Marine Installation Manual
2021-08
4-40
4 Ancillary Systems
4.4 Fuel gas system
X72DF
Free-standing –
Type B tank
Same as the Type A tank, the Type B tank is also designed to be non-pressurised
and will respond to pressure increase by expansion. Consequently, it is also as
important to control the pressure increase. The Type B tank design is based on a
fail-safe concept. From crack monitoring and analysis, the ‘Leak before Failure’
approach of this design means that fatigue is progressive and not catastrophic.
Therefore, it is only necessary to have a partial secondary barrier to hold the cal
-
culated maximum leakage. In the past, the Type B tank, referred to as the Moss
type (see
), was spherical. This curve-designed shape supports the
even distribution of stress across the tank walls. Nowadays however, the Type B
tank (same as the Type A tank) is also available in a prismatic shape, having a
partial secondary barrier.
Figure 4
-
21 Section view of a free-standing Type B tank of moss design
Free-standing –
Type C tank
Type C tanks are designed using conventional pressure vessel codes for pressure
ranges above 2 bar(g). The most common shapes for this type of tank are cylin
-
drical and bi-lobe, which can be either vertically or horizontally mounted de
-
pending on the available space (
). While the cylindrical shape does
not utilise the available space in an optimal way, the bi-lobe shape utilises the
available space more effectively. The intersecting design of two cylinders makes
use of the space between the two single cylinders, which otherwise would be not
be utilised. No secondary barrier is required for Type C tanks, but instead, gas
leakage detectors are placed in the hold space.
Figure 4
-
22 Section view of different free-standing Type C tanks installed below the
deck
SM-0742
Primary
barrier
Drip-tray for
detection leakage
External
insulation
Weather protective
cover
Support tube
for access
Air-gap
Cylinders
Hold space
Hold space
Single cylindrical tanks
Bulkhead
SM-0743
Bi-lobe tanks
