The general accepted rule of thumb is never fill a vessel more than?

• A. 90 percent full
• B. 70 percent full
• C. 60 percent full
• D. 80 percent full

Answer: (D)

Having enough vapor space in a vessel is essential to absorb thermal expansion and the formation of flash gas. If a vessel is filled too full, that headspace is reduced, and during heating, pressure changes, or sudden process shifts can push liquid into lines, flood the suction of compressors, or hinder relief devices from acting properly.

The eighty percent rule gives a practical balance: it leaves about twenty percent of the vessel as vapor space to accommodate expansion and gas formation while still providing useful storage capacity. Filling beyond this reduces the margin for expansion and increases risk of liquid carryover and operational problems, while filling much less is unnecessarily conservative for most applications.