Village extension at the end of a low-pressure town gas district
A registered gas contractor is asked to connect 18 new village houses to the end of an existing low-pressure town gas district. Each house has a declared connected load of 2.5 m3/h, giving 45 m3/h. The existing 90 mm PE main is 600 m long from a district governor rated at 50 m3/h. With present customers, the evening peak is 34 m3/h and the pressure at the last meter falls to 0.9 kPa; the new appliances need 1.0 kPa at inlet. A no-flow reading at the same meter is 1.4 kPa. The network owner's hydraulic model shows the 90 mm main reaching about 18 m/s at that peak, and a 50 mm main that could be looped over part of the route. The developer wants all 18 houses connected before Lunar New Year and asks whether a larger governor alone will solve the problem.
1. Calculate a defensible diversified demand for the extension and judge whether the existing governor and main can meet it. State your assumptions and the additional data you need.
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Connected load is 18 × 2.5 = 45 m3/h, but that is not the design peak. For 18 domestic cooking connections the coincidence factor is unlikely to be 1.0; a defensible first estimate is 0.4–0.6, i.e. 18–27 m3/h, using the network owner's diversity curve rather than a generic factor. Adding the existing evening peak of 34 m3/h gives 52–61 m3/h. The 50 m3/h governor is therefore undersized even at the low end, and the 90 mm main is already at about 18 m/s with 34 m3/h, so the extension will make velocity and pressure drop worse. The measured 0.9 kPa at the critical meter is below the 1.0 kPa appliance minimum; a no-flow reading of 1.4 kPa shows the problem is predominantly distribution pressure loss, not simply a low governor set point. A larger governor alone cannot fix main friction and velocity. Before sizing, obtain the calibrated hydraulic model, actual internal diameter and condition of the main, the governor's lock-up and capacity curve, and a logged 24-hour pressure/flow profile at the critical point. Then test the extension at the peak case, not average demand.
2. Compare reinforcement options and recommend a staged plan that does not compromise the 1.0 kPa appliance minimum.
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Options should be compared on critical-point pressure, velocity, outage risk and time. (1) Replace the governor with a higher-capacity unit and raise its outlet set point only within the safe operating envelope: this may restore pressure at low flow but will not remove the 18 m/s velocity or friction loss at peak. (2) Loop the 90 mm main with the available 50 mm main over a substantial part of the 600 m route. This splits flow, reduces velocity and is likely the most effective hydraulic fix, but the 50 mm leg must be modelled and pressure-tested; a short loop may merely move the bottleneck. (3) Install an intermediate district governor or pressure-management point closer to the new houses. This can hold the local pressure, but requires a safe location, venting, access and isolation, and may not help if the upstream main is already capacity-limited. A staged connection is prudent: complete the loop and governor upgrade first, then connect in tranches while logging pressure at the last existing meter and the furthest new meter during the evening peak. Do not accept the Lunar New Year deadline as a reason to connect all 18 houses before the critical-point pressure is proven above 1.0 kPa with a margin for gauge error and demand variation.
3. What commissioning tests and records should be required before first gas is introduced?
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Before first gas, require: a strength test and tightness test of the new PE and steel sections at the specified pressures and durations; purge to a confirmed combustible gas reading, with purge points controlled and vented safely; governor set-point, lock-up and capacity checks; and a pressure log at the critical points during a controlled peak simulation. Check that service valves, riser isolation, meter connections, cross-bonding and identification are complete. Because this is a live low-pressure district, the switching sequence must avoid over-pressurising existing customers; notify affected customers, use experienced personnel, and keep gas detection and ventilation at excavations and governor vents. Record as-built drawings, test certificates, purge records, governor settings, and the hydraulic model update. The network owner and, where required, the Gas Standards Office should receive the notifications and records before gas is introduced. Any pressure below the appliance minimum at the critical meter is a stop condition, not a commissioning tolerance.