How to size a gas distribution network

1. The input data

Sizing a gas distribution network starts from four families of data:

• Customer demands — the flow required at every delivery point, in Sm³/h (or in kW / kg/h, converted with the gas heating value). Where the project calls for them, a contemporaneity factor and a future-reserve factor apply.
• The supply pressure — the gauge pressure imposed at the source (a city-gate station, a pressure-reduction group, an LPG tank).
• The layout — real pipe lengths along the streets and ground elevations: elevation differences bring in the altitude correction.
• The distributed gas — natural gas, LPG, biomethane or hydrogen blends: composition determines relative density, viscosity and compressibility, which enter the calculation directly.

2. The flow law

The heart of the calculation is the relation between flow and pressure drop for each pipe. The general form, valid from low pressure to transmission mains, is the steady isothermal flow equation for compressible gas:

where R is the pipe's base resistance (length L, internal diameter D — which weighs with the fifth power —, relative density s, temperature T), f is the Darcy friction factor from the Colebrook-White equation on the pipe's real roughness, z the gas compressibility and Q the standard flow. Italian and Spanish practice long used the Renouard formula, an empirical approximation valid in a precise domain; the general law carries no such constraint. Why pressure appears «squared» is explained in the guide on pressure drop.

On a branched network the pipe flows follow from the nodal balance; on a meshed network the split must be solved iteratively (Newton-Raphson nodal method) because each pipe's friction depends on the flow it carries.

3. The checks

A sizing is acceptable when every pipe and every customer respect two families of limits, tied to the pipeline class:

Maximum velocity

Italian distribution practice (UNI 9165 tradition) adopts a ladder by operating pressure band — details in the guide on gas velocity:

Minimum pressure at the customers

Every delivery point must receive the gas above a minimum pressure that depends on the network's class: in low pressure (7th class) Italian practice requires at least 18 mbar at delivery, because the service regulator and the appliances downstream still need to work.

4. Choosing the diameters

The criterion is economic and regulatory at once: for each pipe, choose the smallest commercial diameter that passes every check. «Commercial» is the key word: not a theoretical diameter, but a catalogue pipe (PE 100 SDR 11, steel…) with its real internal diameter and its roughness. The procedure is iterative by nature: changing one diameter changes the drops, which change the pressures, which can flip the checks on other pipes — by hand you proceed by trial, while an automatic solver repeats the calculate → check → substitute cycle to convergence.

5. A worked example

The simplest possible case, with real numbers (it is Case 1 of the validation dossier, developed digit by digit):

6. From the calculation to the report

Sizing ends with the calculation report: input data, methodology with references, full results, check outcomes and the bill of quantities. Gasnetics automates the whole cycle — map drawing, calculation, per-class checks, diameter selection from the catalogue and the PDF report — and every report includes an appendix that recomputes the critical path by hand, so every number stays verifiable.