Class 4 (4th Engineer) SSEP 📅 Oct 2022

Exam Question

During the loading of a petroleum tanker, the incoming cargo forces out from the tank its previous atmospheric contents (air-vapor-inert gas).

(a) Explain

(i) The dangers of venting during the loading via open tank hatches. (5)

(ii) Why the pressure/vacuum valves are not used for venting. (5)

(b) Describe with a line diagram showing a typical 'Inert Gas Sysetm' used for inerting the cargo tanks of oil tankers. State what oxygen content you would expect in the flue gases if good combustion is achieved. What are the other constituents of 'Inert Gas'? (10)

Reference Answer

### (a)(i) The dangers of venting during loading via open tank hatches.
Venting hydrocarbon vapours from a petroleum tanker via open hatches during loading is an extremely hazardous and prohibited practice. The primary dangers are as follows:
1. **Formation of a Flammable Atmosphere:** The displaced vapour, a mixture of hydrocarbons and air/inert gas, is expelled at a low level and low velocity. This allows a large, dense cloud of flammable gas to form and accumulate on deck, especially in calm weather conditions. This gas mixture can easily fall within its flammable range (between the Lower and Upper Flammable Limits - LFL/UFL).
2. **Presence of Ignition Sources:** A vessel's deck, even during controlled operations, presents numerous potential sources of ignition. These include:

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