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


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


–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 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.


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




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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from www.cryostar.com, 2007.

3. Hamworthy Gas Systems, LNG Systems for Marine Appli-

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4. RICHARDSON A.J., AL-SULAITI A.: Construction and per-

formance of the world’s largest ships. Gastech, Bangkok,

March 2008.

5. YONEYAMA H., IRIE T., HATANAKA N.: The first BOG

reliquefaction system on board ship in the world, “LNG

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6. GERDSMEYER K.D.: On-board Reliquefaction for LNG

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2005.

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characterization of the reliquefaction systems installed

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