Because of the real risk of failure of storage tanks and primary piping systems, means must be provided to contain the resulting spills.
- Containment may be a second tank wall around the vessel holding the liquid or a continuous dike designed to hold all of the liquid stored in the largest tank.
- Dikes may be constructed of earth, steel. concrete, or solid masonry.
- They may be square, rectangular, circular, or irregular in shape, conforming to the natural terrain around them.
- Dikes may hold one or many tanks.
- NFPA 30 should be consulted for matters that concern the diking of flammable and combustible liquids.
One common design is to make the dike out of earth, as shown below.
- Dike heights typically range from 3 ft to 6 ft (0.900 m to 1.800 m) above interior grade.
- The width of the dike at the top should be a minimum of 2 ft (0.600 m) unless it is designed to accommodate vehicular traffic.
- The dike slope must he consistent with the angle of repose of the material; otherwise, industry dike slopes are usually 1.5: 1 (i.e.. for every foot of dike height, the toe of the dike is 1.5 ft from its top edge).
Consideration must be given to providing vehicular and personnel access into diked areas. A typical arrangement is shown below.
- The maintenance vehicle ramp should have a maximum grade of 15%.
- It should be located on the side opposite the pump area.
- Personnel access should be by stairways designed according to OSHA standards.
- A single stair-way located at the pump area provides access to a catwalk located on top of the dike.
- Two other stairways provide service access to the dikes, as shown above.
Another dike design commonly found in restricted or tight areas is the concrete wall, shown below.
The location of storage tanks often must be within the battery limits of a process unit, and earthen dikes are not practical for this application.
Double Containment Diking Method
Should it be necessary to store liquefied gas close to a population center, double containment should be considered.
This containment method uses a circular concrete wall surrounded by a conventional earthen dike.
It provides for complete spill containment should the primary dike fail.
Earthen and Concrete Combination Dike
Another method of containment for use with tall concrete dikes is shown in below.
Adding an earthen berm on either side of the concrete wall allows the thickness of the concrete wall to be reduced.
Natural Terrain Diking
Tanks are often located in sloped or hilly areas, where square or rectangular dikes would be impractical.
The designer should consider using the natural terrain for a group of tanks, as shown below.
In addition, roadways and pumping facilities must be located to suit the terrain.
Following figure shows how natural terrain is used to direct spills from spheres of highly volatile liquid to flow to a single sump.
This is done by installing a diversion dike between the spheres to reduce the risk of a fire spreading from one Sphere to the other. As a result of the natural terrain, the spill is directed to its safest point within the dike. When the overall plot plan for a process facility is developed, off-site storage should be set at a lower elevation than the process unit whenever possible.
Using Plant Topography
This approach shown below provides an additional margin of safety in the event of a major rupture.
LNG Tank Containment
Containment of potentially dangerous liquefied natural gas spills is a significant concern to any operating facility. Therefore, extreme safety measures are employed, as shown below.
The inner tank is heavily insulated from the outer concrete containment and is buried in an earthen containment.
Tank buried above grade
Developing a layout for buried storage tanks in an existing chemical plant often poses special problems. it may not be possible or practical to move existing underground obstructions in the area in which the tank must be located. One solution to this problem is to bury the tank above grade in a concrete containment as shown below.
Pumps, maintenance access, and all appurtenances must be installed in the roof of the tank.
Concrete Diversion Canal
Spills that pose a high risk to adjacent facilities must be diverted to a remote holding area.
One method would be to erect a concrete diversion canal as shown below.
Drain valves that are located outside the diked area usually would be closed but would be opened in the event of a critical spill. Sump drain lines discharge into the concrete diversion canal and flow downward to a holding basin.