COMPARISON OF LINE PACK CALCULATION USING ASPEN PLUS DYNAMICS AND BY USING MANUAL CALCULATIONS PREPARED BY: MUHAMMAD WAQAS MANZOOR PROCESS ENGINEER Contact:
[email protected]
LINE PACK
• The quantity of natural gas contained in a certain segment of a pipeline is known as line pack. • It is typically measured in MMSCF.
CALCULATION USING ASPEN PLUS DYNAMICS
• First of all, build a steady state simulation of the pipe line segment, in Aspen PLUS
SPECIFICATIONS OF PIPE SEGMENT IN ASPEN PLUS
SPECIFICATIONS OF PIPE SEGMENT IN ASPEN PLUS
SPECIFICATIONS OF VALVE ‘BV-1’ IN ASPEN PLUS
SPECIFICATIONS OF VALVE ‘BV-1’ IN ASPEN PLUS
SPECIFICATIONS OF VALVE ‘BV-2’ IN ASPEN PLUS
SPECIFICATIONS OF VALVE ‘BV-2’ IN ASPEN PLUS
SPECIFICATIONS OF INLET STREAM ‘NG-1’ IN ASPEN PLUS
EXPORTING SIMULATION TO ASPEN PLUS DYNAMICS
• After that, set ‘Input mode’ to ‘dynamic’ from ‘steady state’.
EXPORTING SIMULATION TO ASPEN PLUS DYNAMICS
• After that, click on ‘File Menu’, and select ‘Pressure Driven’ from ‘Update Aspen PLUS Dynamics Menu’.
EXPORTING SIMULATION TO ASPEN PLUS DYNAMICS
• These 03 files and a folder are created in the same folder where Aspen PLUS ‘.bkp‘ file was previously stored.
SNAPSHOT OF ASPEN PLUS DYNAMICS WINDOW
INITIAL BOUNDARY CONDITIONS IN ASPEN PLUS DYNAMICS
FINAL BOUNDARY CONDITIONS IN ASPEN PLUS DYNAMICS
CONFIGURATION OF PIPE SEGMENT IN ASPEN PLUS DYNAMICS
CALCULATION USING ASPEN PLUS DYNAMICS
CALCULATION OF LINE PACK FROM SIMULATION RESULTS OF ASPEN PLUS DYNAMICS •
Molar concentration of gas within the pipe segment = 0.160589 lbmol/cf
•
Actual volume of pipe segment = 3.14 x {(14.314/12)^2} /4 x 528000 ft = 590042.558 cf
•
Number of Moles of gas contained in the pipe segment = 0.160589 lbmol/cf x 590042.558 cf = 94754.344 lbmol
Calculation of Molar Volume at Standard Conditions: •
Molar Volume (Vmn) of any gas at normal conditions i.e. 0 C & 1 atm, is given by,
•
Vmn = 22.414 Nm3 / kgmole = 22.414 Nm3 x (1 ft /0.3048 m)^3 x 0.454 kgmol/lbmol
•
Vmn = 359.360 Ncf/lbmol
•
Molar volume (Vms) at standard conditions i.e. 60 F (15.55 C) and 14.7 psia (1 atm), is given by
•
359.360 cf x 14.7 psi / 492 F = Vms x 14.7 / 520 F
•
Or, Vms = 379.811 Scf/lbmol
Calculated Line Pack = 94754.344 lbmol x 379.811 Scf/lbmol = 35.988 MMSCF
EQUATIONS USED FOR MANUAL CALCULATION OF LINE PACK • Here Vb represents Line Pack, calculated for a pipe segment having an internal diameter D, length L, and containing natural gas at an average pressure and temperature of Pavg, and Tavg, respectively. • Tb, and Pb represent thr base temperature and base pressure, respectively.
Ref: ‘Gas Pipeline Hydraulics’ by E. Shashi Menon
CALCULATION OF COMPRESSIBILITY FACTOR
Ref: ‘Gas Pipeline Hydraulics’ by E. Shashi Menon
CALCULATION OF AVERAGE PRESSURE AND AVERAGE TEMPERATURE
• Here P1 and P2 are in absolute pressure units. • Average Temperature is typically calculated by taking arithmetic mean of inlet and out temperatures (in absolute units) of the pipe segment.
Ref: ‘Gas Pipeline Hydraulics’ by E. Shashi Menon
MANUAL CALCULATION OF LINE PACK •
Molecular Mass of gas used in Aspen PLUS simulation
= 16.0427 lb/lbmol
•
Specific gravity of gas
•
Pressure of gas at Inlet of pipe segment
= 898.093 psia
•
Pressure of gas at Outlet of pipe segment
= 702.895 psia
•
Average Pressure of gas across the pipe segment =
= 16.0427 / 29 = 0.5531
898.093 + 702.895 −
. .
×
. .
= 804.460 psia = 789.76 psig (1 atm = 14.7 psia) •
Temperature of gas at Inlet of pipe segment
= 74.1135 F = 534.1135 R
•
Temperature of gas at Outlet of pipe segment
= 63.44 F = 523.44 R
•
Average Temperature of gas across the pipe segment
=
•
Average compressibility of gas across the pipe segment
=
.
.
= 528.776 R
.
× .
= 0.90807
.
× .
× .
MANUAL CALCULATION OF LINE PACK •
Base Temperature
= 60 F = 520 R
•
Base Pressure
= 14.7 psia
•
Internal Diameter of pipe segment
= 14.314 inches
•
Total Length of pipe segment
= 100 miles = 528000 ft
•
Line Pack (Vb) calculated using CNGA* method is given by,
•
Vb = 28.798 ×
•
Vb = 34.969 MMSCF
•
Line pack calculated using Aspen PLUS Dynamics = 35.988 MMSCF
•
The calculated Line Pack ‘Vb’ closely matches with the line pack calculated using Aspen PLUS Dynamics.
•
The difference between the two, may be attributed to the difference in physical properties calculation methods and computational methods in Aspen PLUS Dynamics. However, results of Aspen PLUS Dynamics are more accurate.
.
×
. .
×
* California Natural Gas Association
.
× 14.314 × 100
