Download FAMILIARIZATION GAS TANKERS INTRODUCTION

We find that the amount of steam, which is possible to contain per volume unit, remains
constant when the steam’s temperature is equal to the condensation point at the set pressure.
The room cannot absorb more vapour, it is saturated with steam and called “saturated”. If the
piston is pressed outwards, the pressure will still show constant. The conclusion is:
With temperature equal to the condensation point by set pressure, steam is
saturated.
Steam above boiling water is saturated.
Saturated steam with a set temperature has a set pressure. This is called
saturation pressure.
With constant temperature saturated steam cannot be compressed.
This also concerns vapour as saturated steam of other gases. Using the same cylinder
arrangement as before.
The cylinder contains saturated steam, no water. The piston is drawn outward. When no water
exists over the piston no new steam will be supplied underneath. The manometer will now
show reduced (falling) pressure as the steam expands. When saturated steam expands without
supplying new steam, it is called unsaturated steam. The room has capacity to collect more
steam.
Unsaturated steam contains lower pressure than saturated steam at the same temperature. The
unsaturated steam in the cylinder can be made saturated again in two ways. Either by pushing
the piston inward to the originated position, or let the unsaturated steam be sufficiently cooled
down. When the temperature is reduced, the saturation pressure will reduce. Unsaturated steam
will, in other words, have a too high temperature to be saturated with the temperature it
originally had. Therefore, this often is referred to as superheated steam.
2.4
HAZARDS FROM LIQUEFIED GAS
This section deals with the properties common to all or most bulk liquefied gas cargoes. These
cargoes are normally carried as boiling liquids and, as a consequence, readily give off vapour.
The common potential hazards and precautions are highlighted in the following sections.
2.4.1
Flammability
Almost all cargo vapours are flammable. When ignition occurs, it is not the liquid which burns
but the evolved vapour. Different cargoes evolve different quantities of vapour, depending on
their composition and temperature.
Flammable vapour can be ignited and will burn when mixed with air in certain proportions. If
the ratio of vapour to air is either below or above specific limits the mixture will not burn. The
limits are known as the lower and upper flammable limits, and are different for each cargo.
Combustion of vapour/air mixture results in a very considerable expansion of gases which, if
constricted in an enclosed space, can raise pressure rapidly to the point of explosive rupture.