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by Sissel Eng
DNV Technology Services
Dynamic depressurisation calculations
LNG regasification unit
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Agenda
Project background Presentation of results
HYSYS dynamic depressuring unit
Service provided by DNV TS
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Standards and Codes evaluated for LNG regasifications systems
NFPA 59A: Standard for the Production, Storage and Handling of
Liquefied Natural Gas EN 1473: Installation and equipment for liquefied natural gas
design of onshore installations
IGC: International Code for the Construction and Equipment forShips Carrying Liquefied Gases in bulk Code (Gas Code)
API RP 520/521/14C NORSOK
Relevant ISO standards
DNV rules and offshore standards
Other class societies: ABS, Lloyds SIGGTO LNG Operation in Port Areas
IP Guideline/Scandpower guideline
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Background
Compare API methodology with Scandpower Guideline
in general
Investigate thermal effects during depressuring of LNG
processes
Investigate dynamic depressuring utility available inHYSYS version 3.4
Establish a procedure for performing depressuring
calculations in accordance with NORSOK, ISO 13702,
API RP 520, PED
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Typical regasification unit
Two stage heating system
Capacity of one skid: 50-210 tons LNG per hour
Length, width, height: 11 x 4 x 8 meters
Operating pressure: 40 to 130 bara
Locked-in volume approximately 1 ton
Initial liquid inventory, varied from 0 to 100%
No insulation
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Comparison heat absorption models
API heat absorption equation per area (API fire mode)
Heat transfer per area, taken into account radiation,
convection (Stephan-Boltzman fire mode)
4
,,
4 )())(( tTtTThTq OSSOSfrfS +=
82,0000,34 AFq =
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Agenda
Project background Presentation of results
HYSYS dynamic depressuring unit
Service provided by DNV TS
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Comparison API and Stephan-Boltzman
Pressure profile
0
2000
4000
6000
8000
10000
12000
14000
16000
0 200 400 600 800
Time [s]
P
re
s
s
ur
e
k
P
a
API
Stephan-Boltzman
Pressure profile versustime plotted
Initial pressure 60 bara,and 60 0C
Initially 50% liquid filled
Depressuring orificeconstant throughoutsimulations
Graph shows larger
evaporation rate withStephan-Boltzman firemode
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Comparison API and Stephan-Boltzman
Vapor temperatureversus time plotted
S-B fire mode shows
higher temperatures
Which model is correct?
Bulk vapour temperature
-100
0
100
200
300
400
500
600
700
0 200 400 600 800
Time [s]
APIStephan-Boltzman
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S-B compared with experimental values: vapour wall temperature
0
200
400
600
800
1000
1200
0 200 400 600 800 1000
Time (sec)
Walltemperature(degC)
Calculated
Experimental
Experimental values presented by NH/Sintef at FABIG 2003
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Results: Thermal effects
Fire mode selected: StephanBoltzman
Heat input according toNORSOK fire
Orifice sized for colddepressuring. Down to 6.9barg in 15 minutes. Orificesize kept constant throughsimulations
Initial pressure 60 bara, initialtemperature -60 0C
Liquid level varied from 0, 25,50, 75 and to 100% initiallyliquid filled
0
5000
10000
15000
20000
0 200 400 600 800
Time [s]
Pressure[kPa]
@ 0% init liq vol@ 50% init liq vol
@ 100% init liq vol
0
100
200
300
400
500
600
0 200 400 600 800
Time [s]
WallTemperature[degC]
Vap Wall T @ 0% init liq vol
Vap Wall T @ 50% init liq vol
Vap Wall T @ 100% init liq vol
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Results: other parameters reported by Hysys
Remaining mass in vessel
0
100
200
300
400
500
600
700
800
900
0 200 400 600 800
Time [s]
Rem
aining
m
ass[kg]
Vapour mass
Liquid mass
Mass flow out of valveSB Fire Mode
0
500
1000
1500
2000
2500
3000
3500
0 200 400 600 800
Time [s]
MassFlowrate[kg/h]
@ 0% init liq vol@ 50% init liq vol
@ 100% init liq vol
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Agenda
Project background Presentation of results
HYSYS dynamic depressuring unit
Service provided by DNV TS
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HYSYS dynamic depressuring utility
Commercially available
Rigorous thermodynamic
Dynamic depressuringsimulation
Often used for steady
state process simulations
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Services provided by DNV TS
Procedure developed for detailed depressuring
calculations
Utilizing a well established simulation tool
Competence within material data (UTS)
Competence within piping stress
Evaluation of results: risk analysis and consequences
of possible rupture
Evaluation of results: ESD S/D logic and sectioning
philosophy
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