Gas supply systems and network design
What this domain tests
This domain examines how you plan, size and safeguard town gas and LPG distribution systems from the city gate or bulk installation to the consumer meter. The exam expects pressure-tier thinking, demand diversity, hydraulic checks and asset-integrity awareness, not just pipe sizing.
Pressure tiers and topology
- Town gas networks are arranged in tiers: transmission or high-pressure mains, district governor stations, medium-pressure distribution and low-pressure reticulation. Appliance nominal pressures in Hong Kong are commonly about 1.0 kPa for town gas and 2.9 kPa for LPG vapour.
- Governors must maintain outlet pressure within the code and appliance tolerance under varying inlet pressure and demand; lock-up and creep are design checks.
- LPG-air systems are a separate supply route and must not be treated as interchangeable with town gas without checking Wobbe index and flame performance.
Demand, diversity and hydraulics
Connected load is the arithmetic sum of appliance inputs. Design peak demand applies an after-diversity factor, usually derived from utility records, and adds an allowance for future load. Use a design peak (for example a 1-in-20 peak-hour condition), not an annual average. Hydraulic checks use Darcy-Weisbach or the utility's simplified low-pressure equations, with velocity limits to control noise and pressure loss. Materials include PE80/PE100 for buried mains, and coated steel with cathodic protection where pressures or crossings demand it. Valve spacing, isolation philosophy and telemetry belong in the design from the start.
Worked example
A new estate has 200 flats, each with a connected town gas load of 1.5 m3/h. The utility's after-diversity factor is 0.35, and 10% future load is allowed. Diversified flow = 200 x 1.5 x 0.35 x 1.10 = 115.5 m3/h. At 115.5 m3/h the volumetric flow is 0.0321 m3/s. In a 150 mm PE main (area 0.0177 m2) velocity is 1.8 m/s; in a 100 mm main (area 0.00785 m2) it is 4.1 m/s, close to the usual low-pressure guideline and leaving little growth margin. Select 150 mm, then confirm pressure at the furthest governor and at the critical meter.
Traps that cost marks
- Summing connected loads without diversity and producing an oversized, low-velocity network.
- Sizing only for today's load and ignoring future connections, or ignoring the critical node.
- Mixing LPG and town gas calorific values or Wobbe indices in the same calculation.
- Forgetting governor lock-up, minimum inlet pressure, or the effect of a failed regulator on downstream pressure.
- Treating cathodic protection and coating as optional rather than part of the corrosion-management design.