From 5 to 25 m/s depending on pressure: where the Italian practice ladder comes from, and why the «true» velocity is not the standard flow's.
Gas has no water-hammer problem, but velocity remains a design constraint for three concrete reasons:
Distribution practice (UNI 9165 tradition) adopts limits that grow with the operating pressure — the more the gas is compressed, the more slowly it moves at equal standard flow, and the more «room» there is before the unwanted phenomena:
| Operating pressure (gauge) | Maximum velocity |
|---|---|
| above 3.5 bar | 25 m/s |
| 1.5 – 3.5 bar | 20 m/s |
| 1.0 – 1.5 bar | 15 m/s |
| 0.04 – 1.0 bar | 10 m/s |
| up to 0.04 bar (low pressure) | 5 m/s |
They are good-practice limits, not statutory ones: a project may justify its own thresholds (which is why software must keep them customisable, not hard-wired). Their correspondence with the pressure bands of the pipeline classes makes it natural to check them together.
Gas network flows are expressed in Sm³/h — standard cubic metres, referred to atmospheric pressure. But in the pipe the gas is compressed: for the real velocity the standard flow must be expanded to operating conditions,
with z the real-gas compressibility factor. The difference is not academic: at 4 bar gauge the same standard flow occupies about one fifth of the volume, so the real velocity is about five times lower than the one naively computed on Qstd. Checking «at standard flow» fails pipes that actually pass — or, worse, passes low-pressure pipes that do not.
A numerical reference from the validation dossier: in Case 1 (low pressure, DE63, 30 Sm³/h) the operating velocity is 3.92 m/s, within the 5 m/s limit — barely 2% away from the standard one, because at 25 mbar the expansion is minimal. It is climbing in pressure that the gap becomes decisive.
For pressurised mains there is also a criterion independent of the practice ladder: the API RP 14E erosional velocity, ve = C/√ρ with ρ the gas density at operating conditions and C a coefficient chosen by the designer (100 for continuous service in the classic formulation, higher values for clean, non-corrosive gas). It is an additional check typical of trunk mains, where the operating density is high; Gasnetics offers it as an optional control alongside the per-class ladder.
For three practical reasons: noise (whistling in pipes and pressure-reduction stations), erosion and the entrainment of dust and particles that damage regulation and metering equipment, and network stability against demand swings. The limits grow with pressure because compressed gas, at equal standard flow, moves more slowly.
Italian practice (UNI 9165 tradition) adopts: 25 m/s above 3.5 bar, 20 m/s between 1.5 and 3.5 bar, 15 m/s between 1 and 1.5 bar, 10 m/s between 0.04 and 1 bar, 5 m/s in low pressure. They are good-practice design limits, customisable per project.
By expanding the standard flow to operating conditions: Q_eff = Q_std · (P_std/P) · (T/T_std) · z, then dividing by the pipe's cross-section. At 4 bar gauge the same standard flow occupies about one fifth of the volume, so the real velocity is about five times lower than the one computed on the standard flow.