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: m.matyszczak@am.szczecin.pl, l.kaszycki@am.szczecin.pl
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
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 reliquefaccharacteriza-tion 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)
-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 shiftoptimiz-ing 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.
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
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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