International Journal of Emerging Technology and Advanced Engineering Website: www.ijetae.com (ISSN 2250-2459, Volume 2, Issue 6, June 2012)

Application of Distributed Control System in automation of Process Industries Megha Anand S.A. 1, Suprathik Sarkar 2 , Sree Rajendra 3 1

Department of Mechanical Engineering, Malnad College of Engineering, Hassan-573201, Karnataka, INDIA 2

Training Manager, ABB Ltd., Bengaluru-560058, Karnataka, INDIA

3

Associate Professor, Department of Mechanical Engineering, Malnad College of Engineering, Hassan-573201, Karnataka, INDIA Abstract:The operation of modern day process plant This in turn requires new creative solutions to cannot be visualized without the smart looking increase productivity, improve quality and reduce Distributed Control System (DCS) situated in control cost through full utilization of raw materials, rooms. Process plant automation has evolved from equipment and manpower. A systems approach is pneumatics to electronics’ to DCS. Use of DCS leads to needed to conceive these new generation process various advantages like the overall optimization, ease of plant, design, build and operate them. operation and maintenance tasks, easy monitoring of more plant parameters and ensure tighter control on them. The field instrumentation and control system have to be integrated properly and must function optimally to achieve such a goal. The main intent of this paper is to highlight the salient features of the present DCS used in the process industry and the working of it and discuss the future trends in automation of large process plants. Case Studies: This paper also deals with a case study to give a clear view of DCS where it has been applied in process control in Captive power plant.

A large variety of available Distributed Control Systems (DCS) architectures are designed by different vendors therefore the classification of DCS elements as described here, is only a possible classification. II.

A. Domain: Domains are group of stations connected on the network. Bus Converters (ABC) are used to link domains. Domains are classified into real domains and virtual domains. Real or CS domains are domains, which consists of stations that are directly connected on the network. Virtual [1] Domains are domains, which consists of stations that are not directly connected on the network. E.g. AC800M, CS1000, Centum-XL stations, Micro-XL Stations etc.

Keywords: DCS, process industry, field instrumentation

I.

CLASSIFICATION OF DCS ELEMENTS

INTRODUCTION

Today‟s competitive environment have placed very high demands on the process industry to produce products economically and at the same time using latest technologies available in the market, justify the cost effectiveness by investing more time, tools and manpower to achieve optimal controls. In order to maintain a competitive edge and profitable operation in this emerging industrial climate requires not only process plant automation but also total plant management.

B. Field control unit: These are the “lowest ‟‟ hierarchical level of a DCS, the function units of the system are distributed and placed in the field.

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International Journal of Emerging Technology and Advanced Engineering Website: www.ijetae.com (ISSN 2250-2459, Volume 2, Issue 6, June 2012) C. Intermediate Stations:

The units are strong autonomous automation subsystem, whose domain of influence is restricted to a few (1 to 8,16,32) measuring points or control loops. Being functionally autonomous and situated in field, the subsystems are called field stations.

The intermediate functional system units which are placed in between the lowest and the highest hierarchical levels are known as group or supervisory stations. The stations are the autonomous systems elements with the restricted domain of influence to a group of field stations, pertaining to a closed, well defined, and relatively autonomous part of the plant.

The primary function of field station is to:   

Collect and preprocess analog and digital signals. Monitor and log the alarming messages Perform open and closed loop control functions

The most common functions of the intermediate stations are  

With the fore listed functions it is evident that the field stations have to be intelligent, CPU based, autonomous system units, [1] provided by a series of process interface modules, as well as with a number of RAM and PROM blocks. They are connected to the bus with the help of a bus interface coupler.

  

Typical analog inputs and outputs per field station are 4, 8, 16, 48 or 64 and binary inputs and outputs per field station is 2, 4, 8, 16 and 256. In addition to this some fields have special digital inputs such as timers, counters etc. or digital outputs such as pulse and stepper motor output, positioning and motor control etc.

State observation of the process variables Calculation of reference values for control loops at “ lowest ‟‟ level (SV & PV) Tracking if order processing and material and energy balances Efficiency analysis Reporting Data exchange with “higher” level stations (central station)

D. Central Computer Station: Central computer station enables centralized plant monitoring, direct operation on plant instrumentation, and offers important program generation and system diagnostics services. The stations connected to other system through the system bus which can initiate the necessary data transfers. Functions of the central computer control are  

Plant oriented General application

E. Monitoring and Command Facilities: The DCS consists of a series of versatile, intelligent, plant monitoring and command facilities, by the use of which the plant state can be supervised and the process with in the plant directly influenced by the plant operator. Figure2.1: Field Control Unit

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International Journal of Emerging Technology and Advanced Engineering Website: www.ijetae.com (ISSN 2250-2459, Volume 2, Issue 6, June 2012) In most of the present process monitoring systems the principle of defining different windows for monitoring different process parameters and massages are being practiced. In ABB 800xA these are divided into two types they are:

B. Working of Captive Power Plant: Steam is generated in the boiler of the thermal power plant using heat of the fuel burnt in the combustion chamber. The steam generated is passed through steam turbine where part of its thermal energy is converted into mechanical energy which is further used for generating electric power. The steam coming out of the steam turbine is condensed in the condenser and the condensate is supplied back to the boiler with the help of the feed pump and the cycle is repeated.

F. Data Communication Link: The present practice of is to divide the whole plant controls into a number of distinguishable control loops & envisaging standalone controllers for them. All these controllers situated in a centrally located control equipment room are connected together through communication network called as field bus .The main advantage of s field bus as compared to the conventional point to point data transfer links are.   

The function of the boiler is to [4] generate steam. The function of the condenser is to condense the steam coming out of the low pressure turbine. The function of the steam turbine is to convert heat energy into mechanical energy. The function of the condenser is to increase the pressure of the condensate from the condenser pressure to the boiler pressure. The other components like economizer, super heater, air heater and feed waters are used in the primary circuit to increase the overall efficiency of the plant.

Higher information content of the signal to be transferred, including the diagnostic & calibration data Lower cabling expenses Simplified interface of the process control unit. III.

CASE STUDY

A. Process Control of CPP: The power plant is an industrial facility for the generation of electricity. At the centre of nearly all power stations is a generator, a rotating machine that converts mechanical energy into electrical energy by creating relative motion between a magnetic field and a conductor. The energy source harnessed to turn the generator varies widely. It depends on what fuels are easily available and the types of technology that Power Company has accessed to. In automating the captive power plant processes the use of equipment and software, which have not been designed specifically for the industry, can land the user with headaches.

Figure3.1Working of Power Plant

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International Journal of Emerging Technology and Advanced Engineering Website: www.ijetae.com (ISSN 2250-2459, Volume 2, Issue 6, June 2012) C. Captive Power Plant Process Controlled through DCS:

b) Steam Temperature Control: The main aim of this control loop is to maintain the steam temperature at a certain set point. The final steam temperature is measured with the transmitter TT. The output is compared with a fixed set point in the controller TIC. The resultant control signal instructs the spray control valve TV to control flow of water to the attemperator.

The process variables are controlled by DCS through valves, pumps or other motors. A motor has a provision to be either run locally (form field) or from DCS (remote). Once it is in remote mode; the message flashes in the operator screen and further option is available to operate either in auto mode or manual mode.

c)

When it is manual mode, it can be made to start or stop by pressing the on-off buttons in Digivis. When in auto mode, the command is issued from DCS, based on the logic of the process station.

Furnace Pressure Control:

The furnace pressure is measured with the Pressure transmitter PT. This is used as a Process variable (PV) and is then compared with a fixed set point in the controller PIC. The resultant control varies the speed of the motor that controls the pressure (ID fan).

An example, of process control logic is a set point control loop consisting of a pressure sensor, controller, and valves. Some of the areas of control are:

d) Combustion Control: The aim of the combustion control loop is to maintain rated steam pressure during fixed and varying conditions. This is achieved by controlling the firing rate and the air flow rate to have a proper combustion in the furnace.

a) Drum Level Control: The aim of this control loop is to maintain the drum level at normal operating level of the boiler. Middle value of three transmitters (LT1, LT2 and LT3) is selected for drum level control through Level indicator (L1) block.

The main steam pressure is measured by the pressure transmitter PT and is used as a process variable for the controller PIC. This controller generates an output signal based on the difference between the local set point and the process variable. The output of this controller is then passed on to the boiler lading stations HIC-M1, M2,M3,M4 which act as an auto/manual station for rotary feeder for coal.

If reading of any two transmitters deviates by 10% then the controller should be forced to manual mode. If greater than 10% deviation is observed in all the three transmitters then the boiler should be tripped. In case of failure of one transmitter, average of the other two transmitters should be referred for control and in case of failure of two transmitters, the boiler should be tripped.

The air flow controller is a slave controller with the airflow as the Process variable (FI) and total heat demand as remote set point. This remote set point is total combustion air requirement. The air flow controller generates an output signal comparing the remote set point and the process variable. The output is fed to the FD fan to control the net air flow.

The level signal acts as a Process variable (PV) to the controller block LIC. The controller block will vary the percentage opening of the valve LV and hence level control is achieved.

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International Journal of Emerging Technology and Advanced Engineering Website: www.ijetae.com (ISSN 2250-2459, Volume 2, Issue 6, June 2012) e)

given to Pressure indicating controller (PIC) and Temperature indicating controller (TIC) as process variable respectively.

CBD Tank Level Control :

This control loop is used to [3] control the CBD tank level. The CBD tank level is measured by the transmitter LT. this level is fed to a level indicating controller LIC as a measured variable. This measured variable is compared with the fixed set point in the Level indicating controller. The controller then operates the CBD level control valve.

The output of the Pressure indicating controller (PIC) is used to provide to operate the Pressure control valve (PV) to control the pressure of steam. Output of the Temperature indicating controller (TIC) is provided to operate the Temperature control valve (TV) to control the temperature of steam.

Acceptable limit then the corresponding alarm is generated as Low (LAL), Low-Low (LALL), High (LAH) and High-High (LAHH). f)

Similarly for steam header pressure transmitter, the Pressure transmitter (PT) senses the pressure of the steam and the Temperature transmitter (TT) senses the temperature of the steam going into the header. These signals are given to the Pressure indicating controller (PIC) and Temperature indicating controller (TIC) and act as process variables respectively.

Deaerator Level Control:

This control loop is used to control the deaerator storage tank level. The Deaerator level transmitter (LT) senses deaerator tank level and level is fed to a Level indicating controller (LIC) as a measured variable. The measured signal is compared with the fixed set point in the level indicating controller. The resultant control signal operates the deaerator control valve (LV) which controls the deaerator level.

The output of the Pressure indicating controller (PIC) is used to operate the pressure control valve to control the pressure of steam. These are provisions to view the valve position through the Position transmitter (ZT) in the DCS.

g) Deaerator Pressure Control This control loop is [4] used to control deaerator pressure. The deaerator Pressure transmitter (PT) senses the deaerator tank pressure and pressure signal is fed to a Pressure indicating controller (PIC) as a measured variable. The measured variable signal is compared with fixed set point in pressure indicating controller. The resultant control signal operates the Deaerator pressure control valve (PV) to control the deaerator pressure. h)

D. System configuration of DCS:

PRDS Control:

This loop is used to control the pressure and temperature of the steam going to deaerator and steam header. For Deaerator steam, the [3] Pressure transmitter (PT) senses the pressure of the steam and Temperature transmitter (TT) senses the temperature of the steam going to deaerator. These signals are

Figure3.2: System Configuration Drawing

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International Journal of Emerging Technology and Advanced Engineering Website: www.ijetae.com (ISSN 2250-2459, Volume 2, Issue 6, June 2012) All connected to plant server and all controllers are connected connectivity server to It has two redundant servers where both the servers are connected to switch 1 and switch 2. Switch 1 and switch 2 helps to communicated between the server, client and controllers. DCSs may employ one or several workstations and can be configured at the workstation or by an off-line personal computer. Local communication is handled by a control network with transmission over twisted pair, coaxial, or fiber optic cable. A server and/or applications processor may be included in the system for extra computational, data collection, and reporting capability.

A DCS typically [1] uses custom designed processors as controllers and uses both proprietary interconnections and Communications protocol for communication. Input & output modules form component parts of the DCS. The processor receives information from input modules and sends information to output modules. The input modules receive information from input instruments in the process and transmit instructions to the output instruments in the field. Computer buses or electrical buses connect the processor and modules through multiplexer or demultiplexers. Buses also connect the distributed controllers with the central controller and finally to the Human Machine Interface (HMI) or control consoles. The figure shows the simulation of the drum level control.

A typical DCS consists of functionally and/or geographically distributed digital controllers capable of executing from 1 to 256 or more regulatory control loops in one control box. The input/output devices (I/O) can be integral with the controller or located remotely via a field network. Today‟s controllers have extensive computational capabilities and, in addition to proportional, integral, and derivative (PID) control, can generally perform logic and sequential control.

IV.

CONCLUSION

DCS has made the process monitoring and controlling in plants simple as it is an easy to handle powerful system.Process plant operation and maintenance has been simplified with the wide spread use of DCS base control architecture. But with the success of field bus-based architecture in some of the smaller process control industries, it is becoming increasingly obvious that the decision to switch over from conventional DCS centric architecture to a more open field bus-based inevitable.

E. Human Machine Interface:

In future, going by current trends, process automation will be totally based on Artificial Intelligence. Artificial Intelligence seems to be the key technology in the near future, wherein the entire concept of Computer Integrated Manufacturing will be powered by this technology. This has resulted in man power. But reduction in man power has not affected the safe operation of the plant as the processes are being monitored by operating station. As a result; in case of faults, alarms and alerts are being issued without any delay, through the provisions of „horn‟ in Control Builder. Therefore an error message is sent both to field and operating stations and the process is halted by tripping the faulty part.

Figure3.3: Simulation of the Water and Steam Lines

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International Journal of Emerging Technology and Advanced Engineering Website: www.ijetae.com (ISSN 2250-2459, Volume 2, Issue 6, June 2012) Therefore DCS monitored captive power plants function safely and with greater efficiency. REFERENCES [1] ABB Instruction Manual [2] Coal Fired Thermal Power Plant: The Basic Steps and Facts, Written by: johnzactruba• Edited by: Lamar Stonecypher Updated Sep 8, 2011 [3] http://indianpowersector.com/powerstation/thermal-power-plant/ [4] "Thermal Power Plant Simulation and Control" ed. by Damian Flynn IEE Power and Energy Series

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