What is the maximum vapor pressure allowed in a floating roof tank?

• A. 10 psi
• B. 11 psi
• C. 12 psi
• D. 13 psi

Answer: (B)

In a floating roof tank, the vapor space is kept within safe bounds by a breathing vent system. This design sets a maximum allowable vapor pressure to prevent overpressurizing the tank, which could stress the roof seals, risk damage to the roof or shell, or force uncontrolled vapor release. The specified limit for this type of tank is 11 psi, which is the pressure the vents are designed to tolerate and relieve up to. Staying at or below this value ensures safe operation under temperature and vapor‑generation changes. Values below this limit aren’t the maximum allowed, while higher values would exceed the tank’s design and safety margins, creating potential hazards.