What is the max vapor in fixed roof tanks?

• A. .5 psi
• B. 2 psi
• C. 5 psi
• D. 0.05 psi

Answer: (A)

Fixed roof tanks rely on a pressure-relief vent to limit the vapor space pressure inside the tank. As the liquid heats, vapor forms and expands, increasing the pressure in the confined headspace. The vent is designed to open and relieve that pressure at a relatively low level to prevent the tank from exceeding safe stresses on the roof and shell. About 0.5 psi is the typical maximum positive vapor pressure allowed in a fixed roof tank, providing enough relief to handle normal temperature changes without compromising safety.

Higher values like 2 psi or 5 psi would require more aggressive venting and could risk structural damage or uncontrolled release, while an unrealistically low value such as 0.05 psi would cause constant venting even with minor temperature fluctuations. So the commonly accepted maximum is around 0.5 psi.