Fully pressurized tankers that carry LPG to and from smaller gas terminals
These are generally the smallest type of liquefied gas carrier afloat (up to about 5,000 cubic metres, although some are larger) and carry products at ambient temperatures in cylindrical or spherical steel pressure vessels designed to withstand pressures up to 20 bar. They are not fitted with reliquefaction plant and represent a simple cost- effective means of transporting LPGs and chemical gases to the smaller gas terminals.
Today, most fully pressurized LPG carriers are fitted with two or three horizontal, cylindrical or spherical cargo tanks. However, in recent years a number of larger capacity fully-pressurized ships have been built with spherical tanks.
These were the first generation of ships to carry liquefied gases. The ships have a cargo capacity up to ~ 3,500 m3. These ships carry the cargo in spherical or cylindrical steel tanks, designed for a working pressure of 17.5 kg/cm2. This corresponds to the vapour pressure of propane at 45oC, which is the maximum ambient temperature in which the ship is likely to operate. No means of temperature or pressure control is necessary.
The tanks are generally Type C spheres and no secondary barrier is required. A double bottom is constructed for ballast water. The hold space around the cargo tanks does not need to be inerted.
Type 'C' tanks are normally spherical or cylindrical pressure vessels having design pressures higher than 4 barg. The cylindrical vessels may be vertically or horizontally mounted. This type of containment system is always used for semi-pressurised and fully pressurised gas carriers.
Fig:Fully pressurized LPG carrier at sea
Type 'C' tanks are designed and built to conventional pressure vessel codes and, as a result, can be subjected to accurate stress analysis. Furthermore, design stresses are kept low. Accordingly, no secondary barrier is required for Type 'C' tanks and the hold space can be filled with either inert gas or dry air and for fully pressurised tankers normal air may be allowed.
In the case of a typical fully pressurised tanker (where the cargo is carried at ambient temperature), the tanks may be designed for a maximum working pressure of about 18 barg. For a semi-pressurised tanker the cargo tanks and associated equipment are designed for a working pressure of approximately 5 to 7 barg and a vacuum of 0.3 barg.
Advantages of fully pressurized tankers:
i) They are built with ordinary grades of steel as the cargo is carried at ambient temperature and no insulation is required
ii) no reliquefaction plant is required
iii) operations are simpler
i) Due to their shape, the use of underdeck space cannot be optimised
ii) high design pressure requires considerable tank wall thickness, with consequent increase in displacement weight and cost
iii) the weight in tons of cargo carried is lower than for a refrigerated ship of similar size, due to cargo density difference
Fig:Fully pressurized LPG carrier
External links :
Copyright © 2011 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 !