LNG handling |||
LPG handling||| Other Gas products|||
Fire & Safety|||
Emergency response |||
Cargo handling and monitoring equipments for Liquefied Gas carriers
Liquefied gas carriers fitted with cargo,
booster, ballast and stripping pumps, lines, eductors and their associated instrumentation and controls should be in good order and evidence of regular testing should be kept. Instrumentation, valves and pipe work should be clearly marked to indicate their service and where applicable the compartment to which they relate.
Equipment used for Cargo Operations, varies with the type of vessel and its age. Some of the
equipment which may be found on board is listed below. Other items of equipment will be found
on board and the instruction manuals for the equipment and Ship Specific Operating Manuals
should be consulted for details.
- Inert Gas and Dry Air Generator
- Nitrogen Generator
- Cargo / Spray pumps
- Compressor ( High & Low Duty)
- Cargo Heaters (High and Low Duty)
- LNG Vaporizer
- Forcing Vaporizer
- Mist Separator
- Vacuum Pumps
- Fixed Gas detection Systems
- Emergency shutdown system (ESD) and Cargo Tank Protection System
- Ship Shore Link
- Relief Systems
- Cargo Tank Gauging Systems
Inert gas, dry air & Nitrogen generator
Many LNG vessels are equipped with an inert gas generator which may also be used to produce
dry air. The inert gas and/or dry air is used for the inerting and gas freeing of cargo tanks, cargo
pipes and void spaces when required prior to and after a refit or inspection period.
Details of Inert gas, dry air & Nitrogen generator
Cargo & spray pumps
LNG ships are typically fitted with submerged, electric, centrifugal cargo pumps. The motor
windings are cooled by the pumped LNG which also serves to lubricate and cool the pump and
motor bearings. As the LNG serves as both lubricant and coolant, it is critically important that the
pumps are never allowed to run dry, even for short periods.
In addition to main cargo pumps, each tank will also be served by a spray pump. This pump is of
limited capacity, typically around 50m3/hr, and will be used for the following:
- To cool down the liquid header prior to discharging.
- To cool the cargo tank during a ballast voyage prior to arrival at the loading terminal by
discharging LNG to the spray nozzles in the tanks.
- In exceptional circumstances, to pump LNG from the tanks to the vapourisers when forced
vaporisation of LNG to the boilers is required.
- To enable the tanks to be stripped as dry as possible for reasons such as tank entry.
In the case of total cargo pump failure, provision is made for Moss ships to discharge under
On LNG vessels where cargo pumps are 440V supply, insulation test are to be carried out before
arrival in both the loading port and discharge ports. Also during insulation test, air temperature and
humidity must be recorded.
The insulation tests will establish that all pumps are operational and to allow time for the
preparation of emergency pumping arrangements should it be necessary.
On newer LNG vessels, the insulation test should be done before pump removal and after pump’s
re-installation in the tank during DD period and it is not necessary to take readings monthly.
However if cargo tanks are in a gas free condition and pumps have not been operated for some
considerable time, readings should be taken before the pumps are brought into operation again.
The preferred time may be when LNG is being loaded.
Reference should be made to on board documentation for procedures for starting, stopping and
operating cargo and spray pumps, together with the specific arrangements for rigging emergency
Compressor ( High & low duty)
High duty (HD) compressors are installed in the compressor room on deck and are routinely used
for compressing the LNG vapour for return to shore during cargo tank initial cool down, cargo
loading, tank purging and to circulate heated cargo vapour through the tanks during warming up.
Low duty (LD) compressors are installed in the compressor room on deck and are routinely used
for compressing the LNG vapour produced by natural boil-off to a sufficient pressure to be used in
the boilers as fuel.
The HD and LD compressors are normally driven by electric motors or steam, installed in an
electric motor room segregated from the compressor room by a gas tight bulkhead. The drive
shafts penetrate the bulkhead with a gas tight shaft seal.
Cargo heater ( High & low duty)
Steam heated cargo heaters are provided for the following functions:
The heaters are typically heat exchangers of the shell and tube type.
The number of plugged tubes in cargo condensers, heaters or vaporizers should not exceed 25%.
Heating the LNG vapour, delivered by the HD compressors, to the specified temperature for
warming up the cargo tanks before gas freeing.
- Heating the boil-off gas, delivered by the LD compressors, or by free-flow, prior to
supplying it to the boilers or venting to atmosphere.
The LNG vaporiser is a shell and tube type heat exchanger that is used for vaporising LNG liquid
for the following operations:
- Exceptionally, when discharging cargo at the design rate without the availability of a vapour
return from the shore. If the shore is unable to supply vapour return, liquid LNG is fed to
the vaporiser by using one stripping pump or by bleeding from the liquid header. The
vapour produced leaves the vaporiser at approximately –140 degree C and is then supplied to
cargo tanks through the vapour header.
Vapour pressure in the cargo tanks will normally be
maintained at 110kPa abs. (minimum 104 kpa) during the whole discharge operation.
Additional vapour is generated by the tank sprayer rings, the LNG being supplied by the
stripping/spray pump. If the back pressure in the discharge piping to shore is not sufficient
to have a minimum of 300kPa at the inlet to the vaporiser, a stripping/spray pump will be
used to supply liquid to the vaporiser.
- Purging of cargo tanks with vapour after inerting with inert gas and prior to cool down. LNG
is supplied from the shore to the vaporiser via the stripping/spray line. The vapour
produced at the required temperature of +20 degree C is then passed to the cargo tanks.
- Emergency forcing by manual operation. The LNG vaporiser can function as the forcing
vaporiser when the forcing vaporiser has failed.
The forcing vaporiser is used for vaporising LNG liquid to provide gas for burning in the boilers to
supplement the natural boil-off. The LNG is supplied by a stripping/spray pump. LNG flow is
controlled by an automatic inlet feed valve which receives its signal from the Boiler Gas
Each forcing vaporiser is equipped with a temperature control system to obtain a constant and
stable discharge temperature for various ranges of operation. The temperature of the gas
produced is adjusted by spraying a certain amount of bypassed liquid into the outlet side of the
vaporiser through a temperature control valve and liquid injection nozzles.
The mist separator shall prevent liquid from entering the compressors. It receives natural boil off
from the cargo tanks and forced boil off gas from the forcing vaporizer.
On GT96 membrane cargo containment vessels, the vacuum pumps where fitted are used to
evacuate the atmosphere within the primary and secondary spaces in the following cases:
- To replace air with nitrogen for inerting.
- To replace methane with nitrogen for gas freeing before dry docking after there has been a
leakage of cargo.
- To test the tightness of the membranes at regular intervals or after membrane repairs
- When the associated tank is opened up.
- It also helps pull the tank membrane on to the associated supports and insulation when the
cargo tank is not pressurised.
Care must be taken to ensure that the pressure within the primary space is not reduced below that
in the secondary space as there is a danger of distorting the secondary barrier by lifting it off its
supporting insulation. A maximum pressure difference of 3kPa should not be exceeded.
Fixed gas detection systems
There are two types of gas detection system commonly used on board LNG carriers, a sampling
system and a gas detection system incorporating remote heads.
The sampling system draws gas samples from each monitored location into a central analyser
located in a ‘safe’ area. Typically, samples will be drawn from cargo areas in a pre-programmed
sampling sequence and will be passed through an infrared analyser. The system alarms if pre-set
limits are exceeded.
Remote detector heads may also be used to monitor gas concentrations. The signal from
flameproof infrared gas detectors will be passed to a central control unit having visual and audible
Emergency shut down ( ESD) and cargo tank protection
The emergency shutdown (ESD) system is a requirement of the IMO code for the carriage of
liquefied gases in bulk and is a recommendation of SIGTTO. It is fitted to protect both the ship and
terminal in the event of power loss, cryogenic or fire risks, on either the ship or in the terminal.
The system will stop the flow of LNG liquid and vapour by shutting down the pumps and gas
compressors as well as manifold and shipside valves, by the activation of a single control. Shut
down of the cargo system can be initiated either manually or automatically if certain off-limit
Details of cargo emergency shutdown procedure
Ship shore link
Linked ship/shore emergency shut down systems have been recommended by SIGGTO since the
early days of LNG transportation and are now mandated by IMO. The ship and terminal emergency
systems are linked via a ship-shore umbilical that carries ESD, telecommunications and data
As required by IMO, each cargo tank is fitted with two pressure/vacuum relief valves. In addition,
on membrane ships, the primary and secondary insulation spaces around each tank are protected
by two pressure relief valves. On Moss Rosenberg ships, hold spaces around each tank are
The cargo tank relief valves vent to their associated vent mast riser. The valves are of the pilot
operated relief valve type. A cargo tank pressure sensing line relays the pressure directly to the
pilot operating valve. In this manner, accurate operation is assured at the low pressures prevailing
inside the tank.
It is extremely important that the vent mast is checked at regular intervals and drained of any
accumulation of water. This is to ensure that the relief valves operate at their correct settings
which would otherwise be altered if any water were to accumulate in the vent mast and flow onto
the valve assembly.
In addition to the cargo tank and hold or interbarrier space relief valves, each section of the cargo
pipework that can be isolated by two valves will be fitted with an overpressure relief valve.
Arrangements for safely relieving pressure in the lines to the cargo tanks will vary from ship to
Cargo tank gauging systems
All gauging systems used are specifically designed for the extreme low temperatures experienced
on LNG carriers.
Various systems may be fitted to a vessel dependent upon the owner’s specifications and cargo
containment system. There will generally be at least two independent gauging systems fitted to
each tank, in addition to low, high and high-high level alarms
FLOAT ACTUATED GAUGES – these employ a float connected by an invar tape to a tensator spring.
This spring acts as a counter balance system, maintaining a constant tape tension at the float. This
ensures that the float maintains the same level of immersion irrespective of the amount and weight
of the tape paid out. The accuracy of this system is dependent upon tank construction and on the
operating conditions, however the accuracy should remain within 1 cm.
CAPACITANCE TYPE GAUGES – these gauges operate using the variation of electrical capacitance
between two probes when a liquid level changes. A coaxial sensor is installed within a tank, and is
constructed of a number of individual segments, depending upon the height of the tank. As the
liquid level in the tank changes, the capacitance varies.
RADAR TYPE GAUGES – these gauges operate by generating and transmitting radar waves from a
generating device mounted externally on the tank. As the speed of the radar waves is known, if
the time needed by the signal to reach the cargo liquid level, bounce back and be picked up by the
antenna, can be measured accurately, the cargo ullage can be calculated.
ULTRASONIC TYPE GAUGES – these gauges operate in a similar manner to an echo sounder,
where the time taken for a sound wave to be reflected back through the liquid is accurately
measured, and then used to calculate the liquid level.
All cargo measuring systems in use are highly accurate, and form part of the Custody Transfer
System, which is checked and verified by an independent organisation during vessel dry docking
periods. A certificate of accuracy for the system will be issued.
Generally if any ship repairs are carried out on any gauging system, it will be necessary for the
gauge to be re-calibrated and a new certificate issued.
The vessel will carry out and record comparison checks of the various gauging systems in use
during each cargo operation, to enable the early detection of any problems with any of the
systems. Where the completion of these tests reveals any significant errors the Company is to be
advised immediately with a request for attention.
Cargo piping system
Cargo piping systems comprise of the following and to be kept in order :
- Liquid lines ( or headers)
- Vapour lines
- Spray lines
- Fuel gas line
- Vent line
- Inerting / Aeration line
Details of various cargo pipe lines
Cargo pump emergency shut down system
Pump alarms and trips, level alarms, etc., where fitted, should be tested regularly to ensure that they are functioning correctly, and the results of these tests should be recorded
Cargo and ballast system valves;To be kept in order
Cargo system innage gauges;To be kept in order
Remote and local temperature and pressure sensors and gauges Each cargo tank should be provided with at least two devices for indicating cargo temperatures, one placed at the bottom of the cargo tank and the second near the top of the tank below the highest allowable liquid level. The temperature indicating devices should be marked to show the lowest temperature for which the cargo tank has been approved by the administration.
The vapour space of each cargo tank should be provided with a pressure gauge which should incorporate an indicator in the cargo control position.
Fig:Liquefied natural gas ship on sea passage
Cargo tank high level & overflow alarms;To be kept in order
Within 5 days of the ship’s estimated time of berthing, the following checks and tests shall be carried out, and the results recorded. These records are to be made available to the terminal upon request.
(1) Deck water spray line
(2) Water curtain
(3) Gas free condition of hold space
(4) Alarm function of fixed gas detection equipment
(5) Cargo gauging system and alarm set points.
(6) Operation of the emergency shutdown system(ESD) the permitted operation period for emergency shut-down equipment of up to 30 seconds
(7) Operation of cargo system remote control valves and their position indicating systems.
(8) Confirm Cargo transfer emergency stops fully operational and date of last test.
(9) Confirm tank high level and pressure alarms operational.
(10) Confirm that remotely operated manifold valves have been operated through a complete open/closed cycle, functioning and advise valve type(ball,gate, etc)and actual closing time. The corresponding records shall be produced by the master on the ship arrival at berth. Any defects or deficiencies must be reported to the terminal as an addendum to the Pre-Arrival information notice
(11) Deep well cargo pump and booster pump mechanical seals are free of oil leaks.
More Info pages
- Gas measuring equipments calibration procedure
- Cargo piping system
- Connection and disconnection of cargo hoses and hard arms
- Volatile nature of liquefied gases
- How to achieve maximum drainage of liquid during discharge
- The hazards of liquefied gases - Cargo information and safety factors
Procedures for various cargo handling equipment onboard
- Personal protective equipments for people working onboard gas carriers
Procedure for cargo planning in Liquefied Gas Carriers
Details of various cargo handling equipment onboard
Cargo piping layout for LNG carriers
Procedure for commissioning the cargo system
Preparation for Cargo Transfer
Procedure for discussion prior cargo transfer
Procedure for loading Liquefied Gas Cargoes
Procedure for Cargo Conditioning in Liquefied Gas Carriers
Cargo Transfer between Vessels (STS Operation)
Procedure for segregation of Liquefied Gas Cargoes
Procedure for Stripping Liquefied Gas Cargoes
Procedure for Changing Liquefied Gas Cargoes
Displacing with Vapour of the Next Cargo (Purging)
Procedure for Water washing after Ammonia Cargoes
Details of various cargo handling equipment onboard
Custody Transfer Measurement (CTM) System
Records of the calibration of key cargo instrumentation, including temperature and pressure gauges
The high level alarm system
Gas analyzing equipment
Decontamination showers and an eye-wash
Emergency response & contingency plans
Cargo conditioning, reliquefaction and boil-off control requirement for a liquefied gas carrier
Cargo Containment Systems in Liquefied Gas Carriers
cargo emergency shutdown requirement
damage stability guideline for liquefied gas carriers
Various Cargo handling equipments onboard
Cargo hoses connection guideline
Documents accompanying a liquid gas cargo
How LNG transferred from shore to ships cargo tanks ?
Cargo operation guideline onboard a liquefied gas carrier
Cargo piping Systems in Liquefied Gas Carriers
cargo planning requirement
cargo and pumproom safety precautions
cargo stripping guideline
Emergency response for cargo system leaks
Emergency response for cargo tank rupture
Risk of overfilling of cargo tank during loading onboard a liquefied gas carrier
Preparation for cargo transfer
cargo transfer between vessels- safety guideline
// Home page///
LNG handling ///
Sea transport ///
///Emergency response ///
Copyright © Liquefied Gas Carrie.com All rights reserved.
The content published in this website are for general reference only. We have endeavoured to make the information
as accurate as possible but cannot take responsibility for any errors. For latest information please visit www.imo.org .
Any suggestions, please Contact us !
///Links &Resources //