INGENIEUR
pressure throughout the evaporation process
while the vacuum pump remains in operation. The
end of the evaporation phase will be observed on
site by a noticeable decrease in pressure and the
start of next phase.
Final drying: Once the free water has been
converted into water vapour, the majority of it
must be removed from the pipeline in order to
reach the required dryness level. This is achieved
by reducing further the pressure in the pipeline
which will create the effect of drawing the water
vapour out of the pipeline through the vacuum
equipment. Obviously, the more water vapour
removed, the drier the pipeline will become.
The vacuum injection method is based on
the principle that natural gas does not produce a
flammable mixture with air at pressures below 67
mbar.
The effect of pressure on flammability limits
is shown in Diagram 6. At low pressures, natural
gas-air mixtures are not combustible. At high
pressures, the upper limit rises rapidly.
According to the flammability limits under
vacuum conditions, it could be possible to inject
natural gas into the pipeline without any risk
of ignition, provided that, during gas injection,
vacuum conditions are maintained at the interface
front all along the pipeline.
This method was opted for during initial
commissioning of SSGP as pipeline dryness and
purging could be achieved concurrently. The 36”
outlet diameter pipeline with a total length of
about 513 km, was vacuum dried separately in
four different sections using vacuum pumps. At
the end of drying, natural gas was introduced
straight into the vacuumed pipeline.
Diagram 6: Effect of Pressure on Limit of
Flammability of Natural Gas-Air Mixture
<1 psia (67 mbara) Natural Gas-Air
Mixture does not enter flammable area
Diagram 6: Effect of Pressure on Limit of Flammability of Natural Gas-Air Mixture
6
46
VOL
VOL 73
55 JANUARY-MARCH
JUNE 2013 2018