Atmospheric fixed roof tanks

Atmospheric fixed roof tanks built according to API requirements will have a weak seam at the junction of the roof with tank side. If there is internal overpressure, such as an explosion, the seam will part or the roof blows off, leaving the shell in place to retain its contents. The resulting fire will therefore only initially involve the exposure surface of the liquids still in the tank. 

EFR tanks have no vapor space pressure associated with them and operate strictly at atmospheric pressure. IFR tanks, like fixed-roof tanks, can operate at or above atmospheric pressure in the space between the floating roof and the fixed roof. 

Fixed-roof atmospheric tanks require vents to prevent pressure changes which would othei wise result from temperature changes and withdrawal or addition of liquid. API Standard 2000, Venting Atmospheric and Low Pressure Storage Tanks, gives practical rules for vent design. The principles of this standard can be applied to fluids other than petroleum products. Excessive losses of volatile liquids, particularly those with flash points below 38°C (100°F), may result from the use of open vents on fixed-roof tanks. Sometimes vents are manifolded and led to a vent tank, or the vapor may be extracted by a recov-eiy system. 

When the tank was refilled with warm crude oil at 37°C. vapor was pushed out into the space above the floating roof and then out into the atmosphere through vents on the fixed-roof tank (Figure 5-16). This vapor was ignited at a boiler house some distance away. 

Open Tanks These may be used to store materials that will not be harmed by water, weather, or atmospheric pollution. Otherwise, a roof, either fixed or floating, is required. Fixed roofs are usually either domed or coned. Large tanks have coned roofs with intermediate supports. Since negligible pressure is involved, snow and wind are the principal design loads. Local building codes often give required values. 

Table 8-12 summarizes the protection measures that should be provided for vertical atmospheric storage tanks to protect them against fire risks. Fixed or semi-fixed fire protection for cone-roof tanks should be provided as follows ... 

Cone roof tank—an atmospheric storage tank with a fixed cone-shaped roof. 

Cone roof tank with internal floatingcover—an atmospheric storage tank with a fixed cone-shaped roof and an internal floating cover of one of the following designs ... 

Covered floating roof tank—an atmospheric storage tank with a fixed roof and steel internal floating roof of the single-deck pontoon or double-deck design. 

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