Classification by maximum operating pressure is the backbone of the Italian pipeline regulation: here are the seven classes — the «specie» — and what they mean for network calculation.
The Italian technical rule classifies gas pipelines by maximum operating pressure into seven classes (Italian: specie): the first three cover transmission and large adduction, and from the fourth to the seventh you descend through medium pressure down to the low pressure of urban distribution. The decree ties construction requirements to the class (materials, burial depth, distances, testing); design practice also ties to it the two limits that govern the hydraulic calculation: the maximum gas velocity and the minimum pressure to guarantee.
| Class | Operating pressure (gauge) | Max velocity (practice) | Minimum pressure to guarantee |
|---|---|---|---|
| 1st class | above 24 bar | 25 m/s | 24 bar |
| 2nd class | 12 – 24 bar | 25 m/s | 12 bar |
| 3rd class | 5 – 12 bar | 25 m/s | 5 bar |
| 4th class | 1.5 – 5 bar | 20 m/s | 1.5 bar |
| 5th class | 0.5 – 1.5 bar | 15 m/s | 0.5 bar |
| 6th class | 0.04 – 0.5 bar | 10 m/s | 0.04 bar |
| 7th class (low pressure) | up to 0.04 bar | 5 m/s | 18 mbar |
Two reading notes. The velocity limits are the ladder of distribution practice (UNI 9165 tradition), applied by pressure band — the reasoning behind those values is in the guide on gas velocity. And the minimum pressure to guarantee coincides, for classes 1st to 6th, with the lower bound of the band: if the network drops below it, it is effectively operating in the class below, with requirements different from those it was built for.
In low pressure the band's lower bound would be zero — but a customer at nearly no pressure is not served at all. Practice therefore requires guaranteeing at least 18 mbar gauge at the delivery point: downstream of the meter, the service regulator and the appliances (which typically want 20 mbar nominal at the burner, net of the internal installation's losses) still have to work. In most urban low-pressure networks this is the constraint that decides the diameters, well before velocity does.
In a well-set-up calculation the class is not a label: it is the source of the checks. For every pipe the velocity is checked against its pressure band's limit; for every customer the delivery pressure is checked against the declared class minimum. A sizing is acceptable only when both families of checks pass across the whole network — which is why the limits belong inside the calculation engine, not on a separate sheet.
Gasnetics applies this table automatically to the class declared by the designer, highlights non-conformities on the map and reports them; thresholds stay customisable per project, and designers working outside the Italian framework can define their own reference profile.
They are the Italian classification of pipelines by maximum operating pressure, from the DM 16-17 April 2008 technical rule: from the 1st class (above 24 bar) down to the 7th class (up to 0.04 bar, the low pressure of urban distribution). Construction requirements are tied to the class, and in calculation practice so are the velocity limits and the minimum pressures to guarantee.
Italian design practice requires at least 18 mbar gauge at the delivery point in low pressure: the service regulator and the appliances downstream still need to work. The other classes require staying above the lower bound of their own pressure band.
Italian practice (UNI 9165 tradition) adopts a ladder by operating pressure band: 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.