Would like to share some tips in the design and construction of Atmospheric Storage Tanks.
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Apex-down cone bottom tanks should have a drain pipe installed to remove rainwater during construction. The pipe should empty into a water drawoff manhole, rainwater sump, or other location as specified. The inlet to the drain should be per API STD 650.
Bottom leak detection facility: All new tank pads should be equipped with a bottom leak detection facility consisting of a synthetic membrane, a collection sump, and a drain pipe, and specified in accordance with API STD 650, Appendix I. Leak detection is required to prevent soil and ground water contamination. The provisions specified ensure a durable system for long term performance.
Permanent zinc reference electrode: All tanks should be provided with a permanent zinc reference electrode installed per figure above. The zinc anode should be per ASTM B 418, Type II composition.
Zinc Reference Electrode Installation
Concrete Ringwall Details
Pad Construction Apex-Down Cone Bottom Tanks
(1) For Legend Notes 1 through 5, see figure above
(6) 8 in. minimum, increased by predicted differential settlement between the center and the shell of the tank. Increase
this dimension as required when a leak detection system is specified, to ensure that the drain pipe slopes for drainage.
Pad Construction Apex-Up Cone Bottom Tanks (Stone Ringwall Construction Shown)
An increased subbase thickness is required with an impressed current cathodic protection system in order to provide adequate clearance between the tank bottom and the anodes, which could short-circuit the system should the two come in contact with each other. In addition, the increased thickness is required to maximize the throwing power of the individual anodes.
For those subbases utilizing permeable asphalt, there should be a method for determining whether the permeable asphalt is truly permeable because impermeable asphalt may increase the potential for corrosion.
(1) For tanks not requiring ringwalls, all other details remain the same, except general fill replaces the ringwall material and starts at pit grade level.
(2) For tanks less than 30 ft diameter and 30 ft high, the berm width should be decreased to 2 ft.
(3) Specified bottom plate slope (1:48) does not apply for tanks less than 20 ft in diameter.
(4) The crushed stone or gravel (2 in Legend) should extend outward past the annular ring through water test. The necessary amount may be removed to apply the impermeable sand-asphalt mix after the water test.
(5) 1/2 in. thick (min.) asphalt-impregnated board required when oiled sand or permeable asphalt subbase is used.
(1) With cathodic protection, one of the following subbases should be specified (listed in order of preference). The subbase thickness requirement is dependent on the anode configuration.
Without cathodic protection, one of the following subbases should be specified (listed in order of preference).
(2) 4 in. of well graded crushed stone or crushed gravel, size number 7 (ASTM D448 Table 1). Size numbers 6, 56, or 67 are also acceptable, provided that tank pad elevation differences can be met.
(3) 1 in. minimum thickness impermeable sand-asphalt mix. Terminate at the outer edge of the annular plate.
(4) Cohesive or Cohesionless compacted fill.
(5) 12 in. minimum after settlement.
Pad construction should be per the following figures. If flush type nozzles are used, there should be a minimum of 2 in. clearance beneath the flange of the nozzle.
Predicted settlement to be used in establishing foundation pad height should be specified.
Pads equipped with a ringwall are required for the following conditions:
Acceptable materials for stone ringwall construction, below the top 4 in. zone of graded crushed stone or crushed gravel, include any crushed stone or crushed gravel size numbers 2, 24, 3, 357, 4 or 467 (ASTM D 448) that meets the requirements of ASTM D 693 for crushed aggregate. Crushed slag should not be used.
A reinforced concrete ringwall is acceptable, provided a tank shell settlement of 2 in. or less is expected. If site-specific experience has demonstrated satisfactory performance of concrete ringwalls for larger shell settlements, then the use of concrete in lieu of crushed stone can be considered in these cases as well.
The average elevation of finished pads, measured at the tank shell, should be within a tolerance of 1 in. of that shown on the construction drawings.
Tank pad elevation differences, measured at the shell, should not exceed any of the following:
To achieve these tolerances, it may be helpful to either choke surface voids in a crushed stone ringwall with fine aggregate of gradation Size No. 10 (ASTM D 448), or excavate a portion of the crushed stone and place a layer of unreinforced or fiber-reinforced mortar having maximum dimensions of 1 ft. wide by 2 in. thick.
Deviations from the specified slope of the pad within the tank shell should not exceed 1:720 in a horizontal distance of 30 ft or more.