Line Blowing or Line Clearing is a procedure commonly conducted onboard most parcel and chemical tankers either with air or nitrogen supplied at a pressure around 7 bar.
But is it an effective procedure? Is it a safe procedure? Nitrogen or Air?
Experiments in blowing of shore-pipelines show that many factors determine the effectiveness of such operations, but the line-diameter and line construction are by far the most limiting factors.
The greater the line diameter, the greater the ‘stratification’ of the water/air interface.
Stratification is defined as the clear distinction that has been observed visually between the liquid in the lower part of the pipe and the air in the upper part, with a nearly horizontal definite interface occurring between the liquid and air.
In small-diameter pipelines, an air-water interaction (water replaced by air) was observed with a well-defined planar air-water front with the trailing edge of the moving water column leaving a stratified flow behind it.
However, in larger diameter lines, this assumption of this planar air/water front becomes invalid, as the air is now largely confined to the upper part of the pip cross-section with the water occupying the lower part.
In emptying experiments, the air flow on top of the free-surface water continually accelerated until the stratified flow in the line experienced destabilising conditions with the formation of incidental water slugs.
[Source and extracts from: Emptying of Large-Scale Pipeline by Pressurized Air published in the Journal of Hydraulic Engineering December 2012]
A visual example of a simple experiment in line-clearing over a vertical incline can be viewed here:
Is it safe? See the article on Nitrogen or Air Blowing.