HKIE Professional Assessment - Gas Discipline flashcards

33 cards. Read the term and the definition here. The deck at the top of this page is optional.

Wobbe index

Calorific value divided by the square root of relative density. It indicates gas interchangeability, so appliances perform correctly only when the Wobbe index stays within the declared range.

Stoichiometric air requirement

The minimum air needed for complete combustion, calculated from the combustion equation. Methane requires about 9.5 cubic metres of air per cubic metre of gas.

Flammability limits of methane

Methane burns in air between roughly 5% and 15% by volume. Below the lower limit the mixture is too lean, above the upper limit too rich, to ignite.

LPG vapour density

LPG vapour is about 1.5 to 2.0 times heavier than air, so leaks accumulate in low points, basements and pits, unlike lighter natural gas which rises.

Autoignition temperature of methane

About 540 degrees Celsius. It is the temperature at which methane ignites without an external flame, relevant to hot surfaces and equipment selection in hazardous areas.

After-diversity maximum demand

The peak demand of a group of consumers after applying diversity. It falls per consumer as the number of consumers in the group increases, and it sizes distribution mains.

Governor station protection

Pressure regulator stations require fail-safe design with monitor regulators, relief valves, and slam-shut devices to prevent overpressure reaching downstream low-pressure networks.

PE mains for buried gas

Polyethylene pipe is standard for buried medium- and low-pressure gas mains. Steel is used for high pressure and above-ground sections, with cathodic protection where buried.

Utility separation for gas mains

Buried gas mains require minimum cover, specified separation from water, electricity and drainage services, and marker tape or warning tiles above the pipe.

Equivalent length method

Pipe sizing converts fittings and bends into equivalent straight pipe lengths, adds them to the measured run, and applies the allowable pressure drop to the total.

Allowable pressure drop

The permitted loss between meter outlet and the most remote appliance, commonly about 1 millibar for low-pressure domestic installations, so each appliance receives rated pressure.

Corrugated stainless steel tubing

A flexible gas pipe material used in installation pipework. It has restrictions on location, protection and support, and must be installed to the manufacturer's instructions.

Meter outlet isolation valve

An accessible valve immediately downstream of the gas meter, allowing the installation to be isolated. Labels must state the gas type and operating pressure.

Purpose-provided ventilation

Permanent air inlets sized by appliance rating, typically in square centimetres per kilowatt of input, required for open-flued and flueless appliances in addition to adventitious ventilation.

Room-sealed appliance

An appliance that draws combustion air from outside and discharges flue products outside, so indoor air quality is unaffected and ventilation demand is reduced.

Flue terminal clearance

Specified minimum distances from windows, doors, air bricks and adjacent buildings. Terminals must not discharge into enclosed spaces or cause downdraught and recirculation.

Stack effect in flues

Draught produced by the height of the flue and the temperature difference between flue gases and outside air. Flue gas must stay above dew point to avoid condensation.

Hazardous area zones

Zone 0 has continuous gas presence, Zone 1 occasional, Zone 2 short-term. Electrical equipment must be certified to the category matching the zone.

Flammable gas detection alarm

Detectors alarm at a fraction of the lower flammability limit, commonly 10 to 20 percent LFL, and trigger isolation or ventilation before an explosive concentration forms.

Inert gas purging

Displacing air with inert gas, then introducing gas, before commissioning or maintenance. Purge volume and end-point testing must be documented.

Gas Safety Ordinance Cap 51

The principal Hong Kong legislation governing supply, installation and use of gas. Its subsidiary regulations cover gas supply and installation and use.

EMSD gas safety role

The Electrical and Mechanical Services Department is the Hong Kong gas safety authority. Its Gas Standards Office administers registration of gas installers and contractors.

Registered gas installer

A competent person registered under the Gas Safety (Registration of Gas Installers) Regulations for a specific class of gas work, and limited to that class.

LPG maximum filling ratio

Bulk LPG tanks are filled to about 80 to 85 percent liquid by volume, leaving vapour space for thermal expansion and preventing liquid carry-over or hydraulic overpressure.

LPG cylinder store ventilation

Cylinder stores need ventilation at both high and low level, no ignition sources, segregation from oxidisers, and upright secured cylinders, because LPG vapour is heavier than air.

LNG storage temperature

Liquefied natural gas is stored cryogenically at about minus 162 degrees Celsius, requiring low-temperature materials and boil-off gas management.

Strength pressure test

A test at a pressure above design, commonly 1.5 times design or as the code requires, using a suitable medium, to prove pipework integrity before tightness testing.

Tightness leak test

After the strength test, pressure is held for a specified duration after temperature stabilisation, with no measurable drop permitted, to confirm joint integrity.

Combustion analysis at commissioning

Measurement of carbon monoxide, carbon dioxide and excess air in flue gases when appliances are first fired, confirming complete combustion and correct adjustment.

Safety over cost and schedule

A professional duty: where public safety conflicts with cost or programme, safety takes precedence, and the concern must be escalated and documented rather than accepted.

Inspection and test plan

A project document listing inspection and test activities, acceptance criteria, and hold points for witness, forming the basis of contract acceptance for gas installations.

Condensing appliance efficiency

Condensing appliances recover latent heat from water vapour in flue gases, so efficiency can exceed 90 percent on gross calorific value and above 100 percent of gross.

Hydrogen blending limits

Adding hydrogen to natural gas lowers carbon intensity per unit energy, but blend limits depend on end-use appliance tolerance and network material compatibility.