Specifying Sand Bedding to Support Above Ground Storage Tank Corrosion Protection

The ideal sand bedding supports tank corrosion protection and long service life.
Specifying the ideal sand bedding supports tank corrosion protection and a long tank bottom service life.

For new construction tank projects and for retrofits of existing tanks, it is common practice to install clean washed sand as the pad upon which the tank bottom is fabricated. The use of oil sand, crushed stone, asphalt, or other materials directly under the tank bottom should be avoided, as these hinder effective tank corrosion protection. This article provides guidelines to the specifications for the sand bedding materials. These guidelines are based on providing a low corrosivity environment  compatible with cathodic protection to assure a long service life of the tank bottom.

Economic Considerations

For a typical 150-foot diameter tank using 12 inches of sand bedding the quantity of sand that is required is approximately 17,679 cubic feet of sand which is roughly 883 tons. The cost of the sand, including delivery, is a significant cost and far exceeds the cost of cathodic protection– especially if the sand specifications are quite stringent and require sourcing sand that requires significant transportation. Some consideration can be given to relaxing the sand recommendations, even if that warrants increasing the cathodic protection requirements, should there be a significant cost impact to complying with the sand specifications – consult your cathodic protection designer.

Recommended Sand Properties

The table below summarizes the recommended sand properties to support aboveground storage tank corrosion protection.

PropertyRecommended Value
Resistivity (ohm-cm)20,000 – 100,000
pH> 6.5
Chlorides (ppm)< 10
Sulfates (ppm)< 200
Sulfides (ppm)< 0.1
Particle Size100% pass through #4 Sieve

Soil Resistivity

The best proxy for determining corrosivity of sand materials is the electrical resistivity of the sand. Clean washed sand typically has resistivity values more than 20,000 ohm-cm and in some cases can exceed 100,000 ohm-cm. The higher the resistivity of the sand, the lower the corrosivity of the sand; however, when designing cathodic protection, the higher the sand resistivity the greater the impact on the overall system resistance and the electrical power required for the cathodic protection system. API Recommended Practice 651 Cathodic Protection of Aboveground Petroleum Storage Tanks provides the following table classifying resistivity of soil/sand.

Resistivity Range (ohm-cm)Potential Corrosion Activity
<500Very Corrosive
500 to 1000Corrosive
1000 to 2000Moderately Corrosive
2000 to 10,000Mildly Corrosive
>10,000Progressively less corrosive

Soil Resistivity Testing Methods »

Sand pH Range

Measuring pH indicates the hydrogen ion content of a soil. Corrosion of steel is fairly independent of pH when it is in the range of 5.0 to 8.0. The rate of corrosion increases appreciably when pH is < 5.0 and decreases when pH is> 8.0. pH may be determined in accordance with ASTM G 51 or equivalent

Chlorides

Chlorides will affect the resistivity of soil, and act as a depolarizing agent which will increase the current requirement for cathodic protection of steel. Pitting corrosion on steel can begin at chloride levels of 10 ppm. Chloride content may be deter­mined in accordance with ASTM D 512 or equivalent.

Sulfates

Sulfate levels >200 ppm frequently indicate high concentrations of organic matter. Sulfate content may be determined in accordance with ASTM D 516 or equivalent.

Sulfides

Sulfide levels > 0.10 ppm may indicate that sulfates have been reduced by bacteria. Sulfide content may be determined in accordance with EPA 0376.1 or equivalent.

Particle Size

The sand bedding material should be clean of rocks, clumps and other debris and for clean sand capable of passing through a #4 sieve, for washed river sand an alternative acceptable particle size is 100% pass through a 3/8” sieve.

Sand Depth of Cover

For new construction tanks, the typical design is to provide a minimum of 12 inches (30 cm) of sand cover; however, for tank bottom retrofits it is quite common to provide 6 inches of sand (15 cm) for double bottom installations or for applications where the existing bottom is removed, and the top layer of the underlying soil foundation is being removed.


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