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DKL Engineering, Inc.
Sizing of Containment Area
Storage tanks are generally located within a containment area surrounded by a dyke wall or berm. In the event of a tank leak, the contents of the tank will be contained with the dyked area. Containment of the spill is required to prevent the tank contents from escaping into the environment and enable the controlled recovery, treatment or disposal of the spill.
There are different guide lines for sizing the containment area but generally it should be capable of holding a minimum of 110% of the contents of the largest tank in the containment area. It is regarded as an acceptable risk to assume that only one tank will experience a catastrophic failure. Local regulations should be check to ensure that the containment area meets all legislative requirements in terms of its size.
The minimum distance of the dyke wall or berm to the edge of the tank is important to ensure that a tank leak is contained properly. If a leak occurs in the side wall of the storage tank near the bottom of the tank, the liquid will escape in a stream of liquid due to the liquid head above leak point. Calculations should be performed to determine the distance the liquid stream will travel and the distance and height of the berm should be sufficient to contain the liquid steam.
The containment area is generally surrounded by a dyke, berm or some form of containment wall. The height of the wall will depend on the size of containment area. A large area will require a low wall in order to have sufficient volume in the containment area to hold the tank contents. A smaller containment area will need a higher wall in order to achieve the required volume for the containment area.
For small storage areas, the containment wall may be a concrete wall surrounding the tank(s). For large tank farms, the containment wall is generally a earth berm or dyke with sloped sides.
The containment area should be design so any liquid in the area drains to a sump pit located at the low point. Multiple sump pits may be required depending on the size of the containment area.
The sump pit may be equipped with a permanent pump or the owner may choose to use a portable pump to periodically pump out the contents of the sump pit. The contents of the sump pit should be checked to determine if it contains simply water or sulphuric acid. Neutralization of the liquid may be required prior to disposal.
Safe access is required to the containment area for maintenance and operation of the storage tank(s). Access for personnel should permit a quick exit from the containment area in the event of a tank leak. Stair access into and out of the area is preferred over ladder access.
In large tank farm installations, vehicular access may be required. This can be provided by a ramp up to and over the berm. A minimum 3660 mm (12’-0”) wide ramp with a 15o slope provides safe access for normal maintenance vehicles. The ramp should be located in a convenient area which allows access to the entire containment area.
Lining of the containment area is required to prevent spills from contaminating the ground. The lining should be impervious to the liquid and resistant to chemical attack. For small containment areas constructed of concrete a protective coating can be applied to the concrete or the area can be lined with acid resistant brick. For larger containment areas, a flexible membrane is used to line the area.
A material generally used for membranes is polyethylene containing 97.5% polymer and 2.5% carbon black, antioxidants and heat stabilizers. A minimum thickness of 1.5 mm (60 mils) is used. The material will have a smooth finish but may be textured when applied to a sloped surface such as along the berm. The textured surface aids in maintaining the membrane in position.
The membrane is generally installed on a compacted based covered with at least 75 to 100 mm (3 to 4 inches) of sand. The sand provides a cushion to prevent puncture of the membrane from below. The membrane is laid down, anchored and then covered with a layer of Geotextile and then a layer of granular fill (minimum 150 mm (6”)). The granular serves several purposes:
- ballast to prevent movement of the membrane
- protection against the environment and sunlight
- protection against pedestrian and vehicular traffic in the containment area.
Installation of the liner must be done properly to ensure it will serves its intended purpose of preventing spilled liquid from penetrating into the ground. The following steps should be taken to ensure a proper installation.
- Inspect the base to ensure that the sand layer is properly compacted and graded.
- The liner should be installed in a relaxed state
- Minimize handling of the membrane
- Ensure there is sufficient excess material available to accommodate settling and shrinkage
- Ballast should be installed as soon as possible following installation of the liner
- All joints in the membrane shall be completely sealed
- All penetrations shall be completely sealed
The lining shall be inspected visually to ensure it is free of holes, blisters, imperfections, contamination by foreign matter, etc. The seams should be carefully inspected to ensure they are sealed properly.
The integrity of the seam can be tested using a vacuum tester. A vacuum is pulled over a seam and any air in-leakage indicates a faulty seam. Random samples of seam can also be cut out and tested in a laboratory. The area that is cut out is patched.
There are many standards covering the quality of the membrane material. Some of the applicable standards are:
ASTM D570 Water Absorption
ASTM D638 Tensile Strength
ASTM D696 Coefficient of Linear Expansion
ASTM D746 Low Temperature Brittleness
ASTM D751 Hydrostatic Resistance
ASTM D882 Elongation, Modulus of Elasticity
ASTM D1004 Graves Tear Resistance
ASTM D1149 Ozone Resistance
ASTM D1203 Volatile Loss
ASTM D1204 Dimensional Stability
ASTM D1238 Melt Flow Index
ASTM D1505 Specific Gravity
ASTM D1693 Environmental Stress Crack
ASTM E96 Moisture vapour Transmission
Geotextiles are used to separate different granular weights of soils, as filters, draining mediums, reinforcing membranes or even waterproofing for construction
A geotextile similar to Texel # 7618 (Texel, Inc.) or equal is recommended.