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Procedures for the design and construction of Atmospheric Storage Tanks

asked Apr 13, 2016 in Engineering by lampanda (2,450 points)
Would like to share some tips in the design and construction of Atmospheric Storage Tanks.

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answered Apr 13, 2016 by lampanda (2,450 points)

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:

  • Tanks larger than 120 ft in diameter
  • Tanks higher than 48 ft with diameters of 100 ft or larger
  • When cohesion-less soil used as general fill, the minimum size tank requiring a ringwall should be specified.

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:

  • 1:720 in any distance of 30 ft or more.
  • 1 in. between the highest and lowest elevation points.

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.

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answered Apr 13, 2016 by lampanda (2,450 points) | edited Apr 15, 2016 by lampanda

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.

NOTES:
(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.

LEGEND:
(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.

  • Dry Clean Sand: 4 in. minimum thickness below the tank bottom if used in conjunction with a galvanic (sacrificial) anode cathodic protection system.  8 in. minimum thickness if used in conjunction with an impressed current cathodic protection system.
  • Fine Crushed Stone (gravel/limestone): Size number 9 (ASTM D448 Table 1) crushed stone should be used.  The thickness requirements are similar to those required of the dry clean sand.    

Without cathodic protection, one of the following subbases should be specified (listed in order of preference).

  • Oiled Sand:  The sand should entirely pass the 3/8 in. sieve, almost entirely pass the No. 4 sieve, and have no more than 10% passing the No. 200 sieve.  The oil used should be a low corrosivity oil such as 18 gravity railroad fuel oil or 11.5 minimum gravity Bunker C.  A satisfactory mix of oil and sand should maintain its shape when pressed into a vertical plane.  In general, 5 to 10% oil by weight should suffice.  The oiled sand should be compacted by rolling or tamping.  Low viscosity asphalt may be substituted for the oil; however, care should be taken to ensure that the resulting mix is suitable for cold placement.  Oil may also be spray-applied to the sand, provided oil penetration through the full depth of the sand is demonstrated with a test application.  The minimum thickness should be 2 inches.
  • Permeable Asphalt: Aggregate should have gradation No. 8 or 89 (ASTM D448) or equivalent, asphalt cement type AC-20 or equivalent at 2 to 4% by weight.  Compact using light, walk-behind equipment if the pad is equipped with a leak detection membrane.  AASHTO T166 should be used to assess the permeability of the asphalt.  The minimum air void percentage should be 8% for a 9.5 mm mix, 6% for a 12.5 mm mix, and 5% for a 19 mm mix.  The minimum thickness should be 2 inches.

(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.

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answered Apr 15, 2016 by lampanda (2,450 points)

Pad Construction Apex-Down Cone Bottom Tanks

LEGEND:
(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.

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answered Apr 15, 2016 by lampanda (2,450 points)

Concrete Ringwall Details

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answered Apr 15, 2016 by lampanda (2,450 points)

Zinc Reference Electrode Installation

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answered Apr 15, 2016 by lampanda (2,450 points)

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.

  • The requirement for a leak detection system may be waived if the tank contains clean water, a product not fluid at ambient temperature, or if the temperature of the tank is sufficiently high to jeopardize the integrity of the membrane.
  • The membrane should be High Density Polyethylene sheet, 80 mil thickness, unless the stored product necessitates the use of another material.
  • The membrane should be furnished with factory sealed seams if practical.  If field seams are used, these should be made using the extrusion welding or hot wedge methods.  Seams should be 100 percent tested using the vacuum box method and any leaks should be repaired.
  • Where permitted, a concrete pad may be used under small tanks instead of a synthetic membrane for leak detection subject to the following:
    • A continuous slab may be used for closely spaced tanks.
    • The concrete pad or slab should be structurally designed to prevent concrete cracking.
    • The concrete pad or slab should have no joints under the tank.  If a joint cannot be avoided, then a waterstop should be provided to permanently seal the joint.
    • The pad or slab should be provided with groves or slots in the top of the slab/pad or equivalent means to allow a leak to flow quickly out from under the tank and be detected.  A sump should not be placed under the tank.

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.

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