When fuel oil is subjected to a vacuum in excess of how many inches of mercury does it start to come apart?

• A. 5
• B. 15
• C. 20
• D. 10

Answer: (D)

Vacuum lowers the pressure on a liquid, which in turn lowers the boiling points of its components. Fuel oil isn’t a single compound but a blend of hydrocarbons with a range of boiling points. When the vacuum reaches a moderate level, the lighter fractions in that blend begin to boil off while the heavier portion remains liquid. That separation—the lighter vapors escaping and the oil changing consistency—is what people mean by the oil starting to come apart. This is why fuel oil systems are operated at controlled vacuum levels: too much vacuum can cause premature vaporization and separation of the oil’s components.