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How and For What Liquefied Petroleum Gas Reliquefaction Plants Work

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This article covers basic information about Liquefied Petroleum Gas (LPG) Reliquefaction Plant, why reliquefaction are important, systems of reliquefaction, rules and requirements.

Carriage of LPG by Sea

  • 1931 Agnita – build to carry petroleum in cylindrical tanks.
  • 1934 Megara – Modified LR classed oil tanker carries LPG in small pressurised tanks.
  • Late 1930’s to 1950’s – Gradual development, at first coastal USA, then international.
  • 1959 – First semi-pressurised gas ship.
  • 1961 – First fully refrigerated gas ship.

LPG Gas Ship over 10 000 gt – by year

Graph of the grow LPG fleet
LPG Gas Ships above 10 000 gt

LPG Gas Ship between 5,000 and 10,000 gt

Grow of LPG fleet
LPG Gas Ships between 5 000 and 10 000 gt

Why is LPG reliquefaction required?

Boiling point of LPG cargoes

Boiling point of various cargoes at atmospheric pressure:

  • Propylene: -47,7 °C.
  • Commercial propane (2,5 %): -45,2 °C.
  • Propane: -42,1 °C.
  • Ammonia (anhydrous): -33,3 °C.
  • Vinyl chloride (VCM): -13,8 °C.
  • iso-Butane: -11,7 °C.
  • Butylenes: -6,3 °C.
  • Butadiene: -4,4 °C.
  • Butane: -0,6 °C.
Gas temperature-pressure relationship
Temperature – pressure relationship for LPG cargoes

Saturation pressure temperature relationship

If left the cargo will equalise at the saturation pressure associated with the surrounding ambient temperatures.

As an example – take propane as the cargo:

Boiling point-42,1 °CPressure1,013 bar a
IGC Code sea water32 °CPressure11,3 bar a
IGC Code air45 °CPressure15,3 bar a
The International Maritime Dangerous Goods (IMDG) Code requires large (> 1,5 m dia) bare propane cylinders to be designed for:
IMDG Code60,5 °CPressure21,4 bar a

Standard configuration

So what causes the pressure to increase in the cargo tanks?

During transportation:

  • Heat ingress into the cargo tank through insulation.
  • Mechanical energy due to liquid sloshing.

During loading:

  • Heat ingress through tank walls/insulation.
  • Heat from liquid headers, manifolds & loading arms.
  • Heat energy from cargo tank and insulation.

Insulation, boil–off and plant capacity

LPG Carriage Conditions

Fully pressurised – 17,5 bar g (deck tanks):

  • No boil-off, cargo at ambient temperature.
  • Tank not fitted with insulation.

Semi-pressurised – ≅ 4,5 bar g:

  • Restricted boil-off, cargo temperature controlled.
  • Insulation & reliquefaction plant fitted – full or part duty.

Fully refrigerated – 0,25 bar g (at sea):

  • Boil-off rate restricted by insulation.
  • Cargoes at near ambient pressure.
  • Reliquefaction plant capable of maintaining 2 cargoes.

Types of LPG insulation:

  • Natural free flowing – boxed perlite.
  • Natural blanket – Rockwool, glasswool for infill insulation.
  • Man-made foam – polyurethane.
Type of thermal insulation
Thermal insulation

Heat transfer and boil-off rate

Scheme of Type A tank
LPG – Type A Independent Tank. Heat ingress from ambient air and ballast tanks
  • Heat travels in one direction – hot to cold.
  • Modes of heat transfer:
    • Conduction – greatest effect.
    • Convection – free convection most applicable.
    • Radiation – extremely small, may be discounted.
  • Cargo change of phase consideration – heat of vaporization (latent heat of evaporation).
  • Effects of insulation thickness on boil-off rate.
  • Tank volume verses surface area relationship.
Volume – Area: relationship
Sphere volume relationship
Volume – Area: Graph
Prismatic Tank Relationship

Reliquefaction plant capacity

Dependant on pressure temperature relationship:

  • Fully Pressurised – Small Type C no plant.
  • Semi-Pressurised – Type C but partly refrigerated.
  • Fully Refrigerated – Near atmospheric (250 mbar) refrigerated and standby.

Reliquefaction plant utilising cascade system has duty split between reliquefaction and refrigeration circuits.

Current trend is to reliquefy the cargo boil-off gas in a cargo gas system – single or multi stage.

Type C tank volume limited due to scantlings.

Graph of the pressure
Pressure enthalpy diagram for propane
Cargo boil-off rate – input data
Diagram of Aft LPG Cargo Tank – Layout & AreasAreaForward SectionAft Section
Scheme of LPG TankDeckhead424,65347,44m2
Upper Hopper264,87216,71m2
Lower Hopper151,68205,43m2
Pump dome neck41,62m2
Area – subtotal2 547,22 090,9m2
Total area4 638,18m2
Area less bottom3 431,78m2Projected
Cargo boil-off rate results
Cargo Tank Details
Total volume of cargo tanks82 186m3
Total volume available for cargo at 98 % full80 543m3
Weight of cargo carried47 079 437kg
Heat Leakage and Boil-Off Rates
Maximum heat flow – insulation layer only397 920W
Boil-Off rate0,1673%
Maximum heat flow – containment system390 085W
Boil-Off rate0,1640%

System types and requirements

Refrigeration reliquefaction & evaporation

What is refrigeration?

“The process of taking heat energy from a place it is not wanted to a place where it’s dissipation does not matter” (not have an environmental effect)

What is reliquefaction?

“A process where the boil-off vapour is collected, recondensed and returned to the vessel as a liquid”.

Read also: Birth of the Reliquefaction, Design and Operation of the Reliquefaction LPG Plant

Evaporation – conversion of a liquid to vapour.

Evaporative cooling – reduction of heat energy which provides a cooling effect on remaining liquid.

Reefer system
Simple refrigeration system – Reefer system

Reliquefaction plant capacity

Capacity to deal with heat input entering all of the cargo tanks during a loaded voyage:

  • Have a reasonable margin in plant output over maximum load.
  • Due regard to be given to additional capacity to deal with cargo loading conditions.

Suitable standby unit or an alternative means of controlling the cargo pressure/temperature.

Capability to be confirmed by testing. Special RMC (LG) notation will be assigned.

Reliquefaction systems

Classical cascade system – two stage by design:

  • Refrigerant used in the initial stage.
  • Cargo gas used in final stage.

Direct cycle – multi stage (up to 3):

  • Number of stages dependant on cargo.
  • Cargo gas used in each stage.
  • Two or three cylinder compressors used.

Ethylene – separate refrigerant condenser.

Scheme of reliquefaction plant
Reliquefaction plant – Cascade system
Scheme of stage 2
Reliquefaction plant – Cargo gas, 2 stage

Ethylene & ethane reliquefaction systems

Cascade system for ethylene/ethane due to temperature:

  • Cargo gas used in multi-stage system as normal.
  • Refrigerant used in alternative condenser.

Compressor problems due to low suction temperature:

  • Suction temperatures colder than -80 °C.
  • Permanently heated cooling water circulated.
  • Cylinder manufactured from graphite iron casting.
  • Impact test requirements for materials.

Increase in flash gas at expansion device on return of condensate to the cargo tanks.

Work of Reliquefaction Plant
Reliquefaction Plant – Ethylene cascade, 2 stage

Reliquefaction compressors

  • Suction Gas Engineering.
  • Burckhardt (Sulzer).
  • York Refrigeration (Sabroe).
  • Grasso (Grenco Engineering).
Example of reliquefaction compressors
Reliquefaction compressors
Compressor cargo requirements
Discharge temperature set points – to stop polymerisation
Butadiene< 60 °C
Vinyl chloride (VCM)< 90 °C
Other cargoes150/160 °C
Compressor suction pressure limits – to prevent dilution of oil
Butadiene & vinyl chloride (VCM)0,5 bar g
Butane1,5 bar g
Other cargoes5,0 bar g
Butadiene to operate in single stage mode as possibility of condensate forming in the intermediate pressure system
Open table in new window

Vertical cargo condenser

Cargo compatibility
CargoUN NumberAmmonia (anhydrous)1,3-butadiene1,3-Butadiene/Butane mixturesButane (iso and normal)Butenes (Butylene)Diethyl etherDimethylamineEthylamineIsopreneIsopropylaminePropaneLPG (propane/butane mixtures)Commercial propane – < 5%PropyleneVinyl chloride (VCM)Vinyl ethyl ether
Ammonia (anhydrous)1005000000000000000
1,3-Butadiene/Butane mixturesMixture100010100000000
Butane (iso and normal)1011000000000000000
Butenes (Butylene)1012000000000000000
Diethyl ether1155000000000000000
LPG (propane/butane mixtures)Mixtures000000000000000
Commercial propane – < 5 %1978000000000000000
Vinyl chloride (VCM)1086000000000000000
Vinyl ethyl ether (stabilised)1302000000000000000
Open table in new window

Buckhardt (Sulzer) 2-cylinder oil-less compressor

A further option – burning LPG

Chapter 16, paragraph 16.1.1 of the IGC Code states:

  • Methane (LNG) is the only cargo whose vapour or boil-off gas may be utilized in machinery spaces of category A and in such spaces may be utilized only in boilers, inert gas generators, combustion engines and gas turbines.

The USCG Code of Federal Regulations (46 CFR 154.703):

  • Methane (LNG) can be used in boilers, inert gas generators, and combustion engines in main propelling machinery space that use boil-off gas as fuel.

Burning LPG – can it be allowed?

IGC Code states that methane (LNG) is the only cargo whose vapour or boil-off gas may be utilized in machinery spaces of category A.

In accordance with SOLAS Chapter II-2, Regulation 3, paragraph 31.2, Machinery Spaces of Category A are:

  • Which contain internal combustion machinery used for purposes other than main propulsion where such machinery has in the aggregate a total power output of not less than 375 kW.

For US controlled waters; guidance to be obtained from Commandant (G-MSO).

Burning LPG – current proposal

The use of LPG as a fuel in diesel generator engines has already been developed. In conjunction with MAN Diesel (Holeby), Lauritzen Kosan has installed such a system in a series of by LPG gas carriers built in Korea.

The LPG system is considered as an additional fuel:

  • Used within a specific range and at the extremities of the engine’s maximum continuous rating (MCR).
  • The LPG gas forms only a small proportion of the engine’s required fuel load.
  • Currently only proposed to be used to allow “environmentally friendly” incineration of the remaining cargo when changing grades.
Dual fuel system
LPG as additional fuel – Dual fuel system

Lloyd’s Register design involvement, IGC Code and Rule requirements

Lloyd’s Register Rules and other requirements

  • IGC Code – Chapter 7 LPG reliquefaction.
  • Lloyd’s Register Rule requirements for reliquefaction plant – Part 6 Chapter 3.
  • International Statutory Regulations applicable to all ship types, eg SOLAS, etc.
  • USCG requirements for trading to USA Code of Federal Regulations, 46 CFR Section 154 refers.
  • IGC Code – proposal being actioned through IACS and SIGTTO to force the IMO to update the current code.

Rules for Ships for Liquefied Gases and IGC Code

  • Ship Arrangement.
  • Cargo Tank Location.
  • Ship Survival.
  • Cargo Containment.
  • Ship Structure.
  • Materials & NDE.
  • Fire Protection.
  • Cargo Piping.
  • Ventilation.
  • Cargo Handling.
  • Electrical.
  • Instrumentation.
  • Personnel/Operational.
  • Cargo as Fuel (LPG?)

Survey Items – Gas Carriers

Refrigeration & Reliquefaction Plant:

  • Cut-outs and safety stops.
  • Control sequences – unloading gear.
  • Tank temperatures & pressures.

Cargo Tanks and Void Spaces:

  • Condition of insulation and vapour seals.
  • Location and extent of cold spot or ice formations – done from void space.


Independent tanks, type A (MARVS < 0,7 bar)Type A LPG gas carriers will increase in size:

  • Reliquefaction will continue.
  • Volume of boil-off may require cascade systems to be used.
  • Insulation thickness will remain at 120 mm.

Independent tanks, type C (MARVS < 0,7 bar)Type C gas carriers:

  • Smaller “shuttle” tankers will still needed.
  • Bi-lobe and cylindrical tanks used dependant on size.

Use of LPG as fuel – clarification at IGC Code update/rewrite:

  • USCG may be a stumbling block.

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Ноябрь, 03, 2022 552 0
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