Which of the following represents the maximum percent of oxygen, by volume, permitted to be discharged by a ship's inert gas system when operating properly?
• Inert gas system purpose – Why cargo tanks on tankers are kept with very low oxygen levels • Fire triangle – How oxygen percentage affects the ability of hydrocarbon vapors to burn or explode • Relevant standard from 46 CFR and SOLAS for inert gas systems – maximum allowed oxygen content in the inert gas line or in the tank
• Think about the oxygen level at which hydrocarbon vapors can no longer support combustion. Is that closer to normal air (21%) or much lower? • In practice, inert gas systems must give a safety margin below the limiting oxygen concentration. Which choice best reflects a conservative safety margin? • Consider typical values you may have seen for ‘inerted’ tanks on crude oil or product tankers. Which answer is closest to that commonly cited limit?
• Verify what percentage of oxygen by volume is specified in regulations/industry standards as the maximum allowed for inert gas delivered to cargo tanks. • Check that the value you pick is clearly below the minimum oxygen concentration needed for hydrocarbon combustion. • Make sure the choice you select reflects a strict safety standard, not just a small reduction from normal atmospheric oxygen.
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