Fire and Gas System

Fire Suppression System.

The purpose of a fire suppression system is to either put out a fire or stop it from propagating. These systems are often used together with fire alarms and smoke or heat detectors to safeguard people and physical structures. The three main fire suppression system categories use water, inert gases, or various chemical agents to suppress a fire. These systems can also be automatic or manual, depending on whether they require outside intervention to activate. Some fire suppression systems can be hazardous to anyone in the general vicinity, including the danger of asphyxiation associated with pumping inert gases into an area to starve a fire of oxygen.

Water tends to be one of the most commonly used types of fire suppression, and is typically utilized in a sprinkler system. This type of fire suppression system can be either wet or dry. A wet sprinkler system is constantly charged, meaning there is always water in the pipes. This type of fire suppression system can be automatic, and it will activate if the system senses smoke or an excessively high temperature. Dry sprinklers are manual systems that can only be activated after a water source is connected to a stand pipe.

Another method of fire suppression involves filling an area with inert gas. Since fires require oxygen to burn, this can have a smothering effect.These fire suppression systems are commonly used in sensitive areas where water could potentially damage items such as computer equipment or documents. Computer server rooms are one area where inert gas, such as argon, is often used to suppress fires.

Inert gas can suppress a fire without damaging electronics or other equipment, but it can also pose other health and safety hazards. In some cases, an inert gas fire suppression system can lead to asphyxiation. Most of these systems come equipped with alarms to warn personnel to leave the area before the gas is released. Another potential issue with these systems is pressure, since the sudden introduction of a previously compressed gas into a small space can potentially blow out windows or walls.

Both wet and dry chemicals can also be used in fire suppression systems. These systems are typically automatic, though they can also have manual controls to activate the release of fire suppressing chemicals. Some materials can react poorly to the presence of water or inert gases, in which case a dry or wet chemical fire suppression system is often the safest option. Since these chemicals may be hazardous to human health, a delay is often built in

Heat Detector

The heat detector operates by using a matched pair of thermistors to sense heat. One thermistor is exposed to the ambient temperature, the other is sealed. In normal conditions, the two thermistors register similar temperatures, but, on the development of a fire, the temperature recorded by the exposed thermistor will increase rapidly, resulting in an imbalance of the thermistors and causing the detector to change to the alarm state.

Rate of Rise detectors are designed to detect a fire as the temperature increases, but they also have a fixed upper limit at which the detector will go into alarm if the rate of temperature increase has been too slow to trigger the detector earlier.
Fixed heat detectors only change to the alarm state at a preset temperature.

UV/IR (Ultraviolet/infrared) Flame Detector.

UV/IR Detector, which makes use of an ultraviolet radiation sensitive phototube in addition to an infrared detector. This combination provides a flame detector which is highly immune to false alarms. The UV portion of the detector, as described in the previous section, is combined with an infrared detector, which responds to changes in the intensity of infrared radiation. By sensing very specific wavelengths in both the UV and IR spectra and then processing these Signals with a microcomputer, a very high degree of discrimination is achieved. Incorporated in the IR circuitry is a flicker discrimination circuit. This permits the detector to ignore steady IR sources such as hot objects. The inherent flickering of a flame provides the necessary modulation to activate the IR circuit. Since a flame is a copious source of both ultraviolet and infrared radiation, discrimination is provided when both UV and IR emissions are detected. If only UV is detected, as in the case of arc welding, no alarm is given. If only IR is detected, such as a large modulating hot object, no alarm is given. However, if both conditions are met in the correct combination and intensity, as determined by an algorithm in the microcomputer, a fire is identified and the alarm outputs are activated.

Current Level Detector status

0mA Fault

2mA Optical Integrity (io) Fault.

4mA Normal Operation

8mA IR Alarm Only

12mA UV Alarm Only

16mA Pre-Fire Alarm

20mA Fire Alarm

UV ( Ultraviolet) Flame Detector.

Flame Detector which contains an ultraviolet photo tube that responds to radiation in the 185 to 260 nano meter region when radiation from a flame strikes the cathode plate within a UV detector tube, electrons are ejected from the cathode plate. These electrons are accelerated towards the positively charged anode of the tube. They collide with molecules of an ionization gas, with which the tube is filled. This emits more electrons and produces an avalanche condition. More electrons are released which creates a momentary electron flow from the cathode to the anode. This momentary current (pulse) recurs at a rate proportional to the intensity of the UV radiation. This output converted to standard mA output.

IR GAS DETECTOR

The infrared (IR) method of gas detection relies on the IR absorption characteristics of gases to determine their presence and concentration. IR gas detectors consist of an IR light source (transmitter) and light detector (receiver) to measure the intensity both at the absorption wavelength and a non-absorbed wavelength. If gas is present in the optical path, it will affect the intensity of light transmitted between the light source and the detector. This change in intensity provides the data for determining that a specific gas or type of gas is present. This method works only for gases that can absorb infrared radiation. Most hydrocarbon based gases absorb IR radiation at around 3.4 micrometers, which is transparent to both water and carbon dioxide vapors.Combustible IR gas detection can take one of two forms: either the point detector or the open path detector. The primary difference between point detectors and open path is the size of the IR path and its relationship to the gas/vapor sample source. The self-contained point detector has a smaller IR path than the open path detector, and is used to monitor fixed areas of space. The open path detector usually consists of a separate transmitter and receiver, which monitor much larger areas of space. Consequently, in some instances a single open path detector serves the function of multiple point detectors.

Working Principle of Combustible or Flammable Gas Sensor

Many gases and vapors are combustible. The catalytic bead converts the combustible materials to heat. A change in heat is then converted to a change in resistance, which can be measured.

Taking a matched pair of catalytic beads and coating one so that it does not respond to the presence of combustible gases can compare the change in resistance between the two beads. The bead that is coated is called the reference bead and the other is called the active bead. Because the beads are a matched pair, they will respond equally to changes in ambient temperature, humidity, and pressure. This makes the sensor virtually immune to changing environmental conditions.

By connecting one end of each catalytic bead together, a series circuit is formed. This circuit is supplied with a constant current. The voltage drop across each of the beads will be identical in the absence of combustible gases. As combustible material is converted to heat, the resistance of the active bead increases and causing a voltage drop across each bead to be different. This difference is proportional to the amount of combustible gas that is present.

The voltage from the sensor is amplified and fed to an Analog to Digital (A/D) converter and then made available to the microprocessor. The baseline and the gain for the amplifier are set using digital potentiometers. They are adjusted by the microprocessor during calibration.

OPTICAL SMOKE DETECTOR

In one type of photoelectric device, smoke can block a light beam. In this case, the reduction in light reaching a photocell sets off the alarm.

In the most common type of photoelectric unit, however, light is scattered by smoke particles onto a photocell, initiating an alarm. In this type of detector there is a T-shaped chamber with a light-emitting diode (LED) that shoots a beam of light across the horizontal bar of the T. A photocell (light receiving cell), positioned at the bottom of the vertical base of the T, generates a current when it is exposed to light. Under smoke-free conditions, the light beam crosses the top of the T in an uninterrupted straight line, not striking the photocell positioned at a right angle below the beam. When smoke is present, the light is scattered by smoke particles, and some of the light is directed down the vertical part of the T to strike the photocell. When sufficient light hits the cell, the current triggers the alarm.

Auto Ignition Temperature

Auto ignition temperature is the minimum temperature at which a substance will spontaneously ignite in air without any external source of ignition, such as spark or flame.

Ignition Temperature of common liquids and gases.

Name	Ignition Temperature(^OF)
Acetaldehyde	347
Acetic acid	867
Acetone	869
Acetylene	581
Ammonia	1204
Benzene	928
Butane	788
Carbon	1292
Carbon monoxide	1128
Cyclohexane	473
Diethyl ether	320
Ethane	882
Ethyl alcohol	685
Ethyl benzene	810
Ethyl ether	356
Ethylene	842
Hydrogen	932
Gasoline	536
Iso-butane	864
Iso-pentane	788
Isopropyl alcohol	750
Methane	999
Methyl alcohol	867
Naphtha	550
n-Butane	550
n-Heptane	399
n-Hexane	437
n-octane	403
n-pentane	500
Propane	842
Styrene	914
Toluene	896
Xylene-m	982
Xylene-o	867
Xylene-p	984

IONIZATION SMOKE DETECTOR

A typical ionization chamber consists of two electrically charged plates and radioactive source (typically Americium 241) for ionizing the air between the plates .The radioactive source emits alpha particles that collide with the air molecules and dislodge their electrons as molecules loser electrons ,they become positively charged ion. As other molecules gain electrons, they become negatively charged ions. Equal numbers of positive ions and negative ions are created. The positively charged ions are attracted to the negatively charged electrical plate, while the negatively charged ions are attracted to the positively charged p[late. this creates a small ionization current that can be measured by electronic circuitry connected to the plates.(normal condition in the detector)

Particles of combustion are much larger than the ionized air molecules. As particles of combustion enter an ionization chamber, ionized molecules collide and combine with them. Some particles become positively charged & some become negatively charged. As these relatively large particles continue to combine with many other ions, they become recombination centers, and the total number of ionized particles in the chamber is reduced. This reduction in ionized particles results in decrease in the chamber current i.e. sensed by the electronic circuitry monitoring the chamber. When the current is reduced by a predetermined amount, a threshold is crossed and alarm condition is established.

ABBREVIATIONS

AC Alternating Current

ACR Air Circuit Breaker

AGA American Gas Association

AHU Air Handling Unit

ANSI American National Standards Institute

API American Petroleum Institute

ATEX Atmosphere Explosibles

BASEEFA British Approvals Service for Electrical Equipment in

Flammable Atmospheres

BDV Blow down Valve

BMS Building Management System

BSI British Standards Institution

BVQI Bureau Veritas Quality International

CCR Central Control Room

CEN Committee European de Normalization (European Committee for

Standardization)

CENELEC European Committee for Electrotechnical Standardization

CFL Compact fluorescent Lamp

CMS Condition Monitoring Systems

COMP Continues optical monitoring path

CRT Cathode Ray Tube

CRU Condensate Recovery Unit

CSA Canadian Standards Association

CSP Collector and Separation Platform

DACR Digital Alarm Communicator Receiver

DACS Digital Alarm Communicator System

DACT Digital Alarm Communicator Transmitter

DBUS Data Binary Unit System

DC Direct Current

DCS Distributed Control System

DPT Differential Pressure Transmitter

DYNPOS Dynamic Positioning (of rigs and ships)

E&P Exploration and Production

EC European Communities

EECS Electrical Equipment Certification Service (UK)

EEPROM Electrically Erasable Programmable read only memory.

EL Explosive limit

ELCB Earth Leakage Circuit Breaker

EOL End of Line

EOR Enhanced Oil Recovery

ESD Emergency Shutdown System

ESDV Emergency Shutdown Valve

ESP Electric Submerged Pump

EU The European Union

F&G Fire & Gas System

FACP Fire alarm control panel

FCV Flow Control Valve

FD Filament Detector

FD Fire/Flame Detector

FID Flame Ionization Detector

FM Factory Mutual

FM Frequency Modulation

FPD Flame photometric Detector

FPSO Floating Production Storage & Offloading

FT Flow Transmitter

GB(S) Gravity Base Structure

GC Gas Chromatography

GDF Gas Detector Flammable

GDT Gas Detector Toxic

GOR Gas Oil Ratio from the well

GOSP Gas Oil Separation Plant

GTP Gas Treatment Platform

HART Highway Addressable Remote Transducer

HD Heat Detector

HID High Intensity Discharge

HIPPS High Integrity Pressure Protection System

HIPS High Integrity Protection System

HMI Humane Machine Interface

HP High Pressure

HPS High pressure sodium

HPU Hydraulic Power Unit

HVAC Heat Ventilation & Air Conditioning

I/O Input/Out put

I/P Current to Pneumatic

IEC International Electro-technical Commission

ILD Instrument loop diagram

IP Ingress Protection

IR Infra Red

IS Intrinsically safe

ISO International Standards Organization

JB Junction Box

LAN Local Area Net Work

LCD Liquid Crystal Display

LCV Level Control Valve

LED Light emitting diode

LEL Lower Explosive Limit

LHD Linear Heat Detection Cable System

LNG Liquefied Natural Gas

LP Low Pressure

LPG Liquefied Petroleum Gas

LT Level Transmitter

LVDT Linier Variable Differential Transducer

LVDT Linier Voltage Differential Transformer

MBFU Mercury vapor

MBTF Blended mercury vapor

MCB Miniature Circuit Breaker

MCC Motor Control Centre

MCCB Molded Case Circuit Breaker

MICC Mineral Insulated Copper Cable

MODEM Modulator & Demodulator

MOS Metallic Oxide Semi-conductor

MSDS Material Safety Data Sheet

MTBF Mean Time between Failure

NAC Notification Appliance Circuit

NEMA National Electrical Manufacturers Association (US)

NFPA National Fire Protection Association

NGL Natural Gas Liquids

NRTL Nationally Recognized Testing Laboratories (US)

NRTL Nationally Recognized Testing Laboratory

P&ID Piping instrument diagram

P/I Pneumatic to Current

PAGA Public Announcement General Alarm

PCP Progressive Cavity Pump

PCV Pressure Control Valve

PD-Meter Positive Displacement meter

PGP Power Generation Platform algorithm

PID Proportional Integral Derivate control

PIMS Production Information Management System

PLC Programmable Logic Control

PMT photo multiplier tube

POC Pimp of controller (for artificial lift)

POSMOR Position mooring for a floating facility

PPB parts per billion

PPE Personal Protection Equipment

PPM parts per million

PROM Programmable read only memory

PSD Process Shutdown System

PT Pressure Transmitter

PV Process Variable

RAM Random Access Memory

RF Radio Frequency

ROM Read Only Memory

ROV Remote Operated Vehicle (for subsea work over)

RTD Resistance Temperature Detector

RTU Remote Terminal Unit

SAP Systems Applications and Products

SAS Safety and Automation System

SCADA Supervisory Control and Data Acquisition

SD Smoke Detector

SDI Smoke Detector ionization

SDO Smoke Detector Optical

SHE Safety Health Environment

SIL Safety Integrity Level

SOLAS Safety of Life at Sea (convention)

SP Set Point

SRU Sulfure Recovery Unit

SWL Safe Working Load

Ta Tamb Ambient Temperature

TCD Thermal conductivity detector

TCV Temperature control valve

TDSA Time Domain Signal Analysis

TIP Tie in Platform

TLP Tension Leg Platform

TT Temperature Transmitter

TWA Time Wait Average

UEL Upper Explosive Limit

UHF Ultra High Frequency

UL Underwriters Laboratory Inc.

UMS Unmanned Machinery Space Classification (marine =E0)

UPS Uninterrupted Power Source

USM Ultra Sonic Meter

UV Ultra Violet

VCB Vacuum Contact Switch

VCR Vacuum Circuit Breaker

VESDA Very Early Smoke Detection Apparatus.

WHP Well Head Platform