Characterization of the Reliquefaction Systems installed ...
Transcript of Characterization of the Reliquefaction Systems installed ...
Zeszyty Naukowe 28(100) z. 1 83
Scientific Journals Zeszyty Naukowe Maritime University of Szczecin Akademia Morska w Szczecinie
2011, 28(100) z. 1 pp. 83–87 2011, 28(100) z. 1 s. 83–87
Characterization of the Reliquefaction Systems installed on board of the LNG ships
Charakterystyka Systemów Skraplania Gazu BOG stosowanych na statkach do przewozu skroplonego gazu LNG
Marek Matyszczak, Leszek Kaszycki
Maritime University of Szczecin, Institut of Marine Electrical Engineering and Vessel Automation Akademia Morska w Szczecinie, Instytut Elektrotechniki i Automatyki Okrętowej 70-500 Szczecin, ul. Wały Chrobrego 1/2, e-mail: [email protected], [email protected]
Key words: gas LNG, gas BOG, BOG reliquefaction systems, centrifugal compressor, expander, com-
pander, QMAX, QFLEX
Abstract The article refers to BOG reliquefaction plants installed on the new generation LNG ships board.
The construction, principle of operation and properties of the Hamworthy reliquefaction systems MARK I
and MARK III were described. The layout diagrams and the process flow charts of these systems were
enclosed. The construction comparisons of both systems were carried out and technology development
advantages of the reliquefaction systems were presented.
Słowa kluczowe: gaz LNG, gaz BOG, systemy skraplania gazu BOG, sprężarka wirowa, rozprężarka
turbinowa, compander, QMAX, QFLEX
Abstrakt Artykuł dotyczy systemów skraplania gazu BOG stosowanych na najnowszych statkach LNG. Opisano
w nim budowę i zasadę działania dwóch systemów skraplania MARK I i MARK III firmy HAMWORTHY.
Zamieszczono schematy funkcjonalne tych systemów. Scharakteryzowano i porównano konstrukcje i wła-
ściwości obu systemów.
Itroduction
One from most important objective of the safety
and tank management systems is to control the car-
go tank pressure. Both states: the overpressure and
vacuum conditions are very danger for the con-
struction of the LNG ships. In spite of the advanced
technology of the cargo tanks insulation is provid-
ed, it is not possible to quite eliminate the partial
vaporizing of LNG. For new series LNG ships with
membrane type cargo tanks, the boil-off gas
(BOG) generation rate is approximately 0.15% of
tank gross capacity per day (4000–6000 kg/h de-
pend on ship capacity). In order to protect ship con-
struction from over pressuring, the boil-off gas has
to be relieved from tanks and treated onboard. The
typical treatment is consuming the generated BOG
as the fuel of propulsion system. Dual fuel boilers
in steam turbine plant, are the typical consumers.
This solution allows to recover part of the fuel cost,
but some quantity of cargo is lost. The application
of the reliquefaction plant allows to use the slow
turning Diesel engines as more efficient ship pro-
pulsion independent from the tank pressure safety
systems [1, 2].
In the period from 2005 to 2010, QATARGAS
has built over 40 the biggest LNG ships with the
reliquefaction system. These ships, depend on tank
capacity, are called: Qmax (266 000 m3) and Qflex
(217 000 m3). The first time of shipbuilding history,
the liquefaction of the natural gas has been imple-
mented on the LNG ships [5]. The reliquefaction
plants, used on the Qataqrgas LNG ships are being
built by Cryostar and Hamworthy. Qflex ships are
Marek Matyszczak, Leszek Kaszycki
84 Scientific Journals 28(100) z. 1
equipped with Hamworthy reliquefaction systems
MARK I and MARK III, and Qmax ships with
Cryostar system EccoRel [2].
The article refer to description and characteriza-
tion of the Hamworthy BOG reliquefaction systems
installed on the board of the LNG ships.
Description of the BOG reliquefaction systems
Description of the reliquefaction systems Mark I
and Mark III installed by Hamworthy onboard of
the LNG ships is presented below.
The BOG Reliquefaction system MARK I [3, 4, 6, 7]
System Mark I has been installed on the first se-
ries Qflex ships. The main purpose of this system is
to provide cargo tank pressure control by liquefying
all vapour boil-off from cargo tanks during normal
operation and maintaining the tank pressure be-
tween 106 kPa(a) to 112 kPa(a).
The lay-out of the MARK I system is presented
on the figure 1 and it flow chart diagram on the
figure 2.
System MARK I has two independent loops:
Cargo Cycle (BOG Cycle),
Nitrogen Cycle.
Fig. 1. Reliquefaction system BOG MARK I [3]
Rys. 1. System skraplania gazu BOG MARK I [3]
The Cargo Cycle (BOG Cycle) consists the fol-
lowing equipment:
one BOG Pre-heater,
two BOG compressors(duty &standby),
one Plate-fin heat exchanger (part of the Cold
box),
one LBOG Phase separator (part of the Cold
box),
two LNG Transfer pumps.
BOG generated in cargo tanks at temperature
–100C is sent via special gas header to the pre-
Plate-fin Heat Exchanger
Companders
Nitrogen Compressors
Nitrogen Reservoir
BOG Compressors
Condensed BOG Pumps
Separator
Fig. 2. Reliquefaction system BOG MARK I flow chart diagram [7]
Rys. 2. Schemat funkcjonalny systemu skraplania gazu BOG MARK I [7]
Criogenic Plate-fin
Heat Exchanger
Compander
BOG Compressor
Condensed
BOG Pump
Separator
Precooler
To Gas Heater
To Spray Header
To GCU (Gas Combustion Unit)
BOG Compressor
To GCU (Gas Combustion Unit)
–159.3C 650 kPa(a)
5310 kPa(a) 41C
–162.5C 1320 kPa(a)
–170.6C 120 kPa(a) –100C
106 kPa(a)
–120C 104 kPa(a)
–159.5C
420 kPa(a)
–23.5C
450 kPa(a)
–110C 5310 kPa(a)
Characterization of the Reliquefaction Systems installed on board of the LNG ships
Zeszyty Naukowe 28(100) z. 1 85
-cooler where gas is cooled to the temperature
–120C. Precooled BOG is compressed in a two-
-stage, integrally-geared centrifugal compressor to
approximately 450 kPa(a). Compressed BOG enters
a plate-fin heat exchanger where is cooled and con-
densed against the cold nitrogen stream. The Plate-
fin cryogenic heat exchanger and the separator are
assembled into one enclosed unit called the Cold
box. Reliquefied BOG at temperature –159C is
collected in a separator to remove non-condensable
gases, if present. The pressure in the separator
forces the liquefied gas back to the cargo tanks.
Non-condensable gases are transferred for burning
to the GCU (Gas Combustion Unit), or for venting
to the vent mast.
The Nitrogen Cycle consists the following
equipment:
two N2 Companders,
nitrogen water coolers,
Cold box,
N2 Inventory system:
• nitrogen reservoir,
• two nitrogen booster compressors,
• nitrogen drier unit,
• two control valves make-up and spill.
The compander is the basic refrigeration device
in nitrogen cycle. The N2 compander is a three-
stage integrally geared centrifugal compressor with
one expander stage. The refrigeration capacity is
produced by nitrogen compression-expansion cycle.
Nitrogen gas at a pressure 1320 kPa(a) (nitrogen
pressure in a low pressure part of the refrigeration
loop) is compressed to 5310 kPa(a) in compander’s
three stage centrifugal compressor. During the
compression, the nitrogen is cooled to 41C by two
water intercoolers and one water after cooler. The
high pressure nitrogen is led to the “warm” section
at the top of the cold box, where the nitrogen is
cooled in plate-fin heat exchanger to –110C by the
cold nitrogen from low pressure refrigeration loop.
Precooled high pressure nitrogen is forced to the
expander. In the expander high pressure nitrogen
expands down to the pressure about 1320 kPa(a)
and reaches temperature at 162.5C and then it is
routed to the heat exchanger “cold” section at the
bottom of the cold box. The cold nitrogen absorbs
heat from the BOG and warm high pressure nitro-
gen. The nitrogen flows through from bottom of the
cryogenic heat exchanger to the top before it is
returned to the suction side of the compander’s
three-stage compressor.
Refrigeration capacity control (N2 inventory
system). The refrigeration capacity of nitrogen loop
has to be adapted to the varied quantity of the BOG
generated in cargo tanks. In order to keep the com-
pander’s compressors as close as possible to design
conditions (to maximize the cycle efficiency), the
load is mainly controlled by circulating nitrogen
mass flow rate. The relationship between the mass
flow and nitrogen loop refrigeration capacity is
assumed to be linear. In connection with this that
cooling capacity depends on mass flow through the
N2 refrigeration loop, the capacity control is real-
ized by increasing or decreasing nitrogen quantity
in cycle. The operating concept consists in optimiz-
ing the shifting of nitrogen between N2 reservoir
and N2 cycle / loop in order to minimize venting of
dry and clean nitrogen from closed loop. The cool-
ing capacity control system (called N2 inventory
system) comprises the following basic devices:
nitrogen reservoir,
nitrogen booster compressors,
nitrogen drier unit,
two control valves make-up and spill.
When the cooling capacity needs to be increased
(the first stage compressor suction pressure has to
be increased), nitrogen is transferred from N2 reser-
voir via make-up control vale to low pressure part
of refrigeration loop. When the cooling capacity
needs to be decreased (the first stage compressor
suction has to be decreased), nitrogen is transferred
from high pressure part of refrigeration loop via
spill control valve to the N2 reservoir. Because the
mass flow rate of the compander is a direct function
of the pressure at compander’s compressor suction,
this pressure signal is used for cooling capacity
control by N2 inventory system.
The BOG Reliquefaction system MARK III
On the base of experiences collected during
building, gas trials and exploitation of the first se-
ries LNG Qflex ships, equipped with the BOG reli-
quefaction system MARK I, Hamworthy modified
the reliquefaction system. New design called Mark
III system has been installed on the finally series of
LNG ships built for Qatargas. In this solution, the
forcing of the BOG from cargo tanks to the cold
box in the cryogenic temperature conditions is re-
placed for the forcing in under ambient temperature
conditions. The pre-heater instead of the precooler
is installed before BOG compressor. The two-stage
centrifugal cryogenic compressor is replaced for
three-stage centrifugal compressor working under
ambient temperature. During compression the BOG
cooling is applied [3, 8].
The lay-out of the MARK III system is pre-
sented on the figure 3 and it flow chart diagram on
the figure 4.
Marek Matyszczak, Leszek Kaszycki
86 Scientific Journals 28(100) z. 1
The Cargo Cycle (BOG Cycle) consists the fol-
lowing equipment:
one BOG Pre-heater,
two BOG Compressors,
one Plate-fin heat exchanger (part of the Cold
box),
one LBOG Phase separator (part of the Cold
box),
two LNG Transfer pumps.
To enable stable temperatures at the inlet into
the BOG compressor, at level well above cryogenic
conditions, a pre-heater is installed upstream of the
BOG compressor. BOG generated in cargo tanks at
temperature –100C is sent via special gas header
to the pre-heater where gas is warmed to the tem-
perature approximately 37C and then it is led to
Fig. 3. Reliquefaction system BOG MARK III [3] Rys. 3. System skraplania gazu BOG MARK III [3]
Fig. 4. Reliquefaction system BOG MARK III flow chart diagram [7]
Rys. 4. Schemat funkcjonalny system skraplania gazu BOG MARK III [7]
Condensed
BOG Pump
Criogenic Plate-fin
Heat Exchanger
Compander
Compander
Nitrogen Reservoir
Preheater
Nitrogen
Compressor
BOG Compressors
–162C 1000 kPa
–110C
41C 4200 kPa
–158.4C 469 kPa
–165C 232 kPa
–159C 780 kPa
41.0C 800 kPa
111.6C 810 kPa
37C 105 kPa
–100C 106 kPa
41C 4200 kPa
–158.9C
465 kPa
–50C
–50C
Criogenic Plate-fin Heat Exchanger
Compander
BOG Proheater
Separator
LNG Pump
To Spray Header
To Vent Gas Heater
BOG Compressors
To GCU (Gas Combustion Unit)
Characterization of the Reliquefaction Systems installed on board of the LNG ships
Zeszyty Naukowe 28(100) z. 1 87
the three stage integrally-geared centrifugal BOG
compressor. BOG is compressed to the pressure
800 kPa and pre-cooled to temperature 41C by two
water intercoolers and one water after cooler. BOG
compressor forces gas to the plate-fin heat ex-
changer where it is liquefied and sent to the separa-
tor. The gas temperature on the outlet from the heat
exchanger is controlled by temperature control
valve. The separator pressure forces liquefied gas
back to the cargo tanks.
The Nitrogen Cycle consists the following
equipment:
two N2 companders,
nitrogen water coolers,
Cold box,
N2 Inventory system:
• nitrogen reservoir,
• two nitrogen booster compressors,
• nitrogen drier unit,
• two control valves make-up and spill.
At design 100% capacity, the compander’s
three-stage centrifugal compressor compress 90 000
kg/h nitrogen from 1000 kPa to 4200 kPa. During
the 3-stage compression, the gas is cooled to ap-
proximately 41C using water cooled gas coolers in
between each stage. The compressed nitrogen is
divided in two streams. One of them is transferred
to the top of cold box where is pre-cooled to –50C.
The second one as a heating stream is forced via
temperature control valve to the pre-heater of BOG.
From pre-heater the second stream of the nitrogen
is send to the common line with the first one and
then nitrogen enters to the second stage of the cold
box where is finally cooled to temperature –110C.
The cold high-pressure nitrogen is routed to the
expander where is expanded down to 1000 kPa and
it reaches temperature –162C. During the expan-
sion approximately 1000 kW of power is generated.
This is added to the gearbox and thus reducing the
motor power. From the expander cold nitrogen is
sent to “cold” section of the heat exchanger in the
cold box. The nitrogen flowing through heat ex-
changer absorbs heat from the BOG. The low pres-
sure nitrogen leaves the top of the cold box at
a pressure of 960 kPa and a temperature of 39.5C.
From the upper part of cold box the low pressure
nitrogen returns to the suction of the first stage
compander’s compressor completing the closed N2
refrigeration loop.
Conclusions
Conclusions from the comparison of the both
reliquefaction systems MARK I and MARK III are
presented below:
Applying in the reliquefaction system MARK
III the three-stage centrifugal BOG compressor,
working under ambient temperature instead of
cryogenic two-stage compressor used in MARK
I system, allows to reduce the cost and the com-
pressor size.
Inter and after cooling allows to remove heat
with cooling water from heated BOG in the pre-
heater and from BOG during compression.
BOG cooling (during compression) reduce ap-
proximately 15% demand power in the
reliquefaction system.
The three-stage BOG compressor compresses
gas to the higher pressure (800 kPa) in compare
with the two-stage compressor(450 kPa) used in
reliquefaction system MARK I. This allows to
carry out the condensation of BOG at higher
temperature.
The control of the reliquefaction system in con-
dition of the higher pressure and temperature of
BOG are more easy and stable. The expander
stage works in more safety range from the nitro-
gen dew point, what prevents against to enter
liquid phase of BOG into compander.
References
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