HKIE Professional Assessment - Gas Discipline key terms

A measure of the energy input rate of a gas through a fixed orifice, calculated as the calorific value divided by the square root of the relative density. It allows interchangeability assessment between gases.

The amount of heat released when a unit quantity of gas is completely burned. Gross calorific value includes latent heat of water vapour; net excludes it. Used for billing and appliance rating.

The ratio of the density of a gas to the density of air at the same temperature and pressure. Natural gas is lighter than air; LPG vapour is heavier, affecting accumulation and ventilation design.

The range of gas concentration in air within which ignition can occur, bounded by the lower explosive limit (LEL) and upper explosive limit (UEL). Used for gas detector alarm settings.

Lower Explosive Limit: the minimum concentration of gas in air that will support combustion. Gas detectors often alarm at a percentage of LEL, typically 10% or 20%.

The ideal air-to-fuel ratio for complete combustion where all fuel and oxygen are consumed. Deviations lead to excess air or incomplete combustion, affecting efficiency and emissions.

The amount of air supplied beyond the stoichiometric requirement, expressed as a percentage. Necessary for complete combustion but reduces efficiency if excessive.

Measurement of combustion products such as oxygen, carbon dioxide, carbon monoxide and temperature to assess combustion efficiency and safety. Used during commissioning and servicing.

A toxic, odourless gas produced by incomplete combustion. In gas installations, its presence indicates poor combustion or flueing faults and poses a serious health risk.

Combustion where fuel is not fully oxidised, producing carbon monoxide, soot and other pollutants. Causes include insufficient air, poor mixing or flame impingement.

The ability of an appliance to operate safely and efficiently when supplied with a gas of different composition, often assessed using the Wobbe Index and flame speed characteristics.

The pressure exerted by liquefied petroleum gas vapour at a given temperature. It determines storage vessel design pressure and affects vaporisation rate and regulator selection.

Installations storing liquefied petroleum gas in vessels such as bullets or cylinders above a defined quantity, subject to separation distances, bunding and emergency planning requirements.

Liquefied Natural Gas: natural gas cooled to about -162°C for storage and transport. Requires cryogenic materials, vapour management and specific safety systems.

The network of pipes, regulators, meters and associated equipment that conveys gas from the source to the point of use, including transmission, distribution and service lines.

The pipe carrying gas from the distribution main to the consumer's meter or inlet point. Its design considers pressure, flow, corrosion protection and isolation.

A device that reduces and maintains gas pressure at a constant outlet value despite variations in inlet pressure or flow demand. Essential for safe appliance operation.

The arrangement of gas meter, isolation valve, regulator and associated fittings, including location, ventilation and clearances, to ensure safe measurement and control.

The calculation of pipe diameters to deliver required gas flow rates within allowable pressure drop, considering equivalent lengths of fittings, gas density and diversity of demand.

The reduction in gas pressure along a pipe due to friction and elevation changes. Must be limited to ensure adequate pressure at appliances.

The total gas demand of appliances connected to an installation, often calculated using diversity factors. Determines pipe sizing, meter capacity and supply pressure requirements.

A factor applied to the sum of connected gas loads to account for the probability that not all appliances operate simultaneously. Used in pipe and meter sizing.

The supply of fresh air and removal of stale air to maintain indoor air quality and ensure safe combustion. Required for appliances and occupied spaces.

The system for conveying combustion products from an appliance to outside air. Includes flue types, termination clearances, draught diversion and material selection.

The condition of air inside buildings as it affects occupant health and comfort, influenced by ventilation, combustion products, and pollutant sources.

The minimum separation required between gas storage or equipment and buildings, boundaries, or ignition sources to reduce risk of fire or explosion.

The classification of areas where flammable gas may be present, into zones according to likelihood and duration, to ensure selection of suitable electrical equipment.

The use of sensors to detect flammable or toxic gases and trigger alarms or shutdowns. Detector placement depends on gas density and potential leak sources.

A valve installed on a gas service or installation to isolate the gas supply in an emergency. Must be accessible, clearly marked and operable without special tools.

A valve used to shut off gas flow to a section of pipework or an appliance for maintenance or emergency. Must be suitable for the gas type and pressure.

A pressure test to verify the gas-tightness of an installation, typically using air or inert gas, with specified test pressure, duration and allowable pressure drop.

The process of testing and adjusting a gas installation or appliance to ensure safe and correct operation, including tightness testing, purging, and combustion analysis.

The controlled displacement of air or gas from pipework using an inert gas or the fuel gas itself, to avoid explosive mixtures during commissioning or decommissioning.

Hong Kong legislation (Cap. 51) regulating the supply, installation, maintenance and use of gas to ensure safety. Administered by the Electrical and Mechanical Services Department.

Subsidiary legislation under the Gas Safety Ordinance, setting detailed requirements for gas installations, appliances, testing and registration of contractors and workers.

The formal authorisation by the Gas Authority for gas contractors, installers and technicians to carry out specified gas works, ensuring competence and accountability.

A person with the necessary training, qualification and experience to perform specific gas work safely and to the required standard, as defined in regulations.

The principles of honesty, integrity, competence and public safety that guide professional conduct in gas engineering, including disclosure of conflicts and adherence to codes.

A document providing practical guidance on how to comply with legal requirements and achieve good practice in gas installations, maintenance and safety.

The ratio of useful energy output to energy input. In gas systems, improved by condensing boilers, controls, insulation and correct combustion settings.

Gases such as hydrogen, biomethane, or synthetic methane produced with low lifecycle carbon emissions, intended to decarbonise gas networks.