1. Designing a Gas Detection System

2. 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.”

3. 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
Indoors
Outdoors

4. 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

5. 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

6. 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

7. 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

8. Gas Hazards There are three main types of gas hazard Flammable Toxic
Risk of fire and or explosion, e.g. Methane, Butane, Propane
Toxic
Risk of poisoning, e.g. Carbon Monoxide, Hydrogen Sulfide, Chlorine
Asphyxiant
Risk of suffocation, e.g. Oxygen deficiency, Nitrogen, Carbon Dioxide

9. FIRE Flammable Risk Fire Triangle
Three factors are always needed to cause combustion:
A source of ignition
Oxygen
Fuel in the form of a gas or vapour
fuel
oxygen
heat
FIRE

10. Flammable Risk
The operation of a car’s choke illustrates an important part of flammable gas hazards
All flammable gases are only ignitable over their flammable range
Flammable gases tend to be measured in percentage of their explosive Limit (%LEL)
100% v/v gas 0% v/v air
too rich
U.E.L. (upper explosive limit)
flammable range
L.E.L. (lower explosive limit)
too lean
0% v/v gas 100% v/v air

11. 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

12. 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.
1 million balls
1 red ball

13. 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

14. 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

15. 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

16. 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

17. Detector Spacing Indoors, Cont’d.
UL Suggests a 900 Ft2 Ceiling Space Per Smoke Detector
30 Feet
15 Feet
Detector
Detector
References
NFPA 72 E, Standard On Automatic Fire Detectors
Schaeffer, M.J., “The Use of Combustible Detectors in Protecting Facilities from Flammable Hazards,” ISA Transactions, Volume 20, No. 2, Instrument Society of America 1981

18. Indoors Lab Bench 40 feet Door Ceiling Ventilation Natural Gas 20 ft
STORAGE
Lab Bench
Cl2
Door
LN2
Window

19. 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

20. Outdoor Detector Location Guidelines
Possible Entrapment of Leaking Gases and Vapors Within Columns, Low Lying Areas or Confined Spaces
Sources of Ignition and Processes With Fugitive Leak Potential Are Considered for Detector Placement

21. 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.

22. 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

23. Detector Spacing Outdoors
Repeat Outdoor Grid Pattern as Conditions Warrant. Focus on Potential Leak Sources for Additional Detectors if Required
20-30 Feet
10-15 Feet
Detector
Detector

24. 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

25. 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

26. 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

27. Detector Location and Area Coverage Map
IMPORTANT
Detectors should be located close to any potential leak source and between leak source and any potential source of ignition existing at the monitored site.
DETECTOR LOCATED ON
TOP OF TANK NEAR VALVE
OR FLANGE ON FILL LINE
FILL LINE
TANK SUPPORT LEGS
SIDE VIEW OF STORAGE TANK
FILL LINE
OUT FLOW
XNX-Optima
TOP VIEW OF STORAGE TANK
DETECTORS SHOULD BE LOCATED BELOW TOP OF DYKE WALL FOR VAPORS THAT ARE HEAVIER THAN AIR.
Sensor should be approx in above grade. These vapor densities are greater than air.

28. Detector Location and Area Coverage Map
TOP VIEW OF STORAGE TANK
Detector Location and Area Coverage Map
IMPORTANT
Detectors should be located close to any potential leak source and between leak source and any potential source of ignition existing at the monitored site.
DETECTOR LOCATED ON
TOP OF TANK NEAR VALVE
OR FLANGE ON FILL LINE
FILL LINE
TANK SUPPORT LEGS
XNX-Optima
FILL LINE
OUT FLOW
DETECTORS SHOULD BE LOCATED BELOW TOP OF DYKE WALL FOR VAPORS THAT ARE HEAVIER THAN AIR.
Sensor should be approx in above grade. These vapor densities are greater than air.
SIDE VIEW OF STORAGE TANK

29. Detector Location and Area Coverage Map
IMPORTANT: This is intended as a General Application Note and NOT as the sole source of information in determining quantity and location for detector placement. Consult additional resources when developing a monitoring system. Additional information is available for developing Combustible Gas Detection systems, such as: The National Fire Protection Association, NFPA 52 and the Instrument Society of America, ISA-RP12.13-Part II Services are also available from Professional Safety Engineering Firms and should be utilized whenever necessary
DETECTORS SHOULD BE
LOCATED BELOW TOP OF DYKE WALL FOR VAPORS
THAT ARE HEAVIER THAN AIR.
Sensor should be approx in. above grade..
SIDE VIEW OF TANK 4 5 6 7 8
TOP VIEW OF THREE AND TWO TANK AREA
IMPORTANT
Detectors should be located close to
any potential leak source and between
leak source and any potential source of
ignition existing at the monitored site.

30. Detector Location and Area Coverage Map
R.R. SPUR NO. 13
R.R. SPUR NO. 12
TOP VIEW OF UNLOADING FACILITY
IMPORTANT
Detectors should be located close to any potential leak source and between leak source and any potential source of ignition existing at the monitored site.
NOTE: DETECTORS SHOULD BE LOCATED LOW, CLOSE TO GRADE FOR VAPORS THAT ARE HEAVIER THAN AIR.
NOTE: Sensor should be approx.
12-18 in. above grade. These vapor
densities are heavier than air.
SIDE VIEW OF UNLOADING FACILITY I
NOTE: IF PIPING FUNCTIONS OR VALVES ARE ABOVE THE TANK CAR A DETECTOR MAY BE MOUNTED PART WAY DOWN THE WALL TO DETECT FALLING VAPORS FROM A LEAK SOURCE

31. 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
NFPA/ANSI Guides, Standards and Practices
1 Batterymarch Park, Quincy MA
NIOSH: Pocket Guide to Chemical Hazards
Available by FAX Request to (513)
OSHA, CFR 29, Section , Subpart Z“Working in Confined Spaces.”
NIOSH1 Publication
“A Guide to Safety in Confined Spaces.”
NIOSH Publication
“ALERT: Request for Assistance in Preventing Occupational Fatalities in Confined Spaces.”
NIOSH Publication

32. The End Questions?