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Fire and Gas (F&G) System

  • Consists of

· Manual fire call points

· Fire detectors

· Flammable gas detectors

· Toxic gas detectors

· Local Fire alarm panels

· F&G PLC

· Alarms

  • Alarms

· Loud speaker – For unit

· Optical Beacon

· Motor siren

· Strong sound

· For fire only

· Local Bell – Use in building

  • Local fire alarm panels

· Has a mimic panel to know where the fire originated

· Only for buildings, for plants, we rely on DCS.

· Purpose

· For fire brigade

· Because buildings are common alarm we need to know where fire is

· Types

· Addressable

· More diagnostics

· Chain loop topology. If 1 detector fails, the other detectors can till work

· Conventional

· One loop per room

· If 1 detector fails, all other detectors won’t work

  • Fire and Gas Voting

· A method to ensure robustness against failure

· Do IPF study for fire and gas – to be implemented

· Normally use 1oo2D

· Originally 1oo2, If 1 fault become 1oo1. Unlike safeguarding if 1 fault, fail safe

· For automatic, 2oo2

Fire Detection System

  • Type of fires are

· Flaming – From hydrocarbons

· Smoldering – Fire burning without flame

· Flash

  • For early detection one need to detect

· Smoke first

· Flame Second

· Heat last

  • Fire is detected by looking at

· Flame

· Smoke

· Heat

· Human visual

Type

Advantage

Disadvantage

When to use

Flame

Fast detection

Smoke

Early warning

Closed areas, for early warning

Heat

Hazard specific, less nuisance alarms, reliable

Cannot function test, damage replace, Slow response

When rapid fire spread is unlikely,

Person

  • Human Visual

· Manual Call Points

· In process area Installed at side of roads every 100 meters

· In offsite area installed at side of roads every 200 meters

  • Flame based fire detectors

Type

Principle

Advantage

Disadvantage

When to use

Infrared

Detect IR from flame flickering

Open fires

Cannot detect smouldering fires, Cannot detect H2 based fire,

Solar radiation interference

Preferred technology for hydrocarbon

Ultraviolet

Extremely Fast detection, open fires

Heavy smoke will foul the lens.

Welding, flare, black body interference.

UV-IR

Expensive

The best

IR multiple Freq

Visual CCTV

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  • Smoke based fire detectors

· Smoke detectors provide the earliest warning for fires.

· From critical buildings, a concept called VESDA(Very early smoke detection apparatus) is utilized. In our plant, the system is called USSD (Ultra sensitive smoke detection). The concepts of USSD is

· Aspiration units are installed throughout the building. Installed normally on ceilings

· The smokes are sucked and brought to a central unit

· The central unit hosts an optical type smoke detector

Type

Principle

Advantage

Disadvantage

When to Use

Optical / Scattered light type

Smoke deflects beam of light

Good for smouldering fires,

Preferred choice, most common

Ionization

Smoke particles reduce current created from ion flow of radioactive materials

Sensitive to small smoke particles

Must be applied together with heat detectors

Only when ionization type cannot be used. Use in printing workshops, highly flammable places

  • Heat detector types

· Most commonly used inside plants since it’s hazard specific

Type

Principle

Advantages

Disadvantages

When to use

Fusible

Heat will melt a tube containing pressure

Cheapest

False alarms, easily

Continuous-line

Heat will cause insulators to melt and 2 conductors come in contact

Bimetallic

Heat causes metal to bend

  • Control is the art of keeping a measured variable at a desired level
  • How to determine where to put fire detectors?

· Best is use of F&G mapping software

· No standards, just based on principle (owner) study

·

· Installed at most likely place for leaks

· Mechanical seals

· High pressure Flanges

· If gas tend to go up, the detector should be place on top and vice versa

Gas Detection System

  • Flash point is the lowest temperature at which a liquid can form an ignitable mixture in air
  • Installed normally at

· Analyzer house

· Pit

· HVAC air intake for buildings in plant

  • Flammable gas detectors

Type

Principle

Advantages

Disadvantage

When to use

Electro Catalytic

Combustible gas oxidizes and produces heat

Robust, detects any combustible gasses, Cheaper

Catalyst can be poisoned,

Single point detection, fail-dangerous

Use when hydrogen detection is needed

Electro Chemical

Electrolysis of gas creates voltage

PPM Level, Good repeatability,

Cannot be used at high temperature and low humidity,

Must change electrolytes, Not fail safe, Gas specific (H2S, SO2,CO2,NO2, CL2)

Old technology, typically avoided, used mainly for toxic gas (see below)

Point type Infrared Gas Detector

Long service life, background gas does not effect service life, has diagnostic, fail safe

Cannot detect hydrogen

Preferred choice for HC plants.

LOS Infrared Gas detectors

Larger coverage

Expensive, high prone to nuisance alarms

Unit of measure is PPM Meter

  • Control is the art of keeping a measured variable at a desired level
  • The most common control algorithm used is the PID control

  • Hazardous Gas detectors

· 2 categories

· Category 1 – Immediate health effect gasses (H2S, CO, HCL)

· H2S < 10 PPM, more than that is dangerous. 1000 PPM = dead

· HCL < 5 PPM

· Category 2 – Long Term (Vinyl Chloride, benzene, toluene)

·

Type

Principle

Advantages

Disadvantage

When to use

Electro Catalytic H2S gas detector

Combustible gas oxidizes and produces heat

Robust

For H2S detection

Electro Chemical

Electrolysis of gas creates voltage

PPM Level, stability, immune to environment change

Must change electrolytes, Not fail safe, Gas specific (H2S, SO2,CO2,NO2, CL2)

Preferred choice

Semiconductor

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OPEN-PLANT PTY LTD

Perth, Australia

EMAIL

info@open-plant.com
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