Pipeline drying involves reducing the pressure inside pipelines to below the vapor pressure of water using vacuum equipment. This causes water to evaporate from pipelines over several phases. First, pressure is reduced to allow evaporation of free water. Then pressure is further reduced to remove water vapor until the dew point reaches the required dryness level, typically -20°C for gas pipelines. Dryness is tested by purging the line with dry gas and checking that the dew points at both ends equalize at the target value. Vacuum drying ensures pipelines are sufficiently dry for safe operation and to prevent corrosion.

1. Pipeline Drying
• Pipeline must be dried in order for the
delivered product to meet the moisture
specifications.
• Natural gas pipelines are generally dried to
lesser extent to prevent hydrate formation.
• The natural gas industry specifies dryness in
pounds of water per million standard cubic
feet (mscf) of gas.

2. Dew Point
• Temperature at which water vapor begins to
condense out of a gas at atmospheric pressure.
• A high relative humidity indicates that the dew
point is closer to the current air temperature.
• Relative humidity of 100% indicates the dew
point is equal to the current temperature and
that the air is maximally saturated with water.
• When the dew point remains constant and
temperature increases, relative humidity
decreases.

3. Vacuum Drying
• Vacuum Drying is a process in which drying is
carried out at reduced pressure, which lowers the
heat required for rapid drying.
• Corrosion is generally inhibited at relative
humidity(R.H.) levels below 30%, but in presence
of hydroscopic dirt corrosion can occur at R.H.
levels of 20%.
• Therefore, systems should be thoroughly drained
and then vacuum dried to lower than 20% R.H.
• Vacuum Drying consists of 3 phases: Evacuation,
Evaporation and Final Drying.

4. Pressure

5. Phase I- Evacuation
• Pressure in the pipeline is reduced to a level
where evaporation of water starts at ambient
temperature.
• This pressure is similar to the saturated vapor
pressure of free water in pipeline, which depends
upon the ambient temperature of pipeline.
• This pressure is calculated in advance but varies
onsite ,which is noted from the plot of pressure
against time.
• A leak test is performed by stopping the vacuum
equipment for approximately 4 hours, to rectify
any leaks on flanges, hoses or fittings.

6. Phase II-Evaporation
• Once Standard Vapor Pressure is achieved, evaporation starts.
• Pressure is kept constant ,using the vacuum equipment, until all
free water evaporates.
• Evaporation may take several days depending on:
– Amount of water to be evaporated
– Ambient temperature of pipeline
– Size of vacuum equipment
• End of evaporation will be indicated by a decrease in the pressure.
• To ensure if all the free water in the pipeline is evaporated, Soak
Test is performed.
• Vacuum equipment is isolated for at least 12 hours, and any
changes in the pressure are recorded.
• Pressure will remain constant , if all free water has evaporated.

7. Phase III- Final Drying
• To achieve required dryness level, water vapor
must be removed.
• Further Reducing the pressure in the pipeline,
which has a effect of drawing water vapor out
of the pipeline through vacuum equipment.
• The slope of the final drying line must be
observed , if it follows the calculated value.
• A shallower slope indicates the presence of
free water still remaining in the pipeline.

8. Dryness?
• The dryness of a pipeline is measured in terms of dew point which is
measured using an instrument called a mirror hygrometer.
• The water vapor is passed across a polished surface that is slowly cooled
until dew forms.
• The temperature at which the dew forms is the dew point of the water
vapor and is normally expressed in degrees centigrade.
• The drier the air, the lower the temperature at which dew will form.
• In terms of a pipeline being vacuum dried, the lower the pressure in the
pipeline, the lower the dew point will be.
• For example, at a pressure level of 0.26 kPa, the equivalent dew point of
the pipeline would be -10 degree C. If the pressure were further reduced
to 0.104kPa, then the dew point would be -20 degree C.
• For gas pipelines a dew point level of -20 degree C is generally considered
to be adequate and the 0.10 kPa pressure level required to achieve this
dew point is readily attainable using the portable vacuum equipment

10. • To prove the dryness of the pipeline, a dry gas
purge using atmospheric air or nitrogen is carried
out.
• The air is allowed to enter the pipeline through a
valve at the end remote from the vacuum
equipment ,until the pressure has risen to the
SVP equivalent of the target dew point.
• Once this pressure level has been reached, the
vacuum equipment is started and that pressure
level maintained.
• This has the effect of drawing gas through the
pipeline under vacuum at a relatively constant
dew point equal to the final dew point required.

11. Water Vapor Pressure Table
T SVP(kPa) Vapor Density
( Degrees C) (mb x 10^-1) (gm m^3)
-50 0.0039( .039 mb) 0.038
-45 0.0128 0.119
-35 0.0223 0.203
-30 0.0308 0.339
-25 0.0632 0.552
-20 0.1043 0.884
-15 0.1652 1.387
-10 0.2597 2.139
-5 0.4015 3.246
0 0.6108 4.847
5 0.8719 6.797
10 1.2270 9.399
15 1.7040 12.830
20 2.3370 17.300
25 3.1670 23.050
30 4.2430 30.380

12. • The purge gas, will reach the vacuum equipment and be pulled
through it.
• The dew point at both ends of the pipeline is carefully monitored
and compared.
• If there is no free water remaining in the pipeline then the dew
point at the vacuum equipment end will be the same as the dew
point at the remote end.
• If there is any free water present then the dry air passing through
the pipeline under vacuum will absorb the water hygroscopically.
• The dry gas purge operation must then continue to remove the
remaining free water until the dew points at both ends are equal, at
which time purging is discounted.
• The pipeline has now been vacuum dried to the required dew point
level, and the dryness proved.
• Once the dryness has been attained and proved, the pipeline is
ready for commissioning.
• It is possible to introduce the gas directly into the vacuum or to
relieve the vacuum using dry nitrogen