes the development of the allocation list that is representing the hardware used with a corresponding PLC address. The control statement that has been developed should be tested to make sure that it contains no bug.
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Fig. 3: Basic block diagram for the online data acquisition system 6.2 Developed Synoptic Views for this Project
The synoptic view shown in Fig. 4 has been developed in the Conductor NT SCADA [l 11, the synoptic view is very informative as real time values and states of all the devices used in this project is readily available by a simple glance.
6.3 Synaptic view for the parameters of pans This synoptic view shown in Fig. 5 has been developed to simplify the task of the operator so that the latter can easily enter the setpoint. This permits the operator to have full control on the process. The working principle of this synoptic view is explained below.
6 Graphical User Interface Development The pan boiler operators should easily understand the GUI developed so that they can interact fully with the system, which pcrmits full control on the sugar boiling process as per the operator’s choice.
6.4 Developed synoptic view for historical trending of pans
6.1 Real-Time Aspect of the System Most control rooms in industries require PC-based systems that automatically acquire data and display the parameters being monitored. The monitored parameters are relevant to the current operating state of the plant, highlighting potential problem areas to the control room staff and engineering staff. The displays must be informative and easily interpreted at a glance by the operators. The main emphasis concerns the real-time aspect of the system, that
A historical trending sub model was configured for each pan respectively (Fig. 6). This sub model was configured such that the variation of the five main analogue parameters can be displayed with rea1 time fluctuations and can store these fluctuations up to one week. The process manager can thus have knowledge about these fluctuations and the current yielding obtained from the sugar cane being processed at the factory. A11 the synoptic views that have been developed should be tested independently to make sure that each sub models in the synoptic view has been correctly interfaced on the SCADA.
Fig. 4: Combined synoptic view for pan one and pan two
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Fig. 5: Synoptic view for the parameters of pan one
Fig. 6: Historical display
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6.5 Cost analysis
The idea behind this renovation was to reduce the cost of production by decreasing energy consumption, labour and also to optimise the sugar boiling process. We would have to acknowledge the fact that large investments are needed in any re-engineering shift and competitiveness and increased profitability should trail the way. In a standard economic analysis such as for this project, rough estimates must be made. Art economic analysis can be performed on the system together with an economic breakdown of the main categories concerned. This can guide the company or anybody in the assessment of the project feasibility in terms of cost, selectivity and decision-making.
7 Conclusions This automation project will definitely bring along with it benefits which will enable the factory to make maximum profit at minimum cost. Examples of some of the benefits that will be witnessed after the implementation are (1) A decrease in production time, (2) Maximum sucrose yielding from the syrup, that is, lower purity of the final molasses, (3) A decrease in labour cost, and (4) An efficient energy consumption. Furthermore, the aims of increasing the profit, decreasing the cost of production and optimising the production capacity will be attained. This system will clear the need of operators to be pennanently on site, next to the batch vacuum pans and actuating parameters manually as 858
requested by the regulation process. This system will provide the benefits of remote control and automatic regulation fiom a centralised control room action.
References J.V. PiIlay, Essential Sugar Technology by Sugur MiIling Research Instjtute, University o f Natal, 4041
Durban, South Africa. E. Hugot, Handbook Of Cane Sugar Engineering, Elsevier Publishing Co., 1960. John Stenerson, Fundumentals of Programmable Logic Controllers, Sensors, And Communications, . Second Edition, Prentice Hall New Jersey, 1999. Jerry FitzGerald and Alan Dennis, Business Data Communicdons and Nerworking, 6& Edition, Wiley New York, 1999. Douglas E. Comer, Internetworking with TCP/IP, 3rd Edition, Prentice-Hall New Jersey, 1995. W. Bolton, Electronic Conpol Systems in Mechanical Engineering, Addisoq Westley Longman Limited, 3d Edition, 1997. Katsuhiko Ogata, Modern Control Engineering, Prentice-Hall of India, New Delhi, 2000. The FS Duotrac Radio Frequency Probe User Manual. Operation and Maintenance Instruction Manual for Intelligent Pressure Transmitters (LD 300 series). (1 01 AC S6OF Documentation Version-6.2, PLC Manual. [l 11 Conductor NT (Version 4.0) Graphic Editor. [12] hm://www.abb.com/control
