200 bottom selection and design.pdf

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Chevron Corporation 200-1 July 2000 200 Bottom Selection and Design Abstract This section of the manual discusses the design requirements and Company and industry specifications for atmospheric storage tank bottoms. It provides data required to determine the most cost-effective new tank bottom and/or replacement or repair for existing tanks. The advantages and disadvantages of the different types of new designs are addressed. Included is a cost benefit methodology for deter- mining the most cost-effective bottom for a particular site. Leak detection and containment are also discussed. Contents Page 210 Bottom Selection 200-3 211 Single Bottom vs. Double Bottom Characteristic Differences 220 Bottom Design 200-5 221 Bottom Plate Thickness 222 Shell-to-Bottom Joint 223 Reinforcing Pads 230 Bottom Construction 200-9 231 Bottoms for New Tanks 240 Bottom Repair or Replacement 200-10 241 Philosophy 242 Repair Alternatives 243 Bottom Replacement 244 Bottom Replacement Requirements 250 Leak Detection and Containment 200-17 251 Background and Scope 252 Definitions 253 Performance Criteria for Leak Detection and Leak Containment 254 Undertank and Double Bottom Spacer Material Considerations

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Page 1: 200 Bottom Selection and Design.pdf

200 Bottom Selection and Design

AbstractThis section of the manual discusses the design requirements and Company and industry specifications for atmospheric storage tank bottoms. It provides data required to determine the most cost-effective new tank bottom and/or replacement or repair for existing tanks. The advantages and disadvantages of the different types of new designs are addressed. Included is a cost benefit methodology for deter-mining the most cost-effective bottom for a particular site. Leak detection and containment are also discussed.

Contents Page

210 Bottom Selection 200-3

211 Single Bottom vs. Double Bottom Characteristic Differences

220 Bottom Design 200-5

221 Bottom Plate Thickness

222 Shell-to-Bottom Joint

223 Reinforcing Pads

230 Bottom Construction 200-9

231 Bottoms for New Tanks

240 Bottom Repair or Replacement 200-10

241 Philosophy

242 Repair Alternatives

243 Bottom Replacement

244 Bottom Replacement Requirements

250 Leak Detection and Containment 200-17

251 Background and Scope

252 Definitions

253 Performance Criteria for Leak Detection and Leak Containment

254 Undertank and Double Bottom Spacer Material Considerations

Chevron Corporation 200-1 July 2000

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200 Bottom Selection and Design Tank Manual

255 Miscellaneous Design Considerations

256 Design Variations

260 Membrane Design and Selection 200-27

261 Introduction

262 Elastomeric Liner

263 Membrane Materials for Tank Secondary Containment

264 Design and Construction

265 Inspection

266 Approved Manufacturers and Installers

270 References 200-32

July 2000 200-2 Chevron Corporation

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210 Bottom Selection

Tank Bottom SelectionIf the tank is to store petroleum crude or products and if water is present in the contents, then a means for removing the water must be provided. How the water is removed determines the type of bottom. Small tanks where water drawing is not a problem have flat bottoms. Flat bottoms are inexpensive to build and may be supported on a concrete ring or flat pad-type foundation.

Larger tanks usually have a cone up or cone down bottom. The cone up bottom is the more common and is the less expensive of the two. The cone up bottom permits the use of a bottom outlet or waterdraw basin at the edge of the tank. The primary disadvantage of the cone up bottom is that water tends to stand along the edge of the bottom rather than drain to the waterdraw outlet. The standing water can cause severe corrosion on the bottom near the edge and the lower two to three inches of the shell.

The cone down bottom offers better drainage and removal of water since the bottom slopes toward the center of the tank, but installing piping to center of tank is costly and could be difficult. This type of design is not recommended.

Single-slope bottoms are used on tanks where the ability to draw water and clean the tank is important and where the center sump is not practical.

Figure 200-1 summarizes basic configurations for tank bottoms and arrangements for piping and drain connections. The advantages and disadvantages of the different designs are listed. The designs shown deal only with tank structure and do not show any means for corrosion prevention or leak detection such as linings, double bottoms, cathodic protection, etc. See Sections 600 and 250 for information on corrosion prevention and leak detection.

Note Figure 200-1 appears at the end of this section.

Choice of bottom is influenced by: 1) operating requirements for the product to be stored, 2) maintenance considerations, and 3) characteristics of the support soil (unless a piled foundation is to be used). Typical operating requirements include:

• Keeping a layer of water on the bottom in some services.

• Removing water frequently to keep the contents of the tank “dry.” This can important for quality control or when the tank feeds an operating unit.

• Changing service or specifications. In this case, you would want a design twould enable you to completely drain the product in the tank.

Maintenance considerations include accessibility of piping and connections, unside and stockside bottom corrosion, and need for regular cleaning. Section 22details bottom design and Section 300, foundation design.

Chevron Corporation 200-3 July 2000

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200 Bottom Selection and Design Tank Manual

211 Single Bottom vs. Double Bottom Characteristic Differences

Exemptions for Double Bottom TanksThe following are examples of exemptions on the Use of Double Bottoms for aboveground storage tanks (ASTs).

Exemption based on use of a release prevention barrier (RPB). Any tank which is fitted with an RPB or will be fitted with an RPB is exempt from the requirement to use a double bottom. An RPB is an elastomeric, impermeable membrane placed in or under a tank which acts to stop the migration of a leak downward and to direct it to the perimeter of the tank where it can be visually detected. It is essentially equivalent from the perspective of environmental performance to a double bottom tank.

Characteristic Single Bottom Double Bottom

Mean life 25 years Approximately 35 years (a 40-50% increase over single bottoms)

Costs • Initial installation costs are lower than double bottoms

• Higher inspection costs

• Potential for high environmental clean up costs

• Lower operating factor. Repair scope is less.

• Lower total cost of ownership

• Reduced by longer in-service run time intervals

• Lower material costs due to a smaller safety factor against leaks (bottom thickness can be less)

• Reduced inspection costs by reducing inspection frequency

• Minimal environmental cleanup costs

• (Secondary benefits) Can retrofit slopes, coating, etc., for improved product integrity

Leak detection Leaks often not detectable and go on for years resulting in future cleanups

Leaks are detected early and results in minimal, if any, environmental damage

Corrosion Most serious from underside of tank bottom because of:

• Variable soil conditions and chemistry

• Moisture and oxygen variation

• Salts, dirt, debris, scale

Underside attack reduced substantially by changing underside conditions

• Elevated aboveground so less mois-ture

• Clean uniform contact with concrete

• Concrete is a corrosion inhibitor

• Reduces the variance of oxygen concentration, moisture and electro-lytes because they provide a uniform surface, elevating the new steel bottom out of the mud and dirt

• Concrete is itself a corrosion inhib-itor and reduces underside corrosion rates

July 2000 200-4 Chevron Corporation

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Exemptions based on OPCO. All Production tanks are exempt from requirements to apply double bottoms or RPBs.

Exemptions based on liquids stored. The following tanks do not need to be considered for implementation of double bottoms:

• Chemical tanks less than 100 feet in diameter

• Lubricants

• Asphalt tanks

Exemptions based on site. Certain sites such as the Pascagoula Refinery have settling or shifting soil conditions which weigh against the use of a double bottotank or a release prevention barrier. In this case, the entire refinery is exempt frusing double bottom tanks. However, for new tanks or tanks which will be retrofitted with double bottoms, consideration should be given to installation of RPBs. In order to determine whether a site is exempt, an engineering analysis should be performed, documenting the considerations which weigh against theof the double bottom practice. The use of double bottoms should also be evaluin areas of high humidity and/or low water table.

Exemptions based on analysis. Any tank may be exempted from the requirementof a double bottom provided that:

– an analysis is documented which shows why the tank does not need tohave a double bottom, or

– the benefits of the double bottom are so marginal as to not justify the cof the double bottom.

Tanks which may not be exempted. Any tank which stores motor fuels or fuels with MTBE or TAME or pure oxygenates shall be put onto a schedule which implements an RPB or a double bottom.

220 Bottom DesignRefer to Section 210 for the selection of bottom configuration, and Section 300 foundation design.

The design of bottoms for cylindrical storage tanks at atmospheric pressure doeinvolve a direct consideration of the hydrostatic pressure for the design fill heighthe bottom of the tank. The weight of the liquid contained in the tank is assumebe supported by the foundation upon which the bottom rests, and no significantstress is developed in the bottom plates attributable to the hydrostatic pressureother words, the tank bottom serves as a simple liquid-tight membrane, with nosignificant structural strength required. The exception is the outer edge of the bottom under the tank shell, for an annular width of approximately 2 feet. This highly stressed location has been the site of many tank failures. Therefore, the to-bottom welds and the annular ring, when one is required, are the primary desconsiderations.

Chevron Corporation 200-5 July 2000

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This section is a briefing of why we have several different double bottom designs. Although there are several different designs, they are all functionally equivalent. That is, they perform the function of secondary leak detection and containment. All of these designs have the following functional components in them:

• Concrete Spacer

While sand, steel or other media may be used Chevron has chosen concrethe spacer material upon which to install the new or second bottom. This mrial gives good control over the slope of the tank bottom allowing for better water drainage and reduced corrosion due to stagnant water in the tank boThe main reason for the selection of concrete, however, is that it is alkalineit actually reduced corrosion from the underside. In many inspections, we hfound the concrete to extend the life of the tank bottom by a factor of 25 to percent due to reduced underside corrosion attack.

• Corner Lock Design

In the original El Segundo Design, the liner is simply spread out and trimmethe inside diameter of the tank. A caulk bead is then applied between the dshell and the liner to seal the double bottom space and create leak containHowever, Chevron has recognized that the integrity of the system could beimproved by making a continuous bathtub out of the HDPE liner. To do thishave used the CBI PROPRIETARY corner lock method is described as follows.

This design uses a perforated angle iron. To this angle iron is attached a shsegment of HDPE liner. This is extruded into the angle so that the flap can used to heat weld the basic liner to the angle. This provides the same batheffect that the Chevron In-house design does.

• Chevron In-House Design (Batten Strip Design)

In this design we use a steel batten strip to seal the liner to the dead shell. involves using stud welding to bolt the batten strip to the shell. By using thistechnique a bathtub is created by heat seaming all seams up to the point atop of the batten strip. In this system we do not depend on caulking to provtight joint.

All of these systems are of the open design. We do have a few closed systems are described below.

• Open vs. Closed System

Chevron does not support the use of a closed double bottom system in gen(with few rare exceptions). The closed system is defined as one in which thleak detection ports are valved closed and the junction between the dead s(the stub of the shell under the new bottom) and the new bottom is sealed caulking or seal welding. In an open system the leak detection ports are eitopen, or if they have valves, the valves are left open and the area under thdead shell is left open.

July 2000 200-6 Chevron Corporation

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Reasons for not using closed design:

1. Making a weld that would actually seal the space is difficult if not impossible. In most cases there is inadequate room to make the weld. It is therefore of poor quality and would not keep water out and leaks in.

2. Due to the difficulty of making this weld and due to the nature of fillet welds it is prone to having cracks and flaws. As this is the most highly stressed region in the tank, it makes the possibility of a catastrophic failure much more likely. From the principle of fracture mechanics, crack growth or sudden propagation occurs in the presence of flaws and stress. Both are likely with this weld.

It is for this reason that API Standard 650 makes such stringent non-destructive examination requirements for the topside fillet welds in this highly stressed area. It would be basically impossible to verify the integrity of the underside weld at this location.

3. It was originally thought that water and thus corrosion could be minimized on the underside of tank bottoms to reduce underside attack. However, due to the large size of the tank double bottom space and the humidity of the air, if the bottom is sealed into a closed system, water will actually condense on the underside of the tank bottom plates, causing accelerated corrosion. This is similar to the crawl space of a house in which moisture will damage the flooring unless adequate ventilation is provided. For this reason, the open system is superior in that it allows for this ventilation to occur, removing any moisture that does enter the space. Even if the bottom could be perfectly sealed and constructed with no moisture, the concrete itself has moisture which evapo-rates from it and would create a humid corrosive environment were the space not allowed to breathe.

4. The closed system also defeats the purpose of leak detection. By closing the system, a leak cannot be viewed as soon as it occurs. Since leaks tend to start very slowly and increase with time, the best way to protect the environment is to detect leaks as early as possible.

221 Bottom Plate ThicknessThe minimum thickness for bottom plates required by API 650 is ¼ inch exclusof any corrosion allowance that may be required (API 650, Paragraph 3.4.1). Swelded lap joints with full-fillet welds are normally used for bottom plates; however, single-welded butt joints with backing plates are an acceptable altern(API 650, Paragraph 3.1.5.4 through 3.1.5.7) as shown in Figure 200-2. It is preferred that bottom plates over 3/8-inch thick be butt welded.

Annular RingIn general, annular rings are required by API Standard 650 Section 3.5 when stwith tensile strengths of 65,000 psi or higher is used for the lowest shell courseAdditionally, the Company requires annular rings for all 100,000-bbl or greater tanks and tanks greater than 100 feet in diameter, regardless of the strength ofsteel being used.

Chevron Corporation 200-7 July 2000

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200 Bottom Selection and Design Tank Manual

Butt welds with full penetration and complete fusion are required to join the plates that form the annular ring. However, the butt welded plates in the annular ring can be lap welded to the bottom plates. The required thickness of the plates for the annular ring is determined by the hydrostatic stress and thickness of the lowest shell course (API 650, Paragraph 3.5.3), as given in Table 3.1 of API 650.

Width of annular rings must be able to support the column of liquid above it at design fill height, in the event of foundation settlement. The minimum width allowed is 24 inches. However, a greater width may be needed as calculated by the formula given in Paragraph 3.5.2 of API 650. In addition, seismic design requires checking the thickness of the annular ring, as discussed in Section 530.

222 Shell-to-Bottom JointAPI Standard 650 requires double fillet welds for the shell-to-bottom joint (API 650, Paragraph 3.1.5.7), as illustrated in Figure 200-3. The stress in this joint is dependent on the hydrostatic pressure at the design fill height and the diameter of the tank. The minimum size of the fillet welds required by API 650 is based upon the thickness of the lowest shell course, as given in API 650, Paragraph 3.1.5.7. In this manner, the size of the fillet welds is increased in proportion to the hydrostatic pressure acting on the shell-to-bottom joint, without making separate design calculations.

Figure 200-4 illustrates how lap welded bottom plates under the shell are config-ured for the shell-to-bottom joint.

Fig. 200-2 Bottom Plate Welds (From API 650, Figure 3-3A.) Courtesy of the American Petroleum Institute

Fig. 200-3 Bottom-to-shell Joint (From API 650, Figure 3-3A.) Courtesy of the American Petroleum Institute

July 2000 200-8 Chevron Corporation

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223 Reinforcing PadsPads, at least 3/8 inch thick, should be provided under columns that support fixed roofs and under the legs for floating roofs. The pads for floating roof legs should be wide enough to accommodate the maximum horizontal roof movement. Pads at least ¼ inch thick should be installed to serve as wear plates under mixers andnozzles and under any bottom-mounted appurtenances, including gauge wells.pads prevent high local loads from developing in the bottom plates and tend to minimize localized corrosion of the bottom plates. All pads should be welded tobottom plates with continuous ¼ inch fillet welds. See Section 700 for additionaappurtenance design guidelines.

230 Bottom ConstructionThis section covers field installation of steel bottoms.

231 Bottoms for New Tanks

Bottoms Not Requiring Annular RingsThe new bottom sheets are tacked into place, then welded. Watch for excessivoverlapping of plates and grinding down of the upper plate to hide a less-than-ffillet weld. Before welding, check that enough plate extends beyond the outsideedge of the shell radius to meet the specified overlap.

Cone up Bottoms Requiring Annular RingsThe annular ring plate should be installed first. Annular plate must be welded wfull penetration welds. As many plates as can be handled may be back welded single section for installation. Welding these assembled sections together in plarequires the use of backup strips (see Figure 200-5). After installation of the an

Fig. 200-4 Configuration of Lap-welded Bottom Plates Under Shell (From API 650, Figure 3-3B.) Courtesy of the American Petroleum Institute

Chevron Corporation 200-9 July 2000

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ring, the bottom plate is tack welded in place and then welded. The plates should shingle toward the low point, i.e., the outside row of plates should be installed first with the higher center plate row installed last.

Cone down Bottoms with Annular Rings The preferred method of installing a new cone down bottom with annular ring is to install the bottom deck plate first, shingled toward the center (i.e., the row of plates running through the center is placed first). The annular ring is then placed on top of the deck plate with its installation being the same as detailed above. Installing the annular ring first traps a small amount of liquid near the edge of the shell. The finished fillet weld attaching the annular ring to the bottom deck plate should, as a minimum, be equal to the bottom deck plate thickness. If the surface is to be coated, the weld should be ground to a smooth radius.

240 Bottom Repair or ReplacementThis section discusses the justification for replacing a bottom versus a less costly repair. It also gives guidance on the types of replacement bottoms along with the repair methods available and where they are applicable.

241 PhilosophyRepair is recommended over replacement when:

• Corrosion and pitting are not severe and patching or weld repairs can be accomplished economically.

• The maximum depth of unrepaired stockside pits and underside pits will noexceed the plate thickness before the end of the next run. Figure 200-6 giveprocedure for determining the remaining life of a bottom.

Fig. 200-5 Details of Annular Ring Butt Weld and Backup Strip Installation

July 2000 200-10 Chevron Corporation

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• Corrosion and pitting are localized to a specific area (i.e., annular ring corrosion due to water standing around the inside edge of the shell).

• Most of the pitting is underside, and external cathodic protection is being installed to minimize this pitting.

242 Repair Alternatives

Weld Repair and Plate Patching. These methods are for repairing mechanical damage and stockside pitting. Patching is also done to repair openings in the bresulting from turning coupons. The following guidelines are suggested:

1. Repair holes by welding on patches, rather than by spot welding.

2. Before welding, plug holes to prevent moisture from leaking into the tank frunder the bottom. Normally, wood plugs are used, but anything that will stothe seepage long enough to complete the weld all around the patch is acceptable. Preventing moisture leakage keeps the fillet weld on the patch cracking.

Fig. 200-6 Procedure for Determining the Remaining Life of a Tank Bottom

Step 1 Gauge bottom plate thickness in multiple locations where there is no bottom pitting observed on the stockside or indicated on the underside. Average the readings.

Average Reading: 0. inch

Step 2 Gauge the depth of the deepest stockside pitting not to be patched during the shutdown and record.

Deepest Pitting: 0. inch

Step 3 Gauge the depth of the deepest pit on the underside of the bottom by measuring turned coupons.

Deepest Pitting: 0. inch

Step 4 Determine whether the stockside bottom is to be protective coated. If it is, stockside pitting rate in Step 5 is zero.

Yes_____ No_____

Step 5 Determine the following rates:

General Corrosion Rate:Stockside Pitting Rate:Underside Pitting Rate:

0. inch/yr0. inch/yr0. inch/yr

Step 6 Perform the following calculation:Remaining bottom general thickness:Less general bottom corrosion rate X years next operating run:Less deepest unrepaired stockside pitting:Less deepest underside pitting:Less stockside pitting rate X years next operating run:Less underside pitting rate X years next operating run:

= 0.= 0.= 0.= 0.= 0.= 0.

If total is equal to or less than zero, the bottom should be replaced. Total

Chevron Corporation 200-11 July 2000

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200 Bottom Selection and Design Tank Manual

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3. Spot weld pits half way or more through the plate if the pit is not greater than 1 inch in diameter and is surrounded by substantially full thickness material. Shallower pits may be filled with special epoxy compounds, if necessary, prior to the application of internal coatings.

4. Patch pitted areas of larger than 1 inch diameter with pieces of ¼-inch platefillet welded all around. Time can be saved by supplying patch material consisting of random-sized square and rectangular pieces with dimensions12 to 30 inches sheared from scrap plate. Sheared patches must be small enough to pass through the shell manway or existing opening.

Annular Ring Replacement. Water accumulating around the inside edge of the shell can cause accelerated corrosion on the bottom in this area. For tanks ove100 feet in diameter, it is often less costly to replace the annular ring than the ebottom. See Section 300 and API 650 for annular ring design and installation.

Laminate Reinforced Coating. Section 640 discusses the various internal coatinsystems available for tanks. Company Specification COM-MS-4738 is a standaspecification to use for thin film, glass flake, or laminate-reinforced coatings. Because properly applied laminates have some structural strength, they can beeffective tool for prolonging the life of a tank bottom which has moderate undercorrosion. However, they must be used cautiously.

Laminates should not be used in the following situations:

• Where a hole has worn through the bottom plate and it remains unrepaired

• Where the bottom plate will hole through before the end of the next run andleakage can be allowed

• Where general corrosion has caused loss of structural strength in the annuring area. A rule of thumb is not to coat the annular ring if there is a 20% general reduction in plate thickness over any 2-square foot area of the annular ring

Thin Film or Glass Flake Coatings. Thin film or glass flake coatings can be usedin conjunction with bottom repairs or a new bottom to prolong the life of the bottom. They should not be put on over a bottom with severe internal or externcorrosion or pitting.

Section 640 discusses the use of these coatings. Specification COM-MS-4738 fies the materials and application procedures. Section 100 of the Coatings Manual discusses in more detail the factors that affect the type of coatings selected. Thfilm coating is most effective when used with internal cathodic protection. See Specification TAM-MS-3.

External Cathodic Protection. Cathodic protection can be used to stop undersidbottom corrosion of existing tanks. If there is no portland cement concrete slab,asphaltic concrete pavement, or penetration macadam pavement under the tanproperly applied cathodic protection will almost always be effective in preventinfurther corrosion. However, a concrete slab or pavement under the tank may mcathodic protection ineffective.

July 2000 200-12 Chevron Corporation

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An impermeable pavement will prevent the flow of cathodic protective current to the bottom steel. Cathodic protection will be effective where there are permeable areas or breaks in the pavement and will prevent moisture-caused corrosion at these locations. However, cathodic protection cannot eliminate corrosion due to moisture migrating under the tank from permeable to impermeable areas. Similarly, cathodic protection cannot completely control corrosion caused by moisture penetration beneath the tank from the periphery due to breathing. It is very difficult to determine conclusively from short term field tests whether cathodic protection will be helpful for a specific situation. Section 650 and the Corrosion Prevention and Metallurgy Manual discuss cathodic protection in more detail.

Bottom Repairs. New tank bottoms should be installed by cutting horizontal slots in the shell above the existing bottom and slipping in new bottom plates. The new bottom must be joined to the shell with a fillet weld on both the inside and outside. The area between the new and old bottom must be filled with sand or concrete. Refer to Specification TAM-MS-1, Tank Bottom Replacement and Membrane Placement.

Bottom patch plates, when used, should have full fillet welds all around except when they are butted together. Full penetration welds should join patch plates when butted together. When a patch plate is within 6" of the shell, the patch plate shall be “tombstone” shaped. The sides of the patch plate shall intersect the shell-to-botjoint at approximately 90°. When bottom patch plates are added directly below thshell, the shell should be slotted immediately above the old bottom and the platinserted. Fillet weld the patch plate to the shell on both the inside and outside. Acontinuous fillet weld should join the patch plate to the old bottom, including thesection under the shell. For additional information, consult API 653 Section 7.1

243 Bottom Replacement

Specification. A bottom replacement specification, TAM-MS-1, is included in thismanual.

Types of Replacement Bottoms. The considerations in selecting a replacement bottom are generally the same as for new construction. These are discussed inSection 210.

Secondary Containment and Leak Detection Bottoms. If future leakage cannot be tolerated, then a retrofit bottom, which includes secondary containment and detection, should be installed (see Standard Drawing GD-D1120, sheets 1 and membrane (HDPE) liner is placed over the existing steel bottom and overlaid wconcrete slab. The new steel bottom is then placed above the slab. This retrofitdesign works best where you expect minimal bottom settlement. If large settlemis expected, a membrane liner with a sand cushion over it and cathodic protectisystem should be installed. The old steel bottom may need to be removed in thcase due to the amount of storage volume lost to the sand cushion. This approagenerally not recommended because of the sand shifting and causing voids. (SStandard Drawing GD-S1121, sheets 1 and 2). Refer to Section 260 for membrdesign and selection.

Chevron Corporation 200-13 July 2000

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Non-leak Detection Bottoms. An important item to consider when secondary containment and leak detection are not included is that the new steel bottom will be anodic to the old steel bottom. This galvanic effect accelerates corrosion of the new bottom and has produced bottom failures in as little as four years. Therefore, it is essential either to remove the old corroded bottom before putting in the new bottom, or else to provide a good dielectric shield to stop current flow between the two.

An asphalt pad between the old and new bottoms provides a good dielectric shield, but it may not entirely stop water migration to the tank bottom. However, in a retrofit situation, there will be a semi-intact old bottom beneath the asphalt, and most of the tank settling will have already occurred, so the chance for success of asphalt is much greater than in the case of new construction. Therefore, if secondary containment is not required, asphalt may be a viable alternative. See TAM-EF-364 for asphalt pad foundation design.

Replacement Bottom Installation. The replacement bottom plates should be installed in accordance with API 653 and API 650. Generally, the replacement sketch plates (bottom plates upon which the shell rests) or annular ring plates are slid through a slot cut in the shell. The new bottom is continuously welded to the shell, both inside and outside, using fillet welds on the top. Intermittent fillet welds for strength are made between the new bottom and the lower part of the old shell. The weld size should be enough to develop the full strength of the bottom plates in bending. Undercutting at the toe of the fillets should be avoided. Care must be taken to be sure the new pad fully supports the new bottom next to the shell.

Annular ring plates are butt welded together using a 1/8 inch thick compatible backing strip, 2 inches wide, under the joint where it passes through the shell. Inside, the bottom plates are welded with a 1¼ inch lap and a full fillet lap weldfor new API tanks. In either case, it is necessary to notch (rat hole) the shell ovthis joint in the tank bottom to permit the welder to make a good weld through tshell. See Figure 200-5 for details of the annular ring installation in a replacemebottom.

244 Bottom Replacement RequirementsFor a complete description of the requirements for replacing tank bottoms, see commented version of Specification TAM-MS-1, Tank Bottom Replacement, anthe discussion above. Below is a summary of the procedure to follow for tank bottom replacement for small and large tanks.

Small TanksSmall tank bottom replacement is best done by lifting (or jacking up) the tank, placing a prefabricated bottom on the foundation, then lowering the tank to with2 inches of the new bottom, cutting the tank shell just above the old bottom welsliding the old bottom out and then lowering the shell and roof into place. The sis then welded into place and tested.

July 2000 200-14 Chevron Corporation

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Large TanksFor replacing the bottoms of large tanks, follow the steps described below for each of the replacement phases: preparation, bottom-to-shell welding, weld seam testing, and welding of pads and reinforcing plates to bottom.

Preparation PhaseTo prepare the shell for bottom replacement follow these steps:

1. Remove internal appurtenances, supports, and brackets.

2. Cut horizontal slots in the shell. These slots are usually about 5½ to 8½ feelong with 6 inches of shell left between the slots. The height of the slot shoube ¾ inch. The lower face of the slot should be relieved (notched out) for bwelded annular ring backup strips. The bottom edge of the slot will act as aform for the concrete spacer. See Figure 200-2.

3. Weld square C-shaped support clamps (or “dogs”) of heavy steel to the shethat the open area of the “C” allows the new bottom plate to slip through thshell with the required overhang. See Figure 200-2.

4. Install membrane under roof supports. Form around fixed roof supports anwrap floating roof legs as discussed in Specification TAM-MS-1.

5. Install the membrane liner as discussed in Section 260 and shown on DrawGD-D1120.

6. Install the concrete spacer. Complete concrete around supports as discussthe specification.

7. Remove 6-inch spacers between slots, install annular ring through shell sloand install bottom plate.

Relieving Shell over Bottom Plate Weld. A portion of the shell plate directly over the field welded bottom lapped plate or butt welded annular ring joint should benotched in order to permit completion of the weld under the tank shell. Each of lap welded bottom plates or butt welded annular ring joints under the shell shoube inspected before the notch can be welded up. Failure in this weld joint can produce a bottom leak almost impossible to track down. See Figure 200-5.

Bottom-to-Shell Weld SeamMinimum weld thickness is specified in API 650, Paragraph 3.1.5.7. There is noincrease in strength by exceeding the thinner plate thickness dimension with thweld. However, since this particular weld is subject to considerable potential cosion, on cone up bottoms in particular, some extra corrosion allowance in the wis useful.

Procedure. The ideal step-by-step procedure in making and testing the bottom-tshell welds is to weld the inside weld first, leak test the weld by applying diesel or penetrant to the un-welded side and visually inspecting for leakage on the inrior of the shell. The exterior weld is then made. This method ensures a leak-fre

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stockside weld. It should be used wherever the bottom design does not include a thick welded annular ring.

Bottoms equipped with annular rings cannot be welded this way. Making the stock-side weld first causes the annular ring plate to rotate about the bottom edge of the shell. For this reason, the outer weld must be made first and tested before the inner weld is made.

Verify that all traces of diesel oil or penetrant are removed by detergent washing from the opposite side prior to making the weld.

Replacement Bottoms. After the bottom-to-shell weld has been completed and tested, the “dogs” supporting the shell are removed and the tank permitted to sdown on the spacer pad.

Vacuum Testing of Weld SeamsVacuum testing of weld seams is often done as the bottom seam welding progrehowever, this practice is not recommended. Sometimes slag inclusions occur inwelds, particularly at stop and start weld points. Vacuum testing immediately afwelding does not give these inclusions enough time to open up. For this reasonvacuum testing of bottom welds should be delayed for 4 or more days (if possibafter welding. Failure due to hydrogen cracking should be evident after 1 day.

Pads and Reinforcing PlatesAll pads or reinforcing plates welded to the tank bottom should be, as a minimuseal welded all around. No clip, support, bracket, etc., should be welded to the bottom plate without a pad between the item welded on and the bottom plate. Tprecaution avoids concentrated loads that might tear the bottom.

Fig. 200-7 Slot Configuration for Replacement Bottom

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250 Leak Detection and Containment

251 Background and ScopeContamination of surface and subsurface waters by leaks and spills from storage tanks can be prevented by the use of leak detection, leak containment, secondary containment, cathodic protection and internal linings. In this section, leak detection and leak containment are addressed. See Section 650 for details of cathodic protection. Refer to the Coatings Manual for details of coatings and linings.

The Oil Pollution Act of 1990 required EPA to conduct a “liner study” to determiif leak detection and containment can be effectively implemented using liners ineffort to address the problem of surface and subsurface contamination by aboveground storage tank leaks and spills. The American Petroleum Institute hresponded to this call to safeguard the environment by issuing a new AppendixAPI Standard 650, Undertank Leak Detection and Subgrade Protection. In addition, because of various accidents, leaks and spills, the trend in the industry has beeinstall systems aimed at reducing the chance for undetected leaks and spills. Mstates now have some form of regulation that requires undertank leak detectionleak containment.

Undertank leaks, especially small ones, can go undetected for years contaminathe aquifer and accumulating liability for the owner. In large tanks, the thresholdleak detection is about 350 gallons per hour by tank gauging methods. This is considered unacceptable for leak detection. Leak detection methods, leak containment methods, cathodic protection, and linings for new and existing stortanks should be considered where the bottom is being replaced.

Examples of tanks with leak detection/containment are shown in Figure 200-8. These examples are discussed in detail in Section 256.

Note Figure 200-8 is a foldout appearing at the end of this section.

252 Definitions

Leak Detection. Aside from product loss considerations, leaks in ASTs are unacceptable because they may go on for years undetected while contaminating suface waters. Leak detection is the detection of leaks soon after they occur. In thperformance criteria outlined in API 650, Appendix I, a leak must be directed toperimeter of the tank where it shall be capable of detection by visual examinatioOther methods including sensors are acceptable but do not supplant the visualmethod. Also supplementary to the perimeter system of leak detection are all oother methods including ultra sensitive hydrostatic gauging, acoustic emissionstracer compounds blended with stored product, ground penetrating radar, slantdrilling, etc.

Leak Containment. Referred to as Subgrade Protection in Appendix I of 650. Lecontainment is the prevention of leaks from spilling onto the ground. Generally,is meant to apply to ASTs that have an elastomeric liner that prevents leakage

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a tank from entering the groundwater system. Double bottom tanks and tanks with liners that can contain a small fraction of the tank contents usually qualify as having leak containment systems.

Secondary Containment. Secondary containment refers to impounding of the tank contents. Most of the regulations such as NFPA 30 or SPCC require that the dikes be sized to contain the largest AST volume plus some freeboard for rainwater. Secondary containment is not covered in this section.

Cathodic Protection. External cathodic protection (CP) is becoming more and more widespread throughout the industry, both as a means to meet federal and local regulations regarding groundwater protection (some of which are already in place and some of which are still being written), and as a cost-effective method of prolonging the service life of tank bottoms between scheduled shutdowns. Internal cathodic protection is common for corrosion protection of crude tank bottoms or tanks containing water.

For smaller tanks (less than 50 feet in diameter), sacrificial zinc or magnesium anodes are generally used for external CP, and require no maintenance. For larger tanks it is more economical to use mixed metal oxide grids which require some maintenance to keep them in working order. In either case, good tank bottom protection can be had for as little as one to two dollars per square foot of steel protected. External cathodic protection systems are covered by the new API Recommended Practice 651.

Internal cathodic protection is usually provided by aluminum anodes attached to the tank bottom. In order to work properly, the anodes must be submerged in a conductive medium, such as the water layer at the bottom of a crude oil tank.

See Section 650 for further discussion of cathodic protection.

253 Performance Criteria for Leak Detection and Leak ContainmentIn order to satisfy the requirements for leak detection outlined in API 650, Appendix I the following criteria must be adhered to:

1. Leaks through the bottom must be directed to the perimeter where they are visually detectable. If a leak does occur it must be collected.

2. Electronic sensors and detectors may be used but they must be in addition to the requirements for leak detection at the perimeter.

3. Materials used for leak detection must be compatible with the range of products and the stockside temperature ranges and material in contact with the subgrade must be suitable for below grade service.

4. The permeability of the liner shall be less than 10-7 cm (400 mils) per second.

5. The leak barrier shall be of single-piece construction, or the joints shall satisfy leak tightness, permeability, and chemical resistance requirements of the base material. The manufacturer and a complete description of materials used shall be identified to the tank owner.

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6. The installation of sumps and pipes below the tank bottom is acceptable; however, the required leak detection and leak tightness shall be maintained.

254 Undertank and Double Bottom Spacer Material ConsiderationsWhen tank bottoms are replaced most of them are done by the “shell slotting method” referenced to in API 653, 7.10.2.1.2 The details of this construction arshown in Figure 200-9. Using this method requires that a spacer material be plbetween the old and the new bottom. Some options for this material are discusbelow.

Spacer Material for Double BottomsFor double bottom designs a common question is whether concrete or sand shbe used. Although concrete is much more costly on a volumetric basis it has a number of advantages. Concrete has essentially zero void space, whereas sanvoid space of approximately 40 percent. This means that any leak that occurs wcome to the tell-tale holes at the perimeter faster with the concrete system thanthe sand system.

Also, sand as a filler material between the two bottoms may be considered a hazardous material if contaminated with product leaks. The removal of the sandprobably be more difficult. There is also a much greater volumetric quantity of product leakage to remove from a sand layer than from a concrete layer.

Concrete is advantageous from an installation viewpoint because its slope and flatness can be specified and permanently controlled in the construction

Fig. 200-9 Shell Slotting Removal of Existing Bottom

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specifications. It is easy to create an accurate surface with concrete. Sand, on the other hand, is not easy to slope or control because it is too easily disturbed by men and equipment pathways. The concrete acts as a rigid foundation and aids in the process of laying down and forming the bottom plates. This is especially true when forming requires impacts, cutting, and welding operations. Sand is hard to keep out of the weld joints and can contaminate welds. Dragging plates across the sand has caused the plate to dive down through the sand and cut the liner on occasion. The elastomeric liner under the sand can more easily be damaged during construction than if placed under concrete.

Although the concrete system cannot accept a cathodic protection system, the concrete itself is considered to be a factor in inhibiting corrosion since it is alkaline.

Sand should be considered where concrete may not be feasible. This occurs when the tank is relatively large and the soil is subject to settlement. The settling concrete can crack causing failure of the elastomeric liner.

Another problem with concrete can occur when settlement pockets introduced by non-uniform settlement may cause pockets of water to form in the grooves under the tank and accelerate corrosion.

Also, acoustic emissions testing companies claim that a concrete pad makes finding leaks more difficult for them than does a sand pad or filler.

Undertank MaterialsConcrete has most of the advantages that are listed above for double bottom fill materials such as:

• reduced corrosion

• quicker bottom plate layout and installation

• slope and flatness control

• the ability to install leak detection grooves, and

• low void space.

Sometimes, concrete is indispensable as a liner thus fulfilling the need for a leadetection barrier when an elastomeric liner will not suffice. This is the case for htanks. Since the liner should be designed for the stockside temperature which mbe well above any ordinary elastomeric liner design limits, the concrete, if reinforced may be considered a liner.

Sand or soil as an undertank material has the advantage of reduced material coHowever, the washed sand or soil should be selected with minimal amounts of minerals and salts that could accelerate corrosion rates. Sand or soil under a tacan accommodate large amounts of local settlement without any adverse affec

Clay. There are a lot of concerns with the usage of clay. First, it must meet the permeability standards required by API Standard 650, Appendix I. Clay can crawhen dry and lose its properties as a liner. The cracks caused by shrinkage in tclay would allow large quantities of groundwater to migrate up to the bottom of

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liner of the tank causing potential problems. It is recommended that a geotextile fabric be installed between the clay and the liner because the effects of clay shrinkage in direct contact with the liner on the integrity of the liner are not known.

A lot of experience does not exist for clay liners. “Claymax” bentonite liners whihave been used extensively for diking requirements are subject to changing peability when exposed to certain conditions of pH or chemistry. Some states do nallow clay to be used as a liner because of its known expansion and contractionproblems affecting liner integrity. Clay is a poor conductor of electricity when drand therefore will have a variable effect on any cathodic protection systems thamust pass current through the clay. Also, it is hard to visualize a good method oassuring a leak proof joint between the ringwall and the clay when shrinkage contraction occurs.

255 Miscellaneous Design Considerations

Double Bottom TanksThe design engineer should evaluate the condition of the “dead shell” that will ebetween the old bottom and the new bottom. In most cases, the dead shell is incondition with little corrosion except at the very bottom. However, should it be severely thinned or pitted the following may apply. A weakened “dead shell” manot transfer the dead loads or seismic loads to the foundation. It may buckle orwarp. It also might not withstand a build up of hydrostatic pressure that could oshould there be a severe leak in the new bottom.

A common error in the installation of new bottoms is to attempt to install the nebottom inside the tank, fillet welding it to the interior surface of the shell as showin Figure 200-10. This is prohibited by API 653. This type of joint is subject to shrotation and will fail either on first filling or after fatigue of the new fillet weld.

A common question that arises with the design is whether to caulk or weld the underside of the new bottom (See Figure 200-10). Unfortunately, this issue is fafrom simple and involves a number of parameters.

API 650 Appendix I states that welding or caulking a double bottom under the nbottom is required. However, the discussion in I.4.1 of the Appendix requires than analysis and evaluation be performed if the new double bottom is not uniformsupported both inside and outside of the shell.

The intent of the caulk/weld is to seal out moisture which may enter by a numbemechanisms. Rainfall may flow around the new chime and by capillary action migrate into the leak containment space. If the foundation is in a flood area, theflood level may rise above the new tank floor and flood the underside of the newbottom. Slight thermal variation may cause a breathing of moisture laden air ancause a moisture pumping under the tank. Most people agree that this space nebe sealed off from the atmosphere.

A single 3/16 inch fillet weld pass would be the most economical weld to make.However, accessibility to the weld and control of the gap space can often be a

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problem. To really do it right may require the welder to use mirrors, slowing down the weld speed considerably. A proper weld should have a life expectancy approximately equal to that of the service life of the tank. Whether or not this is attained in practice is debatable. Most of our facilities do seal weld this juncture.

To reduce initial capital expenditures, an alternative to seal welding is to seal the joint with caulking. If this choice is considered, Sherwin Williams Steelseam 920-W-974 products are recommended.

Caulking probably does not have the life span of seal welding and is sensitive to surface preparation, flexure of the joint, sunlight, chemical environment effects, etc. However, it is probably cheaper to install on an initial cost basis compared to seal welding the floor to the dead shell. Here are some comparisons between the two methods of sealing this space:

• Seal welding will have a longer life than caulking. The life of the caulking isdependent on a number of factors as mentioned above. With a caulked joinposition of the top shell and dead shell should be monitored periodically to make certain that the tank shell is directly supported by the foundation throthe dead shell.

• Caulking should be used only where concrete is used as a filler material between the old and new bottom. When sand or gravel are used there is tomuch probability that insufficient filler material near the old dead shell will ato load the floor directly at the point of the old dead shell. Any misalignmencould cause failures. This has occurred in some non Company facilities.

Fig. 200-10 Retrofitting an Existing Tank with a New Double Bottom

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• Where high temperatures or varying temperatures of stored product are uscaulking should not be considered because the caulking will tear away as tshell grows radially outward. In this case seal welding would act to keep theupper and lower shells aligned.

• Where large amounts of settlement are expected, caulking should not be usthe seal will be broken.

• After hydrostatic testing, and on first filling, the caulking should be checked its integrity against leaks.

• The effect of uplift caused by seismic events on a welded joint versus caulkjoint is not really known. Further study is needed.

Triple BottomsIn several refineries, not only have second bottoms been installed but additionabottoms including up to as many as four bottoms have been installed. Althoughcertainly possible to add three or more bottoms, it should be realized that if the bottoms ever have to be removed the work will be more costly and difficult. As number of bottoms increases, the likelihood of having to relocate tank appurtenances will increase and, of course, the usable volume of the tank is redAlthough the structural effects are not really understood, the proof that they do seem to be adverse is the large number of operating years experience with thremore bottoms. One problem that has occurred is the oldest bottoms continue todeteriorate, and since the void space is filled with sand it washes out. This causbuckling of the dead shells and results in a very difficult repair job. If it is determined that the second bottom is deteriorating and a new bottom required,consideration should be given to more effective corrosion prevention technique

256 Design VariationsLeak detection/containment may be installed on new tanks or existing tanks. Appendix I of API 650 applies to new installations since the standard is applicato new tank construction only. API 653, “Tank Inspection, Repair, Alteration, anReconstruction,” which applies only to tanks that have been in service makes obrief reference to leak detection/containment: “If a tank bottom is to be replacedconsideration should be given to installing a leak detection (tell-tale) system thawill channel any leaks in the bottom to a location where it can be readily observfrom the outside of the tank” (API 653 Paragraph 2.4.5 Bottom Leak Detection). It was the intent of the API Committee to allow the criteria from Appendix I of API650 to be used for either new or existing construction.

Company practice has generally been to install double bottom tanks on existingtanks where the bottom is being replaced and to use one of the single bottom dwhere a new tank is being installed. This probably has the greatest economic pif all things are considered. However, there are cases where double bottoms arinstalled on new installations. Sometimes this is the result of local regulatory compliance or the opinion that the double steel bottom offers the best, longest lasting means for achieving effective leak detection.

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Figure 200-8 shows most of the designs that are being used by various oil companies today. The figure is divided into “R” figures, meant to cover the retrodesigns where a new bottom is required because the old bottom must be replaand “N” figures, meant to cover the designs that include leak detection for new installations.

Chevron has used the Figure 1R and 1N in the vast majority of cases. These dare proven with over 10 to 12 years experience and have shown no special problems. They are competitive with the other designs from a cost standpoint. Tmanual also has detailed drawings and specifications that are based upon thesdesigns.

Because of the numerous factors and pros and cons associated with each of thvarious designs, it is not a simple task to select the most optimal leak detection/containment system for a given site. Probably the best way is to coordinate the available knowledge by bringing in input from operations, tank building contractors and Company experts.

Designs for New Installations

Design 1N. This design is shown in more detail as “Foundation Type E,” in Standard Drawing TAM-EF-364. It can be used on flat bottom or sloped bottomtanks of any size.

The design incorporates ringwall foundations fitted with a grooved concrete slabThe grooves act to direct the leak to the perimeter of the tank where it can be observed. An 80-100 mil HDPE liner beneath the concrete slab acts to contain the leak.

Since the concrete is slotted, there are strips of tank bottom underside which ain contact with the concrete (it is doubtful that the areas adjacent to the bottom fillet welds are in contact with the concrete either). It has been postulated that tlocations are more prone to corrosion from atmospheric moisture. In some samthat were checked (in tanks with approximately seven years of service) in Richmthis was not observed to be the case. What is far more of a potential problem isbacking up of condensing atmospheric moisture or of ground moisture that canescape because the tell-tale holes are plugged. Debris either in the grooves of leak detection slab or plugged tell-tale holes can cause extremely accelerated corrosion rates on the bottom underside. The slab elevation should be high enoso that the water table or flood level virtually never exceeds it.

One major drawback with this design is that if used on soft or uncompacted soisubject to settlement, the concrete slab can crack. The cracked slab can damagor puncture the liner thus voiding the leak detection system. So far, this design not been tried on soft ground in Chevron. Locations which pump sand under tabottoms to replenish supporting soils are also unlikely to be able to use this des

Designs 2N, 4N, and 6N. Design 2N and 6N are similar except that 6N does not ua ringwall. As an alternate, a crushed rock ringwall maybe used. These designssimilar to foundation type “D” on Company Standard Drawing TAM-EF-364.

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The choice of whether or not to construct a ringwall for a new foundation is best decided on a location-by-location basis. In most locations, a compacted soil ringwall will not withstand erosion from heavy rains and may allow too much settling of the bottom to assure the integrity of the leak detection system. In a few dry locations with hard soils, going without a ringwall may be an alternative. A ringwall also minimizes differential peripheral settlement which causes the need for repairs to be made.

The question of whether or not to use a sacrificial anode system depends on cost. Both systems can be installed in any tank. Using today’s relative costs of zinc or magnesium (for a sacrificial anode system) versus typical mixed-metal anodes (for an impressed current system), the critical tank diameter size is about 35 feet. Tanks of less than about 35 feet diameter are constructed with a sacrificial anode system, while tanks larger than this are generally constructed using an impressed current system.

A definite advantage to designs 2N and 6N is that they incorporate not only leak prevention (the cathodic protection system) but they have leak detection (the tell-tale system).

Designs 3N and 5N. Designs 3N and 5N include clay as a form of liner beneath the tank bottom. These designs can be used for new tanks or for retrofits if the existing tank soil is excavated first. Not very much experience on these designs is available. Locations which might find this design suitable are locations with soft, moist soil where settlement is a problem. Small chemical tank foundations might be able to use these designs.

Design 7N. This design is a reinforced mat foundation extended at the perimeter to act as a ring wall. The middle section relies on a liner and sand to support the bottom.

Designs for Retrofitting Existing Tanks

Design 1R. This design has been used in many locations within Chevron. It is the “Standard Bottom Replacement for Existing Tanks”, which is shown in more deon Standard Drawing GD-D1120.

The concept behind this design is to use the existing bottom four to six inches otank as a form in which to pour concrete. A new bottom is then placed upon theconcrete producing a new tank that is reduced in height by approximately four tinches. The old bottom is generally in pretty bad shape. In order to satisfy the requirements of leak detection/containment, an HDPE liner is placed over the obottom. The liner also acts to insulate the spacer concrete from infiltration of gromoisture and to electrically insulate the new bottom from the old bottom. It is probably a good idea to place a geotextile fabric over the old bottom prior to placement of the liner to protect the liner from damage by the old bottom.

The concrete is typically reinforced with a polypropylene fiber in lieu of welded wire mesh because it is quicker and less costly to install, will not set up galvanicells for corrosion, and does prevent formation of hairline cracks. It also allows installation of thinner sections of concrete than would be possible with welded w

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mesh. In this application, the concrete is not designed for any structural requirements but is simply a spacer.

After the concrete is hardened, leak detection grooves are sawcut into the slab in radial patterns as applicable to the type of sloped bottom being used and dependent on diameter. These patterns are shown on Standard Drawing GD-D-1120.

Design 2R. This design depends on grating, mesh, angles, I-beams, or other structural steel shapes to form the spacer between the old and new bottoms. With this design any leaks and spills should quickly flow to the leak detection points located around the periphery of the tank. With this design hydrocarbon sensors can presumably pick up vapors relatively quickly for sensing leaks.

However, this design has been installed in so few cases in the industry that there is really no experience with it. It was created for relatively small diameter tanks only on concrete foundations. If some moisture accumulates in the space between the bottoms, it is not known if accelerated corrosion will be experienced at contact points with the structural members due to galvanic action.

If this design is chosen, the filler material must be both structurally adequate in itself and configured so as not to overstress any other component of the tank. It must provide the same characteristics as normal tank foundations, i.e., it must provide uniform support to the new bottom as well as transfer the loads uniformly to the old bottom and foundations. At least two potential problems must be addressed. First, the material must have the ability to support the new bottom without buckling or crushing due to the hydrostatic weight of the liquid above. Second, the bending stresses in the new bottom must be limited to prevent cracking. Since lap welded bottom plate construction is subject to failure where the welds are in excessive bending, the bottom plates must be laid out such that the high bending stresses caused by the hydrostatic head do not concentrate in long lengths of the bottom fillet welds.

Designs 3R and 4R. These designs should be considered the backup design to 1R. We have experience with them and they do not seem to present significant problems. It is our opinion that the impressed current anodes are more effective at delivering the required current to all sections of the tank bottom rather than sacrificial anodes systems when the tank diameter exceeds about 35 feet.

If the anodes are covered with sand, they can be exposed or destroyed if the new tank bottom plates are dragged across the sand.

Tank Bottom Selection CriteriaFigure 200-11 lists a number of criteria as they apply to the various designs shown in Figure 200-9. The figure shows that the Company really only has experience with designs 1R, 1N and 2N. However, there are conditions where another design might be appropriate for other reasons. As an example, design 2R for small shop fabricated chemical tanks storing extremely hazardous materials that can be detected with electronic sensors might be appropriate.

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260 Membrane Design and Selection

261 IntroductionThis section discusses the selection and installation of impermeable tank bottom membranes used for secondary containment on both new tanks and old tanks requiring new bottoms. The membranes are installed beneath supporting slabs (slabs are covered in Section 310 of this manual) as shown on Standard Drawings GF-S1121, “Standard Secondary Containment and Leak Detection Details for StoraTanks,” and GF-D1120, “Standard Bottom Replacement for Existing Cone-up aCone-down Bottom Tanks Including Secondary Containment and Leak DetectioThe slabs are grooved on top to channel leaking fluid. The membrane providesimpermeable layer to prevent the leaking fluid from reaching groundwater.

This section discusses only membranes appropriate for tank bottom secondarycontainment. Refer to Section 600 of the Civil and Structural Manual, titled “Ponds and Basins,” for a detailed and complete discussion of geomembranes for otheapplications.

262 Elastomeric LinerThe primary function of this liner is to serve as a release prevention barrier. Theliner directs the leaks to the perimeter, serving as a leak detector while at the stime, the liner prevents groundwater contamination. The liner is a backup systethe original steel bottom, which also serves this function. In addition, the liner functions to prevent galvanic corrosion between the new steel and old steel bot

263 Membrane Materials for Tank Secondary Containment

HDPECompany Standard Drawings GF-S1121 and GF-D1120 specify 80-100 mil thichigh density polyethylene (HDPE) as the appropriate membrane material for tansecondary containment. This recommendation is based the tough physical proprequired of the membrane and on immersion testing of membrane materials in coil and finished product by the CRTC Materials and Equipment Engineering Un(See References [1] through [5] in the Reference Section of this manual.)

California law requires that these membranes be at least 40 mil thick. This requirement is based on the membrane being reinforced with a scrim (such as Hytrel, described below). 80 to 100 mil HDPE is specified because HDPE is unreinforced (no scrim) and the extra thickness is required for strength and toughness. Richmond and El Segundo have used 100 mil HDPE because theyfound that wrinkling caused by thermal expansion is less with 100-mil HDPE thwith 80-mil HDPE during installation.

The proper ordering description is 100-mil thick HDPE liner conforming to EPA standards (see Reference 7 at the end of this section).

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Other MaterialsTwo other membrane materials, “Fueltane” and “Hytrel,” are considered good alternatives and are preferred when design cannot eliminate potential installatioproblems encountered with the thicker, stiffer HDPE. Fueltane (Seaman Corp.)urethane-coated polyester membrane, and Hytrel (DuPont) is a polyester elastomembrane which has been used by the Company for service station piping secondary containment.

Fig. 200-11 Tank Bottom Selection Criteria

Design Number (See Fig. 500-11)

Criteria 1R 2R 3R 4R 1N 2N 3N 4N 5N 6N

Retrofit Design X X X X X X X

New Tank Design X X X X X X X

Cathodic Protection X X X X(1) X(1) X

Concrete Inhibits Corrosion X X

Leak Detection X X X X X X X X X

Reasonable Track Record X X X

Potential Hazardous Waste Disposal X X X X X X X

Moist Soil Conditions Required X X

Stable Soil/Foundation Required X X X X X

Cracking Concrete Can Cause Liner Failure X X

Foundation Washout Potential X

Relatively Small Diameter Only X X X

Releveling Difficult X X X X X X X

Releveling May Damage Leak Prevention X X X

High Number of Barriers to Leakage X X X

Sand Pumping Can Damage Liner or Tank Bottom

X X X X X X

Leak Containment Space Reduces Tank Volume

X X X X

Company Has Experience With This Design X X X

Quick Leak Detection Response X X X

(1) Deep-well cathodic protection possible

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264 Design and Construction

DesignThe membrane layout should be in a pattern that minimizes the number and length of seams. Use the largest pieces that you can from HDPE rolls which come in 22-foot widths and are several hundred feet long. Using odd-shaped, smaller pieces of scrap results in a membrane that resembles a quilt. This patchworking is unacceptable because it increases the number of seams and the total weld length, and also results in weld intersections (often called weld T’s or Y’s due to their appearance) that are much more likely to leak or tear if stressed. On cone up acone down bottoms, the seams should be oriented down-slope to minimize stre

Substrate

New Tanks. The Standard Drawing GF-S1121 for new tanks shows four inches sand or compacted fill as the membrane substrate. This substrate is ideal as it separates the liner from the subgrade and helps protect it from any potential daif the foundation settles or shifts.

Existing Tanks. For retrofitting old tanks, the membrane is often installed over thold steel floor as shown in Standard Drawing GD-D1120. Most often, this provida good, stable substrate for the membrane. However, if the floor is badly corrodand/or riveted, the possibility of puncturing the membrane increases. A high qu10-to-16 ounce geotextile can be deployed under the liner to help protect the linfrom puncture and abrasion. The obvious burrs and edges should be ground fluprior to the installation of the geotextile.

Floating roof tanks have internal support legs that sit on the tank floor if the fluidlevel in the tank is low or empty. For existing tanks with the roof already in placecircular pieces of membrane, roughly two or three feet in diameter, are precut aslid under the roof support legs while the roof is being temporarily supported. Tthe rest of the membrane is installed and welded to the circular pieces under thstands.

InstallationRefer to the Engineering Specifications of this manual for bottom replacement specification TAM-MS-1. This specification also includes membrane placementThe steps to be followed when installing a membrane are summarized below:

Preparation. Before deploying any membrane, remove all debris from the supporting surface. The surface should also be dry.

Cement grout can be used to develop a smooth surface for the liner. A geotextimay be used in the grouted areas beneath the liner to prevent abrasion damag

Membrane Deployment. Deploy the membrane in a pattern to minimize the number and length of the seams. The best method is to unroll the membrane inposition so that all seams are parallel. The membrane can be cut so that it confto the tank circumference. As the membrane is being placed into position, the s

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should be hot air tack welded using a hot air gun (Liester). This will hold the membrane in place until the seams are welded.

The ideal sequence is to install the liner in the afternoon when temperatures are the warmest and the liner will be at its maximum expansion. This procedure will reduce the problem of wrinkles. However, do not fasten the liner to the shell until it cools down to prevent tearing due to shrinkage.

If the liner does wrinkle, it may become necessary to splice the liner and remove the wrinkle. If this is done, repair the liner using an oversized patch that is at least six inches larger than the cut at all locations. Merely welding the cut will not be acceptable because it is impossible to obtain the minimum three inch overlap—required by the specification—at the ends of the cut.

Seam Preparation. Bevel the top, overlying membrane edge at roughly 45 degreThis is necessary on thicker HDPE membranes (80 mil and up) to achieve a gofusion weld. Just prior to extrudate welding, the seams should be lightly sandedground to remove the thin layer of oxide that builds on the surface and then wipor air-blown to remove grindings, dust, or any other contaminants.

Weld Qualification. Prior to production seam welding, the installation technicianmust weld a qualifying test strip. Two test specimens from separate points on thtest strip must be cut and pulled to failure in peel as shown in Figure 200-12.

To pass the peel test, the membrane material, not the weld, must fail. If the welbreaks or if it peels off of the membrane, the test is a failure. The weld must bestronger than the membrane material.

Anchoring and SealingThe liner is anchored with studs around the circumference of the liner near the wall. A sealant is then applied to the membrane which covers the studs and seaedge of the membrane to the tank wall [see Reference 6 at the end of this secti

All of this work should be performed when the liner is cool and shrinkage will nobe a problem.

Fig. 200-12 Peel Test

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265 Inspection

Non-Destructive InspectionThe Company inspector should visually inspect the liner during and after installation. He should verify that:

• the seams are being lightly ground and cleaned prior to welding• the proper testing is being done• the liner is the correct thickness, and • sound construction practices are being enforced.

The installer must vacuum test the seams for pinhole leaks. The vacuum test machine must pull at least 5 psi vacuum. Some installers prefer using a spark twhich is also acceptable. Every inch of weld must be inspected by one of these methods, including patches.

Destructive InspectionDestructive samples are cut from the installed liner and tested in peel as descriabove. The frequency of testing is usually one sample per seam. Often, two destructive samples are also taken from the first 20 feet of seam welding to ensthat the weld guns are operating properly. The holes are patched with a circularpiece of liner welded to the membrane.

266 Approved Manufacturers and Installers

Manufacturers Company specifications should require testing of every roll of liner used to ensuthat we are getting only top quality material. Consult with the CRTC Materials aEquipment Engineering unit for approved manufacturers.

InstallersThe Company does not maintain an official list of approved installers because tquality of the installation depends on the on-site crew installing the system and can vary widely within individual companies. Instead, we give guidance on the experience level required for the site foreman and crew. On large pond projectsusually require two years experience in the specific position (field foreman, CQAforeman, technician, etc.), a predetermined number of square feet installed, anreferences with contacts and phone numbers. For tanks, require a set number oinstallations and ask for references with contacts and phone numbers.

The performance history of a particular company and crew is also very helpful. Obviously, request the good ones and reject the poor ones.

You may also contact CRTC’s Materials and Equipment Engineering Unit for assistance. This Unit is involved with liner jobs and projects throughout the Company and has direct experience with many manufacturers and installers of membranes.

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Standard DrawingsGF-S1121, “Standard Secondary Containment and Leak Detection Details for Storage Tanks.”

GD-D1120, “Standard Bottom Replacement for Existing Cone-up and Cone-doBottom Tanks Including Secondary Containment and Leak Detection.”

270 References1. Cummiskey, B. J., Impoundment Liner Testing - Western Producing Oil

Cleaning Plant, June 30, 1983, Materials and Equipment Engineering Unit File 25.6.

2. Klein, L. J., Storage Tank Containment Membrane Tests—El Segundo, December 7, 1983, Materials and Equipment Engineering Unit File 6.85.

3. Stofanak, R. J., Storage Tank Containment Membrane Tests—El Segundo, April 3, 1985, Materials and Equipment Engineering Unit File 6.85.

4. Rippel, T. E., Permeability Testing Flexible Membrane Liners, February 28, 1986, Materials and Equipment Engineering Unit File 25.06.01.

5. Rippel, T. E., Immersion Testing of Flexible Membrane Liners for SecondaryContainment, May 30, 1986, Materials and Equipment Engineering Unit File 6.85.

6. Kmetz, J. H., Adhesives Testing for Secondary Containment Membrane Systems, December 3, 1987, Materials and Equipment Engineering Unit File 56.1.

7. Environmental Protection Agency, Document EPA-600/2-88/052, Lining of Waste Containment and Other Impoundment Facilities, Appendix K

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Fig. 200-1 Bottom Designs for Storage Tanks

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Fig. 200-8 Tank Bottoms with Secondary Containment, Leak Detection and Cathodic Protection (New or Retrofitted)