HKIE Professional Assessment - Gas Discipline cheat sheet

Hong Kong town gas is manufactured from naphtha and natural gas feedstock and distributed at a declared calorific value; LPG is supplied as an air/LPG mixture or in cylinders and bulk tanks.

Network pressure tiers in Hong Kong practice: high-pressure (transmission), medium-pressure (district), and low-pressure (service to meters); each tier has its own pipe material and jointing rules.

Peak demand estimation uses diversity (after-diversity maximum demand, ADMD) applied to connected load; ADMD falls as the number of consumers in a group rises.

Pipe sizing for distribution mains uses the general flow equation (e.g. Spitzglass, Pole, or Renouard) with allowable pressure drop as the design constraint, not velocity alone.

Network analysis must consider the worst-case scenario: peak hour demand plus fire/emergency load, with n-1 contingency on critical mains and governor stations.

Governor (pressure regulator) stations require fail-safe design, relief or monitor regulators, and lock-up and slam-shut protection against overpressure downstream.

Polyethylene (PE) is standard for buried medium- and low-pressure mains; steel is used for high pressure and above-ground sections, with cathodic protection where buried.

Minimum cover, separation from other utilities (water, electricity, drainage), and marker tape/warning tiles are design requirements for buried gas mains.

Wobbe index = calorific value / sqrt(relative density); it is the interchangeability criterion - appliances operate correctly only when Wobbe index stays within the declared range.

Stoichiometric (theoretical) air for a fuel is calculated from the combustion equation; methane needs 2 mol O2 per mol CH4, i.e. about 9.5 m3 air per m3 gas.

Complete combustion products of methane: CO2 and H2O; incomplete combustion yields CO, which is toxic and is the main indoor hazard indicator.

Flammability limits of methane in air are approximately 5% (LFL) and 15% (UFL) by volume; LPG (propane/butane) LFL is about 1.8–2.1%, making leaks more readily ignitable.

LPG vapour is heavier than air (relative density about 1.5–2.0) and accumulates in low points, basements, and pits; natural gas/town gas is lighter and accumulates at high level.

Excess air (lambda) above 1 is required for practical combustion; CO2 in flue gas and O2 trimming are used to monitor and optimise combustion efficiency.

Autoignition temperature of methane is about 540 °C; flashback occurs when flame speed exceeds burner port velocity, lift-off when port velocity exceeds flame speed.

Domestic installation pipework is sized on the allowable pressure drop from meter outlet to the most remote appliance, using the same flow equations as distribution but with tighter limits.

Equivalent length method: fittings and bends are converted to equivalent straight pipe lengths (using k-factor or tabulated ratios) and added to the measured length.

Typical allowable pressure drop is a percentage of the operating pressure at the meter (e.g. 1 mbar for low-pressure domestic installations), and each appliance must receive its rated pressure at full load.

Installation pipe materials include copper (with appropriate jointing), steel, and corrugated stainless steel tubing (CSST); each has restrictions on location and protection.

Pipes must be protected against corrosion, mechanical damage, and thermal movement; sleeves are required where pipes pass through walls, floors, or cavities.

Gas meters and emergency control valves must be accessible; the meter outlet should have an isolation valve, and a label indicating the gas type and pressure is required.

Appliance connection uses a rigid connection or an approved flexible connector; hose length and location restrictions apply, and hoses must not pass through walls or ceilings.

Manifold and multi-appliance installations require balancing so that opening or closing one appliance does not cause unacceptable pressure variation at another.

Open-flued and flueless appliances require purpose-provided ventilation: permanent air inlets sized by appliance rating (e.g. cm2 per kW input) in addition to adventitious ventilation.

Room-sealed (balanced) appliances take combustion air from outside and discharge flue products outside, avoiding indoor air quality impacts and reducing ventilation demand.

Flue design must ensure adequate draught, correct terminal location away from openings, and prevention of downdraught and recirculation of products of combustion.

Flue terminal clearances are specified relative to windows, doors, air bricks, and adjacent buildings; terminals must not discharge into enclosed spaces.

Chimney/flue draught depends on height and temperature difference (stack effect); flue gas temperature must remain above dew point to avoid condensation in conventional flues.

CO detectors should be installed in rooms containing open-flued appliances and in sleeping areas; they supplement, not replace, correct flueing and ventilation.

Indoor air quality management includes controlling CO, CO2, NOx, and humidity; kitchen extract and general ventilation rates must match appliance and occupancy loads.

Gas safety hierarchy: eliminate ignition sources, isolate and purge before work, test for gas, ventilate, and use intrinsically safe equipment in hazardous zones.

Hazardous area classification for gas installations: Zone 0 (continuous), Zone 1 (occasional), Zone 2 (short-term); electrical equipment must be certified to the corresponding category.

Gas detection: flammable gas detectors (catalytic or IR) alarm at a fraction of LFL (commonly 10–20% LFL) and trigger isolation/ventilation; CO detectors alarm at low ppm thresholds.

Purging: displacement or inert gas purging is required before introducing gas into new pipework and before maintenance; purge volume and end-point testing must be documented.

Risk assessment uses likelihood × consequence; gas-specific controls include emergency shutdown valves, excess flow valves, and pressure protection systems.

Emergency response: raise alarm, isolate supply, ventilate, evacuate, and call the gas utility; no naked flames or electrical switching in a gas-affected area.

Incident investigation follows a structured method (timeline, root cause, barrier analysis) and feeds back into design and maintenance procedures.

The Gas Safety Ordinance (Cap. 51) and its subsidiary regulations govern the supply, installation, and use of gas in Hong Kong; the Gas Safety (Gas Supply) Regulations and Gas Safety (Installation and Use) Regulations are key instruments.

The Electrical and Mechanical Services Department (EMSD) is the gas safety authority; the Gas Standards Office administers registration of gas installers and contractors.

The Hong Kong Gas Safety (Registration of Gas Installers) Regulations require competent persons to be registered for specific classes of gas work.

Codes of Practice issued by EMSD (e.g. on gas installation, LPG, and flueing) provide the technical means of compliance with the Ordinance.

Hong Kong standards reference international standards (e.g. BS, EN, ISO) where local codes are silent; the code of practice takes precedence where there is conflict.

LPG bulk storage and cylinder stores are regulated with separation distances, fire protection, and notification requirements under the Gas Safety Ordinance and Dangerous Goods Ordinance.

Records of installation, testing, and maintenance must be kept and made available for inspection; non-compliance can lead to disconnection and prosecution.

LPG (propane/butane mixture) is stored as a liquid under pressure; bulk tanks are sized for consumption, refill interval, and vapour space, with a maximum filling ratio (about 80–85% liquid fill).

LPG bulk tank installations require separation distances from buildings, boundaries, and ignition sources; these distances depend on tank capacity and are specified in the code.

Cylinder stores require ventilation at high and low level, no ignition sources, and segregation from oxidisers; cylinders must be upright and secured.

LPG vaporisers are used when the natural vaporisation rate is insufficient; they must be protected against overpressure and low temperature (frosting).

LNG is stored cryogenically at about −162 °C; materials must be suitable for low temperature, and boil-off gas management and vapour dispersion are key design issues.

LNG/LPG facilities require gas detection, fire protection (water spray/deluge), emergency shutdown, and exclusion zones for hot work.

Pipework for LPG liquid service must account for expansion, hydraulic shock, and hydrostatic pressure from liquid head; relief valves protect trapped sections.

Strength (pressure) test: pipework is tested at a pressure above operating pressure (commonly 1.5× design or per code) with a suitable medium; pneumatic testing requires extra precautions.

Tightness (leak) test: after strength test, pressure is held and monitored for a specified duration with no measurable drop, allowing for temperature stabilisation.

Purging and commissioning: air is displaced by inert gas then gas; gas is introduced and appliances are commissioned with combustion analysis (CO/CO2, excess air).

Inspection stages include design review, installation inspection, pressure testing, and final commissioning; results are recorded and signed off by a registered installer.

Maintenance regimes include periodic inspection of pipework, flueways, ventilation, and safety devices; appliance servicing includes cleaning, combustion check, and safety device testing.

Non-destructive testing (visual, pressure, and where required, leak detection by soap solution or electronic detector) is used to verify joint integrity.

Records management: test certificates, as-built drawings, and maintenance logs must be retained and updated; they support incident investigation and regulatory inspection.

Professional engineers owe duties to public safety, the client, and the profession; where safety conflicts with cost or schedule, safety takes precedence and concerns must be escalated.

Competence limits: work only within your registered class and experience; seek specialist input for unfamiliar designs, and document assumptions and decisions.

Ethical conduct includes honesty in reporting test results, avoiding conflicts of interest, confidentiality, and refusing to certify work not personally verified.

Project management for gas installations covers scope, programme, cost, risk register, stakeholder interface (utility, contractor, authority), and change control.

Design reviews and hazard studies (HAZOP, HAZID) are used to identify and mitigate risks at concept and detailed design stages.

Contract administration: specifications, method statements, and inspection and test plans (ITPs) define acceptance criteria and hold points for witness.

Continuing professional development and knowledge of current codes are professional obligations; outdated practice must not be used without justification.

Combustion efficiency is improved by matching excess air, maintaining clean heat exchangers, and using condensing appliances that recover latent heat (efficiency above 90% on gross calorific value).

Gross vs net calorific value: condensing appliances can exceed 100% of gross efficiency because latent heat of water vapour is recovered.

Hydrogen blending into natural gas networks reduces CO2 intensity per unit energy; blending limits depend on end-use appliances and network materials.

Biomethane (upgraded biogas) is a drop-in renewable gas; injection requires quality specification for CO2, H2S, water, and calorific value.

Carbon accounting for gas installations includes scope 1 (direct combustion), scope 2 (purchased electricity), and scope 3 (supply chain); efficiency measures reduce scope 1.

Demand-side measures: appliance selection (high-efficiency boilers, heat pumps for low-grade heat), controls, insulation, and heat recovery.

Environmental impacts of gas projects include NOx and CO emissions, land use for LNG/LPG facilities, and lifecycle greenhouse gas assessment; mitigation is part of design.