0.2 Properties of Natural Gas, Fuel Gas Classification & Combustion Basics

ACS CCN1 Core Domestic Natural Gas Safety Assessment Exam

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1 Chapter 0: Introduction & ACS CCN1 Overview

0.1 ACS CCN1 Assessment Format, Open-Book Strategy & Gas Safe Path 0.2 Properties of Natural Gas, Fuel Gas Classification & Combustion Basics

2 Chapter 1: Gas Safety Legislation & Regulations

1.1 Gas Safety (Installation and Use) Regulations 1998 (GSIUR) — Operative Duties 1.2 Gas Safe Register Rules, Competence Categories & Operative ID Cards 1.3 Landlord Gas Safety Duties & CP12 / Safety Check Records (Regulation 36) 1.4 Building Regulations (Part J), HASWA & Health and Safety on Site

3 Chapter 2: Tightness Testing & Let-By Testing

2.1 Gas Meter Types (U6/G4/E6), Emergency Control Valves (ECVs) & MIVs 2.2 Let-By Testing Procedures & Electronic / U-Tube Manometer Setup 2.3 Tightness Testing Procedures for Natural Gas (IGEM/UP/1B) 2.4 Permissible Pressure Drops, Calculations & Tracing Gas Leaks

4 Chapter 3: Gas Rating, Pressure & Calorific Value

3.1 Gas Pressure Standards: Standing, Working & Meter Outlet Pressures 3.2 Gas Rating Domestic Appliances — Imperial & Metric Meter Calculations 3.3 Gross Calorific Value (39.5 MJ/m³), Gross/Net Heat Input & Efficiency 3.4 Burner Pressure Adjustments, Injector Sizing & Gas Rate Troubleshooting

5 Chapter 4: Pipework Sizing, Installation & Purging

4.1 Gas Pipework Materials: Copper, Steel, PE & Corrugated Stainless Steel (CSST) 4.2 Pipe Sizing Calculations & The 1 mbar Maximum Pressure Drop Rule 4.3 Pipework Jointing, Sleeving, Corrosion Protection & Concealed Routes 4.4 Main Equipotential Bonding (within 600mm) & Direct Purging (IGEM/UP/1B)

6 Chapter 5: Ventilation Requirements for Domestic Gas

5.1 Ventilation Fundamentals & BS 5440-2 Standard Requirements 5.2 Ventilation Calculations for Open-Flued Appliances (5 cm²/kW Rule) 5.3 Ventilation for Flueless Appliances, Cookers & Appliance Compartments 5.4 Adventitious Air, Free Area Grille Sizing & Ventilation Defects

7 Chapter 6: Flue Types, Siting & Spillage Testing

6.1 Flue Classifications: Open-Flued (Type B), Room-Sealed (Type C) & Flueless (Type A) 6.2 Flue Terminal Siting, Clearance Distances & Protection (BS 5440-1) 6.3 Flue Flow Testing Procedures & Smoke Pellets Usage 6.4 Spillage Testing Procedures under Normal & Adverse/Worst-Case Conditions

8 Chapter 7: Combustion Analysis & Carbon Monoxide Safety

7.1 Complete vs Incomplete Combustion & Flame Picture Analysis 7.2 Electronic Flue Gas Analysers (FGA) & BS 7967 Testing Protocols 7.3 CO/CO₂ Ratio Action Levels (BS 7967) & GIUSP Actions 7.4 Carbon Monoxide Poisoning Symptoms, Causes & CO Alarm Rules (BS EN 50291)

9 Chapter 8: Emergency & Unsafe Situations (GIUSP / RIDDOR)

8.1 Gas Industry Unsafe Situations Procedure (GIUSP / IGEM/G/11) Overview 8.2 Immediately Dangerous (ID) Situations: Definition, Actions & Warning Labels 8.3 At Risk (AR) Situations & Advising Not to Current Standards (NCS) 8.4 Gas Escape Actions (0800 111 999), Isolation & RIDDOR Reporting Regulations

0.2 Properties of Natural Gas, Fuel Gas Classification & Combustion Basics

Key Takeaways Explain This Topic AI

Natural Gas belongs to the 2nd Family of fuel gases, consisting of ~95% Methane (CH4) with a Relative Density of approximately 0.6 (lighter than air).

The explosive limits of Natural Gas in air range from 5% (Lower Explosive Limit - LEL) to 15% (Upper Explosive Limit - UEL).

Complete combustion of 1 m³ of Natural Gas requires approximately 9.6 to 10 m³ of air (~2.0 m³ of pure oxygen), producing CO2, H2O, N2, and heat.

Incomplete combustion occurs due to lack of oxygen or flame chilling, producing toxic Carbon Monoxide (CO), soot (C), and dangerous unburnt fuel.

Natural Gas is naturally colourless and odourless; it is odorised with Mercaptan compounds to allow human detection at 1% concentration in air (20% of LEL).

Last updated: July 2026

0.2 Properties of Natural Gas, Fuel Gas Classification & Combustion Basics

Quick Summary: Understanding the chemical composition, physical properties, and combustion behaviour of Natural Gas is fundamental to diagnosing gas appliances, setting burner pressures, and ensuring ventilation safety. Natural Gas is a 2nd Family fuel gas composed mainly of Methane ( C H 4 CH_4 C H 4 ​ ) , possessing a Relative Density of ~0.6 (making it lighter than air) and a flammability range between 5% and 15% by volume in air.

Fuel Gas Families & Classification (BS EN 437)

In the United Kingdom and Europe, fuel gases are classified into three distinct families based on their origin, Wobbe Index, and combustion characteristics as defined in BS EN 437 :

1st Family Gases (Manufactured / Town Gas): Produced historically by coal gasification. High Hydrogen content (~50% H 2 H_2 H 2 ​ ), Relative Density of ~0.4 to 0.5, gross Calorific Value around 18–20 MJ/m³. Highly toxic due to inherent Carbon Monoxide content. Obsolete in UK public distribution networks.

2nd Family Gases (Natural Gas): Extracted from geological reserves under the North Sea or imported via pipelines and Liquefied Natural Gas (LNG) terminals. Consists primarily of Methane ( C H 4 CH_4 C H 4 ​ , ~95%) , with minor traces of Ethane, Propane, Nitrogen, and Carbon Dioxide. Gross Calorific Value ranges from 37.5 to 40.0 MJ/m³ (10.4–11.1 kWh/m³).

3rd Family Gases (Liquefied Petroleum Gas - LPG): Hydrocarbon gases stored liquid under pressure. Comprises Commercial Propane ( C 3 H 8 C_3H_8 C 3 ​ H 8 ​ ) and Commercial Butane ( C 4 H 10 C_4H_{10} C 4 ​ H 10 ​ ) . They have high Calorific Values (Propane ~95 MJ/m³, Butane ~120 MJ/m³) and are significantly heavier than air.

Property / Feature 1st Family (Town Gas) 2nd Family (Natural Gas) 3rd Family (Commercial Propane) 3rd Family (Commercial Butane)

Primary Chemical Constituent Hydrogen ( H 2 H_2 H 2 ​ ) / C O CO CO Methane ( C H 4 CH_4 C H 4 ​ ) Propane ( C 3 H 8 C_3H_8 C 3 ​ H 8 ​ ) Butane ( C 4 H 10 C_4H_{10} C 4 ​ H 10 ​ )

Relative Density (Air = 1.0) ~0.40 - 0.50 ~0.60 (Lighter than air) ~1.50 (Heavier than air) ~2.00 (Heavier than air)

• Gross Calorific Value (MJ/m³) ~18.0 - 20.0 ~37.5 - 40.0 ~95.0 ~120.0
• Flammability Limits in Air ~4.0% - 40.0% 5.0% - 15.0% 2.0% - 10.0% 1.8% - 9.0%

Air Required for Complete Comb. ~4.0 m³ air / m³ gas ~9.6 - 10.0 m³ air / m³ gas ~24.0 m³ air / m³ gas ~31.0 m³ air / m³ gas

Physical Properties of Natural Gas ( C H 4 CH_4 C H 4 ​ )

Natural Gas possesses several critical physical attributes that directly dictate safety procedures and installation design:

Relative Density (Specific Gravity): Natural gas has a relative density of approximately 0.6 relative to dry air (Air = 1.0). Because it is lighter than air, escaping natural gas rises vertically and accumulates at high levels or ceiling voids. In well-ventilated rooms, it dissipates rapidly upward. (In contrast, LPG with SG 1.5–2.0 sinks to low levels, basements, and drainage gullies).

Odorisation: Natural gas in its raw state is completely colourless and odourless. To ensure rapid leak detection, gas transporters add an odorant compound (typically Mercaptan or Dimethyl Sulphide). This odorant is formulated so that gas can be readily detected by smell at a concentration of 1% gas in air —which represents just 20% of the Lower Explosive Limit (LEL) .

Toxicity: Unlike old 1st Family Town Gas, clean Natural Gas is non-toxic. However, it acts as an asphyxiant in high concentrations by displacing atmospheric oxygen.

Flammability Limits: Natural gas will only ignite or explode when mixed with air in specific proportions:

Lower Explosive Limit (LEL): 5% gas in air. Below 5%, the mixture is too "lean" (insufficient fuel) to burn.

Upper Explosive Limit (UEL): 15% gas in air. Above 15%, the mixture is too "rich" (insufficient oxygen) to ignite.

Stoichiometric Mixture: Approx. 9.5% to 10% gas in air , where perfect chemical combustion occurs.

Combustion Chemistry: Complete vs. Incomplete

Combustion is a rapid exothermic chemical reaction between fuel gas and oxygen. Domestic appliances use atmospheric air, which consists of approximately 21% Oxygen ( O 2 O_2 O 2 ​ ) and 79% Nitrogen ( N 2 N_2 N 2 ​ ) .

Complete Combustion

Complete combustion occurs when there is an adequate supply of oxygen, thorough mixing of fuel and air, and sufficient flame temperature without flame chilling. Methane ( C H 4 ) + 2 Oxygen ( 2 O 2 ) + [ N 2 ] → Carbon Dioxide ( C O 2 ) + 2 Water Vapour ( 2 H 2 O ) + [ N 2 ] + Heat \text{Methane } (CH_4) + 2 \text{ Oxygen } (2O_2) + [N_2] \rightarrow \text{Carbon Dioxide } (CO_2) + 2 \text{ Water Vapour } (2H_2O) + [N_2] + \text{Heat} Methane ( C H 4 ​ ) + 2 Oxygen ( 2 O 2 ​ ) + [ N 2 ​ ] → Carbon Dioxide ( C O 2 ​ ) + 2 Water Vapour ( 2 H 2 ​ O ) + [ N 2 ​ ] + Heat

Key facts for complete combustion:

1 m³ of Methane requires 2 m³ of pure Oxygen .

Since air is only 21% Oxygen, 1 m³ of Methane requires ~9.6 to 10 m³ of atmospheric air .

Products of complete combustion are non-toxic: Carbon Dioxide ( C O 2 CO_2 C O 2 ​ ) , Water Vapour ( H 2 O H_2O H 2 ​ O ) , inert Nitrogen ( N 2 N_2 N 2 ​ ) , and heat.

Incomplete Combustion

Incomplete combustion occurs when there is insufficient primary or secondary oxygen, poor air/gas mixing, damaged burner ports, or when the flame impinging directly onto a cold surface (flame chilling). C H 4 + O 2 ( restricted ) → C O + C ( soot ) + C O 2 + H 2 O + Heat CH_4 + O_2 (\text{restricted}) \rightarrow CO + C (\text{soot}) + CO_2 + H_2O + \text{Heat} C H 4 ​ + O 2 ​ ( restricted ) → CO + C ( soot ) + C O 2 ​ + H 2 ​ O + Heat

Products of incomplete combustion include:

Carbon Monoxide ( C O CO CO ): A lethal, colourless, odourless, tasteless gas.

Carbon / Soot ( C C C ): Black deposits on heat exchangers, burner ports, and flue terminals.

Aldehydes & Hydrocarbons: Pungent smelling intermediate breakdown products.

Carbon Monoxide ( C O CO CO ) Toxicity & Flue Gas Analyzer Ratios

Carbon Monoxide ( C O CO CO ) is extremely dangerous because it binds to red blood cell haemoglobin with an affinity 200 to 300 times greater than oxygen , forming Carboxyhaemoglobin ( C O H b COHb CO H b ) . This prevents oxygen transportation to critical organs, causing tissue hypoxia, brain damage, and death.

Symptoms of C O CO CO Poisoning

Early symptoms mimic flu without fever: headaches, dizziness, nausea, fatigue, confusion, and breathlessness. Exposure to 800 ppm of C O CO CO causes severe headache and dizziness within 45 minutes and collapse within 2 hours.

Combustion Performance Benchmarks

Registered operatives use electronic Flue Gas Analysers (FGAs) conforming to BS EN 50379 to measure the ratio of C O CO CO to C O 2 CO_2 C O 2 ​ in appliance flues: Combustion Ratio = C O (ppm) C O 2 (%) × 10 , 000 \text{Combustion Ratio} = \frac{CO \text{ (ppm)}}{CO_2 \text{ (\%) } \times 10,000} Combustion Ratio = C O 2 ​ (%) × 10 , 000 CO (ppm) ​

Normal Clean Combustion Ratio: Typically below 0.0040 .

Action Required / Unsafe Threshold: A ratio exceeding 0.0080 indicates severe incomplete combustion requiring immediate appliance shutdown and investigation under the Gas Industry Unsafe Situations Procedure.

Flame Types & Burner Characteristics

Aerated (Bunsen-Type) Burners: Primary air is drawn into the burner body via a venturi prior to ignition. Produces a distinct blue inner cone and a lighter blue outer mantle. High flame temperature, clean combustion, no soot formation.

Non-Aerated (Neat Gas / Diffusion) Burners: No primary air is mixed before the burner head; combustion air is drawn entirely from surrounding secondary air. Operates with a softer flame. If secondary air is restricted, the flame turns yellow/luminous, generating heavy soot ( C C C ) and high C O CO CO .

Worked Calculation Scenario: Combustion Air Volume

Scenario: A gas operative is assessing an open-flued natural gas boiler with a heat input rating requiring a gas consumption rate of 2.4 m³/h of Natural Gas. Calculate the minimum volume of atmospheric air required each hour to achieve complete combustion.

Formula / Ratio: 1 m³ of Natural Gas requires 9.6 m³ to 10 m³ of atmospheric air.

Calculation: Air Required = 2.4 m 3 /h × 9.6 m 3 air/m 3 gas = 23.04 m 3 /h of air \text{Air Required} = 2.4 \text{ m}^{3}\text{/h} \times 9.6 \text{ m}^{3}\text{ air/m}^{3}\text{ gas} = 23.04 \text{ m}^{3}\text{/h of air} Air Required = 2.4 m 3 /h × 9.6 m 3 air/m 3 gas = 23.04 m 3 /h of air Air Required (Upper limit) = 2.4 m 3 /h × 10.0 m 3 air/m 3 gas = 24.0 m 3 /h of air \text{Air Required (Upper limit)} = 2.4 \text{ m}^{3}\text{/h} \times 10.0 \text{ m}^{3}\text{ air/m}^{3}\text{ gas} = 24.0 \text{ m}^{3}\text{/h of air} Air Required (Upper limit) = 2.4 m 3 /h × 10.0 m 3 air/m 3 gas = 24.0 m 3 /h of air

Conclusion: The installation room and ventilation openings must supply between 23.04 and 24.0 m³/h of fresh atmospheric air continuously during boiler operation to prevent incomplete combustion and C O CO CO creation.