Page 275 - Ship Construction.DJ Eyres 6Ed

Ch23-H8070.fm Page 264 Wednesday, October 18, 2006 7:38 AM

23
Liquefied Gas Carriers

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งานห้องสมุด ศูนย์ฝกพาณิชย์นาวี
A large number of ships are in service which are designed to carry gases in
liquid form in bulk. Most of these ships are designed to carry liquefied
petroleum gas (LPG) whilst a much smaller number of ships are designed
to carry liquefied natural gas (LNG).

Liquefied Petroleum Gas (LPG)

LPG is the name originally given by the oil industry to a mixture of petro-
leum hydrocarbons principally propane and butane and mixtures of the
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two. LPG is used as a clean fuel for domestic and industrial purposes.
These gases may be converted to the liquid form and transported in one of
three conditions:

(1) Solely under pressure at ambient temperature.
(2) Fully refrigerated at their boiling point (−30 °C to −48 °C).
(3) Semi-refrigerated at reduced temperature and elevated pressure.

A number of other gases with similar physical properties such as ammo-
nia, propylene and ethylene are commonly shipped on LPG carriers. These
gases are liquefied and transported in the same conditions as LPG except
ethylene which boils at a much lower temperature (−104 °C) and which is
therefore carried in the fully refrigerated or semi-refrigerated condition.

Liquefied Natural Gas (LNG)

LNG is natural gas from which most of the impurities such as sulphur and
carbon dioxide have been removed. It is cooled to or near its boiling point
of −165 °C at or near atmospheric pressure and is transported in this form
as predominantly liquid methane. Methane has a critical pressure of
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45.6 kg/cm at a critical temperature of −82.5 °C, i.e. the pressure and
temperature above which liquefication cannot occur, so that methane can
only be liquefied by pressure at very low temperatures.