Nitrogen and Drying

Four methods of pipeline drying compared

There is no universally best drying method. There is a method that suits this line, this residual water, this dew point criterion and this schedule.

What drying has to achieve

After dewatering, a line still holds water as films, droplets and pockets at low points. Drying removes what remains until the gas leaving the line is dry enough to meet an agreed criterion, usually expressed as an outlet dew point.

The criterion exists because residual water causes corrosion, hydrate formation and product contamination in service.

Dry air

Air passed through desiccant dryers and swept through the line. Widely available and relatively inexpensive.

Slower than nitrogen on lines with substantial residual water, and it introduces oxygen, which matters where the line will carry hydrocarbons or where corrosion is a concern.

Hot air

Heating raises the vapour-carrying capacity of the sweep gas substantially, so evaporation proceeds faster.

The trade-off is equipment, fuel and the practical limits of heating a long line. It works best on shorter runs where the heat is not all lost to the ground.

Vacuum

Reducing pressure lowers the boiling point of the residual water so that it evaporates at ambient temperature, and the vapour is drawn off.

Very effective on lines with limited residual water and good integrity; slow where the residual is large, and it demands a line that will hold vacuum, which not all will.

Nitrogen

An inert, dry sweep with no oxygen introduced. It suits lines that must not see oxygen, lines going straight into hydrocarbon service, and situations where the drying window is short.

Cost per unit volume is the constraint, and inventory is calculated with the real-gas law, PV = ZnRT, R = 10.7316 psia·ft³ per lb-mol per °R.

Choosing

Line must not see oxygen, or goes straight to hydrocarbon service → nitrogen
Large residual water and a long line → dry air, accepting the time
Short run, aggressive schedule → hot air
Tight dew point, limited residual, sound line → vacuum

In practice methods are combined: bulk removal one way, final approach to criterion another.

Standing noteThis is educational reference material, not engineering advice. Regulations are revised continually; the editions in force at the time of a project govern, and every project requires its own qualified engineering analysis and the operator’s governing specifications.

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From Proper Pigging, the hydrostatic and nitrogen test field book, and the HydroTech certification courses built on it.