DM Plant FYP Report

8/12/2019 DM Plant FYP Report

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Chapter 1

INTRODUCTION

This project DESIGN OF CONTROL SYSTEM FOR A DE-MINERALIZEDWATER PLANT entails maintaining pH factor of raw water & for the removal ofminerals in order to obtain de-Mineralized water. In the Plant build in this project, acid& base mix up with the raw water according to the need of the solution & performneutralization. This plant is basically a part of many industries including oil and gas,chemical industry, silicon industry, pharmaceuticals etc. Our intention of selecting thisproject was to be able to know how DM water plant works. Secondly it will lead us togain knowledge about industry and how things work over there. This project has thefollowing features:

A pH sensor is used to monitor & control pH factor of raw water.

Conductivity sensor is used to monitor conductivity of raw water. Level sensors are sensing eight discrete levels from 0-100% in tanks.

An alarm system containing buzzers is used to indicate acid & base level dropfrom their respective minimum levels.

Solenoid valves are used to control liquid flow in the entire process.

A stirrer is used to mix the solution in order to achieve stable values.

The controlling device used in this project is Programmable Logic Controller(PLC). The control system IOs are shown below:

Acid valve

pH sensor Buzzers

Level Detector Water Pump

Conductivity sensor Inlet/Outlet valves

Base valve

Figure 1.1 I/Os of de-Mineralized Water Plant Logic Controller

As indicated by above IOs block, PLC will have inputs from differentcomponents and will generate the desired outputs to perform different tasksaccordingly.

Our project is divided into four different modules which are as follows:

PLC


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1.1 RAW WATER INJECTION

In this module, raw water from any source is to be injected into the mainprocess tank (de-Mineralizer). This is achieved by sensing level of water in the de-

Mineralizer by water level detector. Upon sensing a low level, water injection isstarted with the help of water pump through the Inlet valve.

Figure 1.2 Water Injection System

1.2 pH & CONDUCTIVITY CONTROL SYSTEM

In this module, the pH & conductivity sensors are deployed inside the de-Mineralizer where they will generate different voltages according to the pH &conductivity of water. These sensors are connected to the PLC where a mappingbetween pH/conductivity values & voltages is calculated. According to the valuesmapped by PLC & set process parameters; PLC will control acid & base valves inorder to vary pH & conductivity.

Figure 1.3 pH & Conductivity Control System

De-Mineralizer

pH sensor

Conductivity

sensor

PLC

Acidcontainer

Acid

valve

Basecontainer

Basevalve

De-Mineralizer

Water

Level

Detector

Water Pump

PLC

Inlet Valve

Raw

Water


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1.3 ACID/BASE LEVEL DETECTOR

Level sensors are installed for liquid level indication in Acid & Basecontainers. The purpose of these sensors is to visually indicate current liquid level

in containers & signal an alarm if a container reaches its critical level.

Figure 1.4 Acid/Base Level Indicators & Alarm System

1.4 DRAINING & MIXING SYSTEM

As pH & conductivity sensors are slow to respond to abrupt change invalues, the mixture must be continuously stirred. For this purpose, a stirrer isinstalled in the de-Mineralizer which starts working as soon as the main processtank is filled & stops whenever the required pH & conductivity values are attained.

When the desired pH & conductivity values for the solution are achieved, itis necessary that the solution is drained in order to restart the whole process withnew raw water. The draining of the solution is automatically done as part of theprocess. Apart from the automatic draining of the solution, a manual drain switchis also installed which can be used in emergency or whenever impurities aregreater than the tolerance level.

Figure 1.5 Liquid Draining & Mixing System

PLCAcid

container

Acid level

indicator

Basecontainer

Base level

indicator

Alarm

PLC

De-Mineralizer

Stirrer

Level indicator

Drain Switch

Raw Water

De-Mineralized

Water


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Chapter 2

BACKGROUND LITERATURE STUDY

2.1 PROGRAMMABLE LOGIC CONTROLLER (PLC)

2.1.1 Introduction

A PLC (Programmable Logic Controller) is an industrial computerused to monitor inputs, and depending upon their state make decisions basedon its program or logic, to control (turn on/off) its outputs to automate amachine or a process.

It uses a programmable memory for the internal storage of instructionsby implementing specific functions such as logic sequencing, timing, counting,and arithmetic to control, through digital or analog input/output modules,various types of machines or processes. Some of the advantages anddisadvantages of a PLC are as follows.

ADVANTAGES DISADVANTAGES

Rugged and designed to withstand

vibrations, temperature, humidity,

and noise.

Too much work required in connecting

wires. Therefore difficulty when

replacement is required.

Have interfacing for inputs and outputs

already inside the controller.

Difficulty in finding errors; requiring

skillful work force.

Easily programmed and have an easily

understood programming language.

When a problem occurs, hold-up time is

usually long.

Table 2.1 Advantages & Disadvantages of PLC

2.1.2 System Block Diagram

Since a PLC is a type of computer therefore it has all the basic parts ofa computer a central processing unit, memory, input and output interfacing.A typical PLC block diagram is shown below:


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Figure 2.1 Block Diagram of PLC

2.1.2.1 Central processing unit (CPU)

It is the control portion of the PLC. It interprets the programcommand retrieved from the memory and acts on those commands.This unit is a microprocessor based system. It allows arithmeticoperations, logic operators, block memory moves, computer interface,local area network functions etc.

2.1.2.2Memory

The system generally consists of a RAM and a ROM. The ROMmemory contains the program information that allows the CPU tointerpret and act on the ladder program stored in the RAM. This iswhere information is stored on the status of input and output devices

and the values of timers and counters and other internal devices.

2.1.2.3 I/O interfacing

Inputs monitor field devices, such as switches and sensors. Theinput section can accept discrete or analog signals of various voltagelevels. Discrete inputs present the CPU a signal of either 1 or 0.Whereas analog inputs are passed through an analog to digitalconverter and present the CPU an input voltage as a binary numbernormalized to the maximum count available from the unit.

The output unit provides either sinking (supplying ground) or

sourcing (providing a voltage) as a discrete or analog voltage, which in-turn controls other devices, such as motors, pumps, solenoid valves etc.

CPU

Memory

(ROM & RAM)

Digital

INPUTS

Analog

Digital

OUTPUTS

Analog


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2.2 PLC OPERATION

The first thing that a PLC does when it starts functioning is update I/O.That is all discrete I/O states are recorded and transferred to the I/O units

respectively. Each I/O point has associated with it a one bit input image register.PLC is generally configured in 16-bit word. This means that one word of memorycontains states of 16 input points of the PLC.

2.2.1 Input Relays

The relays are connected to the outside world. They physically existand receive signals from switches, sensors etc. Typically they are not relaysbut rather they are transistors

2.2.2 Internal Utility Relays

The relays do not receive signals from the outside world nor do theyphysically exist. They are simulated relays and are what enables a PLC toeliminate external relays. From programming standpoint all internal relayshave one coil and as many NO (normally open) and/or NC (normally closed)contacts as the programmer needs.

2.2.3 Counters

They do not physically exist. They are simulated counters and they canbe programmed to count pulses. Typically these counters can count up, downor both up and down. Since they are simulated they are limited in theircounting speed.

2.2.4 Timers

They also do not physically exist. They come in many varieties andincrements. The most common type is an on-delay type. Others include off-delay and both retentive and non-retentive types. Increments vary from 1msthrough 1s.

2.2.5 Output Relays

These are connected to the outside world. They physically exist andsend on/off signals to solenoids, lights, etc. Basically they are transistors.

2.3 PLC COMMUNICATION

2.3.1 Serial Communication

Serial communication is when data is transmitted one bit at a time. Adata word has to be separated into its constituent bits for transmission and thenreassembled into the word when received. Serial communication is used for


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transmitting data over long distances. It is used for the connection between acomputer and a PLC to transfer the program.

RS-232 communications is the method of PLC to external devicecommunications. There are two types of RS-232 devices normally used:a) DTE - Data Terminal Equipment and a common example is a computer.b) DCE - Data Communications Equipment: common example is a modem.PLC usually has a DCE device.

Figure 2.2 DB9 Male Serial Communication Cable

Figure 2.3 DB9 Female Serial Communication Cable


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2.3.2 RS-232 Communication

RS-232 is an asynchronous communications method. We use a binarysystem to transmit our data in the ASCII format. PLCs serial port is used for

transmission/reception of the data, it works by sending/receiving a voltage,With RS232, normally, a 1 bit is represented by a voltage -12 V, and a 0 by avoltage +12 V.

2.4 PLC PROGRAMMING

2.4.1 Introduction to Ladder Logic

Ladder logic uses graphic symbols similar to relay schematic circuitdiagrams. Ladder diagram consists of two vertical lines representing the powerrails. Circuits are connected as horizontal lines between these two verticals.The symbols which are used in ladder program are as follows

--------||------------ Normally open switch--------| |------------ Normally closed switch--------( )------------ Output / Relay--------[ ]------------ Special functions

The features of ladder diagram are as follows

Power flows from left to right.

Output on right side cannot be connected directly with left side.

Contact cannot be placed on the right of output.

Each rung contains one output at least.

Each output can be used only once in the program. A particular input output can appear in more than one rung of a ladder.

The inputs outputs are all identified by their addresses, the notationused depends on the PLC.

2.5 T100MD2424+ PLC

The PLC that we have used is DCI Technologies T100MD+2424 PLC.


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Figure 2.4 Programmable Logic Controller (PLC)

2.5.1 Features

The standard features of the PLC are as follows:

4 to 8 channels of 10-bit Analog Inputs.

1 to 2 channels of 8-bit Analog outputs.

2-channel programmable Motion Controllers for controllingstepper motors up to 20,000 pulses-per-second.

2-channel Pulse Width Modulated (PWM) outputs.

2-channel 32-bit High Speed Counters (HSC) counts up to10,000 Hz.

4-channel Interrupt Inputs.

2-channel pulse measurement inputs capable of measuringfrequency and pulse-width of incoming pulses up to 10,000 Hz.

Real time Clock/Calendar for programming multiple scheduledON/OFF events.

6016 Words (16-bit) of EEPROM Program Memory.

1700 Words (16-bit) of programmable EEPROM for usersdata.

Built-in 16 channels of PID-computation engines let T100MD+PLCs directly provide Proportional, Integral and Derivative(PID) type digital control.

One Independent RS232 port for connection to a host PC forprogramming or monitoring.

One independent RS485 port for networking or for connecting


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to external peripherals such as LCD display and RS485-based analog I/O cards, etc.

Watch-Dog Timer (WDT) which resets the PLC if the CPUmalfunctions due to hardware or software error.

2.5.2 Specifications

The specifications of this PLC are as follows;

It operates at 12 or 24VDC.

All digital inputs and outputs have NPN transistors i.e. they are active low.

The I/O scan time is 2ms whereas program scan-time is 10us.

It has 64 internal counters.

512 internal timers.

64 internal timers with a resolution of 0.01sec.

8 sequencers with 32 steps each. A DCE communication Port for RS-232 communication.


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Chapter 3

SYSTEM ARCHITECTURE AND DESIGN

The design of this project has three major parts. First is the interfacing of theinput devices to the controller. Second is the interfacing of all the loads with thecontroller. And third is to make it a stand-alone system and provide it with necessaryand different levels of voltage supplies. The overall architecture of the system isshown below. The output of various sensors will be given as input to theProgrammable Logic Controller (PLC). Then according to the logic developed thePLC will control the outputs consequently.

Figure 3.1 System Diagram

3.1 OUTPUT INTERFACING

The output configuration of the Programmable Logic Controller is suchthat it is active low, i.e. whenever an output pin of the controller is activated itsvoltage shifts from 12 volts to 0 volts. The high and low regions are defined as:

High Region: 9-12 volts

Low Region: 0-2.5 volts.

The special feature of PLC is that it has built in opto-couplers. An opto-coupler is a passive device for branching or coupling an optical signal. Also itworks as active low which means that when the output is high, there is a virtualground at the output port of the PLC. So nothing can be driven/run directly fromthe PLC. So there is a need to introduce an output circuit that will run the outputdevice. One thing to keep in mind is that it should provide high voltage when there

ProgrammableLogic

Controller

(PLC)

pH Sensor

ConductivitySensor

De-Mineralizer

level indicator

Acid/Base

level indicator

Valves

Stirrer

Water Pump


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is a virtual ground at the output port and zero when there is a high voltage atoutput port. Two scenarios are mentioned below;

Figure 3.2 Scenario 1: ON Condition

Figure 3.3 Scenario 2: OFF Condition

3.1.1 Hardware

Therefore to make the output circuitry or load compatible with thecontroller we designed a circuit. The schematic of which is:

Figure 3.4 Output Interfacing

The two scenarios of the operation are explained as under.

One when the PLCs output is high, means there is no output (as thePLC works as active low). At this time there will be a voltage of about 10volts at the output terminal. This voltage will act as the input signal to theoutput circuit. Next to it is the zener diode in the reverse bias condition.There will be a voltage drop of about 6.2 volts, thus providing 3.8 volts tothe base of transistor Q1. As a result Q1 will be switched on. A virtual


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ground will appear at the collector of Q1. This will turn OFF the transistorQ2. And there will be no Output at the end.

Figure 3.5 OFF Condition

The second way is that when the PLCs output is low, means thereis some output. It will act like there is no signal at the input of the outputcircuit. So Q1 will not work thus all the voltage will arrive at the collector.As a result Q2 will start working and 12 Volts are received at the outputterminal.

Figure 3.6 ON condition


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3.1.2 Parts List

PART DESCRIPTION QUANTITY

Zener Diode 6.2 V 1

PNP 2N3906 Transistor 1

NPN 2N2222 (or MPS2222A) Transistor 2

1 KResistor 3

10 KResistor 2

Table 3.1 Components detail of Output Circuit

3.2 INPUT INTERFACING

As with the output configuration, the input configuration is also active low.That is, the input of the controller is activated or triggered only when a low signalis given. The ranges of the high and low regions are the same as the output

High Region: 9-12 volts.

Low Region: 0-2.5 volts.

Therefore the outputs from the sensor are taken from those componentswhich give a zero voltage drop when a sensor or any other circuit is activated. Wemodeled the input circuits in such a way to achieve the low voltage required totrigger the controllers input.

3.3 DE-MINERALIZED WATER PLANT CONTROL SYSTEM

The system basically had three different modules namely the input, theoutput and the controller itself. All these three modules had different requirementsof voltages and currents to operate independently. The voltage requirements forthese modules are summarized as follows:


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MODULES VOLTAGES REQUIRED

pH Sensor +9 V,-9 V, Grd

Conductivity Sensor +24 V, Grd

Level Indicators + 5 V, Grd

Valves 220 VAC, Grd

Pump 220 VAC, Grd

Stirrer, Buzzers +5V, Grd

Table 3.2 Voltages Required for Each Module

We used a step-down transformer 220/18 VAC & the direct 220 VAC line inorder to acquire these voltages. The specifications of the transformer are as follows:

Multiple voltages

24 V

12 V

9 V

-9 V

5 V all with 3 Amps

The current ratings of the I/O modules and the PLC are summarized as under;

SENSORS Current ratings

pH Sensor: 0.15 A

Conductivity Sensor: 0.15 A

Acid Level Sensor: 0.50 A

Base Level Sensor: 0.50 A

De-Mineralizer Level Sensor: 0.50 A

Total current = 1.80 A


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PLC Voltage/Current ratings

12 V/ 2A

Output Circuit current ratings

No. of circuits = 10

Current rating = 0.1 A/ circuit

Total current = 1 A

Output devices

Stirrer current rating = 0.8A

Number of Valves = 4

Valves current rating = 0.35A/valve = 1.4A

Number of Buzzers = 2

Buzzers current rating = 0.2A/buzzer = 0.4A

Water Pump = 1 A

Total = 4.0 A

3.4 REALIZATION

This project can be implemented in any industry where de-Mineralizedwater is used. It is a completely stand-alone system. Besides that it can also beimplemented in universities and offices for drinking water purposes. This projectwas realized on a small scale just for demonstration purposes. For large scaleimplementation, all the valves, pumps & the stirrer must be of higher ratings &capacities.


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Chapter 4

IMPLEMENTATION

The implementation process had both hardware and software parts involved. Inthis chapter there is a detailed discussion on each of the modules that were completedand realized.

4.1 pH CONTROL SYSTEM

In this module our main agenda is to maintain pH factor of water bycontrolling the addition of acid or base in the main process tank through the acid &base valves. The sensor will be hanged from the top of the tank through a solidPVC pipe of 1 diameter. This sensor will detect the pH of water & pass the pHvalue to PLC after every 30 seconds.

Figure 4.1 Block Diagram of the pH Control System

The basic principle is that the pH sensor probe measures voltages producedin the solution. These voltages are amplified by the operational amplifier used inthe circuit & passed on to the PLC for further action. These received voltages aremapped onto the pH scale of 0-14 by the PLC according to the programmed scale.Based on these calculations, if the desired pH factor is greater than 7 & calculatedcomes out to be less than 7, then the PLC opens the base valve in order to increase

the pH factor. On the other hand, if the desired pH factor is less than 7 &calculated comes out to be greater than 7, then the PLC opens the acid valve inorder to decrease the pH factor. All other conditions are dealt with accordingly.

As soon as the desired pH factor is achieved, the PLC stops the wholeoperation & starts draining the solution. Once the whole solution is drained, thewhole process restarts immediately.

PLC

De-Mineralizer

pH Sensor

Acid Tank

Acid Valve

Base Tank

Base Valve


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4.1.1 Hardware

The sensor used in this project includes a probe from the HANNAinstruments. The reason for using this probe is its accuracy & market name

of HANNA instruments all over the world. The pH sensor circuit includesthe pH probe, a dual J-FET IC TL082, two 9V batteries & a female BNCconnector. The circuit diagram of the pH sensor is shown below:

Figure 4.2 pH Sensor Circuit Diagram

4.1.1.1Working

The pH probe is inserted in the main process tank fromthe top through a solid PVC pipe of 1 diameter. The probe isheld tight so that during stirring of the solution, there is nochance of damage to the probe. The probe reads voltagesproduced in the tank. These voltages are of the order ofmicro volts. Due to this reason, TL082 is used in order toscale these voltages to milli volts range. These readings arepassed to the PLC through one of its analog inputs. The timing

difference between two consecutive readings from the probe isminimum 30 seconds after mixing the solution. This is becausethe probe is too much sensitive to voltage changes on even themicro volts scale. After 30 seconds of continuous stirring, thesolution produces constant voltages which are accurate.

One thing to remember is that a pH probe needs to becalibrated before being used in calculations. These calibrations


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are done by using standard buffer solutions. For our project wehave used three buffer solutions of values 4.01, 7.01 & 10.01from the HANNA instruments. These calibrations help to verifythe pH readings of different solutions. The graph below shows

the relation between pH values & voltages measured by the pHsensor circuit from standard buffer solutions.

Figure 4.3 pH as a Function of Voltage

4.1.1.2Parts list


pH Probe 1

9V battery 2

TL082 1

BNC Connector (female) 1

Table 4.1 Components detail of pH sensor


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4.1.2 Software

Our ladder programming that we used to control the pH factor ofthe raw water is given below.

Figure 4.4 Ladder Programming for pH Sensor

Figure 4.5 Custom Functions for pH Sensor Ladder Logic

4.1.2.1pH sensor

The ladder program for the pH sensor consists of twoparts & includes timers of 30 seconds & 2 seconds along with twocustom functions namely pH & Switching. The detail description ofthe logic is discussed below.

The first half of the ladder circuit includes a 30 secondstimer & the custom function named as pH as shown below.


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Figure 4.6 pH Sensor Ladder Logic Description

This half of the ladder logic diagram includes two circuits:first circuit invokes pH custom function every 30 seconds while thesecond circuit reloads the timer after it overflows.

One thing to bear in mind is that we have set the value ofpH to 8.5 because it is the WHO standard for de-Mineralized water.Another reason is that the oil & gas purification process requireswater with a pH of 8.5 on scale.

From the pH custom function, it is clear that the pH sensorcircuit is connected to the analog channel#1 of the PLC. The PLCreads value from the pH sensor every 30 seconds on its analogchannel#1. After getting the value, this value is scaled for anaccurate reading of up to one decimal place. If the value obtained isless than 8, the PLC opens the base valve. If the value obtained isgreater than 8, the PLC opens the acid valve. These calculations areshown in Fig 4.5 in pH custom function.

The second half of the ladder circuit includes a 2 secondstimer & the custom function named as Switching as shown below.

Figure 4.7 pH Sensor Ladder Logic Description

This half of the ladder logic diagram includes two circuits:first circuit invokes Switching custom function every 2 secondswhile the second circuit reloads the timer after it overflows. Thepurpose of Switching custom function is to close down the acid orbase valves after every 2 seconds as shown in Fig 4.5.


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4.2 CONDUCTIVITY CONTROL SYSTEM

In the conductivity module our main agenda is to maintain conductivity ofwater by defining a range of conductivity. The sensor will be hanged from the top

of the tank through a solid PVC pipe of 1 diameter. This sensor will detect theconductivity of water & pass the observed value to PLC after every 30 seconds.

Figure 4.8 Block Diagram of the Conductivity Control System

The basic working principle is that external 18 VAC is applied to the twoouter poles (electrodes) of the sensor. The produced voltage is measured betweenthe two inner poles. This voltage is directly proportional to the conductivity of thesolution. These received voltages are mapped onto the conductivity scale by thePLC. The range set for conductivity is 0.0-2.0 uS/cm.

4.2.1 Hardware

The conductivity sensor used in this project includes four poles(electrodes) design of stainless steel.

Figure 4.9 Four Pole Design Conductivity Sensor

De-Mineralizer

PLC

Conductivity

Sensor

HMI

Measuring

Electrodes

Drive

Electrodes

ACVoltage

Source

ACVoltage

Measurement


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4.2.1.1Working

The conductivity electrodes are inserted in the mainprocess tank from the top through a solid PVC pipe of 1diameter. The electrodes are held tight so that during stirring of

the solution, there is no chance of damage to the electrodes. Theinner electrodes read voltages produced in the tank. Thesereadings are passed to the PLC through one of its analog inputs.The timing difference between two consecutive readingsfrom the electrodes is minimum 30 seconds after mixing thesolution. This is because the electrodes are too much sensitiveto voltage changes in the solution. After 30 seconds ofcontinuous stirring, the solution produces constant voltageswhich are accurate.

4.2.2 Software

The ladder programming done for the calculation of conductivity ofwater is discussed below.

Figure 4.10 Ladder Programming for Conductivity Sensor

Figure 4.11 Custom Functions for Conductivity Sensor Ladder Logic


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4.2.2.1Conductivity sensor

The ladder program for the conductivity sensor consists oftwo parts & includes timers of 30 seconds & 2 seconds along with

two custom functions namely Cond & Switching. The detaildescription of the logic is discussed below.

Figure 4.12 Conductivity Sensor Ladder Logic Description

This half of the ladder logic diagram includes two circuits:first circuit invokes Cond custom function every 30 seconds whilethe second circuit reloads the timer after it overflows.

One thing to remember is that we have set the range ofconductivity from 0.0 uS/cm to 2.0 uS/cm because it is the WHOstandard for de-Mineralized water.

From the Cond custom function, it is clear that theconductivity sensor circuit is connected to the analog channel#2 ofthe PLC. The PLC reads value from the conductivity sensor every30 seconds on its analog channel#2. After getting the value, thisvalue is scaled for an accurate reading of up to one decimal place. If

the value obtained is not within the range, the PLC opens the acidvalve as clear from Fig 4.11 in Cond custom function.

The second half of the ladder circuit includes a 2 secondstimer & the custom function named as Switching as shown below.

Figure 4.13 Conductivity Sensor Ladder Logic Description

This half of the ladder logic diagram includes two circuits:first circuit invokes Switching custom function every 2 secondswhile the second circuit reloads the timer after it overflows. Thepurpose of Switching custom function is to close down the acid orbase valves after every 2 seconds as shown in Fig 4.11.


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4.3 LEVEL MONITORING SYSTEM

In the level monitor module our main aim is to continuously have a checkon liquid level in all the three tanks. The level sensor visual display is installed on

the front of each tank & the sensor itself is inserted into the tank. Each sensor willdetect eight different discrete levels of liquid in the tank & pass the observed levelto PLC immediately.

Figure 4.14 Block Diagram of the Level Monitoring System

The basic working principle is that a common ground is provided to thelowest placed steel rod on the level strip. Whenever water reaches a certain levelwhere a rod is placed, the rod shorts out with the common ground thus providing acomplete flow path. This shorting out of the rod gives a voltage which is easilydetectable. The sensors used in this project are discretized at eight different levels,giving a percentage difference of approximately 13% between two consecutivelevels.

For generating a conscious about the low level of acid or base in theirrespective tanks, buzzers are installed for the acid & base tanks. These are low

powered buzzers operating at just 5 volts. The purpose of these buzzers is to alarmthe supervisor about the critical quantity of acid or base left in their respectivetanks. These alarms start buzzing whenever there is a level drop of acid or basequantity from the minimum level of 12% in their respective tanks.

PLC

De-Mineralizer

Level Sensor

Acid Tank

Acid LevelSensor

Alarm

Base Tank

Alarm

Base LevelSensor


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4.3.1 Hardware

The sensor used in this project includes nine stainless steel rods of0.5 length. The reason for using this sensor assembly is its simplicity &

low cost. The circuit diagram of the level sensor is shown below:

Figure 4.15 Circuit Diagram of the Level Sensors

4.3.1.1Working

The steel rods are attached to a hard PVC pipe at equaldistance from each other. The pipe is tightly bonded with the innerside of tank so that there is no chance of reading errors. All theserods of each sensor are wired to a digital input of the PLC.Whenever a rod is shorted out with the ground, a high is receivedby the PLC (PLC being active low). A level change will be detectedonly if the rod at that level & all rods below it become shorted

1

2

3

4

5

6

7

8

J1

CONN-SIL8

1

2

J4

CONN-SIL2

DIGITAL INPUTS[1] DIGITAL OUTPUTS

1

2

3

4

5

6

7

8

J2

CONN-SIL8

1

2

3

4

5

6

7

8

J3

CONN-SIL8

BUZZ_A

BUZZER

BUZZ_B

BUZZER

ACID LEVEL SENSOR BASE LEVEL SENSOR DEMIN LEVEL SENSOR

DIGITAL INPUTS[2] DIGITAL INPUTS[3]


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simultaneously. This is logical because the liquid cannot attain ahigher level without crossing the lower level.

Whenever the liquid level drops below the lowest level, thebuzzers attached to the digital outputs of the PLC are signaled &

they start generating alarms for the supervisor. This is an indicationthat the acid or base is about to end that can halt the process in afew minutes.

4.3.2 Software

The ladder programming done for the monitoring of liquid level indifferent tanks is discussed below separately.

Figure 4.16 Ladder Programming for Level Sensors

Figure 4.17 Custom Function for Acid Level Sensor Ladder Logic


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Figure 4.18 Custom Function for Base Level Sensor Ladder Logic

Figure 4.19 Custom Function for de-Mineralizer Level Sensor Ladder Logic


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4.3.2.1Level sensor

The ladder program for the level sensors consists of a singlecircuit with a 0.5 seconds clock & three parallel custom functionsnamely Acid_lvl, Base_lvl & DM_lvl. After every 0.5 seconds, the

three custom functions are invoked & level of the three tanks ismeasured. The logic behind the level detection is simple: if a steelrod at a random level is shorted along with all the rods below it tothe ground, then the level is detected. The shorting will only occurwhen water reaches that level.

From the custom functions, it is clear that the PLC iscontinuously monitoring its digital inputs. Whenever a combinationof the levels is received, PLC displays the appropriate level on theLCD. In case of acid & base level sensors, buzzers are installed thatwill start sounding alarm as soon as the level drops below 12% ofthe total volume.

4.4 MAIN SWITCHING CONTROL SYSTEM

In the main switching module our main aim is to continuously monitor & controlthe switching of valves, pump & stirrer. The main switching panel is installed onthe front of the de-Mineralizer tank. The switching panel controls the switching ofinlet valve, outlet valve, acid valve, base valve, AC pump & stirrer based on thefeedback from different sensors & some preset conditions.

Figure 4.20 Block Diagram of the Main Switching Control System

PLC

De-Mineralizer

Stirrer

pH Sensor

Conductivity

Sensor

Level Sensor

Base Tank

Acid Tank

Acid Valve

Base Valve

Inlet Valve

Outlet ValveAC Pump


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4.4.1 HardwareThe main switching panel includes 24VDC relays & mechanical

buttons for power ON & OFF. All valves, pump & stirrer are connected tothe power sources through these relays.

Figure 4.21 Circuit Diagram of the Main Switching Control System

4.4.1.1 Working

The relays used in the main switching panel are 24 VDC, 5pin relays. One of the two pins of the relay coil is connected to 24VDC source. The second pin is connected to the PLC. In case ofvalves & the AC pump, the NO (Normally Open) pin of the relay isconnected to 220 VAC. In case of stirrer, the NO pin is connectedto 5 VDC. The remaining two pins of all relays are left open.

As PLC is active low on its output terminal i.e. PLC gives24 V for logic 0 & 0 V for logic 1. Therefore all relay coils are

permanently connected to a 24 VDC source. This is becausewhenever PLC gives logic high on any of its output pin, the voltageencountered is 0 volts. So for energizing the coil & completing thecircuit, we have connected the one end of all relay coils to 24 VDCsource. On the other hand, a relay will never switch when PLCgives logic low (24 V) because both ends of the coil will be at samepotential thus blocking the electric path.

RL1(NC)

RL1TEXTELL-KBE-24V

RL2TEXTELL-KBE-24V

RL3TEXTELL-KBE-24V

RL4TEXTELL-KBE-24V

RL5TEXTELL-KBE-24V

RL6TEXTELL-KBE-24V

RL3(NC)SWITCHING PANEL

STIRRERAC PUMP OUTLET VALVE

INLET VALVE ACID VALVE BASE VALVE

1234567

J1SIL-100-07

DIGITAL OUTPUTS

PLC


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4.4.1.2 Parts list


24 VDC Relays 6

2 pin MOLEX 7

2 pin Cream Shell 5

ON/OFF Switch 2

Table 4.2 Components detail of Switching Panel

4.4.2 Software

Below is the ladder logic implemented for the switching panel of the

de-Mineralized water plant.

Figure 4.22 Ladder Programming for Switching Panel

Figure 4.23 Custom Function for Switching Panel Ladder Logic


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4.4.2.1 Switching panelThe ladder program for the switching panel consists of a 0.5

seconds clock & a custom function named Switch.After every 0.5 seconds the Switch custom function is

invoked. In this function, two digital inputs are constantlymonitored by the PLC namely High_lvl & Low_lvl. These twolevels are the high & low levels of the de-Mineralizer tankrespectively. Whenever the water level reaches its lowest value, thePLC switches ON the AC pump & the inlet valve. On the otherhand, whenever water level reaches its maximum value, the PLCswitches OFF the AC pump & the inlet valve & starts the stirrer.

On the other hand, the PLC is also continuously monitoringthe pH & conductivity analog inputs. Whenever the pH valuereaches 8.5, the PLC turns off the acid & base valves. If the pHvalue is other than 8.5, the PLC switches acid & base valves

according to the logic defined in the custom function.


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Chapter 5

TESTING & VALIDATION

Before implementing each module the proposed design was first tested throughsoftware. Only after getting satisfactory results the design was put into practical use.

The logic that was developed and transferred to the PLC was first tested on theTRILOGI Simulator to ensure that it worked as planned.

5.1 SOFTWARE TESTING

The TRILOGi has a built-in simulator for test and debugging. Beforetransferring the whole program to the controller; the program was tested through thesimulator. A shot screen of the testing process is shown below:

Figure 5.1 Simulator Screen 1

It shows the various operations being carried out by the PLC. The redhighlighted icons indicate that these inputs and outputs are currently activated. Inaddition to this the timer counter and relay icon numbers are also highlighted when thecontroller activates them.

Similarly the screen shot shown below shows the values of different variablesused by the programmer from A to Z. it also shows the time, date and the analog


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values from the A/D and D/A convertors of the PLC. The display area shows the itemsthat will be displayed on the LCD which is connected to the PLC.


The values of the Data Memory (DM) elements from 0-4000, the contents ofthe strings used by the programmer from A$ to Z$ and the values of the relays, timersand counter bits are also displayed and checked through the simulator. The screenshots are shown in the same order.



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5.1.1 SERIAL COMMUNICATION SETUP AND TEST

To test that the COM port is working properly or not, a testcommand is sent to the PLC which in return sends its ID number back. Thetest command is IR*. The PLC replies with IR*01 if the connection iscorrect. Else it sends a message of no response from the PLC. Both thescenarios are shown below.


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Figure 5.6 Serial Port Testing 1

Figure 5.7 Serial Port Testing 2


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Chapter 6

CONCLUSION & RECOMMENDATIONS

6.1 CONCLUSION

In the implementation of this project Design of Control System for a de-Mineralized Water Plant it was really necessary to develop a strong understanding ofthe various component used in this project. Many different ways can be deployed tomake the designing and automation more efficient and time effective. The superPLCs are more applicable to industrial automation and mechanization where amultitude of operations are to be conducted in a short time interval.

Such prototype designs can help industries to train there new entrants before

even putting them into practical field. The miniature size of this project as comparedto the actual plants used in industries can help engineers visualize the working of thewhole plant. On the other hand, such small scale plants can be implemented in ruralareas where healthy drinking water is not available. Besides the usage of plant asdrinking water plant, such plants can be applied in rural areas for the need of smallscale industries.

In short this project can go a long distance in acquiring various objectives forpractical & industrial applications which can truly make some major step forward inachieving a power saving and secure environment around us.

6.2 RECOMMENDATIONSSome of the future advancements which can be done in this project are

1) A Pre-Carbon filter can be applied so that the free chlorine & oxygen mixed in

the raw water can be removed before passing the water to de-Mineralizer.

2) A boiler can be attached to the output of the plant so that processed water can

be passed to it, leading to a more industrially optimized setup.

3) Cationic & Anionic beds can be attached to the plant so that the pH maintained

water is passed to it, leading to an efficient removal of cations & anions from

the water.

4) Parameters such as TDS, Hardness, and Salinity of water can also be

maintained so that a more refined & optimized industrial product is furnished.


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REFERENCES

LINKS

T100MD+ PLCs SOFTWARE DOCUMENTATION

http://www.tri-plc.com/documentlist.htm#TRILOGI

SIMPLEST POSSIBLE pH Meter

http://www.66pacific.com/ph/simplest_ph.aspx

CONDUCTIVITY THEORY & PRACTICE

http://www.tcd.ie/Biochemistry/courses/js_conductivity.pdf

PRINCIPLES AND HARDWARE OF ELECTROLYTIC CONDUCTIVITY

M EASUREM ENTS

http://www.alliancets.com/site/files/408/.../White_Paper_-_Conductivity.pdf

pH THEORY

http://www.en.wikipedia.org/wiki/PH

ELECTRICAL CONDUCTIVITY THEORY

http://www.en.wikipedia.org/wiki/Electrical_conductivity

DE-MINERALIZED WATER

http://www.lenntech.com/demi-water.htm

DEMIN WATER FAQs

http://www.lenntech.com/demi-water-faq.htm

T100MD2424+ PLCs DATASHEET

http://www.tri-plc.com/t100md2424.htm

BOOKS

Process Control Systems (Applications, Design, & Tuning) by F. G. Shinskey(2000), McGraw Hill, Singapore, 393-404 & Appendix A

Process Control Instrumentation Technology by Curtis D. Johnson (2000),

Prentice-Hall, USA, 1-40, 47-97, & 220-222


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APPENDIX A.1

PIPING & INSTRUMENTATION DIAGRAM (P&ID)


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APPENDIX A.2

PROCESS FLOW DIAGRAM (PFD)


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APPENDIX A.3

CAUSE & EFFECT DIAGRAM (C&ED)

ISHIKAWA DIAGRAM or FISHBONE DIAGRAM


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APPENDIX A.4

FIELD DEVICES

pH PROBE (HI 1333)

CONDUCTIVITY PROBE (InLab731-2m)


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VALVES (2W-160-15)

PUMP (AI-S)

STIRRER MOTOR (DME38S6HPB)


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APPENDIX A.5

CONNECTIONS (PLC & FIELD DEVICES)

Device Connection Type Pin #pH Sensor Analog ADC(1)

Conductivity Sensor Analog ADC(2)

Acid Level Sensor(Lowest Level)

Digital OUT 11

Base Level Sensor(Lowest Level)

Digital OUT 12

Water Level Sensor

(Lowest Level)

Digital OUT 14

Water Level Sensor(Highest Level)

Digital OUT 15

Acid Valve Digital OUT 3Base Valve Digital OUT 4Inlet Valve Digital OUT 5

Outlet Valve Digital OUT 6AC Pump Digital OUT 1

Stirrer Digital OUT 2

Acid Tank Buzzer Digital OUT 7Base Tank Buzzer Digital OUT 8

DM Plant FYP Report

Documents