Fuel oil begins to come apart when subjected to a vacuum exceeding how many inches of mercury?

• A. 8 inches Hg
• B. 10 inches Hg
• C. 12 inches Hg
• D. 6 inches Hg

Answer: (B)

When you pull a vacuum on fuel oil, you’re lowering the pressure above the liquid. Fuel oil is a mix of hydrocarbons with a range of boiling points, so if the pressure gets low enough, the lighter fractions can flash into vapor. Once the vacuum reaches about ten inches of mercury, those lighter components begin to vaporize more readily, causing the oil to separate into vapor and liquid phases, form bubbles, and lose its smooth, continuous flow. This makes the oil unstable and harder to pump or atomize.

So, the threshold around ten inches of mercury is the point where the oil starts to come apart under a vacuum. Below that, the oil stays in a single liquid phase; above that, vaporization and separation become noticeable, leading to flow and combustion issues.