1

Smoke & Gas detection Monitoring and Control using PLC

(Programmable Logic Controller):

ACKNOWLEDGMENT:

This Project has been developed with the involvement and assistance

of several people who are responsible in the completion of this project.

Hereby we would like to acknowledge the effort of all the sources

(lecturer and our project guide) which were incorporated in our project.

We are thankful to the Head of our department Dr. M.P. Soni who has

been a source of inspiration to us right from the inspection of our

project, providing us with valuable suggestions and key notes in our

project seminar. His unending quench for maximizing the scope and

opportunities for the betterment of students has been inspiring to us to

do better in our project. We are also grateful to our project guide

Mr. Mohd. Abdul Muqeet for his valuable suggestions. His profound

knowledge on the subject of PLC and its Applications has been helpful

throughout. We owe our deepest gratitude to our faculty adviser

Mr. Shaik Abdul Qadeer for his immense faith in our project and also

for his support in providing us with technical as well as nontechnical

assistance. We would also like to thank our Lab Technician

Mr. A. David.F.Krishow for his support. Our project would not have

been possible without the support, guidance and assistance of

Mr. Ahmed M.A and Mr. Hanumantha Rao.C from Avidus

Engineering Pvt. Ltd, who has shown relentless efforts in educating

us throughout the process in various ways apart from working with us

in making our project success.

Lastly we would like to offer our regards to all those who have

supported us in any respect during the completion of our project.

ABSTRACT

Monitoring and Controlling of Smoke emission and Gas Leakage using PLC

2

The Project is to study Smoke and Gas detector Systems with PLC

(Programmable Logic Control).

Smoke Detector System:

Smoke detector system is essential for Industrial and Commercial

facilities. It detects accidental fire and spray water on the source of fire

to quench it. They include different type of smoke detectors, both

addressable and conventional and Fire Alarm Control Panel (FACP). In

the current project in place of FACP, Programmable Logic Controller

(PLC) used for Logic development.

Gas Detector System:

Gas detector system is essential for detecting flammable and

poisonous gases in Industrial facilities and HVAC (Heating Ventilation

Air Conditioning) systems.

Programmable Logic Controller:

Programmable Logic Controller is a digitally operated electronic

system, designed for use in an industrial environment, which uses a

programmable memory for the internal storage of user-orientated

instructions for implementing specific functions such as logic

sequencing, timing, counting, & arithmetic to control, through digital or

analog inputs & outputs, various types of machines or processes. Both

the PLC & its associated peripherals are designed so that they can be

easily integrated into an industrial control system & easily used in all

their intended functions.

Incoming control signals, or inputs, interact with instructions specified

in the user ladder program, which tells the PLC how to react with the

incoming signals. The user program also directs the PLC on how to

control field devices like motor starters, pilot lights, & solenoids. A

signal going out of the PLC to control a field device is called an Output.

TABLE OF CONTENTS

CHAPTER PAGE NUMBER

Monitoring and Controlling of Smoke emission and Gas Leakage using PLC

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ACKNOWLEDGEMENT 1

ABSTRACT 2

Chapter 1: Introduction 9

1.1Project Overview 9

1.2 Introduction to Smoke Detector 12

1.2.1 Conventional type

1.2.2 Addressable type

12

1.3Introduction to Gas Detector 15

1.3.1 Gas hazarda

1.3.2 Typical areas that require gas detection

1.3.3 Gas detection

1.3.4 Location of Sensors

15

18

22

26

1.4 Control systems and Mitigation 27

1.4.1 Fire alarm Control Panels 30

1.5 PLC(PROGRAMMABLE LOGIC CONTROLLER) 44

1.5.1 Introduction to PLC

1.5.2 PLC Advantages and Disadvantage

44

45

1.5.3 Allen Bradley PLC Micrologic 1200 Controller 46

CHAPTER 2: HARDWARE DESCRIPTION 52

2.1 Block Diagram 52

2.2 Wiring Diagram 55

2.3 Smoke Sensor 56

Monitoring and Controlling of Smoke emission and Gas Leakage using PLC

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2.3.1 Product Introduction

2.3.2 Product profile

56

56

2.4 Gas Detector 57

2.4.1 Sensitivity

2.4.2 Specifications

2.4.3 Gases Detected

57

58

58

2.5 Pump, Solenoid Valve, Fan, Relays 58

CHAPTER 3: PLC PROGRAMMING 60

3.1 Introduction 60

3.2Programming Languages 63

3.3 Ladder Logic Structure 67

3.4 Ladder Logic Programming Basic Instructions 69

CHAPTER 4:LOGIC DEVELOPMENT 73

4.1 Electrical circuit-Logic diagram relationship 77

4.2 Introduction 78

4.3 Ladder Diagram Symbols 78

4.4 Developing a ladder diagram 81

4.5 Automatic Mode of Operation 83

4.6 Ladder Diagram Analysis 85

4.7 Developing the PLC Program Logic 86

4.8 Power Supply Section 87

4.9.1Full Wave Rectifier 87

Monitoring and Controlling of Smoke emission and Gas Leakage using PLC

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4.9.2 Full Wave Bridge Rectifier 89

CHAPTER 5: PROJECT PICTORIAL REPRESENTATION 95

5.1 Project PLC Programming Pictorial Representation

101

CHAPTER 6: ADVANTAGES AND LIMITATIONS 106

6.1 Advantages 106

6.2 Limitations 107

6.3 Conclusion 108

BIBLOGRAPHY 109

LIST OF FIGURES:

Figure Page number

Fig 1: Inside view of Smoke Detector 10

Fig 2: Observation Relays 11

Fig 3: Smoke Detector Installation 12

Fig 4: Installation and placement 14

Monitoring and Controlling of Smoke emission and Gas Leakage using PLC

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Fig 5: Horn & Beacon 28

Fig: 6 Wiring Diagram 32

Fig: 7 Addressable FACP 33

Fig: 8 System Functions 37

Fig 9: Fire alarm panel, showing drill switch 38

Fig 10: Allen Bradley PLC Architecture 49

Fig11: Block Diagram 52

Fig 12: Project Perspective-1 53

Fig 13: Project Perspective-2 54

Fig 14: Wiring Diagram 55

Fig 15: LPG Gas Detector 57

Fig 16: PLC Rack 61

Fig 17: PLC Operatation 62

Fig 18: Basic Components of SFC Lanuguage 65

Fig 19: Basic Components of FBD Language 67

Fig 20: Timer & Counter 70

Fig 21: Three types of Logic representation 72

Fig 22: Positive Logic 74

Fig 23: AND Symbol 75

Fig 24: OR Symbol 76

Fig 25: Inverting Circuit 77

Fig 26: Reset Operation 77

Fig 27: Electrical Interlock Circuit 78

Fig 28: Symbol used Ladder Diagrams 80

Fig 29: Automatic Control of a pressurized water

tank

82

Fig 30: Full Wave Rectifier Circuit 88

Monitoring and Controlling of Smoke emission and Gas Leakage using PLC

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Fig 31: Diode Bridge Rectifier 90

Fig 32: Positive Half-Cycle 90

Fig 33: Negative Half-cycle 91

Fig 34: Typical Bridge Rectifier 91

Fig 35: Full-wave Rectifier with Smoothing

Capacitor

92

Fig 36: Full wave Rectifier 93

Fig 37: Project Model with PLC 95

Fig 38: Project Model View-1 96

Fig 39: Project Model View-2 97

Fig 40: PLC Trainer 98

Fig 41: Project Model View-3 99

Fig 42: Project Model View-4 100

Fig 43: Programming Screen Shot-1 101

Fig 44: Programming Sreen Shot-2 102

Fig 45: Programming Screen Shot-3 103

Fig 46: Programming Screen Shot-4 104

Fig 47: Programming Screen Shot-5 105

Chapter1: Introduction:

1.1 Project Overview:

The Project is the study of Smoke and Gas detector systems with PLC

(Programmable Logic Controller).

The Project includes Smoke and Gas detectors, fan, pump, and

solenoid valve activation. The logic is developed in the Allen Bradley

PLC (Programmable Logic Controller).

Monitoring and Controlling of Smoke emission and Gas Leakage using PLC

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A Smoke detector was installed on a chamber, when it senses the

smoke; it actuates a water pump and solenoid valve to spray water at

the smoke emanating area to extinguish if any fire is there.

A Gas detector was installed in a chamber. When it detects the gas, it

actuates a exhaust fan to remove all the poisonous or flammable gas

from the chamber.

The smoke detector powered by 9 V battery and gives analog signal of

15mA when it detects the smoke. It is connected to PLC

(Programmable Logic Controller) as analog input.

The Gas detector powered by 9v battery and gives potential free

contact (Normally open NO) as digital input to PLC (Programmable

Logic Controller). When it detects the gas the NO contact closes.

1.2 Introduction to Smoke Detector

Smoke detector

A smoke detector is a device that detects smoke, typically as an

indicator of fire. Commercial, industrial, and mass residential devices

issue a signal to a fire alarm system, while household detectors, known

as smoke alarms, generally issue a local audible or visual alarm from

the detector itself.

Smoke detectors are typically housed in a disk-shaped plastic

enclosure about 150 millimeters (6 in) in diameter and 25 millimeters

(1 in) thick, but the shape can vary by manufacturer or product line.

Most smoke detectors work either by optical detection (photoelectric)

or by physical process (ionization), while others use both detection

methods to increase sensitivity to smoke. Sensitive alarms can be used

to detect, and thus deter, smoking in areas where it is banned such as

toilets and schools. Smoke detectors in large commercial, industrial,

and residential buildings are usually powered by a central fire alarm

system, which is powered by the building power with a battery backup.

Monitoring and Controlling of Smoke emission and Gas Leakage using PLC

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However, in many single family detached and smaller multiple family

housings, a smoke alarm is often powered only by a single disposable

battery.

Fig1: Inside view of Smoke Detector

Inside a basic ionization smoke detector. The black, round structure at

the right is the ionization chamber. The white, round structure at the

upper left is the piezoelectric buzzer that produces the alarm sound

An ionization type smoke detector is generally cheaper to manufacture

than an optical smoke detector; however, it is sometimes rejected

because it is more prone to false (nuisance) alarms than photoelectric

smoke detectors.[2][3] It can detect particles of smoke that are too small

to be visible. It includes about 37 kBq or 1 µCi of radioactive

element americium-241 (241Am), corresponding to about 0.3 µg of the

isotope. The radiation passes through an ionization chamber, an air-

filled space between two electrodes, and permits a small,

constant current between the electrodes. Any smoke that enters the

chamber absorbs the alpha particles, which reduces the ionization and

interrupts this current, setting off the alarm.

An alpha emitter, has a half-life of 432 years. Alpha radiation, as

opposed to beta and gamma, is used for two additional reasons: Alpha

particles have high ionization, so sufficient air particles will be ionized

Monitoring and Controlling of Smoke emission and Gas Leakage using PLC

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for the current to exist, and they have low penetrative power, meaning

they will be stopped by the plastic of the smoke detector or the air.

Obscuration is a unit of measurement that has become the standard

definition of smoke detector sensitivity. Obscuration is the effect that

smoke has on reducing sensor visibility; higher concentrations of

smoke result in higher obscuration levels.

Typical smoke detector obscuration ratings[9]

Type of

DetectorObscuration Level

Ionization 2.6–5.0% obs/m (0.8–1.5% obs/ft)

Photoelectric 6.5–13.0% obs/m (2–4% obs/ft)

Beam 3% obs/m (0.9% obs/ft)[citation needed]

Aspirating0.005–20.5% obs/m (0.0015–6.25%

obs/ft)

Laser 0.06–6.41% obs/m (0.02–2.0% obs/ft)[10]

Fig 2: Observation Relays

Commercial smoke detectors are either conventional or analog

addressable, and are wired up to security monitoring systems or fire

alarm control panels (FACP). These are the most common type of

detector, and usually cost a lot more than a household smoke alarms.

They exist in most commercial and industrial facilities, such as high

rises, ships and trains. These detectors don't need to have built in

alarms, as alarm systems can be controlled by the connected FACP,

which will set off relevant alarms, and can also implement complex

functions such as a staged evacuation.

1.2.1Conventional

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The word "conventional" is slang used to distinguish the method used

to communicate with the control unit from that used by addressable

detectors whose methods were unconventional at the time of their

introduction. So called “Conventional Detectors” cannot be individually

identified by the control unit and resemble an electrical switch in their

information capacity. These detectors are connected in parallel to the

signaling path or (initiating device circuit) so that the current flow is

monitored to indicate a closure of the circuit path by any connected

detector when smoke or other similar environmental stimulus

sufficiently influences any detector. The resulting increase in current

flow is interpreted and processed by the control unit as a confirmation

of the presence of smoke and a fire alarm signal is generated.

1.2.2Addressable

Fig 3: Smoke Detector Installation

An addressable Simplex smoke detector

This type of installation gives each detector on a system an individual

number, or address. Thus, addressable detectors allow an FACP, and

therefore fire fighters, to know the exact location of an alarm where

the address is indicated on a diagram.

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Analog addressable detectors provide information about the amount of

smoke in their detection area, so that the FACP can decide itself, if

there is an alarm condition in that area (possibly considering day/night

time and the readings of surrounding areas). These are usually more

expensive than autonomous deciding detectors

Standalone smoke alarms

The main function of a standalone smoke alarm is to alert persons at

risk. Several methods are used and documented in industry

specifications published by Underwriters Laboratories [12]  Alerting

methods include:

Audible tones

Usually around 3200 Hz due to component constraints (Audio

advancements for persons with hearing impairments have been

made;

85 dB A  at 10 feet

Spoken voice alert

Visual strobe lights

110 candela output

Monitoring and Controlling of Smoke emission and Gas Leakage using PLC

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Fig:4 Installation and placement

In new construction, minimum requirements are typically more

stringent. All smoke detectors must be hooked directly to the electrical

wiring, be interconnected and have a battery backup. In addition,

smoke detectors are required either inside or outside every bedroom,

depending on local codes. Smoke detectors on the outside will detect

fires more quickly; assuming the fire does not begin in the bedroom,

but the sound of the alarm will be reduced and may not wake some

people. Some areas also require smoke detectors in stairways,

main hallways and garages.

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1.3Introduction to Gas Detector

What is gas?

The name gas comes from the word chaos. Gas is a swarm of

molecules moving randomly and chaotically, constantly colliding with

each other and anything else around it. Gases fill any available volume

and due to the very high speed at which they move will mix rapidly

into any atmosphere in which they are released.

Industrial processes increasingly involve the use and manufacture of

highly dangerous substances, particularly flammable, toxic and oxygen

gases. Inevitably, occasional escapes of gas occur, which create a

potential hazard to the industrial plant, its employees and people living

nearby. Worldwide incidents, involving asphyxiation, explosions and

loss of life, are a constant reminder of this problem.

In most industries, one of the key parts of any safety plan for reducing

risks to personnel and plant is the use of early-warning devices such as

gas detectors. These can help to provide more time in which to take

remedial or protective action. They can also be used as part of a total,

integrated monitoring and safety system for an industrial plant.

1.3.1Gas Hazards

There are three main types of gas hazard: Flammable, Toxic and

Asphyxiant

Flammable Gas Hazards

Combustion is a fairly simple chemical reaction in which oxygen is

combined rapidly with another substance resulting in the release of

energy. This energy appears mainly as heat – sometimes in the form of

flames. The igniting substance is normally, but not always, a

Hydrocarbon compound and can be solid, liquid, vapor or gas.

However, only gases and vapors are considered in this publication.

Monitoring and Controlling of Smoke emission and Gas Leakage using PLC

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Flammable Limit

There is only a limited band of gas/air concentration which will produce

a combustible mixture. This band is specific for each gas and vapor

and is bounded by an upper level, known as the Upper Explosive Limit

(or the UEL) and a lower level, called the Lower Explosive Limit (LEL).

At levels below the LEL, there is insufficient gas to produce an

explosion (i.e. the mixture is too ‘lean’), whilst above the UEL, the

mixture has insufficient oxygen (i.e. the mixture is too ‘rich’). The

flammable range therefore falls between the limits of the LEL and UEL

for each individual gas or mixture of gases. Outside these limits, the

mixture is not capable of combustion. The Flammable Gases Data in

section 2.4 indicates the limiting values for some of the better-known

combustible gases and compounds. The data is given for gases and

vapors at normal conditions of pressure and temperature. An increase

in pressure, temperature or oxygen content will generally broaden the

flammability range. In the average industrial plant, there would

normally be no gases leaking into the surrounding area or, at worst,

only a low background level of gas present. Therefore 5 Flammable

Gas Hazards Flammable Limit There is only a limited band of gas/air

concentration which will produce a combustible mixture. This band is

specific for each gas and vapor and is bounded by an upper level,

known as the Upper Explosive Limit (or the UEL) and a lower level,

called the Lower Explosive Limit (LEL). the detecting and early warning

system will only be required to detect levels from zero percent of gas

up to the lower explosive limit. By the time this concentration is

reached, shut-down procedures or site clearance should have been put

into operation. In fact this will typically take place at a concentration of

less than 50 percent of the LEL value, so that an adequate safety

margin is provided.

Monitoring and Controlling of Smoke emission and Gas Leakage using PLC

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However, it should always be remembered that in enclosed or

unventilated areas, a concentration in excess of the UEL can

sometimes occur. At times of inspection, therefore, special care needs

to be taken when operating hatches or doors, since the ingress of air

from outside can dilute the gases to a hazardous, combustible mixture.

Flammable Gas Properties Ignition Temperature:

Flammable gases also have a temperature where ignition will take

place, even without an external ignition source such as a spark or

flame. This temperature is called the Ignition Temperature. Apparatus

for use in a hazardous area must not have a surface temperature that

exceeds the ignition temperature. Apparatus is therefore marked with

a maximum surface temperature or T rating.

Toxic Gas Hazards

Some gases are poisonous and can be dangerous to life at very low

concentrations. Some toxic gases have strong smells like the

distinctive ‘rotten eggs’ smell of H2S. The measurements most often

used for the concentration of toxic gases are parts per million (ppm)

and parts per billion (ppb). For example 1ppm would be equivalent to a

room filled with a total of 1 million balls and 1 of those balls being red.

The red ball would represent 1ppm. More people die from toxic gas

exposure than from explosions caused by the ignition of flammable

gas. (It should be noted that there is a large group of gases which are

both combustible and toxic, so that even detectors of toxic gases

sometimes have to carry hazardous area approval). The main reason

for treating flammable and toxic gases separately is that the hazards

and regulations involved and the types of sensor required are different.

Hygiene Monitoring

The term ‘hygiene monitoring’ is generally used to cover the area of

industrial health monitoring associated with the exposure of

employees to hazardous conditions of gases, dust, noise etc. In other

Monitoring and Controlling of Smoke emission and Gas Leakage using PLC

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words, the aim is to ensure that levels in the workplace are below the

statutory limits. This subject covers both area surveys (profiling of

potential exposures) and personal monitoring, where instruments are

worn by a worker and sampling is carried out as near to the breathing

zone as possible. This ensures that the measured level of

contamination is truly representative of that inhaled by the worker.

1.3.2Typical Areas that Require Gas Detection

There are many different applications for flammable, toxic and oxygen

gas detection. Industrial processes increasingly involve the use and

manufacture of highly dangerous substances, particularly toxic and

combustible gases. Inevitably, occasional escapes of gas occur, which

create a potential hazard to the industrial plant, its employees and

people living nearby. Worldwide incidents involving asphyxiation,

explosions and loss of life, are a constant reminder of this problem.

In most industries, one of the key parts of the safety plan for reducing

the risks to personnel and plant is the use of early warning devices

such as gas detectors. These can help to provide more time in which to

take remedial or protective action. They can also be used as part of a

total integrated monitoring and safety system for an industrial plant.

Oil & Gas

The oil and gas industry covers a large number of upstream activities

from the on and offshore exploration and production of oil and gas to

its transportation, storage and refining. The large amount of highly

flammable Hydrocarbon gases involved are a serious explosive risk

and additionally toxic gases such as Hydrogen Sulfide are often

present.

Typical Applications:

Exploration Drilling Rigs

Production Platforms

Onshore oil and gas terminals

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Refineries

Typical Gases:

Flammable: Hydrocarbon gases

Toxic: Hydrogen Sulfide, Carbon Monoxide

Typical Applications:

Around the boiler pipe work and burners

In and around turbine packages

In coal silos and conveyor belts in older coal/oil fired stations

Typical Gases:

Flammable: Natural Gas, Hydrogen

Toxic: Carbon Monoxide, SOx, NOx and Oxygen deficiency.

Waste Water Treatment Plants

Waste Water Treatment Plants are a familiar site around many cities

and towns.

Sewage naturally gives off both Methane and H2S. The ‘rotten eggs’

smell of H2S can often be noticed as the nose can detect it at less than

0.1ppm.

Typical Applications:

Digesters

Plant sumps

H2S Scrubbers

Pumps

Typical Gases:

Flammable: Methane, Solvent vapors

Toxic: Hydrogen Sulfide, Carbon Dioxide, Chlorine, Sulfur Dioxide,

Ozone.

Boiler Rooms

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Boiler Rooms come in all shapes and sizes. Small buildings may have a

single boiler whereas larger buildings often have large boiler rooms

housing several large boilers.

Typical Applications:

Flammable gas leaks from the incoming gas main

Leaks from the boiler and surrounding gas piping

Carbon Monoxide given off badly maintained boiler

Typical Gases:

Flammable: Methane

Toxic: Carbon Monoxide

Hospitals

Hospitals may use many different flammable and toxic substances,

particularly in their laboratories. Additionally, many are very large and

have onsite utility supplies and backup power stations.

Typical Applications:

Laboratories

Refrigeration plants

Boiler rooms

Typical Gases:

Flammable: Methane, Hydrogen

Toxic: Carbon Monoxide, Chlorine, Ammonia, Ethylene oxide and

0Oxygen deficiency

Tunnels/Car Parks

Car Tunnels and enclosed Car Parks need to be monitored for the toxic

gases from exhaust fumes. Modern tunnels and car parksuse this

monitoring to control the ventilation fans. Tunnels may also need to be

monitored for the build up of natural gas.

Typical Applications:

Monitoring and Controlling of Smoke emission and Gas Leakage using PLC

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Car tunnels

Underground and enclosed car parks

Access tunnels

Ventilation control

Typical Gases:

Flammable: Methane (natural gas), LPG, LNG, Petrol Vapor.

Toxic: Carbon Monoxide, Nitrogen Dioxide

Principles of Detection

Many people have probably seen a flame safety lamp at some time

and know something about its use as an early form of ‘firedamp’ gas

detector in underground coal mines and sewers. Although originally

intended as a source of light, the device could also be used to estimate

the level of combustible gases- to an accuracy of about 25-50%,

depending on the user’s experience, training, age, colour perception

etc. Modern combustible gas detectors have to be much more

accurate, reliable and repeatable than this and although various

attempts were made to overcome the safety lamp’s subjectiveness of

measurement (by using a flame temperature sensor for instance), it

has now been almost entirely superseded by more modern, electronic

devices.

Nevertheless, today’s most commonly used device, the catalytic

detector, is in some respects a modern development of the early flame

safety lamp, since it also relies for its operation on the combustion of a

gas and its conversion to carbon dioxide and water.

A further improvement in stable operation can be achieved by the use

of poison resistant sensors. These have better resistance to

degradation by substances such as silicones, sulfur and lead

compounds which can rapidly de-activate (or ‘poison’) other types of

catalytic sensor.

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To achieve the necessary requirements of design safety, the catalytic

type of sensor has to be mounted in a strong metal housing behind a

flame arrestor. This allows the gas/ air mixture to diffuse into the

housing and on to the hot sensor element, but will prevent the

propagation of any flame to the outside atmosphere. The flame

arrestor slightly reduces the speed of response of the sensor but, in

most cases the electrical output will give a reading in a matter of

seconds after gas has been detected. However, because the response

curve is considerably flattened as it approaches the final reading, the

response time is often specified in terms of the time to reach 90

percent of its final reading and is therefore known as the T90 value.

T90 values for catalytic sensors are typically between 20 and 30

seconds.

1.3.3Gas Detection

In general, gas detection is divided into combustible gas detection and

toxic gas detection. This is a broad separation that breaks down in

some cases, e.g. some gases are both toxic and combustible in the

concentrations expected. Historically there has also been a separation

in technology between combustible and toxic detection. Below are

some of the issues you need to consider when choosing gas detectors.

Most devices used in the oil and gas industry are set to detect

methane (CH 4) or hydrogen sulphide (H2S). Many detectors show cross-

sensitivity; i.e. a detector for detecting one gas will also detect

another, at different readings. So at the time of purchase it is

important to specify the gas that is to be detected and consider other

gases that may be present that may affect the readings. The nature of

the gas should be considered – e.g. H2S is heavier than air, methane

rises, propane sinks. However they may not behave like that under a

high pressure discharge. Altitude affects the readings of some

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detectors. Portable personal gas detectors, set for multiple gasses may

be used in areas where toxic gases may be present.

Combustible Gas Detection

Two mainstream technologies are available – infra-red absorption and

catalytic types. Other types are available and in development; e.g.

metal oxide semiconductor sensors. Detection methods from the field

of analyzers may cross over to meet gas detection needs.

Point detectors are calibrated against the lower explosive limit (LEL) of

a certain gas, frequently methane. The lower explosive limit for

methane mixed in air is achieved at a 5% concentration. Typical alarm

settings are 20% LEL and 60% LEL. Confusion can arise as these levels

are traditionally labelled low gas and high gas, whereas control

instrument engineers would use the term high alarm and high-high

alarm.

Open path gas detectors are calibrated in LEL metres (LELm). This

setting has evolved as an analogue with the LEL range used in point

detectors.

Infra-red Absorption Combustible Gas Detection

The technology uses the absorption characteristics of the hydrocarbon

molecules to infra-red light. The more hydrocarbon molecules are

present, the higher the absorption of infra-red radiation. More than one

type of hydrocarbon gas may be detected.

This technology is more expensive than catalytic detection, but it is

used for many applications as it doesn’t need field calibration and

proof test intervals are considerably better (longer) than for catalytic

types. Speed of response is quicker than for catalytic types. The

measured value doesn’t drift unlike catalytic detectors. And unlike

catalytic types, the detector doesn’t need oxygen for operation,

Point infra-red gas detectors

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Point detectors record the gas concentration at the detector location.

They need to be placed where a release of gas is considered possible.

They can be placed remotely and connected to the sampling location

by tubes, with air sucked across the detecting chamber. Consideration

needs to be given to the extra detection time added by the transit time

down the tube.

Example uses: Detection in confined spaces, specific locations, air

inlets etc.

Open path infra-red gas detectors

Open path gas detectors have a separate transmitter and receiver.

Manufacturers quote up to 200m range, but in practice smaller

distances are used, due to climatic and practical mounting

arrangements.

Detectors should be mounted rigidly to avoid misalignment between

the transmitter and receiver, both statically and due to vibrations.

Current devices will detect more than one hydrocarbon gas. New

devices are in development that are tuned to a particular gas. Different

versions of these can also detect H2S.

Example uses: Migration detection, pipe rack monitoring.

Catalytic Gas Detectors

Catalytic detectors rely upon burning gas in a sintered chamber. For

this reason they are only available as a point detector or as part of a

multi-point aspirating system.

Various technologies are available – chemical cell and semiconductor

point detectors; open path (Laser) gas detection is in development.

Monitoring and Controlling of Smoke emission and Gas Leakage using PLC

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Many different types of gas can be detected. Cross-sensitivity to

different gases other than those being looked for needs to be given

careful attention.

Response times of detecting and testing frequencies need careful

attention.

Chemical cell types require sensor replacement at intervals

determined by the environment. Semiconductor cells are also affected

by their environment and may need to be ‘kept awake’ by exposure to

the detected gas. New products are in development that are less

susceptible to these limitations.

Infra-red single gas open path detectors are at an advanced stage of

development. These offer the important advantages of fast response

and high reliability.

Example uses: H2S from sour oil wells or processing plant; carbon

monoxide from burning products and Co2 (Carbon Dioxide) build up

Calibration

The most common failure in catalytic sensors is performance

degradation caused by exposure to certain poisons’. It is therefore

essential that any gas monitoring system should not only be calibrated

at the time of installation, but also checked regularly and re-calibrated

as necessary. Checks must be made using an accurately calibrated

standard gas mixture so that the zero and ‘span’ levels can be set

correctly on the controller.

Codes of practice such as EN50073:1999 can provide some guidance

about the calibration checking frequency and the alarm level settings.

Typically, checks should initially be made at weekly intervals but the

periods can be extended as operational experience is gained. Where

two alarm levels are required, these are normally set at 20-25% LEL for

the lower level and 50-55% LEL for the upper level.

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Today, there are a number of ‘one-man’ calibration systems available

which allow the calibration procedures to be carried out at the sensor

itself. This considerably reduces the time and cost of maintenance,

particularly where the sensors are in difficult to get to locations, such

as an off-shore oil or gas platform. Alternatively, there are now some

sensors available which are designed to intrinsically safe standards,

and with these it is possible to calibrate the sensors at a convenient

place away from the site (in a maintenance depot for instance).

Because they are intrinsically safe, it is allowed to freely exchange

them with the sensors needing replacement on site, without first

shutting down the system for safety.

Maintenance can therefore be carried out on a ‘hot’ system and is very

much faster and cheaper than early, conventional systems.

1.3.4 Location of Sensors

‘How many detectors do I need?’ and ‘where should I locate them?’ are

two of the most often asked questions about gas detection systems,

and probably two of the most difficult to answer. Unlike other types of

safety related detectors, such as smoke detectors, the location and

quantity of detectors required in different applications is not clearly

defined.

The placement of detectors should be determined following the advice

of experts having specialist knowledge of gas dispersion, experts

having knowledge of the process plant system and equipment

involved, safety and engineering personnel. The agreement reached

on the location of detectors should also be recorded.

Detectors should be mounted where the gas is most likely to be

present. Locations requiring the most protection in an industrial plant

would be around gas boilers, compressors, pressurized storage tanks,

cylinders or pipelines. Areas where leaks are most likely to occur are

valves, gauges, flanges, T-joints, filling or draining connections etc.

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There are a number of simple and quite often obvious considerations

that help to determine detector location:

To detect gases that are lighter than air (e.g. Methane and Ammonia),

detectors should be mounted at high level and preferably use a

collecting cone. To detect heavier than air gases (e.g. Butane and

Sulfur Dioxide), detectors should be mounted at a low level. Consider

how escaping gas may behave due to natural or forced air currents.

Mount detectors in ventilation ducts if appropriate. When locating

detectors consider the possible damage caused by natural events e.g.

rain or flooding. For detectors mounted outdoors it is preferable to use

the weather protection assembly. Use a detector sunshade if locating a

detector in a hot climate and in direct sun. Consider the process

conditions. Butane and Ammonia, for instance are normally heavier

than air, but if released from a process line that is at an elevated

temperature and/or under pressure, the gas may rise rather than fall.

Detectors should be positioned a little way back from high pressure

parts to allow gas clouds to form. Otherwise any leak of gas is likely to

pass by in a high speed jet and not be detected. Consider ease of

access for functional testing and servicing. Detectors should be

installed at the designated location with the detector pointing

downwards. This ensures that dust or water will not collect on the front

of the sensor and stop the gas entering the detector. When siting open

path infrared devices it is important to ensure that there is no

permanent obscuration or blocking of the IR beam. Short term

blockage from vehicles, site personnel, birds etc can be

accommodated. Ensure the structures that open path devices are

mounted to are sturdy and not susceptible to vibration.

1.4Control Systems and mitigation

Mitigating actions

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Actions may range from alerting a control room operator, release of

extinguishants to a complete plant shut down to sounding an

evacuation alarm. The control room operator may be part of the

control loop perhaps by being required to decide whether to initiate a

general alarm or plant shut-down.the ozone depleting effects of Halon

1301 and its almost complete ban has led to a review of the general

use of automatically released exinguishants and an increase in

emphasis on early warning of fire and manual interventions. This

emphasis is, however, company and country specific.

Start water fire pumps

Frequently fire pumps are started as a precautionary measurement

detection of fire or on a manually initiated fire indication. Fire pumps

can normally or stopped locally to the fire pump. Fire pump control

logic- sequencing is sometimes performed by a fire and gas system. It

can also be implemented in dedicated fire pump controllers supplied

by a fire pump supplier.

A manual start fire pump push-button should be provided in the control

room wired directly to the fire pump control panel.

Initiate Plant Alarms

Plant alarms can be automatic or manual and can be wired directly the

fire and the gas system or form part of a general, high integrity public

address system.

Consideration should be given to an alarm hierarchy and local zoning.

For example is it necessary to evacuate a complete site based local

alarm in an instrument room? visual repeats of audible alarms may be

needed in noisy areas. Some jurisdictions require all audible alarms to

be accompanied by a visual indication such as an integrated flashing.

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beacon(srobe) . Colours of visual indications need to be established at

an early stage to ensure that they don’t clash with other indicators .

The chopice of colours affects the distance covered by a device of

given power.

Fig 5: Horn & Beacon

For audible alarms, consideration needs to be given to how many

different sounds are required and how many sounds the plant

personnel can distinguish. Voice messages offer greater flexibility in

conveying messages, but may not be so effective in multilingual

projects or comprehensible to off-site personnel.

If alarm signaling is via another system, it needs to be established

which system has control over facilities such as system resetting,

inhibiting and silencing.

Choice of interface could be hard wired, secure serial or a combination

using remote I/O (inputs and outputs) co-located at the alarm signaling

system.

Audible and visual alarms can be initiated in the following ways:

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Automatically on detection of fire, perhaps requiring more than one fire

detector to operate(voting) automatically on detection of gas, perhaps

voted automatically after a delay to allow for manual intervention

manually by the control room operator manually by manual call point

in the field other combinations of the above

1.4.1Fire alarm control panel

A Siemens MXL fire alarm control panel (top) and graphic annunciator.

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A Fire Alarm Control Panel (FACP), or Fire Alarm Control

Unit (FACU), is the controlling component of a Fire Alarm System. The

panel receives information from environmental sensors designed to

detect changes associated with fire, monitors their operational

integrity and provides for automatic control of equipment, and

transmission of information necessary to prepare the facility for fire

based on a predetermined sequence. The panel may also supply

electrical energy to operate any associated sensor, control,

transmitter, or relay. There are four basic types of panels: coded

panels, conventional panels, addressable panels, and multiplex

systems.

Conventional Fire Alarm Control Panels

Conventional panels have been around ever since electronics became

small enough to make them viable. conventional panels are used less

frequently in large buildings than in the past, but are not uncommon

on smaller projects such as small schools, stores, restaurants, and

apartments.

A conventional Fire Alarm Control Panel employs one or more circuits,

connected to sensors (initiating devices) wired in parallel. These

sensors are devised to dramatically decrease the circuit resistance

when the environmental influence on any sensor exceeds a

predetermined threshold. In a conventional fire alarm system, the

information density is limited to the number of such circuits used.

To facilitate location and control of fire within a building, the structure

is subdivided into definite areas or zones. Floors of a multistory

building are one type of zone boundary.

An Initiating Device Circuit connected to multiple devices within the

same "zone" of protection, effectively provides 2 bits of information

about the zone corollary to the state of the circuit; normal, or off

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normal and alarm or quiescent. The state of each Initiating Device

Circuit within a zone displays at the Fire Alarm Control Panel using

visible indications called Annunciators.

These Annunciators may employ a graphical representation of the

Zone boundaries on a floor plan (Zone map) using textual descriptions,

illuminated icons, illuminated sections, or illuminated points on the

map corresponding to Initiating Circuits connected to the Fire Alarm

Control Panel.

For this reason, slang often inaccurately refers to initiating circuits of a

Fire Alarm Control Panel as Zones.

Larger systems and increasing demand for finer diagnostic detail

beyond broad area location and control functions expanded the control

by Zone strategy of conventional systems by providing multiple

initiating circuits within a common Zone, each exclusively connected to

a particular type of initiating device, or group of devices. This

arrangement forms a device type by Zone matrix whose information is

particularly suited to the Tabular Annunciator In multistory buildings

employing a Tabular Annunciator for Example; rows of indicators

define the floors horizontally in their stacked relationship and the type

of device installed on that floor displays as columns of indicators

vertically aligned through each floor. The intersection of the floor and

device indicators provides the combined information. The density of

information however remains a function of the number of circuits

employed.

Even larger systems and demands for finer diagnostic and location

detail led to the introduction of addressable fire alarm systems with

each addressable device providing specific information about its state

while sharing a common communication circuit. Annunciation and

location strategies for the most part remain relatively unchanged.

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Fig: 6 Wiring Diagram

A wiring diagram for a simple fire alarm system consisting of two input

loops (one closed, one open)

Releasing panels

Releasing panels are capable of usings solenoids to disperse fire-

fighting chemical agents such as halon or water from piping located

throughout a building. A releasing panel usually will have a manual

abort switch to abort an accidental release which could damage

property or equipment. Releasing capability can be part of both

addressable or conventional panels.

Addressable Fire Alarm Control Panels

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Fig 7: Addressable FACP

A Simplex 4100U InfoALARM addressable fire alarm control panel: note

the voice-evacuation microphone built into it

Signaling Line Circuits

Addressable Fire Alarm Control Panels employ one or more Signaling

Line Circuits, slang - usually referred to as loops or SLC loops -

ranging between one and thirty. Depending on the protocol used, a

Signaling Line Circuit can monitor and control several hundred devices.

Some protocols permit any mix of detectors and input/output modules,

while other protocols have 50% of channel capacity restricted to

detectors/sensors and 50% restricted to input/output modules. Each

SLC polls the devices connected, which can number from a few devices

to several hundred, depending on the manufacturer. Large systems

may have multiple Signaling Line Circuits. [1] [2]

Each device on a SLC has its own address, and so the panel knows the

state of each individual device connected to it. Common addressable

input (initiating) devices include

Smoke detectors

Heat Detectors (Rate of Rise and Fixed Temperature)

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Manual call points  or manual pull stations

Notification appliances  (Simplex systems with TrueAlert signals

only)

Responders

Fire sprinkler system  inputs

Switches

Flow control

Pressure

Isolate

Standard switches

Addressable output devices are known as relays and include

(Warning System/Bell) Relays

Door Holder Relays

Auxiliary (Control Function) Relays

Relays are used to control a variety of functions such as

Switching fans on or off

Closing/opening doors

Activating fire suppression systems

Activating notification appliances

Shutting down industrial equipment

Recalling elevators to a safe exit floor

Activating another fire alarm panel or communicator

Mapping

Also known as "cause and effect" or "programming", mapping is

the process of activating outputs depending on which inputs

have been activated. Traditionally, when an input device is

activated, a certain output device (or relay) is activated. As time

has progressed, more and more advanced techniques have

become available, often with large variations in style between

different companies.

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Zones

Zones are usually made by dividing a building, or area into

different sections. Then depending on the specific zone, a certain

amount and type of device is added to the zone to perform its

given job.

Groups

Groups contain multiple output devices such as relays. This

allows a single input, such as a smoke detector or MCP, to have

only one output programmed to a group, which then maps to

between two to many outputs or relays. This enables an installer

to simplify programming by having many inputs map to the

same outputs, and be able to change them all at once, and also

allows mapping to more outputs than the programming space for

a single detector/input allows.

Boolean logic

This is the part of a fire panel that has the largest variation

between different panels. It allows a panel to be programmed to

implement fairly complex inputs. For instance, a panel could be

programmed to notify the fire department only if more than one

device has activated. It can also be used for staged evacuation

procedures in conjunction with timers.

Networking

The principle of networking involves connecting several panels

together to form a system. Inputs on one panel may activate

outputs on another, for example, or the network may allow

monitoring of many systems. Networking is often used in

situations where one panel is not large enough, or in multiple-

building situations. These are often done with manufacturers'

"top of the line" control panels.

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Although quasi-standards exist that allow panels from different

manufacturers to be networked with each other, they are not in

favor with a lot of companies. One of the most common protocols

used BAC net which is used for various type of industrial

networks.

More recently, some panels are being networked with

standard Ethernet, but this is not yet very common. Most

organizations choose to create their own proprietary protocol,

which has the added benefit of allowing them to do anything

they like, allowing the technology to progress further. However, a

bridging layer between the proprietary network and BACnet is

usually available]

Networking may be used to allow a number of different panels to

be monitored by one graphical monitoring system.

Monitoring

In nearly every state in the USA, the International Building

Code requires fire alarm and sprinkler systems to be monitored

by an approved supervising station.

A fire alarm system consists of a computer-based control

connected to a central station. The majority of fire alarm systems

installed in the USA are monitored by a UL listed or FM Global

approved supervising station.

These systems will generally have a top level map of the entire

site, with various building levels displayed. The user (most likely

a security guard) can progress through the different stages. From

top level site → building plan → floor plan → zone plan, or

however else the building's security system is organised.

A lot of these systems have touch screens, but most users tend

to prefer a mouse (and a normal monitor), as it is quite easy for a

touch screen to become misaligned and for mistakes to be made.

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With the advent of the optical mouse, this is now a very viable

option.

System functions

Fig 8: System Functions

There are many functions on a fire alarm panel. Some of these are:

System reset

This resets the panel after an alarm condition. All initiating devices are

reset, and the panel is cleared of any alarm conditions. If an initiating

device is still in alarm after the system is reset, such as a smoke

detector continuing to sense smoke, or a manual pull station still in an

activated position, another alarm will be initiated. A system reset is

often required to clear supervisory conditions. A system reset does not

usually clear trouble conditions. Most trouble conditions will clear

automatically when conditions are returned to normal.

On UK and most US panels, a "Silence" or "Acknowledge" is usually

required before a "System Reset" can be performed.

Acknowledge

This function, also abbreviated to "ACK", is used to acknowledge an

abnormal situation such as an alarm, trouble or supervisory. The

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acknowledge function tells the panel that building personnel or

emergency responders are aware of the alarm, trouble, or supervisory

condition. Acknowledging the alarm or trouble condition also normally

silences the panel's own sounder, but does not silence any Notification

Appliances.

Fig 9: Fire alarm panel, showing drill switch

Drill

Also known as "manual evacuation" or "evacuate". On panels that have

this function, the drill function activates the system's notification

appliances, often for purposes of conducting a fire drill. Using the drill

function, an alarm is normally not transmitted to the fire department or

monitoring center. However, building personnel often notify these

agencies in advance in case an alarm is inadvertently transmitted.

Walk test

Walk test allows the functional testing of the system's devices without

the assistance of additional people at the control panel itself. It is also

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designed to allow initiating devices to be tested without setting off the

building's alarms. Most panels offer the option for a silent walk test (no

alarms activate) or an audible walk test (alarms activate for a brief

period when a device is initiated). A system trouble is typically

generated while the panel is in walk test mode. On European panels,

this is usually an engineer-only function and cannot be activated by a

user.

Signal silence

Also known as "alarm silence" or "audible silence". Depending on the

configuration of the alarm system, this function will either silence the

system's notification appliances completely, or will silence only the

audible alarm, with strobe lights continuing to flash. Audible silence

allows for easier communication amongst emergency responders while

responding to an alarm. This can also be used during construction as a

means of a preliminary test, before the final full test.

Lamp test

Also known as "flash test". This button is known to have become

obsolete, but is still used on many panels. This function is used to

check the condition of the LEDs themselves. A "Lamp Test" button is

required by code on multi-zone panels installed in Canada. Many

panels do a lamp test when the system is reset.

Alarm circuit supervision

Various forms of alarm circuit supervision have been used to indicate

trouble with an alarm circuit. Possible alarm circuit faults on a two wire

circuit include one of the conductors being shorted to ground, open

circuit (conductor continuity break), or a short circuit between the

conductors. Also the circuits could be tampered with by having an

external AC or DC voltage applied with various duty cycles or

waveforms. There are a number of US patents that address this issue

and some have been implemented in available system products. One

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of the first to address this issue was Patent No. 3,588,890 "Resistance

Sensing Supervisory System" issued on June 28, 1971 and assigned to

General Motors Corporation. General Motors used this supervision on

all circuits installed in GM plants starting in 1970.[3] An improvement to

this basic "Resistance Sensing Supervisory System" can be obtained by

providing a pulsed or time dependent variable voltage applied to the

alarm circuit and is addressed in US patent numbers 4,030,095 [1] and

4,716,401 [2].

Panel alerting

Many panels today have the capability of alerting building personnel of

a situation which can arise into a potentially serious problem. Fire

alarm panels indicate an abnormal condition via a solid or flashing LED.

Some panels also contain a small sounder, used in conjunction with the

visual alert. A number of indicators are shown below. Note that not all

fire alarm panels have all of these indicators.

Alarm

Also known as "Fire" or "General Alarm". This indicator is lit when an

alarm condition exists in the system, initiated by smoke

detectors, heat detectors, sprinkler flow switches, manual pull stations,

manual call points, or otherwise. Along with the indicator on the panel,

notification appliances, such as horns and strobes, are also activated,

signaling a need to evacuate to building occupants. In an alarm

condition, the fire alarm panel indicates where the alarm originated.

The alarm panel can be reset once the device which initiated the alarm

is reset, such as returning the handle of a manual pull station to its

normal position.

Audible silence

The Audible Silence indicator is used in conjunction with the "Alarm"

indicator. It indicates that the fire alarm panel is still in an alarm

condition, but that notification appliances have been silenced. While

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41

the alarm is silenced, other functions in an alarm condition continue to

operate, such as emergency service for elevators, stairway

pressurization, and ventilation functions. A new alarm initiation while

the alarm is silenced will take the panel out of Audible Silence and

reactivate the notification appliances.

Report

Also known as "Brigade Called". This indicator is activated when

emergency responders have been automatically notified by the fire

alarm system. A variant of this LED known as "kissoff" activates when

the monitoring center replies back to the panel, indicating a successful

communication. Requirements vary depending on jurisdiction

regarding whether a direct connection to the fire department is

required, optional, or prohibited. If a connection to the fire department

is optional, or is prohibited, a fire alarm system is often connected to a

monitoring center at the building owner's discretion.

Drill

Also known as "Manual Evacuation" or "Evacuate". On panels

containing this function, the "Drill" indicator shows that the alarm

condition was activated from the fire alarm panel, often in order to

conduct a fire drill. When an alarm is initiated for a drill, the fire

department or monitoring company is usually not notified

automatically. However, building personnel preparing to conduct a fire

drill often will provide advance notice of a drill to the fire department

and monitoring center in case an alarm is unintentionally transmitted.

Pre alarm

This LED is often used in conjunction with a two-stage system, in which

the panel requires two devices to be activated (and/or a predetermined

time limit to run out after one device is activated) in order to go into

full alarm.[4] This is mostly used in areas where false alarms are a

common problem, or in large applications (such as hospitals) where

Monitoring and Controlling of Smoke emission and Gas Leakage using PLC

42

evacuating the entire building would not be efficient. The prealarm LED

is lit when one device has tripped. The prealarm LED may also be used

if an analog smoke detector registers low levels of smoke in the

detection chamber, but not enough to trigger a full alarm.[5] Depending

on the system's layout, the NAC's may or may not activate for

prealarm conditions. In a two-stage system, the NAC's are typically

coded to a special first-stage coding, or in some situations where a

loud alarm signal could be disruptive, chimes will activate. If there is a

voice evacuation system, it will usually instruct building occupants to

await further instructions while the alarm is being investigated.

Priority 2 alarm

Also known as "Security". This LED is common on top-of-the-line

intelligent panels. This LED can only activate if there is a secondary

device hooked into the "Priority 2 Alarm" terminals. This secondary

device could be a security system, building management system, or

another fire alarm control panel. Depending on how the panel is

programmed, the panel's alarms may or may not activate when a

condition like this is present.

Trouble

Also known as "Fault" or "Defect". When held steady or flashing, it

means that a trouble condition exists on the panel. Trouble conditions

are often activated by a contaminated smoke detector or an electrical

problem within the system. Trouble conditions are also activated by a

zone being disabled (disconnected from the system), a circuit being

disabled, low power on the backup battery, the disabling of a

notification appliance, the ground faults, or short or open circuits.

Usually the alarm panel's sounder will activate if a trouble condition

exists, though older systems would sometimes activate a bell or other

audible signal connected to the panel. In a trouble condition, the panel

displays the zone or devices causing the condition. Usually, the

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"Trouble" indicator goes out automatically when the situation causing

the trouble condition is rectified, however in some systems (EST) the

panel must be reset to clear the trouble alarm. Some panels have

more specific indicators such as 'Trouble-PSU' which shows when the

panel itself is compromised and 'Trouble-Bell' ('Sounder fault' on UK

panels) which shows that the sounders are not functioning correctly.

On most panels, an acknowledge button is pressed to turn off the

panel's buzzer.

Supervisory

This signal indicates that a portion of the building's fire protection

system has been disabled (such as a fire sprinkler control valve being

closed and, consequently, a sprinkler tamper switch being activated),

or, less frequently, that a lower priority initiating device has been

triggered (such as a duct smoke detector). Depending on the system's

design, the supervisory point may be latching, meaning the panel must

be reset to clear the supervisory condition, or non-latching, meaning

the indicator automatically goes out when the condition has cleared.

However, some panels require a reset regardless of whether the

supervisory point is latching or non-latching.

AC power

Also known as "Normal". When this indicator is lit, power is being

provided to the system from the building's electrical system, and not

from the backup battery. When an AC power condition changes, the

Trouble indicator comes on and the AC power indicator goes off and

the screen alerts building personnel of a power failure. If the AC power

indicator is lit without any other indicators also lit, then the system is

in a normal condition. If no LEDs are lit, there is no power source

feeding the panel.

DC power

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This is used to tell the operator that DC power (batteries) are being

charged or used. While using DC power, the system remains in a

trouble condition.

Highrate

This LED is on when there are unusual power-line conditions.

1.5Programmable Logic Controller (PLC)

Programmable logic control or PLC is the most commonly used

industrial automation technique in the world. It is universally

applied for factory automation, process control and

manufacturing systems. Programmable logic control originated

from the creation of computerized versions of relay control

systems used to control manufacturing and chemical process

systems. The programming is done using a special technique

called ladder logic, which allows sequences of logical actions to

be set up, inter-linked and timed. A standard task in logic

control is batch control and sequencing in a process system.

1.5.1 PLC Introduction

A PLC or Programmable Logic Controller is a user friendly,

microprocessor specialized computer that carries out control

functions of many types and levels of complexity. Its purpose is

to monitor crucial process parameters and adjust process

operations accordingly. It can be programmed, controlled and

operated by a person unskilled in operating computers.

Essentially, a PLC's operator draws the lines and devices of

ladder diagrams with a keyboard onto a display screen. The

resulting drawing is converted into computer machine language

and run as a user program.

PLC will operate any system that has output devices that go on

and off (Discrete, or Digital, outputs). It can also operate any

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45

system with variable (analog) outputs. The Programmable Logic

Control can be operated on the input side by ON/OFF devices or

by variable (analog) input devices.

Control engineering has evolved over time. In the past humans

was the main method for controlling a system. More recently

electricity has been used for control and early electrical control

was based on relays. These relays allow power to be switched

on and off without a mechanical switch. It is common to use

relays to make simple logical control decisions. The

development of low cost computer has brought the most recent

revolution, the Programmable Logic Controller (PLC). The

advent of the PLC began in the 1970s, and has become the

most common choice for manufacturing controls.

Programmable Logic Controllers have been gaining popularity

on the factory floor and will probably remain predominant for

some time to come. Most of this is because of the advantages

they offer.

1.5.2 PLC Advantages and Disadvantages

Flexibility: One single Programmable Logic Controller can easily

run many machines.

Correcting Errors: In old days, with wired relay-type panels,

any program alterations required time for rewiring of panels and

devices. With PLC control any change in circuit design or

sequence is as simple as retyping the logic. Correcting errors in

PLC is extremely short and cost effective.

Space Efficient: Today's Programmable Logic Control memory

is getting bigger and bigger this means that we can generate

more and more contacts, coils, timers, sequencers, counters and

so on. We can have thousands of contact timers and counters in

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46

a single PLC. Imagine what it would be like to have so many

things in one panel.

Low Cost: Prices of Programmable Logic Controlers vary from

few hundreds to few thousands. This is nothing compared to the

prices of the contact and coils and timers that you would pay to

match the same things. Add to that the installation cost, the

shipping cost and so on.

Testing: A Programmable Logic Control program can be tested

and evaluated in a lab. The program can be tested, validated and

corrected saving very valuable time.

Visual observation: When running a PLC program a visual

operation can be seen on the screen. Hence troubleshooting a

circuit is really quick, easy and simple.

1.5.3 ALLEN BRADLEY PLC MICROLOGIX 1200 CONTROLLER.

AB PLC TYPES:

Allen Bradley PLCs, the standard by which other PLCs are measured.

From the very inception of the idea of the programmable logic

controller the Allen Bradley PLC's were there. Thirty years of history

and experience is involved in every Allen Bradley programmable logic

controller that you get, helping you to move forward, to exert powerful

and expert control of your devices. .

The Programmable logic controller was designed to provide you with

the control solutions that you need for your remote mechanics. The

Allen Bradley controllers offer some key ways to solve your problems

and to give you what you need.

Solving the challenges of manufacturing and lowering your costs is the

business of every business. Improving your output, increasing the

quality and the flexibility that you have is your secondary aim. Allen

Bradley programmable logic controllers are expert at those things.

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They bring to your business a highly customizable integrated solution

to the control of your systems and devices. Your PLC will be up the

challenges that you face when you select the Allen Bradley

Programmable logic controller.

The Allen Bradley Programmable logic controllers help you in both time

and money saving, as well as offering you a faster start-up. Your new

fast start up time is a result of products that are pre-integrated--

designed to fit together like pieces of a well oiled puzzle. From the

beginning to the end of the operation, the maintenance will be far less

and the need for programming will be minimal.

Allen Bradley is a part of Rockwell Automation Integrated

Architecture and offers controllers that are suitable for drives, for

motion, and for process controlling. No matter what you need, if you

have to have high performance or value based in your programmable

logic controller system, you will find just the right controller with the

Allen Bradley programmable logic controllers.

The many different offerings from Allen Bradley-Rockwell Automation

Integration include the NetLinx, the Kinetix, and the Logix. All of these

will offer you maximum capabilities, easy use, reuse capacity of

program, flexibility in the communications system and fast easy use so

that you can spend less of your company's time and money on the

entire setup process.

Allen Bradley uses five different types of programmable logic

controllers. These different types of PLCs perform specialized

functions.

Pico Controllers are simple, as well as flexible and small, performing

logic, counting, time and clock operations..

MicroLogix PLCs are a cost-effective solution for micro-control that

will expand as needed.

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SLC 500s are small, modular programmable controllers that are

chassis-based. It is often the choice for I/O and power supply functions.

PLC-5 is the most popular Allen Bradley PLC and can be found

worldwide, providing flexibility in networking, I/O and programming

and being suitable for a wide variety of applications. .

1758-RTU is a programmable logic controller designed for rugged and

harsh environments as a Remote Terminal Unit (RTU).

From conception to implementation, any Allen Bradley programmable

logic controller results in cost savings, increased productivity and

satisfied clients.

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Monitoring and Controlling of Smoke emission and Gas Leakage using PLC

System Overview

The MicroLogix 1200/1762 system provides functionality between the MicroLogix

1000/1761 and MicroLogix 1500/1764 systems, using the proven MicroLogix and

SLC family architecture. The 6K-word memory provides for a maximum program

of 4K words and maximum data of 2K words with 100% data retention. An

optional

memory module provides program and data backup with program upload and

download capability. The optional real-time clock enables time scheduling of

control

activities. The flash upgradeable operating system lets you upgrade system

software

without replacing hardware.

Product Design

The MicroLogix 1200 controller and expansion I/O modules provide a modular,

rackless control system designed for ease of installation and maintenance. Each

MicroLogix 1200 controller includes a processor, built-in I/O, and power supply.

Expansion I/O modules install to the right of the controller. Cables built into the

I/O modules provide connection to the adjacent I/O module or controller.

Controllers and I/O modules can be mounted either on a panel or on a DIN-rail.