A fixed roof tank is a type of storage tank, used to store liquids, consisting of a cone- or dome-shaped roof that is permanently affixed to a cylindrical shell. Newer storage tanks are typically fully welded and designed to be both liquid- and vapor-tight. Older tanks, however, are often riveted or bolted, and are not vapor tight. A breather valve (pressure-vacuum valve), commonly installed on many fixed roof tanks, allows the tank to operate at a slight internal pressure or vacuum. This valve prevents the release of vapors during very small changes in temperature, barometric pressure, or liquid level. [1] Fixed roof tanks without breather valves will generally be freely vented; [2] thus the emissions from a fixed roof tank can be non-trivial. Gauge hatches, sample wells, float gauges, and roof manholes provide accessibility to these tanks, and also act as potential sources of volatile emissions. [3] [4]
Storage tanks in general, and fixed roof tanks in particular, containing organic liquids can be found in many industries, including:
Fixed roof tanks are typically used with products that have relatively low volatilities and flash points. [5] Using a fixed roof tank to store a product with high volatility can lead to product losses as vapors escape the enclosure. [6]