Desingning a Gas Detection System ( Honeywell )

  • Designing a Gas Detection System

  • Why Gas Detection?

  • Safeguard Life and Property.

–Provide Early Warning of Hazardous Conditions.

–Provide Opportunity for Evacuation and Notification from Re-entry

–Provide Time for Intervention and Correction.

  • Trigger Facility Protection Systems.

–Ventilation, Water Mist, Fire Suppression.

  • Satisfy Local Fire Code and Provide Insurability.

  • Address Real and Perceived Safety Concerns.

  • Note: Gas Detection is “Recommended Practice,” “Required by Code,” or “Required by Law.”

  • Points to Consider

  • Understand the application

  • Identify potential danger points

  • Establish design goals

  • Determine gas characteristics

  • Profile the plant and potential release scenarios

  • Other elements in selecting gas detection systems

  • Actual placement of detection



  • Understand The Application

  • The gases to be monitored

–Toxic (STEL, TLV, TWA)

–Combustible (LEL, UEL)

–Exposure limits

–Density and Other Gas Properties

  • Local and federal regulations

–Uniform Fire Code

–Code of Federal Regulations

–Local Fire Marshal

  • Identify Potential Danger Points

–Release Points – sensors should be placed as close as possible to potential leak sources.

–Seals and flanges, fittings and welds

–Expansion joints and gaskets

–Engine combustion

–Storage, loading and unloading areas

–Runoff areas

–Decomposing materials

–Receptor Points – a gas detection notification system should protect any person, property or equipment that may come in contact with harmful gases.

–Wind direction

–Ventilation systems

–Run off areas

–Confined spaces

–Communities and facilities

  • Establish Design Goals

Initiate a response based on an early warning of a potential problem.

Notification or annunciation – method of warning

Ventilation control

Process shutdown

Evacuation and emergency response

Amount of confinement – over pressurization and accumulation

Run-up distance – speed of flame increases with distance

Amount of congestion or obstacles

Fuel quantity and mixing

Margin of safety – distance between leak source and receptors

Plant safety process

Insurance requirements

Determine Gas Characteristics

  • LEL, UEL, Toxicity

  • Vapor density

–Density differences with temperature

  • Cryogenic liquids, flammable liquids

–Low density gases displacing ambient density gases (helium vs. oxygen)

–Gases under pressure will condense in areas where vented first

  • Gases changing composition – dry ice

  • Toxicity vs. flammability (MTBE 40 PPM 1.6% LEL)

  • Hydrolyzed (BF3, F2)

  • Pyrolyzed (NF3)

  • Flash point – the lowest temperature at which a liquid can form an ignitable mixture in air near the surface of the liquid. The lower the flash point, the easier it is to ignite the material.

  • Rate of evaporation and dispersion characteristics

  • Gas mixing (fuel, oxygen, ignition source)

  • Oxygen enriched environments

  • Gas Hazards

There are three main types of gas hazard


–Risk of fire and or explosion,
e.g. Methane, Butane, Propane


–Risk of poisoning,
e.g. Carbon Monoxide, Hydrogen Sulfide, Chlorine


–Risk of suffocation,
e.g. Oxygen deficiency, Nitrogen, Carbon Dioxide

  • Flammable Risk

  • Fire Triangle

Three factors are always needed to cause combustion:

1.A source of ignition


3.Fuel in the form of a gas
or vapour

  • Flammable Risk

  • Toxic Risk

  • 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

  • Others are completely odourless like Carbon Monoxide

  • Toxic Risk

  • The measurement most often used for the concentration of toxic gases is parts per million (ppm).

  • 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.

  • Profile the Plant and Potential Release Scenarios

Gas sensors should be placed to ensure that a quantity of gas will past by them in all normal release scenarios.

  • Identify physical features of plant

  • Identify ventilation tracks

  • Identify escape routes

  • Protect entrances to areas

  • Mark escape routes

  • Identify wind directions

  • Other Elements

  • Accessibility for calibration and maintenance

  • Wiring and installation

  • Environmental conditions

  • EMI and RFI

  • Alarm levels

  • Exposure limits

  • Oxygen levels – some toxic gas electrochemical sensors require a minimum oxygen level to function. All catalytic bead combustible detectors require oxygen to work.

  • Be aware of poisoning and inhibiting factors

  • Interior Detector Placement Guidelines

  • Operate Detectors Within their Temperature Limits. Use Sample Draw or Duct Mount Configurations When Needed

  • Water, Moisture, Dust and Dirt May Affect Performance. Minimize Exposure and Protect From Adverse Conditions

  • Locate Detectors With Respect to Grade, Floor, or Operating Level – According to Building Design, HVAC System, Characteristics of Potential Leak

  • When Monitoring Specific Equipment, Place Detectors Near (12 Inches) Pump, Seal, Tank, Valve, etc.

  • Sensitivity of Detector Depends on Proximity to Leak. Adjust Alarms if Earlier Annunciation is Required

  • Mount Detectors Securely, Independent of Vibration, With Weather Shield Facing Downward

  • Conduct Smoke Trace Behavior Studies If in Doubt

  • Detector Spacing Indoors

  • There are Few Published Guidelines and No Standards Indicating Area or Volume Effectively Protected By a Diffusion Sensor. There is a Corollary in Fire Protection

  • UL Suggests a 900 Ft2 Ceiling Space Per Smoke Detector, Which is a 30 Ft. Square or 15 Ft. Radius

  • Using This Base Guideline, the Total Number of Detectors Must be Based on

–Gas Dispersion Characteristics and Air Movement

–Potential Leak Source Locations and Characteristics,

–Sources of Ignition Locations

–Interior Space Division by Walls or Barriers

–Economics of the Procurement

  • Detector Spacing Indoors, Cont’d.

  • UL Suggests a 900 Ft2 Ceiling Space Per Smoke Detector

  • Indoors

  • Outdoor Detector Location Guidelines

  • Use same considerations outdoors as indoors.

  • Consider Angle and Direction of Prevailing Wind

  • The Orientation of Structures and Surrounding Terrain with Regard to Shielding Affects

  • The Proximity of Large Quantities of Toxics to Personnel and Equipment, Which May Require Added Detectors to Isolate the Two

  • Outdoor Detector Location Guidelines

  • Outdoor Detector Location, Cont’d.

  • Heavier Than Air Gases or Vapors: Vapor Density >1

–The Preferred Location for Detectors is ~18 Inches Above Grade.  For Liquid Spills, As Close to the Vapor/Liquid Interface as Possible, and Still Allow for Detector Calibration

  • Lighter Than Air Gases and Vapors: Vapor Density <1

–The Preferred Location for Detectors is About 6 to 8 Feet Above Grade or Operating Level, With Special Attention Being Paid to Air Currents, Structures, Roofed Areas, Etc.

  • Detector Spacing Outdoors

  • Few Guidelines Exist for Detector Placement

  • An Increased Grid Density is Used Outdoor Presumably Because of the Greater Potential for Leak Dilution

  • Thus, More Detectors Are Required, and Potential Leaks Are Encircled, to Account for Wind Shifts

  • Detector Spacing Outdoors

  • General Location Considerations

  • Toxic Gases and Vapors:

–Identify Potential Leak Sources, Work Areas, and Exit Points.  Understand Where People are Performing Their Work and Place Detectors Between Probable Release Points and the Work Area

  • General Location Considerations

  • Allow Access for Sensor Calibration and Replacement.

–Sensors Have a Finite Life – Calibrate and Maintain Regularly!

  • Always Locate Detectors Using “Local Conditions Knowledge,” and Lighter or Heavier Than Air Principles

  • General Location Considerations

  • Locate Detectors Within Their Temperature Rating

–  Avoid Exposure to Sources of High Radiant Heat

  • Keep Detectors Away From Moisture and Chemicals

–  Avoid Vibration and Mechanical Shock Hazards

  • Observe Recommended Wiring and Tagging Practices

  • Use Shielded Cable Whenever Possible and Follow Correct Grounding Practices (NFPA 70)

  • Observe Proper Detector Mounting Orientation

  • Detector Location and Area Coverage Map

  • Detector Location and Area Coverage Map

  • Detector Location and Area Coverage Map

  • Detector Location and Area Coverage Map

  • Publications to Reference

  • Chemical Weekly, 2008, “Key Considerations when Designing a Gas Detection System”

  • ISA Recommended Practices

  • ACGIH: Annual TLV and BEI Guide

–1330 Kemper Meadow Drive, Cincinnati OH 45240-1634

  • NFPA/ANSI Guides, Standards and Practices

–1 Batterymarch Park, Quincy MA 02269-9101

  • NIOSH: Pocket Guide to Chemical Hazards

–Available by FAX Request to (513) 533-8573

  • OSHA, CFR 29, Section 1910.1000, Subpart Z“Working in Confined Spaces.”

–NIOSH1 Publication 80-106

  • “A Guide to Safety in Confined Spaces.”

–NIOSH Publication 87-113

  • “ALERT: Request for Assistance in Preventing Occupational Fatalities in Confined Spaces.”

–NIOSH Publication 86-110

  • The End


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