Nitrogen and Drying
Nitrogen inventory, and doing the gas law properly
Nitrogen is ordered by the load and consumed by the cubic foot. The gap between those two units is where jobs run short.
The relation
Inventory is calculated with the real-gas law:
P V = Z n R T R = 10.7316 psia·ft³/(lb-mol·°R)
with pressure absolute, temperature in degrees Rankine, and Z the compressibility factor. Nitrogen has a molecular weight of 28.01 lb per lb-mol.
Why Z matters
At low pressure Z is close to unity and the ideal gas law is adequate. At the pressures used for packing, blanketing and pressure testing it is not.
Ignoring Z produces an estimate that is short, and it is short in the direction that stops work: you discover it when the last trailer is empty and the line is not yet at pressure.
Standard conditions, and the tariff trap
Volumes are quoted at standard conditions, commonly 14.7 psia and 60 °F. Some tariff and custody contexts use 14.73 psia.
The difference is small per unit and material across a large job. Confirm which basis a quotation uses before comparing two of them.
Building the estimate
- Line volume from internal area and length, using actual wall thicknesses
- The pressure the line must reach, absolute, not gauge
- Temperature at the conditions expected during the work, not annual average
- Z at that pressure and temperature
- Purge or sweep requirements on top of the static inventory
- A margin for losses, venting and the inevitable second attempt
Where it is calculated in practice
The same arithmetic sits in the reference section of this site and in the HydroTech Pocket Engineer, which carries the nitrogen volume and pipeline fluid volume calculations.
It is worked by hand on the engineering workshop, because engineers who have done it once by hand catch the errors a calculator will happily produce.
From Proper Pigging, the hydrostatic and nitrogen test field book, and the HydroTech certification courses built on it.