Hamworthy QGII operation manual 2007

Hamworthy Gas Systems AS

Project Title:

QGII QFLEX
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02 13.09.2007

As-built

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01 25.06.2007

Reissued before Gas trial

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Document Title:

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QGII Operation Manual

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400573-R-HGS-69299


Hamworthy Gas Systems AS
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QGII Operation Manual

Revision:
Effective Date:

01
25.06.2007

Table of Contents
1

Introduction........................................................................................................ 8
1.1

PURPOSE .................................................................................................................................. 8

1.2

CONTACT ADDRESS ................................................................................................................... 8

1.3

REQUIRED QUALIFICATIONS OF PERSONNEL ................................................................................. 8

1.4

REFERENCED DOCUMENTS AND DRAWINGS.................................................................................. 8

1.5

ABBREVIATIONS AND TERMINOLOGY ........................................................................................... 9

1.6

SAFETY PRECAUTIONS ............................................................................................................. 10

1.7

WARRANTY ............................................................................................................................. 10

2.1

TECHNICAL DATA ..................................................................................................................... 11

2.2

PROCESS CONSUMPTIONS ....................................................................................................... 11

3.1

GENERAL DESCRIPTION ............................................................................................................ 12

3.2

BOIL-OFF LOOP ....................................................................................................................... 14

2

Main Data ........................................................................................................ 11

3

Technical Description ...................................................................................... 12

3.2.1
3.2.2

3.2.2.1
3.2.2.2
3.2.2.3

3.3

PRE-COOLER .............................................................................................................. 15
BOG COMPRESSOR (TWO OFF).................................................................................... 16

BOG-Compressor lube oil system.................................................................................18
Bulk head seal for BOG-compressor ............................................................................19
BOG Compressor shaft seal gas system ......................................................................19

3.2.3 COLD BOX .................................................................................................................. 20
3.2.4 SEPARATOR................................................................................................................ 21
3.2.5 LNG PUMP SKID WITH VENT GAS HEATER ...................................................................... 22
3.2.6 GAS ANALYSER ........................................................................................................... 25
N2 R-LOOP ............................................................................................................................. 26
3.3.1

3.3.1.1
3.3.1.2
3.3.1.3
3.3.1.4
3.3.2
3.3.3

3.3.3.1

N2 COMPANDERS ........................................................................................................ 27

N2-compander lube oil system......................................................................................30
N2-compander seal gas system....................................................................................30
N2-compander seal air system......................................................................................30
N2 coolers .....................................................................................................................30
COLD BOX .................................................................................................................. 30
N2 BOOSTER COMPRESSOR SKID WITH DRYER .............................................................. 31

N2 dryer.........................................................................................................................32

3.4

3.3.4 N2 RESERVOIR ............................................................................................................ 33
HARDWARE LAYOUT ................................................................................................................ 34

4.1

GENERAL ................................................................................................................................ 35

4.2

RELIQUEFACTION MODE (RELIQ MODE)...................................................................................... 36

4

Operation Stages ............................................................................................ 35

4.2.1
4.2.2

RELIQ PLANT OFF ........................................................................................................ 36
RELIQ PLANT STANDSTILL ............................................................................................ 36

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4.3

4.2.3 RUNNING STANDBY...................................................................................................... 36
4.2.4 FREE FLOW ................................................................................................................. 36
4.2.5 NORMAL RELIQ ........................................................................................................... 36
4.2.6 VENT GAS MODE.......................................................................................................... 37
4.2.7 EXCESSIVE BOG MODE ............................................................................................... 37
GCU MODE ............................................................................................................................. 37

4.4

4.3.1 GCU STANDSTILL........................................................................................................ 37
4.3.2 GCU MODE ................................................................................................................. 37
SPECIAL OPERATING SCENARIOS .............................................................................................. 38
4.4.1
4.4.2

5

BOG COMPRESSOR OPERATION DURING WARMING UP OF THE CARGO TANKS .................. 38
BOG COMPRESSOR OPERATION DURING GAS FREEING / INERTING OF THE CARGO TANKS . 38

Operation procedures...................................................................................... 39
5.1

PREPARATIONS FOR START ...................................................................................................... 39

5.2

5.1.1 SELECT TANK PRESSURE CONTROL SETTINGS ............................................................... 39
5.1.2 SELECT LEAD EQUIPMENT ............................................................................................ 39
5.1.3 VERIFY VALVE POSITIONS ............................................................................................. 39
PLANT OPERATION FLOW CHART ............................................................................................... 39

5.3

RELIQ READY .......................................................................................................................... 39

5.4

RELIQ. MODE START................................................................................................................. 39

5.5

RELIQ STANDBY....................................................................................................................... 39

5.6

RELIQ STOP............................................................................................................................. 39

5.7

GCU MODE START ................................................................................................................... 39

5.8

GCU MODE STOP..................................................................................................................... 39

5.9

AFTER PROCESS SHUT DOWN ................................................................................................... 39
5.9.1
5.9.2

5.9.2.1
5.9.2.2

N2 COMPANDER PROCESS SHUT DOWN ........................................................................ 39
BOG COMPRESSOR SHUT DOWN .................................................................................. 39

From operation with one BOG compressor running. ....................................................39
From operation with two BOG compressors running ....................................................39

5.9.3 N2 BOOSTER COMPRESSOR SHUT DOWN ...................................................................... 39
5.9.4 LNG PUMP SHUT DOWN ............................................................................................... 39
5.10 OPERATION DURING FAILURE .................................................................................................... 39
5.10.1 PRE-COOLER LIQUID SUPPLY CONTROL VALVE RL-818 (25TV103) ................................ 39

5.10.2 SEPARATOR LIQUID LEVEL CONTROL VALVE RL-823 (25LV040) .................................... 39
5.10.3 SEPARATOR PRESSURE CONTROL VALVE RG-826 (25PV030) ....................................... 39
5.10.4 VENT 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 DAMAGE TO COLD BOX ................................................................................................ 39
5.11 OTHER OPERATION FEATURES .................................................................................................. 39
5.11.1 MANUAL CAPACITY CONTROL OF BOG COMPRESSORS.................................................. 39
5.11.2 MANUAL CONTROL OF BOOSTER COMPRESSORS ........................................................... 39
5.11.3 CHANGING BETWEEN LEAD AND LAG N2 COMPANDER .................................................... 39
5.12 HANDLING OF RAPIDLY INCREASING TANK PRESSURE ................................................................. 39

6

Emergency Procedures................................................................................... 39
6.1

EQUIPMENT EMERGENCY SHUTDOWN ........................................................................................ 39

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6.2

START-UP AFTER EMERGENCY SHUTDOWN ................................................................................ 39

7.1

PREVENTIVE MAINTENANCE SCHEDULE ...................................................................................... 39

7.2

PREVENTIVE MAINTENANCE ...................................................................................................... 39

7.3

7.2.1 BOG COMPRESSOR SKID ............................................................................................. 39
7.2.2 N2 COMPANDER SKID .................................................................................................. 39
7.2.3 LNG PUMP ................................................................................................................. 39
7.2.4 GAS ANALYSER ........................................................................................................... 39
7.2.5 N2 DRYER................................................................................................................... 39
7.2.6 N2 BOOSTER COMPRESSOR ......................................................................................... 39
FAULT SYMPTOMS & ERROR MESSAGES .................................................................................... 39

7.4

7.3.1 PRE-COOLER .............................................................................................................. 39
7.3.2 BOG COMPRESSOR .................................................................................................... 39
7.3.3 COLD BOX .................................................................................................................. 39
7.3.4 SEPARATOR................................................................................................................ 39
7.3.5 LNG PUMP ................................................................................................................. 39
7.3.6 VENT 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 BUTTERFLY VALVES .................................................................................................... 39
7.3.12 BALL VALVES ............................................................................................................. 39
7.3.13 CONTROL VALVES ....................................................................................................... 39
PURGING PROCEDURE ............................................................................................................. 39

7

Maintenance .................................................................................................... 39

7.4.1
7.4.2

7.4.2.1
7.4.2.2
7.4.2.3
7.4.2.4
7.4.2.5
7.4.3

INTRODUCTION ............................................................................................................ 39
DEPRESSURIZING AND PURGING OF N2 CYCLE .............................................................. 39

Depressurizing of N2 Compander no. 1 (40KX100) .....................................................39
Depressurizing of N2 Compander no. 2 (40KX200) .....................................................39
Depressurizing of N2 Booster train no. 1 (40KX104)....................................................39
Depressurizing of N2 Booster train no. 2 (40KX204)....................................................39
Depressurizing and purging of complete N2 Cycle.......................................................39
DEPRESSURIZING 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

VACUUMING ................................................................................................................ 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

Spare Parts...................................................................................................... 39

8
8.1

SPARE PARTS DELIVERED WITH THE RELIQUEFACTION SYSTEM ................................................... 39

8.2

VALVE SPARES ........................................................................................................................ 39

FLOW METERS ................................................................................................................................. 39

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8.3

DIFFERENTIAL PRESSURE TRANSMITTERS ................................................................................. 39

8.4

PRESSURE TRANSMITTERS ....................................................................................................... 39

8.5

LEVEL SWITCH ......................................................................................................................... 39

8.6

PRESSURE SWITCH .................................................................................................................. 39

8.7

TERMOSTAT ............................................................................................................................ 39

8.8

TEMPERATURE SWITCH ............................................................................................................ 39

8.9

POSITION SWITCH .................................................................................................................... 39

8.10 MANOMETERS ......................................................................................................................... 39
8.11 TEMPERATURE ELEMENTS ........................................................................................................ 39

9
10

Disposal instruction ......................................................................................... 39
Drawings and Documents ............................................................................... 39

10.1 DRAWINGS .............................................................................................................................. 39
10.2 DOCUMENTS ........................................................................................................................... 39
10.3 SUB SUPPLIER DOCUMENTATION ............................................................................................... 39

11
12
13
14
15
1

Appendix A - Valves Position .......................................................................... 39
Appendix B - BOG compressor log sheets ..................................................... 39
Appendix C - N2 compander log sheets ......................................................... 39
Appendix D - N2 booster compressor log sheets ........................................... 39
Appendix E - Manual control of N2 supply...................................................... 39
Introduction...................................................................................................... 39

1.1

PURPOSE OF MANUAL OPERATION ............................................................................................ 39

1.2

ERROR IN 40PV055 AND 40PV060 .......................................................................................... 39

2.1

IMPORTANT GENERAL NOTES .................................................................................................... 39

2.2

INCREASING INVENTORY ........................................................................................................... 39

2.3

REDUCING INVENTORY ............................................................................................................. 39

2.4

2.3.1 RUNNING COMPANDER DISCHARGE PRESSURE < 36 BARG .............................................. 39
2.3.2 RUNNING COMPANDER DISCHARGE PRESSURE > 36 BARG .............................................. 39
SUPPLY OF NITROGEN FROM SHIP SYSTEM ................................................................................. 39

2.5

MOVING NITROGEN FROM STANDBY COMPANDER TO RESERVOIR ................................................. 39

2

Procedures ...................................................................................................... 39

16

Appendix F - Cause & effect matrix ................................................................ 39

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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 39. Condensate & drain................................................................................... 39

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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

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Introduction

The User Manual contains the basic information that is necessary in order to operate the plant in a
safe and secure manner.

1.1 Purpose
The purpose of this User Manual is to inform the operator on how the plant is designed, its function,
technical data, operation, maintaining, faultfinding and any dangers or hazards that may arise/occur
during operation.
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

Telephone

+47 815 48500

Telephone 24 hrs +47 6610 9660
Telefax

+47 815 48520

Email

Internet

/>
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.

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1.5 Abbreviations and terminology
The following abbreviations and terminology are used throughout this document:
ASV

Anti Surge Valve

Bar a

Absolute Pressure In Bar

Bar g

Gauge Pressure In Bar

BOG

Boil-Off Gas

BOR

Boil-Off Rate

CCR

Cargo Control Room

Cryogenic

Low Temperature Cooling

DACS

Distributed Alarm And Control System

DGV

Diffusor Guide Vanes

ESD

Emergency Shutdown

EMR

Electrical Motor Room

GCU

Gas Combustion Unit

HD

High Duty

HGS

Hamworthy Gas Systems

IGV

Inlet Guide Vane

Laden Voyage

Loaded With Gas

Lag

Standby

LBOG

Liquefied Boil-Off Gas (Condensate)

LNG

Liquefied Natural Gas

MCC

Motor Control Room

N2

Nitrogen

N2 compander

N2-Compressor/Expander Unit

PFD

Process Flow Diagram

PSV

Pressure Safety Valve

RS

Reliquefaction System

R-loop

Reliquefaction Loop

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1.6 Safety precautions
In this user manual the following symbols are used:

Disregard of directions in cautions can result in damage to equipment
CAUTION
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.

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Main Data

2.1 Technical data
Description

Value

Ambient temperature range

HOLD

Cargo tank pressure

0.03 to 0.12 bar g.

Lube oil, BOG compressor

Volume : 650 litre (first filling 700 litre).
Type : ISO VG 46.

Lube oil, N2 compander

Volume : 2000 litre
Type : ISO VG 46.

Lube oil volume, N2 booster compressor

Volume : 23 litre
Type : Anderol 1200N

Refer to the drawing PFD 400573-P-HGS-32675 “Process Flow Diagram” for the design operating
conditions.

2.2 Process Consumptions
Refer to the consumption lists in chapter 10 for further details.

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Technical Description

3.1 General description
The description in this chapter outlines the process of the Moss RS LNG Reliquefaction System
installed on a LNG carrier.
The main purpose of the reliquefaction system is to provide a cargo tank pressure control by liquefying
all vapour boil-off from the cargo tanks during normal ship operations and thus protecting the cargo
tanks from over-pressurising.
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.

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

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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).

•

Auxiliary systems.

The plate fin cryogenic heat exchanger and the separator are assembled in one enclosed unit called
the cold box.
Refer to the document 400573-I-HGS-62791 “QGII Control System Functional Description” for further
details.

Figure 3. Boil-off Loop arrangement

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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 tubefinned 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.

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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).

•

Junction box (11).

•

Oil demister (2).

•

Core unit (12).

nd

•

Actuator for DGV 2 stage (3).

•

Gear box (13).

•

Interconnection gas pipe (4).

•

Main oil pump (14).

st

•

Actuator for DGV 1 stage (5).

•

Coupling (15).

•

Oil cooler (6).

•

Lube oil system. See below.

•

Aux. oil pump (10).

Refer to document Atlas Copco Instruction manual for further details.

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Figure 5. BOG compressor assembly left

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Hamworthy Gas Systems AS
Project no:

QGII Operation Manual

400573

Project Doc no: 400573-R-HGS-69299
Revision:

01

Effective Date:

25.06.2007

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.

Page 18 of 124


Hamworthy Gas Systems AS
Project no:

QGII Operation Manual

3.2.2.2

400573

Project Doc no: 400573-R-HGS-69299
Revision:

01

Effective Date:

25.06.2007

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

Page 19 of 124

Hamworthy Gas Systems AS
Project no:

QGII Operation Manual

Project Doc no: 400573-R-HGS-69299
Revision:

01

Effective Date:

25.06.2007

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
than this rate.

Figure 9. Cold box

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.

Page 20 of 124

400573


Hamworthy Gas Systems AS
Project no:

400573

Project Doc no: 400573-R-HGS-69299

QGII Operation Manual

Revision:

01

Effective Date:

25.06.2007

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
GCU.

Figure 10. Cold box w/ separator

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.

Page 21 of 124


Hamworthy Gas Systems AS
Project no:

400573

Project Doc no: 400573-R-HGS-69299

QGII Operation Manual

Revision:

01

Effective Date:

25.06.2007

3.2.5 LNG pump skid with vent gas 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.

Figure 11. Pump bearing heater

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

Page 22 of 124


Hamworthy Gas Systems AS
Project no:

400573

Project Doc no: 400573-R-HGS-69299

QGII Operation Manual

Revision:

01

Effective Date:

25.06.2007

The LNG pump skid unit consists of
the following main equipment:
•

Pump (1).

•

Pump bearing heater (2)

•

Vent gas heater (3).

•

Control panel (4).

•

Flow element (5) with
transmitter (6).

Figure 13. Flow element w / transmitter

Page 23 of 124


Hamworthy Gas Systems AS
Project no:

QGII Operation Manual

Project Doc no: 400573-R-HGS-69299
Revision:

01

Effective Date:

25.06.2007

Figure 14 LNG pump skid
Refer to document 400573-I-HGS-96657 Technical Manual for further details.

Page 24 of 124

400573


Hamworthy Gas Systems AS
Project no:

QGII Operation Manual

400573

Project Doc no: 400573-R-HGS-69299
Revision:

01

Effective Date:

25.06.2007

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
BOG N2 content.
2. Input during capacity
verification
3. Input to the R-loop capacity
control.

Figure 15. Gas analyser

Page 25 of 124