Short answer
Sizing commercial gas pipe means finding the smallest diameter that carries the required flow over the run length without exceeding the allowable pressure loss. You need the gas flow (m³/h) for each section, the effective length including fittings, and the pressure loss budget. For low pressure natural gas the Pole formula gives a good approximation: Q = 0.0071 × √(h × d⁵ ÷ (s × L)), with Q in m³/h, h pressure loss in mbar, d internal diameter in mm, s relative density (about 0.6) and L length in metres.
Step 1: flow per section
Convert each appliance's gross heat input to m³/h (gross kW ÷ about 10.6–10.9 kWh/m³ depending on calorific value). Add the flows downstream of each section, applying diversity where appliances genuinely will not run together.
Step 2: effective length
Measure the pipe length and add equivalent lengths for fittings — elbows, tees and valves each add resistance. A simple allowance used on small installations is to add around 0.5 m per elbow or tee for smaller sizes, more for large diameters; detailed design uses manufacturer or standard tables.
Step 3: check the size
Example: 40 m³/h over 30 m effective length with a 1 mbar budget. Try 50 mm (actual bore about 53 mm): Q = 0.0071 × √(1 × 53⁵ ÷ (0.6 × 30)) ≈ 0.0071 × √(418,195,493 ÷ 18) ≈ 0.0071 × 4,820 ≈ 34 m³/h — too small. Try 65 mm (bore about 69 mm): ≈ 0.0071 × √(1,564,000,000 ÷ 18) ≈ 66 m³/h — adequate.
Check against the meter and regulator
The pipe can only deliver what the meter and regulator can pass. Confirm the meter's rated capacity exceeds the diversified load, and that the meter regulator can hold outlet pressure at that flow.
Figures are typical for UK natural gas. The current edition of the relevant IGEM standard and the manufacturer's instructions always take precedence on site.
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