INGENIEUR
Diagram 7: Method 2 - Natural gas injection as propellant of a pig train confining a nitrogen batch
been filled with the inert gas and the specified
end-point reached.
To minimise mixing, the purge velocity in
the pipeline should be high enough to prevent
stratified flow while creating turbulent flow.
This method was opted for sectional pipeline
re-commissioning post sectional repair as no
pigging facility was available.
3.2 - Inert purge using inert slug to separate
natural gas and air
Commissioning a natural gas pipeline includes
displacing the residual dry air used during pre-
commissioning for dewatering and drying. This
must occur before gas is introduced to begin
normal transport.
Purging the air is carried out by using a
nitrogen batch sized to avoid mixing air and
natural gas. Purging must avoid formation of
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a flammable air-gas mixture at the interface
between the two.
Formation of flammable mixtures during
purging can be prevented with the inert gas
without having to fill the entire length of the
isolated section of pipeline with it.
This is accomplished by maintaining a quantity
of inert gas known as a slug between the air
and combustible gas while the two are being
interchanged.
The slug of inert gas travels through the pipe as
a separate mass preventing the mixing of air and
combustible gas. The slug must be long enough
to allow for shortening because of mixing with air
on one end of the slug and combustible gas at the
other end.
The diluting of a gas-air mixture with inert
components such as nitrogen and carbon dioxide
changes the limits of flammability. Both the lower