Drainage and Dikes – Drainage and impounding or diversion dikes shall be provided in accordance with Chapter 2 of NFPA 30. The toe of an impounding dike shall be located a minimum of 15 m from the shell of tanks 46 m in diameter and larger containing flammable or combustible products. For non-flammable products at ambient condition, NFPA-30 can be followed. Minimum height for impounding dikes shall be 2 meters. Impounding dike basins shall be graded away from tanks to fire sealed and valved drains that extend through the dike wall. Each drain valve shall be clearly labeled and located outside the dike with overflow into the plant drainage system. Indicating valves shall be used. The minimum size of the sealed drain pipe will be the diameter of the largest pipe to or from the tank.
Unless a diversion dike system is being utilized, separate impounding dike enclosures shall be provided for each tank that is 45 m or greater in diameter and stores crude oil, and for tanks in hot oil [over 93 °C] or slop oil service. Separate impounding dike enclosures shall be provided for each tank that is 60 m or
greater in diameter and stores Class I liquids other than crude oil. Intermediate dikes shall meet the requirements of NFPA 30.
Drainage and Dikes | Safety Considerations for Plant Layout Levels
If spacing permits, a diversion dike system may be used. Diversion dike systems shall be graded away from tanks and shall provide routing to a safe location. It should consist of open channels draining to a retention area in a safe location. It should have capacity equal to the greater of the following:
a. The volume of the largest diked enclosure that would be required for the protected tanks if individual diking were provided.
b. Eighty cubic meters.
Dike walls shall be located away from low-pressure storage tanks to provide sufficient natural ventilation and to prevent flammable vapor accumulations. The diked enclosure shall be sized to contain at least 50 percent of the largest tank capacity. In locations where climatic conditions result in prolonged periods
during which pressure storage temperatures may be below the boiling point, the diked enclosure shall be sized to contain 100 percent of the tank capacity. For refrigerated and cryogenic storage, secondary containment for 100 percent of the tank capacity shall be provided by a dike or by a double containment
shell. A dike sized for the maximum probable piping spill shall be provided if a double containment shell is used.
All tanks (other than those which have to be individually diked) and spheres, built in common dikes, shall not be located in more than two rows in any direction. There will not be more than four tanks built in a common dike, this to ensure easy access for fire-fighting vehicles/equipment.
Single Tank Dikes
a. Crude tanks in any diameter must be individually diked.
b. Hot Oil, FCC Charge and Slop Tanks of any diameter, must be individually diked.
Tanks containing the same type of product, other than those listed above, may be located within a common dike. These to be limited to a maximum of four tanks.
Walls of all diked areas must be of solid masonry or concrete. Concrete slabs fitted over earthen dikes are permissible. All dikes must be designed to be liquid tight and to withstand a full hydrostatic head, with a minimum height of 2 m from the interior grade level.
Intermediate dikes must not be more than 50% of the height of the main impounding area dike.
Impounding Dike Basins; A slope from the tank, of not less than 1% shall be provided for at least 15 m, or to the dike base, whichever is less.
To permit easy access for fire fighting vehicles and/or equipment, there must be a minimum width of 6 m around each impounding dike. Only piping to/from individual tanks is permitted inside of the diked areas. Non-associated pipeways and all pumps must be located outside of the diked areas.