Diagram 8: Method 3.2 - Limit of flammability of fuel gas-air mixture when diluted with various
percentages of nitrogen and carbon dioxide
and higher limits of flammability are raised by
addition of the diluents as shown in Diagram 8.
Purging of pre-commissioning dry air from
natural gas pipelines demonstrates the reliability
of a simple nitrogen-slug shortening calculation
model in estimating the seal volume requirements
and the feasibility of using an inert slug when
purging large-diameter, long transmission
pipelines.
A slow purge of less than the minimum
allowable purge velocity may permit excessive
mixing and stratification of air and combustible
gas and should be avoided due to the potential for
ignition from solid particles inside the pipe scale.
The greater velocity and greater turbulence result
in a lower chance rate of creating a long section of
combustible gas-air mixture.
To minimise mixing, the purge velocity in
the pipeline should be high enough to limit
stratification of the gases and create turbulent
flow at the interface of the gas and air.
The initial length of the slug is dependent on
the length and size of the pipeline and the velocity
of the slug within the pipeline.
Low-pressure operation maintains a steady
flow at higher velocities. Low pressure purging
with hydrocarbon gas also helps to increase drying
efficiency.
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