TABLE OF CONTENTS 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).
LPG is the name originally given by the oil industry to a mixture of petroleum hydrocarbons principally propane and butane and mixtures of the 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:
Solely under pressure at ambient temperature.
Fully refrigerated at their boiling point (-30 C to -48 C).
Semi-refrigerated at reduced temperature and elevated pressure.
A number of other gases with similar physical properties such as ammonia, 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.
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 45.6 kg/cm 2 at a critical temperature of -82.5 C,...
