Steel Tanks

Steel tanks serve as primary storage and process vessels in municipal water and wastewater treatment facilities, providing containment for raw water, treated water, chemical solutions, and biosolids. These welded steel structures utilize gravity flow and hydrostatic pressure principles to maintain system hydraulic grade lines while offering flexible capacity management. Typical municipal installations range from 100,000 to 5 million gallons with detention times of 4-24 hours depending on application. The key trade-off involves higher initial capital costs and ongoing corrosion protection requirements versus superior structural strength and customization capabilities compared to concrete alternatives.

• Raw Water Storage: Steel tanks provide 1-7 day storage capacity (0.5-15 MG) between intake and treatment processes. Selected for rapid construction, structural reliability, and ability to handle variable water quality. Connected upstream from intake screens/pumps, downstream to clarification processes.
• Finished Water Storage: Elevated and ground-level steel tanks store treated water (0.25-5 MG typical) for distribution system pressure maintenance and fire protection. Chosen for structural integrity, seismic resistance, and ability to maintain water quality with proper coatings. Connected downstream from disinfection, upstream to distribution pumps.
• Process Chemical Storage: Steel tanks store liquid chemicals like sodium hypochlorite (5,000-50,000 gallons), ferric chloride, and polymers. Selected for chemical compatibility with proper linings, structural durability, and secondary containment integration. Connected between chemical feed systems and process injection points.
• Digester Gas Storage: Steel spherical or cylindrical tanks store biogas (10,000-100,000 cubic feet) from anaerobic digesters. Chosen for pressure rating capability, corrosion resistance with proper coatings, and safety system integration. Connected downstream from digesters, upstream to cogeneration or flare systems.

Daily Operations: Operators monitor tank levels via SCADA systems, checking for proper fill/drain cycles and overflow conditions. Visual inspections include checking for leaks, coating deterioration, and proper ventilation. Water quality sampling performed at designated sample ports, with particular attention to chlorine residuals in finished water storage and temperature stratification in large tanks.

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Maintenance: Annual interior inspections when tanks are drained, typically during 5-10 year coating renewal cycles. Confined space entry procedures mandatory with atmospheric monitoring, ventilation, and rescue standby. Maintenance includes cathodic protection testing, valve exercising, and instrumentation calibration. Coating repairs require specialized contractors and 2-4 week outages for large tanks.

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Troubleshooting: Common issues include coating failure (chalking, blistering after 15-20 years), foundation settlement causing shell distortion, and mixing system failures in chemical storage. Early warning signs include water quality changes, visible rust staining, and level instrument drift. Structural issues manifest as shell buckling or roof ponding.

• Shell Structure: Carbon steel plates (3/16" to 1/2" thick) welded in courses, sized per API 650 standards. Selection based on hydrostatic pressure, seismic loads, and service requirements. Municipal tanks typically 20-120 feet diameter.
• Foundation System: Concrete ring wall or full slab foundation with anchor bolts. Designed for soil bearing capacity, seismic forces, and tank settlement. Ring walls typical for tanks >50 feet diameter.
• Roof Assembly: Cone, dome, or floating roof depending on application. Fixed roofs for potable water (aluminum or steel), floating roofs for chemical storage to minimize vapor space. Includes access hatches, vents, and instrumentation penetrations.
• Coating Systems: Interior epoxy or polyurethane coatings for potable water service, specialized chemical-resistant linings for process applications. Exterior coatings provide corrosion protection. Coating selection critical for 20-30 year service life.
• Appurtenances: Inlet/outlet nozzles, level instrumentation, overflow systems, mixing equipment, and cathodic protection. Sized based on hydraulic requirements and process needs.

• Volume Requirements: Storage capacity: 0.1-15 MG for typical municipal applications; Peak hourly demand factor: 1.5-2.5x average daily flow; Fire flow reserve: 500-3,000 GPM for 2-4 hours (varies by population); Emergency storage: 1-3 days average demand
• Structural Design: Internal pressure: 0-50 psi for ground storage, 50-150 psi for elevated; Wind load: 90-140 mph basic wind speed per ASCE 7; Seismic design: Site-specific per ASCE 7, typically 0.1-1.5g; Live load: 20-40 psf roof loading; Snow load: 10-70 psf depending on location
• Material Specifications: Steel grade: ASTM A36 (36 ksi yield) or A572 Grade 50 (50 ksi yield); Plate thickness: 1/4" to 2" depending on height and diameter; Corrosion allowance: 1/8" minimum for potable water service; Coating systems: 75-year design life with proper maintenance
• Dimensional Parameters: Diameter: 20-200 feet for ground storage; Height: 16-40 feet for ground storage, 100-200 feet total for elevated; Freeboard: 2-4 feet minimum above high water level

• Ground vs. Elevated Storage Configuration: Ground storage suitable when system pressure >40 psi available, site elevation adequate for gravity distribution. Elevated required when pressure boosting >$200,000 or insufficient site elevation. Wrong choice impacts $500K-2M in pumping infrastructure costs. Need: hydraulic grade line analysis, site topography, distribution system modeling.
• Single Large vs. Multiple Smaller Tanks: Single tank economical <5 MG capacity (~$150/1000 gal vs. $200/1000 gal for multiple). Multiple tanks provide operational flexibility, redundancy during maintenance. Critical for systems >20,000 population. Wrong choice affects regulatory compliance, emergency response capability. Need: population served, redundancy requirements, maintenance access.
• Interior Coating System Selection: NSF-61 epoxy suitable for 15-20 year service ($2-4/sq ft). Polyurethane systems last 25-30 years ($4-6/sq ft). Wrong selection doubles lifecycle coating costs. Potable water requires NSF-61 certification. Need: water quality, temperature range, maintenance budget, access constraints.
• Foundation Type and Soil Bearing Capacity: Concrete ringwall adequate for bearing capacity >3,000 psf. Mat foundation required <2,000 psf or expansive soils. Pile foundation needed <1,000 psf. Wrong choice causes settlement, structural failure. Need: geotechnical investigation, groundwater elevation, frost depth.

• Primary: Division 46 - Water and Wastewater Equipment
• Section 46 71 13 - Potable Water Storage Tanks

• Material/Equipment Verification: Steel mill certifications and coating compliance documentation; NSF-61 certification for potable water contact surfaces; AWWA D100/D103 compliance verification
• Installation Requirements: Foundation tolerance ±1/4" over tank diameter; Crane access for lifting operations; Temporary weather protection during coating application
• Field Challenges: Wind restrictions during erection (typically <25 mph); Coating cure time weather dependency
• Coordination Issues: Structural/civil coordination for anchor bolt placement; Lead times: 16-24 weeks typical for municipal projects

• CST Industries - Aquastore glass-fused-to-steel tanks, dominant in municipal water storage applications
• DN Tanks - Specializes in bolted steel with NSF-61 linings, popular for smaller municipalities
• Caldwell Tanks - Focus on elevated storage with Pedesphere designs
• Tank Connection - RTP (rolled, tapered panel) technology for ground storage, strong municipal references including recent installations in Texas and Ohio

• Prestressed Concrete Tanks - Lower lifecycle cost for large capacity (>2 MG), 20-30% higher initial cost but 40+ year service life
• Glass-Fused-to-Steel - Premium option with superior corrosion resistance, 15-25% cost premium justified for aggressive water chemistry
• Welded Steel with Advanced Coatings - Standard municipal choice, balanced cost/performance for typical 0.5-5 MG applications

Foundation preparation is critical - budget 15-20% more than contractor estimates for proper concrete work. Establish relationships with local steel erectors experienced in water tanks; many general contractors lack this specialty. Consider factory-applied coatings over field application to reduce weather delays. Specify witness testing for coating thickness and holiday detection during construction rather than relying solely on contractor QC.