When fuel oil is subjected to a vacuum in excess of what measurement in inches of Hg does it start to come apart for the second time mentioned?

When fuel oil is subjected to a vacuum, it can begin to experience structural changes and separation at certain pressure thresholds. Specifically, when a vacuum exceeds 10 inches of mercury (Hg), fuel oil starts to break apart due to the vapor pressure of the liquid. This phenomenon occurs because as the pressure decreases, the boiling point of the liquid decreases as well, leading to rapid evaporation and the potential for the liquid to become aerated or develop vapor pockets.

It's important to understand this vacuum threshold, as exceeding it can result in operational issues within HVAC systems that use fuel oil, including cavitation or a drop in efficiency. Monitoring and maintaining proper vacuum levels is crucial to ensure the integrity of the fuel oil and the functionality of the heating system.