● ●
● ●
Evacuation: A vacuum is created by removing
air from within the pipeline. During this phase, the
pressure in the pipeline is reduced to a level where
the ambient temperature of the pipeline will cause
the free water to boil and change to water vapour.
This pressure level corresponds to the saturated
vapour pressure (SVP). A vacuum is exerted on the
system, which removes the air contained in the
system. When the vacuum is sufficient, any free
water in the system will begin to boil.
Evaporation: Once the saturated vapour
pressure has been reached then evaporation of
the free water into water vapour will commence.
The vacuum system is held constant until the
water has completely evaporated. This is known
as the "hold" phase. This phase may take a long
time to complete depending on the amount of
water to be evaporated, the size of the vacuum
equipment, and the ambient temperature of the
pipeline. The pipeline pressure will show constant
Commercial specification of the gas to be
produced; and
Completion of pipeline drying which may
also be done during commissioning.
METHOD 1 - Natural gas injection under
pipelines vacuum conditions without the
use of pigs
The vacuum-drying process, similar to the air-
drying process, physically removes all the water
from the pipeline. In air drying it is blown out and
in vacuum drying it is extracted by reducing the
pressure in the pipeline to the saturated vapour
pressure (SVP) for the ambient temperature, which
will cause the water to vaporise. It can then be
removed from the pipeline as a gas via a vacuum
pump. The vacuum drying is divided into three
separate phases:
Diagram 5 : Method 1 - Vacuum Drying Operation Diagram &
Tables
SVP (mbara) Vapour Density (g/m³)
-45
-35
-30
-25
-20
-15
-10
-5
0
5
10
15
20
25
30 0.128
0.223
0.308
0.632
0.1043
1.652
2.597
4.015
6.108
8.719
12.27
17.04
23.37
31.67
42.43 0.119
0.203
0.339
0.552
0.884
1.387
2.139
3.246
4.847
6.797
9.399
12.83
17.30
23.05
30.38
Water Vapour Pressure & Vapour Density
(Pipeline Rules of Thumb Handbook, E.W Mc Allister)
Open
T (°C)
Vacuum Drying Opera1on
(Pipeline Rules of Thumb Handbook , E.W Mc Allister)
Pf Desired Target Dryness
(-20 deg C dewpoint)
1013
1000
800
600
400
200
100
80
60
40
30
23.4
20
15
12
10
8
6.11
4
2
1
800
600
400
200
100
80
40
10
4
Measuring Range & Vacuum Pressure
Range
760
750
600
450
300
150
75
60
45
30
22.5
17.53
15
12
9
7.5
6
4.58
3
1.5
0.75
600
450
300
150
75
60
30
7.5
3
H2O Boiling (°C)
100
99.6
93.5
85.5
75
60.1
45.7
41
36.4
29
24
20
17.2
14
9.7
7
3.8
0.0
- 5
-13
- 20.3
- 22.7
- 25.4
- 29.3
- 36
- 42
- 43.7
- 50
- 60.5
- 67
Boiling Point of Water at various Vacuum Levels
Diagram 5: Method 1 - Vacuum Drying Operation Diagram & Tables
45