How much nitrogen does it take to pig a pipeline?

Step one: the line volume

The geometric volume is V = A × L, with A = π/4 × ID². A 12-inch standard wall line, 10 km long, holds 0.0730 m² × 10,000 m = 730 m³. That number is the same whatever you put in it.

Step two: convert to what the supplier sells

Nitrogen is bought in standard cubic metres — the volume it would occupy at 15 °C and 1.01325 bar. Gas in a pressurised line is compressed, so one line volume costs many standard volumes:

V_std = V_line × (P_abs / P_std) × (T_std / T) × (1 / Z)

For nitrogen at normal pigging pressures Z is within a percent or two of 1, so the pressure ratio does nearly all the work. That same 12-inch line, filled at 8 bar g, holds about 6,500 Sm³.

What you hold is not what you consume

Two different numbers get called "the nitrogen quantity", and mixing them up is how jobs run short:

• Peak inventory — the gas in the line at any moment. That is the table above. It decides the bottle bank or the size of the liquid nitrogen storage.
• Total consumed — everything that passes through the line over the run. If the receiving end vents while the pig travels, you pay for roughly a line volume at run pressure for each pass, plus purge and hold. A dewatering run followed by a drying run followed by a nitrogen packing hold can be three to five times the single-fill figure.

If it comes as liquid

One litre of liquid nitrogen makes about 0.69 Nm³ of gas. A 20,000-litre road tanker is therefore roughly 13,800 Nm³ — a little over two fills of the 12-inch line above at 8 bar g. Vaporiser capacity, not tanker capacity, usually limits how fast you can deliver it, and a vaporiser that ices up delivers cold gas that reads as a flow shortfall.

When compressed air will do instead

Air is free; nitrogen is not. Air is normally acceptable for hydrotest dewatering and for cleaning runs on a line that has never been in hydrocarbon service and will be purged before commissioning. Nitrogen is used where an oxygen-free atmosphere is required — a line going straight into hydrocarbon service, a line being packed and left, or anywhere the specification calls for an inert hold. If the line will hold gas for weeks, dew point matters as much as purity: wet air in a carbon steel line starts corroding immediately.