Water Storage Tanks

NOTE This standard covers the water storage tank as a product: the shell, bottom, roof, and their supports from the top of the foundation up, the riser and the inlet and outlet piping inside the tank, the flexible connection at the foundation, the overflow, drain, manways, hatches, vents, ladders, platforms, handrails, and fall protection anchorages mounted on the tank, the mixing system where one is selected, the mechanical level indicator, the stilling well and the connections that serve the level instrument, the fittings and sleeves that serve the cathodic protection system, the obstruction lighting and antenna mounts, the inside and outside coating systems, the weld inspection and the hydrostatic test, and the disinfection of the completed tank before it is placed in service. (1.1)

NOTE Five constructions are covered, each under its own AWWA standard: the welded carbon steel ground storage reservoir, standpipe, and elevated tank under AWWA D100, the composite elevated tank whose steel tank stands on a concrete pedestal under AWWA D107, the factory-coated bolted carbon steel tank under AWWA D103, the wire- and strand-wound circular prestressed concrete tank under AWWA D110, and the tendon-prestressed concrete tank under AWWA D115. The construction type is the first selection on the datasheet, and the articles for each construction apply only where that construction is selected. (1.2)

NOTE This standard is read with Common Work Results ProcessCommon Work Results for Process EquipmentResolves to the current adopted revision.sync/common-work-results-process, the umbrella for the process discipline, which carries the submittal register, the process equipment data sheet, manufacturer qualifications, the drinking water contact certifications, the factory test witnessing policy, the manufacturer's field services, nameplates, the anchorage basis, the vibration and airborne noise acceptance, startup, the field test rules, warranty, and the spare parts basis. Nothing in those provisions is restated here. (1.3)

NOTE The capacity, the dimensions, the elevations, and the foundation of each tank are the Engineer of Record's design and are recorded on the drawings; the structural design of the shell, the roof, and the anchorage to the stated loads is a delegated design the tank manufacturer performs and seals. This standard specifies what the tank is and how it is proven, and it adds no design requirement of its own. (1.4)

NOTE The following are governed elsewhere and are outside this standard. (1.5)

NOTE A tank is designed, built, coated, and tested as one thing, and the warranty on it is only as good as the single party who can be held to it. A shell fabricated by one firm, erected by a second, and coated by a third produces a leak at a field weld that each of the three attributes to the other two; the single-responsibility requirement exists so that the Owner has one firm to call. (4.1.3)

NOTE A wire-wound or strand-wound tank is prestressed by a machine the tank builder owns and a procedure the builder developed, and the design assumes the losses, the spacing, and the tension that machine delivers. The tank cannot be designed by one firm and wound by another without the design being redone for the second firm's equipment, which is why the design and the construction are held together. (4.3.3)

NOTE The reviewing agency approved the project on the basis of the tank the design describes, and the Recommended Standards for Water Works carry provisions on vents, overflows, access openings, drains, and disinfection that the agency's inspector checks at the end. A tank that satisfies this standard and the AWWA standard can still fail a state criterion on a vent screen or an overflow termination, and the departure is cheaper to find before the plate is cut. (4.5.2)

NOTE Where the height of an elevated tank requires a determination of no hazard from the Federal Aviation Administration, the determination is obtained by the Owner before the tank is released for fabrication, and the obstruction marking and lighting it imposes are shown on the electrical drawings. (4.5.3)

NOTE Finished water is the service of nearly every tank bought under these AWWA standards, and it is the service that brings the drinking water certifications, the disinfection, and the state program review, which is why it is the default. Raw water and settled water relax the certification but bring suspended solids that settle on the floor and a water that is not chlorinated, so the interior is cleaned more often and the mixing matters less. Reclaimed water carries a residual that attacks some elastomers and an identification requirement in purple. Backwash supply water sits full for long periods between short, fast draws, which sizes the outlet and the vent. A fire protection tank under this standard is built to AWWA D100 because the Owner wants one tank construction on the site, and NFPA 22 then still governs its capacity, its heating, and its connections. (5.1.2)

NOTE A welded steel ground reservoir holds the most water for the least money where the site has the elevation, and its roof and shell are field welded and field coated. A standpipe is a ground tank taller than its diameter, which buys head on a flat site at the cost of a lower volume that is useful, because the water below the pressure zone's minimum grade line is never drawn. An elevated tank puts the whole volume at the operating head; a multi-column tank is the lowest-cost elevated structure at small capacities and the one with the most steel to paint, a single-pedestal tank hides the riser and the piping inside a fluted column with a dry interior that can hold a pump room, and a composite tank carries the steel bowl on a concrete pedestal that needs no exterior coating and is the construction at the larger elevated capacities. A bolted tank arrives as factory-coated panels, is erected without field welding or field coating, and is bought where the coating has to be factory controlled or the site cannot support a welding crew; its seams are gasketed and bolted, which is also where it leaks when it leaks. A wire-wound prestressed concrete tank has no coating to maintain, a floor and wall that are concrete, and a life that is measured against the cover coat over the wire; a tendon-prestressed tank places its prestress in ducts in the wall rather than in a winding outside it. The construction follows the capacity, the head, the site, and the Owner's maintenance practice, so no type is the norm across projects. (5.2.2)

NOTE The capacity, the dimensions, and the elevations are the hydraulic design of the system the tank serves, settled in the hydraulic analysis before the tank was specified, and the tank is built to them rather than selected from them. The design data table carries them in one place so that the manufacturer, the foundation designer, and the coating applicator read the same numbers. (5.3.3)

NOTE A self-supporting cone or dome puts nothing inside the tank to coat or to interfere with a mixer and spans a ground tank to a diameter the plate thickness allows; a column-supported cone spans a larger diameter with thinner plate at the cost of interior columns, their footings, and the coating of all of it. An ellipsoidal or torispherical roof is the roof of an elevated tank, where the shell and roof are one pressure-rated shape. An aluminum geodesic dome spans any diameter with no interior support and no coating, and it is bolted to a steel or concrete shell through a tension ring that has to be designed with the shell. A concrete dome is the roof of a prestressed concrete tank, cast against a form and prestressed at its ring; precast flat roof panels suit a tank whose roof carries traffic or equipment, at the cost of joints to seal. An open top is the arrangement of a raw water or backwash tank that the state program allows uncovered, and it is never the arrangement of a finished water tank. (5.4.2)

NOTE A tank is a nonbuilding structure in the eyes of the building code, and the AWWA standards supply the design method the code then accepts; the loads themselves come from the code and the site, not from the AWWA standard, which is why the general notes carry them and the calculations cite them. (6.1.3)

NOTE Seismic Use Group III is the group of a tank that has to be in service after the earthquake, which is the tank that supplies fire protection or the only storage of a system; it carries the highest importance factor and the most steel. Group II is the group of a tank whose failure is a substantial hazard to the public but whose water is not needed in the first hours after the event. Group I is every other tank. The group is the Engineer of Record's decision about the role of the tank in the system, and it is written here so that the calculations and the drawings agree on it. (6.2.3)

NOTE Whether a tank needs anchors is the outcome of an analysis, not a preference: a short, wide ground tank in a low seismic area stays down by its own weight, and a tall standpipe or any tank in a high seismic area does not. The field is derived from that analysis so that the foundation designer receives the anchor pattern and the uplift together, and the engineer who wants anchors regardless selects them. (6.3.4)

NOTE A ringwall carries the shell on concrete and the bottom plate on compacted sand or stone inside it, which is the foundation of most ground steel tanks because it is the least concrete that keeps the shell edge level and the bottom drained; a full slab carries the whole bottom on concrete and is chosen where the subgrade cannot be trusted, where a steel bottom is to be inspected from below through a leak detection layer, or where a prestressed concrete tank is built with its floor as a structural slab. Piles go under either where the bearing is not there. A multi-column tank bears each leg on its own footing, and a pedestal tank bears on one mat under the column. The foundation follows the subgrade and the construction type, so no foundation is the norm across projects. (6.4.2)

NOTE The foundation designer needs the loads and the bolt pattern before the foundation can be drawn, and the tank manufacturer needs the foundation to be level before the shell can be plumb; the two depend on each other across a contract boundary, and the survey before erection is the handoff. A tank set on a foundation that is out of level by more than the manufacturer's tolerance has a shell that cannot be plumbed and a bottom that does not drain. (6.4.7)

NOTE A36 is the plate every tank shop stocks and the material of most ground tanks under the allowable stress design basis, which is why it is the default; A283 is a lower-strength plate some shops prefer for thin courses; A516 is a pressure vessel plate with a notch toughness that AWWA D100 requires in thicker courses and at low design temperatures, and it is the plate of the elevated tank; A572 Grade 50 is the higher-strength plate the alternative design basis is written around, where thinner courses are bought with more inspection. (7.1.4)

NOTE A coated and cathodically protected tank is designed with no corrosion allowance on the premise that the coating is maintained, and that is the basis of most steel tanks; a 1/16 in. allowance on the bottom and the first course buys life for the plate the coating fails on first, at the bottom corner where the sediment lies, and a 1/8 in. allowance is bought by an Owner who expects to recoat late. The allowance follows the Owner's maintenance practice, so the field carries no default. (7.1.7)

NOTE The allowable stress basis of Section 3 is the design of nearly every ground tank and standpipe, with plate stresses that admit spot radiography and the mill-run steels, which is why it is the default. The alternative design basis of Section 14 uses higher allowable stresses on steels of known toughness with a full radiography program and tighter tolerances, and it buys thinner plate on a large tank at the cost of the inspection and the fabrication control it requires. (7.2.2)

NOTE A wet riser stores water in the column of a multi-column tank or the fluted pedestal of a single-pedestal tank, puts the inlet and outlet at the base where they are easy to reach, and carries a column of water that freezes from the outside in on a still winter night unless the tank cycles or the riser is heated; a dry riser keeps the water in the bowl and runs the inlet and outlet pipe up an access tube, so that the pipe is inside a space that can be heated and inspected, at the cost of a pipe that has to carry the head and a pipe support that has to follow the tank's movement. (7.3.3)

NOTE Glass fused to the steel at the factory is inert to the water, is not recoated in the life of the tank, and chips where it is struck, so its repair at a chip is a patch with an epoxy that does not match it; a thermoset epoxy is applied and cured in the same factory oven process, is repairable with a compatible epoxy, and carries a shorter life than glass in immersion; galvanizing is the coating of a bolted tank in raw water or utility service where neither the cost of glass nor the appearance of epoxy is wanted, and it is not the coating of a finished water tank without a lining. (8.1.4)

NOTE A steel floor on a slab keeps the floor off the ground and the floor seams bolted like the shell, and it is the floor of a glass-coated tank in finished water service, where every surface the water touches carries the same coating; a steel floor on a granular base inside a ringwall is the lower-cost arrangement of a raw water or utility tank; a concrete floor with the first shell course embedded in a starter ring removes the floor seams and the floor coating, puts a concrete surface in contact with the water, and depends on the embedded joint for its tightness. (8.1.7)

NOTE An encapsulated carbon steel bolt presents a polymer cap to the water and a plated shank to the seam, which is the fastener AWWA D103 is written around and the lowest cost; a stainless bolt removes the cap that can be lost and the plating that can fail, at a higher price and with an isolating washer against the coated panel; Type 316 is bought where the water is high in chlorides or the tank is at the coast. (8.1.9)

NOTE AWWA D110 names four core walls. A Type I wall is cast in place and prestressed vertically as well as circumferentially, with no steel diaphragm, and it is tight by its concrete alone, which puts the whole watertightness on the placement and the vertical prestress. A Type II wall is shotcrete built up over a steel diaphragm that is the water barrier, the lightest wall and the one most dependent on the shotcrete nozzleman. A Type III wall is precast panels set on the floor and joined with a steel diaphragm, which moves the wall concrete into a plant and shortens the site schedule. A Type IV wall is cast in place around a steel diaphragm, the heaviest wall with a barrier that is not the concrete. Each is a licensed builder's system, and the type a project gets follows the builder it can procure as much as the engineering. (9.1.5)

NOTE A bonded tendon is grouted into its duct after stressing, so that a break in the strand at one point loses the prestress only locally and the tendon is protected by the grout; an unbonded tendon runs in grease inside a sheath, can be restressed or replaced, and carries its whole force from anchorage to anchorage, so that its corrosion protection is the sheath and the anchorage cap. (9.1.7)

NOTE A free sliding base lets the wall shorten under the prestress and move with temperature without bending at the floor, so the wall carries hoop stress alone, and it depends on a waterstop at the joint and on bearing pads that keep working; a hinged base adds seismic cables that hold the wall against sliding in an earthquake while letting it rotate, which is the base of most tanks in a seismic area; a fixed base restrains the wall at the floor, which puts a bending moment at the base that the vertical prestress or reinforcement has to carry, and it removes the joint that could leak. (9.1.9)

NOTE The cover coat is the only thing between the prestressing wire and the weather, and the wire is a high-strength steel that fails by corrosion before it fails by load. One inch is the thickness AWWA D110 requires over the outer layer; a thicker cover is bought at a coastal or deicing-salt site, where the chloride reaches the wire through the thinner cover within the life of the tank. (9.1.11)

NOTE A common inlet and outlet is one pipe through the floor, which is the arrangement of nearly every elevated tank and of a ground tank that floats on the system, and it is the arrangement under which the water that came in last goes out first and the oldest water sits at the far side; a separate inlet extended into the tank and pointed away from the outlet turns the volume over and is the arrangement a water quality model asks for; separate pipes on opposite sides suit a tank that is filled by a pump and drawn by gravity on a different main. The arrangement follows the hydraulics and the water age the Owner has to manage, so the field carries no default. (10.1.2)

NOTE Steel pipe is coated with the tank interior as one system and is the piping of most steel tanks; ductile iron is the piping the site main brings to the tank and is run inside a concrete tank where there is no steel interior to match; stainless steel is bought where the interior piping is to outlast the coating cycle without being recoated with it, at the highest price and with an isolating flange where it meets carbon steel. (10.1.4)

NOTE An external overflow down the shell is visible, is reached from grade, and is the arrangement of most ground tanks; an internal overflow through the floor hides the pipe inside the tank, where it is coated with the interior, and emerges at the foundation, which is the arrangement of an elevated tank and of a tank whose exterior is to be clean; an overflow into a drainage structure through an air gap takes the discharge where the site drainage can carry it, and the air gap is what keeps the drainage structure from backing into the tank. Every arrangement ends in the open air above a splash pad or an inlet, screened and flapped, and never in a direct connection to a sewer or a storm drain. (10.2.2)

NOTE An overflow is the one opening in the tank that is open to the outside all the time, and it is where the insects, the birds, and the vandal arrive; the screen, the flap valve, and the air gap are each a barrier against a different one of those, and the state program's inspector looks at all three. (10.2.6)

NOTE Two shell manways on opposite sides let a ventilation fan be placed at one while the painter enters by the other, and they are the arrangement of nearly every ground tank; one is the arrangement of a small tank and of an elevated tank whose bowl is entered from the riser. A 30 in. manway passes a worker in a harness and a blasting hose with margin, which is why it is the default where AWWA D100 permits 24 in.; 36 in. is bought where equipment such as a mixer or a dive crew's gear is to pass. (10.3.3)

NOTE A ground tank's vent is reached by a ladder and its screen can be fine enough to exclude insects; an elevated tank's vent is reached by nobody, and a fine screen frosts shut on the first cold, humid night and pulls the roof in on the next drawdown, which is why the elevated tank takes a coarse bird screen set inside the vent where it cannot be reached. The pressure and vacuum relief is the backstop for both. (10.4.3)

NOTE A ladder safety system travels with the climber on a rail or a cable and arrests a fall within inches, needs a compatible sleeve the Owner's staff carry, and is the system most tank specifications now carry on every ladder, which is why it is the default; a fall arrest anchorage at the top of the ladder requires a self-retracting lifeline to be rigged before each climb and is the arrangement where the Owner's practice is built around it; a spiral stairway puts a stair with handrails around the pedestal or the shell, which is climbed without equipment and is the choice of an Owner whose operators climb the tank often, at a cost that exceeds the ladder by an order of magnitude. (10.5.2)

NOTE A locked shield over the lower rungs is what keeps the tank from being climbed by anyone with a key missing, and it is the provision of nearly every tank outside a fenced plant, which is why it is the default; a ladder that starts above reach needs a removable section stored somewhere and carried to the tank, which keeps the tank unclimbable and the operator's climb slower; relying on the fence is the position inside a plant where the tank stands among other structures behind a locked gate. (10.5.5)

NOTE A full perimeter handrail lets the roof be walked by anyone without equipment and is the arrangement of a ground tank whose roof carries equipment and is visited for the vent, the hatch, and the mixer; a partial handrail protects the area where the work is and leaves the rest of the roof to a tied-off worker, which is the arrangement of an elevated tank where a perimeter rail would be the most visible thing in town; anchorages alone are the position for a dome too steep to walk and for a tank where everyone who goes up is trained and equipped. (10.5.7)

NOTE A tank is the tallest structure at most plants and the one visited least, and every fall from one in the record happened to someone who climbed it alone with the equipment that was in the truck. The climbing protection and the anchorages are furnished with the tank because the tank manufacturer is the only party who can weld them to the shell and certify what they hold. (10.5.9)

NOTE Passive mixing directs the inlet jet so that the inflow momentum stirs the tank, costs nothing to run, and works only when the tank fills at a rate and from a position the nozzle was designed for; a submersible mixer stirs the tank regardless of the fill cycle, keeps a winter tank from stratifying and icing and a summer tank from losing its residual at the top, and brings a motor, a cable, and a lift point into a confined space; pumped recirculation does the same work from a pump at the base where it can be maintained, at the cost of the pump and the pipe. No mixing is the arrangement of a tank that turns over daily by its own operation. The provision follows the water age the system model shows, so the field carries no default. (10.6.2)

NOTE A float, a cable, and a target on a graduated board tell an operator at the gate what is in the tank with nothing powered and nothing to calibrate, and it is the indicator on nearly every ground tank and standpipe, which is why it is the default; it is omitted on a tank whose level is read only at the plant and on an elevated tank where the board would be unreadable. (10.7.2)

NOTE The connections for the level instrument are on the tank because they are welded or cast into it and coated with it; the instrument is not, because it is selected with the rest of the plant's instruments and read by the plant's control system. The split keeps the tank manufacturer from choosing a transmitter and the instrument supplier from cutting a hole in a coated shell. (10.7.5)

NOTE The cathodic protection of the interior wetted surface under AWWA D104, NACE SP0196, or NACE SP0388, and of the steel bottom exterior, is designed, furnished, and commissioned under Cathodic ProtectionCathodic ProtectionResolves to the current adopted revision.sync/cathodic-protection; this standard furnishes only the openings, fittings, insulation, and mounts that system needs on the tank, and the list above is what the sibling asks for. (10.8.2)

NOTE An antenna lease arrives after the tank is built more often than before, and a mount welded to a coated shell after the fact is a holiday in the coating and a load the calculations never saw; the mounts the drawings show are furnished with the tank so that the shell is reinforced for them and coated once. (10.9.4)

NOTE The coating of a water tank is specified with the tank rather than with the plant's coatings because AWWA D102 ties the system to the AWWA D100 fabrication: the shop primer is applied before the plate leaves the shop, the field seams are left bare for welding and coated afterward, the holdback is a fabrication dimension, and the coating warranty is the tank warranty. The application quality rules are the same as every immersion lining's, and those live in High Performance CoatingsHigh-Performance CoatingsResolves to the current adopted revision.sync/high-performance-coatings. (11.1.3)

NOTE A two-coat epoxy at 8 to 12 mils is the inside system of the last several decades, applied with conventional equipment and repaired in the field by anyone; a third coat buys thickness and a second chance at holidays on a roof underside that sees condensation; a zinc-rich primer under the epoxy sacrifices itself at a holiday and extends the time to first rust on a tank whose owner recoats late; a plural-component 100 percent solids epoxy builds its thickness in one pass with no solvent to flash in a confined tank and is the system of most new large tanks; a plural-component polyurethane does the same with a faster cure and a tolerance for the shell flexing of an elevated tank, and both need the heated plural-component equipment and the applicator qualified for it; an epoxy with a polyurethane topcoat is a system some Owners specify for the roof underside and the vapor space where ultraviolet enters at the vent. The system follows the tank size, the climate, and the Owner's recoating practice, so the field carries no default. (11.2.2)

NOTE Near-white blast cleaning is the preparation AWWA D102 requires beneath every inside system and the one the applicator's blasting rate and the inspector's visual standards are built around, which is why it is the default; white metal is bought under a zinc-rich primer or a plural-component system whose manufacturer publishes it as the condition of the warranty. (11.2.4)

NOTE The thickness is set by the system: a two-coat epoxy reaches 8 to 12 mils, a three-coat epoxy or a zinc-primed system 12 to 16, and a plural-component system 20 to 40, and a project writes the thickness its selected system's manufacturer publishes for immersion rather than a number of its own. Below the published thickness the holiday count rises at every weld and edge; above it a solvent-borne epoxy traps solvent and a plural-component system merely costs more. (11.2.6)

NOTE An alkyd system is the lowest first cost and the shortest life, chalking in a decade of sun and recoated with another alkyd; an epoxy and polyurethane system holds its gloss and color for fifteen years or more and is the system of most tanks whose appearance matters; a zinc-rich primer under it sacrifices itself at every scratch and extends the time to first rust on a tank in a wet or coastal climate; a fluoropolymer finish over the same primer and intermediate holds color for two decades on a tank that carries a logo the town will look at every day, at the highest first cost; a waterborne acrylic system is recoated over an existing acrylic without blasting and meets the strictest volatile organic compound rules. The system follows the climate and the Owner's recoating cycle, so the field carries no default. (11.3.2)

NOTE Commercial blast cleaning is what AWWA D102 requires beneath the outside systems and what an exterior coat on an atmospheric surface needs, which is why it is the default; near-white is bought beneath a zinc-rich primer, whose manufacturer publishes it as the warranty condition; power tool cleaning at the field seams over a shop-blasted plate is the arrangement where a site cannot be abrasive blasted for its neighbors, and it leaves the field seams with a shorter life than the plate. (11.3.4)

NOTE A shop primer protects the plate through shipping and erection and leaves the field seams and the finish to the applicator on the site, which is the sequence of most welded tanks; coating everything in the field after erection puts one applicator and one blast on the whole tank, which suits a tank erected quickly in a dry season and makes the blast and the containment the Contractor's; coating everything in the shop and repairing the field seams is the sequence of a bolted tank by definition and of a welded tank shipped in large sections, and it leaves the field seams with a coat applied under field conditions inside a shop-coated tank. (11.4.2)

NOTE The holdback is the strip of bare plate the welder needs so that the primer does not burn into the weld and the heat does not lift the primer beside it; too narrow and the weld is contaminated and the primer cracks, too wide and the field applicator is blasting plate that could have been primed in the shop. The width follows the plate thickness and the welding process, so the field carries no default. (11.4.4)

NOTE A bolted tank is coated once, at the factory, and the whole point of the construction is that the coating is never applied in the field; the field repair at a chip is a different material applied under different conditions, and an inspector who finds the chips before the tank is filled finds them while they are still reachable. (11.5.4)

NOTE An elastomeric coating bridges the hairline cracks of a dome and sheds the water that would otherwise find a crack above the stored water, and it is recoated on a cycle like a steel tank's exterior; a fluid-applied membrane does the same with a thicker film and a longer cycle, at a higher cost and with a surface that is walked on less easily; exposed concrete is the position where the dome is designed and prestressed to stay uncracked and the Owner accepts the staining of a concrete roof. (11.6.3)

NOTE An uncoated wall is the finish of most prestressed tanks, because the cover coat is the protection and a coating over it hides the cracks an inspector wants to see; an elastomeric coating adds a water-shedding film over the cover coat at a coastal or deicing-salt site; a textured architectural coating is bought where the tank is seen from the street and the Owner wants a color. (11.6.5)

NOTE Spot radiography at the frequency the design basis requires is the inspection the joint efficiency of that basis assumes, and it is what nearly every tank under the allowable stress basis receives; full-length radiography of the vertical seams finds the defect that the spot missed and is what the alternative design basis requires and what an Owner buys on a tall standpipe whose vertical seams carry the most stress; ultrasonic examination reads a weld without a radiation source, which lets the inspection proceed while other trades are on the site, and it depends on the examiner's hand in a way a film does not. (12.1.3)

NOTE AWWA D100 places the inspection with the manufacturer and gives the purchaser the right to be there, which is how most tanks are inspected and the arrangement under which the film is the manufacturer's record; an agency the Owner retains reads the film for the Owner and is paid by the party that wants the defect found; an agency the Contractor retains is independent of the manufacturer and inside the Contract Sum, and its report arrives through the Contractor. (12.1.6)

NOTE Twenty-four hours at the overflow level is the hold AWWA D100 and D103 are written around and the one nearly every steel tank specification carries, which is why it is the default; a prestressed concrete tank is held longer because its concrete absorbs water for days before the loss stabilizes, and an Owner who wants the loss measured rather than observed holds a week. (12.2.2)

NOTE A welded steel tank is tight or it is not: AWWA D100 allows no leakage, and a weep at a seam is a weld to be repaired, so the criterion for a welded tank is zero. A bolted tank is judged the same way after its bolts are retorqued, with a seam that still weeps resealed. A prestressed concrete tank absorbs water into its concrete and loses a measurable volume through evaporation and through the damp patches on its wall that AWWA D110 and D115 accept, so those standards allow a small measured loss with no visible flow, and the project writes the loss the standard for its construction type permits. The criterion follows the construction, so the field carries no default. (12.2.4)

NOTE The hydrostatic test proves the tank and loads the foundation for the first time at once, and the settlement survey is as much the point of the test as the leak inspection: a ringwall that settles unevenly under the first fill is found while the tank can still be drained and the foundation corrected, and a tank whose shell is out of plumb after the first fill is a tank whose foundation moved. (12.2.10)

NOTE Filling the whole tank with chlorinated water holds every surface at the disinfection concentration for the contact time and is the method of a small tank, because the water it uses and then has to dechlorinate is the whole tank; spraying the surfaces with a strong solution puts the chlorine where the surfaces are and uses a few hundred gallons, so it is the method of most large tanks and the one that puts a crew in the tank with the chemical; chlorinating a fraction of the volume at high strength and then filling over it disinfects the floor and the lower shell by contact and the rest by dilution, which suits a tank whose roof and upper shell were disinfected by spray before the floor was finished. The method follows the tank size, the water available, and the purveyor's practice, so the field carries no default. (12.3.2)

NOTE A tank is the one structure on the site that is erected to a tolerance of fractions of an inch in a hundred feet by a crew working in the wind, and the tolerances are the manufacturer's because the manufacturer's design assumed them; a shell out of round by more than the design allows carries a bending stress the calculations never saw, and the plumb survey before the roof goes on is the last time it can be corrected. (13.8)

NOTE A one-year coating warranty matches the tank warranty and is what the coating manufacturers and applicators give without negotiation; a two-year warranty covers the first full freeze and thaw and the first summer of ultraviolet on the exterior, and it is the period many utilities now carry; a longer period is bought on a plural-component system or a fluoropolymer finish whose manufacturer offers it, and it is conditioned on the inspection record this standard requires. The period follows the system and the Owner's practice, so the field carries no default. (15.2)

NOTE The first-anniversary inspection drains the tank once inside the warranty, puts the applicator and the manufacturer on the floor with the Owner, and finds the blister, the holiday at a weld, and the weeping seam while the party who warranted them is still obliged to fix them, which is why nearly every tank specification carries it and why it is the default; omitting it is the position of an Owner whose system cannot spare the tank for a day in its first year. (15.4)

NOTE The touch-up coating is the part used first, at the scratch the first maintenance visit leaves, and it is bought with the tank because the batch that matches the tank's color is the one on the tank; the gaskets and the screens are the parts replaced on a schedule; the flap valve, the climbing sleeve, the float assembly, and the bolted tank hardware are bought where the Owner's stores are far from the tank and the part is one the manufacturer does not stock in the Owner's region. (16.2)