Dikes, storage tanks

A small, deep diked area around a storage tank is safer than a large, shallow one, as the evaporation rate is lower and the area of any fire is smaller. 

At Qatar, the liquid came out with such force that it spilled over the dike wall. Conventional dike walls also have the disadvantage that a large area of liquid is exposed to the atmosphere if a leak occurs. For these reasons it is now usual to surround cryogenic storage tanks with a concrete wall, about 1 m from the tank and the full height of the tank. If the tank is not made from crack-resistant material, then the concrete wall should be designed to withstand the effects of a sudden release of liquid. 

A 26,000 gallon outdoor vertical storage tank contains benzene. The tank is not insulated, but has a dike around it tvith a volume equal to one-and-a-half times the volume of the tank. Size 15 diameter X 20. Tank does not have weak roof. Temp. = 60°F, pressure = 14.7 psia. 

A facility stores NFPA (National Fire Protection Association) Class 1B liquid in a diked atmospheric storage tank equipped with an external floating roof. 

K. Vertical storage tanks should be individually diked. If nal, capacity in single dike should not exceed 25,000 bbls. Far horizontal slomge tanks, maximum is 400,000 gallons per group, with 100 between groups, or other suitable arrangement. 

Each of the styrene storage tanks will have a dike around it that is capable of containing the tank s contents when it is full. 

This accident was attributed to the lack of design protection to prevent the backup of ammonia into this storage tank. It also appears that mitigation techniques were not part of the system (deluge systems, dikes, and the like). 

A major factor in location of storage tanks within a tank farm is the topography of the tank farm area. The slope of the natural topography can be used to assist in the drainage requirements for a diked area and minimized the accumulation of spilled liquids near a storage tank. Diversion dikes or curbing can be used to divert spillage so it runs off remotely to a safe location. 

Drainage slopes within tank areas should ensure that any spills are drained away from tanks, manifolds or piping. Small fires that can occur in gutters or drains around tanks weaken connections to the storage tank and release the contents of the tank. Any gutter encircling the tank should be located at a safe distance from the tank and drain basins should not be located under tank mixers, major valves or manway entrances to the tank. The diked areas should be provided with an impervious surface that is will collect liquids towards a drainage collection point. 

Materials, particularly combustibles, or equipment placed inside storage tank dikes. 

Pumps and other equipment are generally not provided inside a storage tank area, but are located in their own curbed area adjacent to the dike. 

F9II, Standard for Diked Aboveground Storage Tanks... 

A double-wall AST of steel fulfills the same function as a diked AST with rain shield. Double-wall designs consist of a steel wrap over a horizontal or vertical steel storage tank. The steel wrap provides an intimate, secondary containment over the primary tank. One such design is the Steel Tank Institute s F921 standard, based upon... 

Flare stacks that safely bum organic material released into vent headers are an example of a prerelease mitigation measure. The flare destroys the hazardous organic material before it reaches the environment. A dike around a storage tank is an example of a postrelease mitigation measure. The dike contains the release in a small area, reducing the total evaporation rate from the spill and so reducing the impact of the release. 

Secondary containment enclosures other than double-wall construction have been widely used to control vessel leaks. Bunkers built around underground storage tanks are common examples of this approach to preventing releases to the atmosphere and into the soil around the tank, to minimize potential for ground water contamination. Use of tank-high dikes with covers or roofs is another example of secondary containment that limits postrelease emissions. 

To demonstrate the effectiveness of diking as a postrelease mitigation measure, an accidental release of carbon disulfide from a vertical storage tank will be evaluated. In the example, isopleths to the ERPG-2 concentration of 50 ppm (AIHA, 1992) will be considered. 

Figure 7.1. Storage tank with dike (carbon disulfide and n-pentane).

In this example the use of foam, which was described in Section 3.3.1, will be demonstrated as a postrelease mitigation agent. An accidental release of n-pentane, a flammable material, from a storage tank into a diked area, and a hazard zone to a concentration of one half the lower flammable limit (LFL/2) will be evaluated. For n-pentane the LFL is 1.4% by volume in air. Then the LFL/2 will be 0.7% by volume in air (GPSA, 1972). 

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