Hamworthy QGII operation manual 2007

38 4.4.1 BOG COMPRESSOR OPERATION DURING WARMING UP OF THE CARGO TANKS .... 3.2 Boil-off loop The boil-off gas from the cargo tanks are collected in a header vapour main, pre-cooled in

Project Title:

QGII QFLEX

The contents of this document are Hamworthy’s intellectual property and shall be treated as confidential Information This

document shall not be shown nor given to any 3 rd party without written acceptance by Hamworthy Gas Systems AS

Rev Effective Date Description Made

Table of Contents

1 Introduction 8

1.1 P URPOSE 8

1.2 C ONTACT ADDRESS 8

1.3 R EQUIRED QUALIFICATIONS OF PERSONNEL 8

1.4 R EFERENCED DOCUMENTS AND DRAWINGS 8

1.5 A BBREVIATIONS AND TERMINOLOGY 9

1.6 S AFETY PRECAUTIONS 10

1.7 W ARRANTY 10

2 Main Data 11

2.1 T ECHNICAL DATA 11

2.2 P ROCESS C ONSUMPTIONS 11

3 Technical Description 12

3.1 G ENERAL DESCRIPTION 12

3.2 B OIL - OFF LOOP 14

3.2.1 P RE - COOLER 15

3.2.2 BOG C OMPRESSOR ( TWO OFF ) 16

3.2.2.1 BOG-Compressor lube oil system 18

3.2.2.2 Bulk head seal for BOG-compressor 19

3.2.2.3 BOG Compressor shaft seal gas system 19

3.2.3 C OLD B OX 20

3.2.4 S EPARATOR 21

3.2.5 LNG PUMP SKID WITH VENT GAS HEATER 22

3.2.6 G AS ANALYSER 25

3.3 N2 R- LOOP 26

3.3.1 N2 COMPANDERS 27

3.3.1.1 N2-compander lube oil system 30

3.3.1.2 N2-compander seal gas system 30

3.3.1.3 N2-compander seal air system 30

3.3.1.4 N2 coolers 30

3.3.2 C OLD BOX 30

3.3.3 N2 B OOSTER C OMPRESSOR SKID WITH DRYER 31

3.3.3.1 N2 dryer 32

3.3.4 N2 RESERVOIR 33

3.4 H ARDWARE L AYOUT 34

4 Operation Stages 35

4.1 G ENERAL 35

4.2 R ELIQUEFACTION MODE (R ELIQ MODE ) 36

4.2.1 R ELIQ PLANT OFF 36

4.2.3 R UNNING STANDBY 36

4.2.4 F REE FLOW 36

4.2.5 N ORMAL RELIQ 36

4.2.6 V ENT GAS MODE 37

4.2.7 E XCESSIVE BOG MODE 37

4.3 GCU MODE 37

4.3.1 GCU STANDSTILL 37

4.3.2 GCU MODE 37

4.4 S PECIAL OPERATING SCENARIOS 38

4.4.1 BOG COMPRESSOR OPERATION DURING WARMING UP OF THE CARGO TANKS 38

4.4.2 BOG COMPRESSOR OPERATION DURING GAS FREEING / INERTING OF THE CARGO TANKS 38 5 Operation procedures 39

5.1 P REPARATIONS FOR START 39

5.1.1 S ELECT TANK PRESSURE CONTROL SETTINGS 39

5.1.2 S ELECT LEAD EQUIPMENT 39

5.1.3 V ERIFY VALVE POSITIONS 39

5.2 P LANT OPERATION FLOW CHART 39

5.3 R ELIQ READY 39

5.4 R ELIQ MODE START 39

5.5 R ELIQ STANDBY 39

5.6 R ELIQ STOP 39

5.7 GCU MODE START 39

5.8 GCU MODE STOP 39

5.9 A FTER PROCESS SHUT DOWN 39

5.9.1 N2 C OMPANDER PROCESS SHUT DOWN 39

5.9.2 BOG C OMPRESSOR SHUT DOWN 39

5.9.2.1 From operation with one BOG compressor running .39

5.9.2.2 From operation with two BOG compressors running 39

5.9.3 N2 B OOSTER COMPRESSOR SHUT DOWN 39

5.9.4 LNG PUMP SHUT DOWN 39

5.10 O PERATION DURING FAILURE 39

5.10.1 P RE - COOLER LIQUID SUPPLY CONTROL VALVE RL-818 (25TV103) 39

5.10.2 S EPARATOR LIQUID LEVEL CONTROL VALVE RL-823 (25LV040) 39

5.10.3 S EPARATOR PRESSURE CONTROL VALVE RG-826 (25PV030) 39

5.10.4 V ENT GAS TEMPERATURE CONTROL VALVE RG-847 (25TV038) 39

5.10.5 R- LOOP LOADING CONTROL VALVE RN-848 (40PV055) 39

5.10.6 R- LOOP UNLOADING CONTROL VALVE RN-843 (40PV060) 39

5.10.7 D AMAGE TO COLD BOX 39

5.11 O THER OPERATION FEATURES 39

5.11.1 M ANUAL CAPACITY CONTROL OF BOG C OMPRESSORS 39

5.11.2 M ANUAL CONTROL OF BOOSTER COMPRESSORS 39

5.11.3 C HANGING BETWEEN LEAD AND LAG N2 COMPANDER 39

5.12 H ANDLING OF RAPIDLY INCREASING TANK PRESSURE 39

6 Emergency Procedures 39

6.1 E QUIPMENT E MERGENCY SHUTDOWN 39

6.2 S TART - UP AFTER EMERGENCY SHUTDOWN 39

7 Maintenance 39

7.1 P REVENTIVE MAINTENANCE SCHEDULE 39

7.2 P REVENTIVE MAINTENANCE 39

7.2.1 BOG COMPRESSOR SKID 39

7.2.2 N2 COMPANDER SKID 39

7.2.3 LNG P UMP 39

7.2.4 G AS ANALYSER 39

7.2.5 N2 DRYER 39

7.2.6 N2 BOOSTER COMPRESSOR 39

7.3 F AULT SYMPTOMS & ERROR MESSAGES 39

7.3.1 P RE - COOLER 39

7.3.2 BOG COMPRESSOR 39

7.3.3 C OLD BOX 39

7.3.4 S EPARATOR 39

7.3.5 LNG PUMP 39

7.3.6 V ENT GAS HEATER 39

7.3.7 N2 COMPANDER 39

7.3.8 N2 BOOSTER PUMP 39

7.3.9 N2 RESERVOIR 39

7.3.10 N2 DRYER 39

7.3.11 B UTTERFLY V ALVES 39

7.3.12 B ALL V ALVES 39

7.3.13 C ONTROL V ALVES 39

7.4 P URGING P ROCEDURE 39

7.4.1 I NTRODUCTION 39

7.4.2 D EPRESSURIZING AND PURGING OF N2 C YCLE 39

7.4.2.1 Depressurizing of N2 Compander no 1 (40KX100) 39

7.4.2.2 Depressurizing of N2 Compander no 2 (40KX200) 39

7.4.2.3 Depressurizing of N2 Booster train no 1 (40KX104) 39

7.4.2.4 Depressurizing of N2 Booster train no 2 (40KX204) 39

7.4.2.5 Depressurizing and purging of complete N2 Cycle 39

7.4.3 D EPRESSURIZING AND PURGING OF BOG CYCLE 39

7.4.3.1 Depressurizing and purging of Precooler (25VA100) and BOG Compressor no 1 (25KX100) 39 7.4.3.2 Depressurizing and purging of BOG compressor no 1 (25KX100) 39

7.4.3.3 Depressurizing and purging of BOG compressor no 2 (25KX200) 39

7.4.3.4 Depressurizing and purging of complete BOG cycle 39

7.4.4 V ACUUMING 39

7.4.4.1 Vacuuming of complete N2 Cycle 39

7.4.4.2 Warm purging of N2 companders 39

7.4.4.3 Vacuuming of complete BOG Cycle 39

8 Spare Parts 39

8.1 S PARE P ARTS DELIVERED WITH THE R ELIQUEFACTION SYSTEM 39

8.2 V ALVE SPARES 39

F LOW M ETERS 39

8.3 D IFFERENTIAL P RESSURE T RANSMITTERS 39

8.4 P RESSURE T RANSMITTERS 39

8.5 L EVEL SWITCH 39

8.6 P RESSURE SWITCH 39

8.7 T ERMOSTAT 39

8.8 T EMPERATURE SWITCH 39

8.9 P OSITION SWITCH 39

8.10 M ANOMETERS 39

8.11 T EMPERATURE ELEMENTS 39

9 Disposal instruction 39

10 Drawings and Documents 39

10.1 D RAWINGS 39

10.2 D OCUMENTS 39

10.3 S UB SUPPLIER DOCUMENTATION 39

11 Appendix A - Valves Position 39

12 Appendix B - BOG compressor log sheets 39

13 Appendix C - N2 compander log sheets 39

14 Appendix D - N2 booster compressor log sheets 39

15 Appendix E - Manual control of N2 supply 39

1 Introduction 39

1.1 P URPOSE OF MANUAL OPERATION 39

1.2 E RROR IN 40PV055 AND 40PV060 39

2 Procedures 39

2.1 I MPORTANT GENERAL NOTES 39

2.2 I NCREASING INVENTORY 39

2.3 R EDUCING INVENTORY 39

2.3.1 R UNNING COMPANDER DISCHARGE PRESSURE < 36 BARG 39

2.3.2 R UNNING COMPANDER DISCHARGE PRESSURE > 36 BARG 39

2.4 S UPPLY OF NITROGEN FROM SHIP SYSTEM 39

2.5 M OVING NITROGEN FROM STANDBY COMPANDER TO RESERVOIR 39

16 Appendix F - Cause & effect matrix 39

List of Figures

Figure 1 Plant general arrangement Boil-off loop in red, N2 loop in green 12

Figure 2 MOSS RS reliquefaction system Boil-off loop in red, N2 loop in green 13

Figure 3 Boil-off Loop arrangement 14

Figure 4 Pre-cooler assembly 15

Figure 5 BOG compressor assembly left 17

Figure 6 BOG compressor assembly right 18

Figure 7 Bulk head seal for BOG-compressor 19

Figure 8 BOG compressor seal gas system 19

Figure 9 Cold box 20

Figure 10 Cold box w/ separator 21

Figure 11 Pump bearing heater 22

Figure 12 Vent Gas Heater 22

Figure 13 Flow element w / transmitter 23

Figure 14 LNG pump skid 24

Figure 15 Gas analyser 25

Figure 16 N2 arrangement N2 loop in green 26

Figure 17 N2 compander right 28

Figure 18 N2 compander left 29

Figure 19 N2 booster compressor skid 31

Figure 20 N2 dryer 32

Figure 21 Dew point transmitter 32

Figure 22 N2 reservoir 33

Figure 23 DACS Hardware Layout 34

Figure 24 Overview of the different operating modes 35

Figure 25 Reliq BOGC GMS 39

Figure 26 Reliq Booster comp main 39

Figure 27 Reliq.plant main 39

Figure 28 Plant operation flow chart 39

Figure 29 Reliq booster comp main 39

Figure 30 Reliq plant main 39

Figure 31 Reliq Plant Main 39

Figure 32 Reliq Booster Comp Main 39

Figure 33.Reliq plant Main 39

Figure 34 Reliq Plant Main 39

Figure 35 Oil level indicator 39

Figure 36 Pressure drop across filter & refill of oil 39

Figure 37 Oil pressure to el motor 39

Figure 38 DGV 39

Figure 40 Oil level indicator 39

Figure 41 Refill of oil 39

Figure 42 Pressure drop across filter 39

Figure 43 IGV 39

Figure 44 Heating cable 39

Figure 45 Gas analyser instrument panel 39

Figure 46 Gas analyser 39

Figure 47 N2 dryer 39

Figure 48 Oil level, filling and filter 39

Figure 49 Pressure and temperature 39

Figure 50 Drain of demister 39

Figure 51 Oil separators 39

The operator must adhere to all instructions and recommendations from this User Manual including corresponding User Manuals from sub-suppliers It is therefore imperative that the User Manual is ready available to all plant operators

1.2 Contact address

Address

Hamworthy Gas Systems AS Solbraaveien 10, NO-1383 Asker Postal Addr.: PO box 144, NO-1371 Asker Norway

1.3 Required qualifications of personnel

Only qualified personnel are allowed to operate the plant Qualified personnel shall fulfil the following:

• General training and experience

• Specific training on the LNG reliquefaction plant

• Familiar with basic work safety and accident prevention regulations,

• Read and familiarised themselves with this user manual and accompanying documentation The user manual is designed to provide qualified personnel with information for the day-to-day operations and maintenance tasks on the delivered equipment

1.4 Referenced documents and drawings

All documents and drawings referred to in this document are listed in chapter 10

1.5 Abbreviations and terminology

The following abbreviations and terminology are used throughout this document:

Lag Standby

N2 Nitrogen

1.6 Safety precautions

In this user manual the following symbols are used:

CAUTION

Disregard of directions in cautions can result in damage to equipment

NOTE Text set off in this manner brings attention to additional information or

specific instructions

1.7 Warranty

Warranty or liability claims in connection with personal injuries or damage to equipment or the plant do not apply if the injury or damage is due to one of the following:

• Faulty use or operation of the plant

• Faulty installation, commissioning, operation or maintenance of the plant

• Continuous operation of the plant after a fault has been identified

• Information in this User Manual concerning transportation, storage, installation,

commissioning, and maintenance has not been followed

• Changes made to the plant without Hamworthy Gas Systems AS approval

• Insufficient condition monitoring of wear parts

• Repair work has not been done according to supplier’s specifications and requirements

• Non-genuine spare parts have been used

• Unqualified personnel operating, maintaining, using or installing the plant

• If the LNG reliquefaction plant is operated outside its operating limits

2 Main Data

2.1 Technical data

The system consists of to main loops:

• The boil-off loop (in red) Refer to chapter 3.2 for more details

• The N2 loop (in green) Refer to chapter 3.3 for more details

Figure 1 Plant general arrangement Boil-off loop in red, N2 loop in green

Figure 2 MOSS RS reliquefaction system Boil-off loop in red, N2 loop in green

3.2 Boil-off loop

The boil-off gas from the cargo tanks are collected in a header (vapour main), pre-cooled in a heat exchanger, compressed in a two-stage centrifugal compressor (BOG compressor), cooled and condensed in a large multi-pass heat exchanger (part of the cold box)

The BOG loop consists of the following main equipment:

• BOG-compressors, one duty and one standby (1)

• LNG return pump, one duty and one standby (2)

• BOG pre-cooler, one off (3)

• Plate-fin cryogenic heat exchanger, one off (part of the cold box) (4)

• LBOG phase separator, one off (part of the cold box) (5)

3.2.1 Pre-cooler

The purpose of the pre-cooler is to

ensure that the compressor discharge

temperature remains constant in order

to protect the cold box from large

temperature changes and

consequently damage due to thermal

stresses

The pre-cooler consists of a

tube-finned heat exchanger that is installed

inside a vertical separator

Figure 4 Pre-cooler assembly

The pre-cooler is designed to cool the incoming BOG from -100°C to -120°C It is also designed to separate liquid droplets from the vapour to protect the compressor from liquid and thus from becoming damaged

The pre-cooler is controlled based on discharge temperature of the BOG compressor The BOG is cooled by vaporising LBOG on the inside of the heat exchanger tubes while the BOG flows over the outside of the tubes The vaporised LNG (now gas) then will mix with the pre-cooler inlet BOG

A high level trip signal in the pre-cooler will shut down the BOG compressor(s), and close the LNG supply valve (24TV103) if activated

The pre-cooler consists of the following main equipment:

• Tube-finned heat exchanger installed in a pressure vessel

• Demister pad installed inside the pressure vessel

Refer to the document 400573-I-HGS-96657 Technical Manual for more details

3.2.2 BOG Compressor (two off)

The BOG-compressor is a two stage centrifugal compressor with diffuser guide vanes (DGV) on each stage for controlling the capacity

One running compressor (0% -100% BOR):

When one BOG compressor is running, the suction pressure controller will modulate the recycle valve and the DGV’s, to control the pressure in the cargo tanks

If the cargo tank pressure increases above the set point, the recycle valve closes and the DGV’s will open in a split range arrangement The DGV’s will not start to open until the recycle valve is fully closed The BOG

compressor is running at 100% capacity when the DGV’s are fully open

When the suction pressure drops below the set point, the DGV’s will close and then the recycle valve will open a split range arrangement The recycle valve will not start to open until the DGV’s are fully closed unless overridden by the anti surge control function

Two running compressors (100 – 150% BOR)

When two BOG compressors are running at the same time, one master suction controller is controlling the pressure in the cargo tanks The lead compressor suction controller will then take the function as master controller, where the lag compressor controller will be disabled The output signal of the master controller is sent to both compressors, which will then be operating at the same capacity (load

sharing) The transition between single control and master control with two compressors running is done by a time-based ramp

One compressor unit consists of the following main equipment:

• Electric main driver (1)

• Oil demister (2)

• Actuator for DGV 2nd stage (3)

• Interconnection gas pipe (4)

• Lube oil system See below

Refer to document Atlas Copco Instruction manual for further details

Figure 5 BOG compressor assembly left

Figure 6 BOG compressor assembly right

3.2.2.1 BOG-Compressor lube oil system

The electric pump (10) maintains the oil pressure at start-up, shutdown and trip The gear driven oil pump (14), maintains the pressure while the compressor is running A steam heater is installed to keep the oil temperature within the limits when the machine is at standstill A fresh water-cooled heat exchanger (6) is installed to cool the oil when the machine is running

3.2.2.2 Bulk head seal for BOG-compressor

The BOG-compressors require both

air and N2 as seal gas for the

bulkhead

The N2 is used as an emergency

supply if the air supply fails

Air is used as bulkhead seal since

there is air on both sides of the seal

(compressor room and motor room)

There is no leakage detection

However, there is a trip on low

pressure

Figure 7 Bulk head seal for BOG-compressor

3.2.2.3 BOG Compressor shaft seal gas system

Shaft seals are employed to seal the

space between impeller back-face

and gearbox N2 is used as seal gas

for the shaft seal

This seal is designed to keep process

gas from entering the shaft seal and

thus destroying it, and to keep

dangerous gases from entering the

atmosphere or the gear

Figure 8 BOG compressor seal gas system

3.2.3 Cold Box

The compressed boil-off gas

transferred from the BOG-compressor

is cooled and condensed in a large

multi-pass heat exchanger

The temperature in the heat

exchanger is lowest at the bottom

(-164°C) and highest at the top (slightly

above the cooling water temperature)

The low pressure nitrogen flows

through from bottom of the cryogenic

heat exchanger to top before it is

returned to the suction side of the first

stage compressor on the N2

compander

The heat exchanger temperature is

allowed to change at a maximum rate

of 1°C per minute The expander

bypass valve will protect the cold box

temperature from changing quicker

The cold box consists of the following equipment:

• Plate Fin Heat exchanger (1)

• Separator (2), refer section 3.2.4

Refer to document 400573-I-HGS-96657 Technical Manual for further details

3.2.4 Separator

LBOG and non-condensable gases

are separated in the separator where

the liquid phase is returned to the

cargo tanks by the differential

pressure (only when the BOG

compressors are operating) between

the separator and cargo tank The

gas phase is vented to the GCU or

returned to the cargo tanks

During free flow mode, the BOG

compressors are not operating In

this mode the LBOG is returned to

the tanks by the LNG return pump

The amount of vent gas will increase

with increasing nitrogen content in

the BOG

A three-way valve directs vent gas

either to the GCU through the vent

line or alternatively back to the cargo

tanks via a separate vapour-return

line

The three-way valve has an

automatic and manual mode:

Automatic mode

In the automatic mode the valve

normally directs vent gas to the

cargo tanks If the cargo tank

pressure increases above its set

point, the GCU is activated and the

valve directs the vent gas to the

Manual mode

In the manual mode the operator decides where to direct the vent gas However, if the cargo tank pressure increases above its set point, the control system ensures that the valve directs the vent gas

to the GCU

The separator is a part of the cold box

Refer to document 400573-I-HGS-96657 Technical Manual for further details

3.2.5 LNG pump skid with vent gas heater

Figure 11 Pump bearing heater

LNG Pump

The LNG transfer pump (1) will be

operated automatically during free

flow based on the separator liquid

level The LNG pump has a variable

frequency drive that regulates the

pump capacity depending on the level

controller in the separator The pump

will run on minimum speed until the

valve has opened completely

If the flow is too small for the LNG

pump to handle, the level will be

controlled by switching the pump on

and off based on a high and low level

signal from the level measurement

The unit is a 2 X 100% pumps

Vent gas heater

The vent gas is heated to ambient

temperature in a vent gas heater (3)

upstream the GCU

The outlet temperature is controlled to

ensure that the temperature stays

within the GCU operating limitations

Figure 12 Vent Gas Heater

The LNG pump skid unit consists of

the following main equipment:

• Pump bearing heater (2)

• Vent gas heater (3)

Figure 14 LNG pump skid

Refer to document 400573-I-HGS-96657 Technical Manual for further details

3.2.6 Gas analyser

The gas analyser take off is located at

the system battery BOG inlet limit It

measures the CH4 (methane) content

in the Boil Off Gas

The Gas Analyser serves the

following functions:

1 Adjust the BOG compressor

surge lines according to the

is routed back to the compressor suction side

Capacity control of the loop is done by inventory control, i.e letting nitrogen in or out from the R-loop The main components of the R-loop are:

• N2 compander (3-stage Compressor / Expander), two off (1)

• Cold box, one off, refer chapter 3.2.3 (2)

• Nitrogen booster compressor, one duty and one stand by (3)

• Nitrogen dryer, two off (4)

• Nitrogen reservoir, one off (5)

3.3.1 N2 companders

The RS is designed for one N2 compander operating (lead) whilst the other is in standby (lag) Parallel operation is not possible The two companders are isolated from each other with double isolation valves All isolation valves have an interlock system using limit switches

The lead N2-compander is not allowed to start before all the double shutdown valves isolating the unit are open

In addition, all the shutdown valves isolating the stand by compander must be closed

N2-If the stand by N2-compander is to be started the situation is correspondingly opposite

The unit consists of the following main equipment:

• Flexible water pipes (1)

• Main driver (2)

• Bull gear shaft (3)

• Gear box cover (4)

• Oil cooler for the main driver (13)

• Double oil filter (14)

• Instrument rack (15)

• Control panel (16)

• Intermediate cooler bundle 1 (17)

• Intermediate cooler bundle 2 (18)

• Gear driven oil pump (main) (19)

• Gear box (20)

• Expander nozzle actuator (21)

• Stages beside gear box (22)

• Oil Cooler gear box (23)

Refer to Atlas Copco Instruction manual for further details

Figure 17 N2 compander right

Figure 18 N2 compander left

3.3.1.1 N2-compander lube oil system

Refer to figures 16 and 17 when reading this section

The auxiliary electric oil pump (11) maintains the oil pressure at start-up, shutdown and trip, and a gear box driven oil pump (19) maintains the pressure while compressor is running An electrical heater (9) is installed to keep oil temperature within its limits when the machine is at standstill A fresh water-cooled heat exchanger (13) is installed to cool the oil while the N2-compander is operating

The oil system is monitored with by level, temperature and pressure

When the auxiliary oil pump is started, the oil demister fan starts automatically and cannot be stopped manually

3.3.1.2 N2-compander seal gas system

Nitrogen shall only be used as seal gas and it is taken from the refrigerant loop

3.3.1.3 N2-compander seal air system

Instrument air to be used in order to seal off lubrication oil

3.3.3 N2 Booster Compressor skid with dryer

The purpose of the N2 booster system

is to replenish the R-loop with N2 due

to N2 leakages from N2 compressor

seals The compressors will also

operate during the loading and

unloading of the R-loop refrigeration

system

Figure 19 N2 booster compressor skid

The N2 booster compressors will try to maintain a pressure in the N2 reservoir based on a set point with a dead band This is done by switching on one compressor when the lower limit of the dead band has been reached and switching it off when the upper limit of the dead band is reached The set point

in the reservoir will change in accordance with the R-loop first stage compressor suction pressure (40PT201)

The unit is a 2 X 100% system

The unit consists of the following main equipment:

• N2 Dryers (1), refer to 3.3.3.1 below

• N2 Booster Compressors (2)

• Electric Motors (3)

Refer to document 400573-I-HGS-96657 Technical Manual for further details

3.3.3.1 N2 dryer

The unit is an absorption dryer with

alternating absorption and

regeneration phases

The N2 will be alternately dried in one

of the absorbers while the other

absorber is being regenerated This

procedure ensures continuous

operation

Condensate, oil aerosols and dirt

particles are removed in the pre-filter

The N2 then flows from the bottom to

the top through the lower shuttle valve

and to one of the absorbers

The dried N2 passes through the

upper shuttle valve to the afterfilter to

the N2 booster compressor suction

The unit is a 2 X 100% system

• Front panel with controller (9)

• Dew point meter (10)

Figure 21 Dew point transmitter

3.3.4 N2 reservoir

Figure 22 N2 reservoir

The N2 reservoir’s purpose is to:

• Store nitrogen in order to perform capacity control

• Provide N2 to the main loop to cope with shaft seal leakages

• Hold sufficient volume to prevent PSV’s to lift during power trips, etc

3.4 Hardware Layout

The control system is a dual processor-based system with 100% redundancy with both local and remote control capabilities The RS control system is a stand-alone system with its own operator station placed in the electro motor room The stand-alone system is connected to the DACS for the ship

Figure 23 DACS Hardware Layout

4 Operation Stages

4.1 General

Figure 24 Overview of the different operating modes

In order to avoid controlled venting of BOG from the cargo tanks the tank pressure will be controlled in one of two superior modes; the “Reliq mode” or the “GCU mode” Either of these modes contain different sub-modes as illustrated in Figure 24 above As far as possible the Reliq mode shall be given priority over the GCU mode

4.2 Reliquefaction mode (Reliq mode)

4.2.1 Reliq plant off

“Reliq plant off” represents the situation when all subsystems associated with the reliquefaction plant are inactive This situation will typically take place when the system is started up for the first time, or after gas freeing of the ship

4.2.2 Reliq plant standstill

The basal functions are initiated by pushing the “reliq on” button The plant will then have access to utilities such as power, the nitrogen supply system is initiated, and the reliq plant is the “reliq plant standstill” mode

4.2.3 Running standby

The LNG RS can be set in a position where it is cooled down and prepared for operation without sending any LBOG back to the cargo tanks The R-loop will then be in operation, but does only provide sufficient cooling capacity in order to compensate for the heat ingress to the system via cold box walls, piping, etc

For stable operation in running standby it is necessary to have liquid present in the separator The start-up from warm conditions will therefore require that the BOG section of the cold box has access to BOG from the cargo tank, in order to be able to build up a liquid level in the separator and to avoid sub-atmospheric as a result of density changes during cooling The cold box BOG inlet valve (RG-825) must consequently not be closed during the cool down phase

When the liquid in the separator has reached normal liquid level low, and the temperatures in the lower parts of the cold box have stabilized, the cold box BOG inlet valve (RG-825) can be closed, and running standby operation has been reached

As the level increases in the separator the LBOG forced return pump 25PA101 is started up from the level control system The pump will have sufficient head for bringing the LBOG from near-atmospheric pressure in the separator to the bottom of the cargo tanks

4.2.5 Normal reliq

Normal reliquefaction occurs when the tank pressure control is performed by one BOG compressor and the BOG compressor capacity is less than or equal to 100% of maximum flow

In normal reliq mode, the separator level control does not involve the LBOG forced return pump, as the BOG compressor will provide sufficient head for bringing the LBOG back to the cargo tanks When it comes to the ability of handling rapid changes in BOR from the cargo tanks, the response time of the R-loop is significantly longer that of the GCU To be able to quickly ramp up from low to high capacity, the GCU may be set to operate in parallel with the R-loop while the R-loop capacity is ramped up to take the entire load by itself When this R-loop capacity has been reached the GCU will

be stopped

4.2.6 Vent gas mode

If the BOG is rich on nitrogen, only partial reliquefaction will be applied to the incoming flow In normal reliq mode the non-condensed gas (top product from separator 25VA101), from now on denoted as

“vent gas”, will be directed back to the cargo tanks In vent gas mode the vent gas will instead be redirected towards the GCU by changing the output flow direction of three-way valve RG-827 Note that the vent gas will consist of approximately 70 mole% nitrogen and 30 mole% methane, and must therefore be treated different than normal methane-rich BOG by the GCU

4.2.7 Excessive BOG mode

When one BOG compressor is incapable of controlling the cargo tank pressure alone, it is necessary

to start-up the lag BOG compressor This is what is referred to as the excessive BOG mode When two BOG compressors operate in parallel, the excess BOG which can not be handled by the

reliquefaction system will be sent to the GCU

If the GCU is out of operation, the gas flow that otherwise would have been handled by the GCU must instead be taken care of by controlled venting

Excessive BOG conditions can take place due to non-equilibrium tank conditions (e.g insufficient cool down of tanks before loading) or rough weather

4.3 GCU mode

4.3.1 GCU standstill

The GCU standstill mode is equivalent to the reliq plant standstill mode in that the GCU will have access to all required utilities (instrument air, power, etc.) The GCU has not received any start signal and does not have access to process gas (BOG or vent gas)

The operator can manually initiate the reliq mode from the GCU mode The load (BOG flow) will then gradually be transferred from the GCU to the reliquefaction plant, and the GCU stopped when all BOG

is handled by the R-loop

Note that if the BOG compressor system is inoperable, the cargo tank pressure must be controlled by

a combination of free-flow to the GCU via the recycle lines of the HD compressors and controlled venting

If the R-loop is out of operation and the BOR exceeds that which can be handled by the GCU,

controlled venting must take place in parallel with the GCU operation

4.4 Special operating scenarios

Additional to the above operating scenarios the BOG compressors will be used for the following special purposes:

• BOG compressor operation during warming up of the cargo tanks where the suction temperature can be high

• BOG compressor operation during inerting of the cargo tanks

Note that these cases will take place very seldom, maybe only every dry docking or if a tank has to be inspected between dockings

To run in the system in these special operating modes, it must be selected on the screen This

selection shall be password protected to avoid that this mode can be selected during normal operation

by mistake

4.4.1 BOG compressor operation during warming up of the cargo tanks

Prior to the inerting, the cargo tanks need to be warmed up Therefore the High Duty (HD)

compressors will then be used to circulate BOG gas through the gas heater back in to the cargo tank until the tanks are warm During the warm up the volume of the vapour will increase and thus create a higher pressure in the tanks The pressure in the cargo tanks will then be kept constant using the BOG compressors, which will transfer the BOG to the GCU During this operation scenario the suction temperature of the BOG Compressor will rise from -120 °C up to max 40°C The gas will be BOG throughout the sequence

4.4.2 BOG compressor operation during gas freeing / inerting of the cargo tanks

After the cargo tanks have been warmed up, the inerting of the cargo tanks can be started The inert gas will enter the cargo tanks at the bottom Since the inert gas has a higher mol weight (30,42 kg/kmol) than the BOG (16,04 – 18,19 kg/kmol), the inert gas will remain at the bottom of the cargo tank pushing the BOG to the top of the tank (piston effect) The BOG will then be absorbed the BOG compressor, compressed and sent to the GCU

At the top of the “pistion” a layer of BOG mixed with N2 will move towards the top of the tank and finally enter the BOG compressor When this mixed layer enters the BOG compressor, the discharge temperature of compressor stage 1 and 2 will raise because of the increasing mol weight When the discharge temperature stage 2 reaches 130°C, the compressor will shut down

(25TAHH107/25TAHH207) The rest of the gas must then be removed from the cargo tanks in free flow operation by other means than the BOG compressor

During both these special operating scenarios the pre-cooler 25VA100 is not available because LNG

is not available for cooling Therefore, if the compressor differential pressure moves close to the surge line and the anti surge valve (25FV113/213, RG-809/810) starts to operate (recycling), the BOG compressor will trip on high inlet temperature 25TAHH106/TAHH206 or outlet temperature

25TAHH107/25TAHH207

When the BOG compressor is started for warming up of the cargo tanks or inerting, the recycle valve 25FV113/213 must be closed The GCU control valve must therefore be open to avoid that the warm gas is returned to the compressor suction

When the BOG compressor is delivering gas to the GCU, the discharge pressure of the compressor must be kept at such level, that the operating point remains within the compressor performance curve and the compressor is not operated in stone wall

5.1.1 Select tank pressure control settings

Figure 25 Reliq BOGC GMS

1 Select one of the three tank pressure control settings (1)

• Ballast gauge

2 Define set points for tank pressure controller 25PIC001

3 Define initial start values for output of tank pressure controller 25PIC001, track values (1)

4 Define BOG auto start limit (2)

5 Define BOG auto stop limit (3)