Page 74 - International safety guide for oil tankers and terminals

INTERNATIONAL SAFETY GUIDE FOR OIL TANKERS AND TERMINALS

of the gas mixture as it issues from the opening becomes greater so that
there is a greater tendency for the plume to sink.
requirements.รศึกษาเท่านั้น---
At low concentrations, therefore, a flammable zone similar in outline to
that shown in Figure 2.3 (c) is to be expected, but it is likely to be small
atmosphere. Effects of this kind occur, for example, in gas freeing. If fixedย์นาวี
because of the relatively small amount of hydrocarbon gas. As the
concentration increases, the flammable zone tends to assume such
shapes as depicted in Figures 2.3 (b) and 2.3 (a) as the increasing
density exerts its influence. In addition, the overall size of the zone
becomes greater due to the greater rate of emission of hydrocarbon gas.

2.5.4.5 Cross-Sectional Area of the Opening
งานห้องสมุด ศูนย์ฝกพาณิช
The area of the opening through which the hydrocarbon gas/air mixture
issues determines, for a given volumetric rate of flow, the linear flow
velocity and hence the efficiency of the mixing of the plume with the

turbo-blower fans are used, the mixture is usually vented through a
standpipe with a cross-sectional area small enough to give a high velocity
and to encourage dispersion in the atmosphere. When using small
portable blowers, which normally have to be operated against a low back
pressure, it is usual to exhaust the gas through an open tank hatch. The
outflow velocity is then very low with the outlet close to the deck;
circumstances that encourage the gas to remain close to the deck.
ึ
---ใช้เพื่อกา
2.5.4.6 The Design of the Vent Outlet
The design and position of a vent outlet must comply with current SOLAS

In certain operations, such as gas freeing, vapour may be vented from the
tank through apertures other than these designated tank vents.

2.5.4.7 Position of the Vent Outlet
If vent outlets are situated near structures such as accommodation
blocks, the shape of the flammable zone is influenced by turbulence
produced in the air as it passes over the superstructure. A diagram
illustrating the kind of eddies formed is given in Figure 2.4. This shows
how, on the upwind side, there are downward eddies below a level
indicated by the line X-X and how, above and in the lee of the structure,
there is a tendency for turbulent air to form eddies close to the structure.

These movements can adversely affect the efficient dispersion of
hydrocarbon gas.

If the exit velocity from an opening near a structure is high, it can
overcome the influence of eddies.

For example, Figure 2.5 (a) shows the flammable zone from a tank
opening situated only about 1.5 metres upwind of an accommodation
block; the plume is almost vertical and only just touches the
accommodation block. However, a somewhat lower rate of venting would
have resulted in serious impingement of the zone upon the
accommodation block.

Figure 2.5 (b) illustrates the effect of an additional opening which doubles
the amount of gas released. Partly as the result of eddies and partly due

40 © ICS/OCIMF/IAPH 2006