Nitrogen and Drying
Purging, packing and blanketing: three different nitrogen jobs
Three operations, one gas, three completely different acceptance criteria. Conflating them is how a line ends up nominally inerted and actually not.
Purging
Purging displaces one atmosphere with another — air out and nitrogen in before hydrocarbon introduction, or hydrocarbon out and nitrogen in before entry or maintenance.
Success is measured by composition at the outlet, sampled, not assumed. The controlling variable is velocity, because too slow a purge stratifies and stops displacing.
Packing
Packing fills a line or section with nitrogen to a working pressure, usually to hold it in a known state, to support a pressure test, or to keep it ready for service.
Success is a pressure held over time. Inventory is calculated with the real-gas law, and the volumes at pressure are considerably larger than an ideal-gas estimate suggests.
Blanketing
Blanketing maintains a low positive nitrogen pressure to keep air and moisture out of a line or vessel that is otherwise idle.
Success is continuity. A blanket that lapses because nobody was topping it up has done nothing, and a line that has drawn air back in through a lapsed blanket has to be re-purged.
Why the distinction matters commercially
The three jobs have different gas consumption profiles, different durations and different equipment on site. A scope that says only “nitrogen services” leaves all three ambiguous, and the ambiguity surfaces as a variation.
- Purging: high rate, short duration, sampling equipment
- Packing: high volume at pressure, inventory-driven
- Blanketing: low rate, long duration, monitoring and top-up
Specify it properly
Say which operation, at what pressure, to what acceptance criterion, measured how, for how long, and who confirms it. Six clauses that prevent most of the arguments this work produces.
From Proper Pigging, the hydrostatic and nitrogen test field book, and the HydroTech certification courses built on it.