Welded Steel Tanks

Welded steel tanks provide liquid storage and process containment for municipal water and wastewater treatment facilities, constructed from steel plates joined by continuous welding to create leak-proof vessels. These tanks function through structural steel framework supporting welded shell plates, with capacities ranging from 10,000 gallons to over 10 million gallons depending on application. Typical municipal installations achieve 40-50 year service life with proper corrosion protection and maintenance. The primary trade-off involves higher initial capital cost and ongoing corrosion protection requirements compared to concrete alternatives, though steel offers superior constructability for large-diameter applications exceeding 100 feet.

• Clearwell Storage: Welded steel tanks serve as finished water reservoirs downstream of filtration, providing contact time for disinfection and storage for distribution demand. Selected for precise volume control (typically 0.5-5 MG capacity) and ability to maintain positive pressure

• Chemical Feed Storage: House liquid chemicals like sodium hypochlorite, ferric chloride, and polymer solutions in 5,000-50,000 gallon tanks. Chosen for corrosion resistance with proper coatings and ability to handle specific gravity variations

• Backwash Water Storage: Store filter backwash water for reuse or equalization, typically 100,000-500,000 gallons. Selected for structural integrity under rapid fill/empty cycles and ability to handle suspended solids

• Sludge Thickening: Gravity thickeners concentrate primary and secondary sludge before dewatering, ranging 20-100 feet diameter. Chosen for controlled retention time and structural stability under varying solids loading

Daily Operations: Operators monitor tank levels via SCADA or local gauges, checking for proper fill/drain cycles and level alarm functionality. Adjust inlet/outlet valve positions based on plant demand and process requirements. Visual inspection of exterior coating condition, foundation settlement, and access ladder/platform integrity during routine rounds.

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Maintenance: Annual interior inspections require confined space entry procedures with atmospheric testing, ventilation, and rescue standby. Coating touch-up every 3-5 years, full recoating every 15-20 years. Cathodic protection systems need biennial testing. Level instrumentation calibration quarterly. Requires millwright skills for mechanical components, coating contractor for major refinishing work.

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Troubleshooting: Shell corrosion appears as rust staining or pitting, indicating coating failure requiring immediate attention. Foundation settlement causes shell distortion and potential leak paths. Typical service life 30-50 years

• Tank Shell: Carbon steel plates welded to API 650 standards, typically 1/4" to 1/2" thickness depending on height and diameter. Sized using hydrostatic pressure calculations with 1.5 safety factor. Selection based on design pressure, seismic requirements, and corrosion allowance

• Foundation Ring: Concrete ring wall supporting tank bottom, designed for soil bearing capacity and frost protection. Typically 18-36" wide by 24-48" deep. Critical for preventing differential settlement and maintaining tank geometry

• Roof Structure: Fixed cone, dome, or floating roof depending on application. Fixed roofs handle 20-40 psf snow loads; floating roofs prevent vapor losses in chemical storage. Material selection considers span requirements and access needs

• Coating System: Interior/exterior protective coatings matched to stored media. NSF-approved epoxy linings for potable water, chemical-resistant coatings for process applications. Typical systems include primer, intermediate, and finish coats totaling 12-20 mils thickness

• Inlet/Outlet Nozzles: Flanged connections sized for design flow rates, typically 6"-48" diameter. Include vortex breakers, baffles for mixing prevention, and multiple elevation taps for operational flexibility

• Volume Requirements: 0.1 to 10 MG typical range for municipal applications, sized for 4-24 hour detention time based on process requirements. Clearwell storage typically 1-6 hours peak demand

• Operating Pressure: Atmospheric to 150 psig for most municipal applications. Ground storage tanks operate at 0-5 psig. Elevated tanks designed for 25-75 psig operating pressure with 150 psig test pressure

• Wall Thickness: 1/4" to 1" for tanks up to 40 feet diameter. Larger tanks require 3/8" to 1-1/4" based on API 650 calculations. Bottom plates typically 1/4" minimum

• Foundation Loading: 62.4 pcf for water plus 10-15% safety factor. Soil bearing capacity 3,000-6,000 psf typical for tank foundations. Wind and seismic loads per ASCE 7

• Geometry Constraints: Height-to-diameter ratios of 0.5:1 to 2:1 optimize cost and structural stability. Standard diameters: 20', 30', 40', 60', 80', 100', 120' for municipal applications

• Access Requirements: 24" minimum manways, 4" minimum drain connections, overflow 6" minimum diameter. Mixing systems require 8-12" nozzles for equipment installation

• What detention time and peak flow conditions determine tank volume? Municipal clearwells require 1-6 hours storage at peak day demand (1.5-2.5x average). Contact tanks need 30-60 minutes at peak hour flow (2-4x average). Insufficient volume causes pressure drops and treatment bypassing. Need 20-year demand projections and peaking factors

• Should the tank be ground-supported or elevated based on system hydraulics? Ground tanks work with system pressures above 40 psig and available land area >100' x 100'. Elevated tanks required when system pressure <30 psig or limited footprint. Wrong choice costs $200,000-500,000 in pumping infrastructure or land acquisition

• What corrosion protection strategy matches water quality and service life? Potable water tanks need NSF-approved epoxy linings ($3-5/sq ft) for 20-25 year life. Raw water applications use coal tar epoxy ($2-4/sq ft) for 15-20 years. Cathodic protection adds $15,000-30,000 but extends life 5-10 years. Need water quality analysis including chlorides, pH, and temperature

• How do seismic and wind loads affect foundation and anchorage design? Seismic zones 3-4 require base isolation or flexible connections adding 15-25% to foundation costs. Wind speeds >100 mph need guy wires or increased wall thickness. Wrong assumptions cause catastrophic failure or expensive retrofits

• Primary: Division 46 23 13 - Welded Steel Water Storage Tanks

• Secondary: Division 33 16 13 - Water Storage Tanks (civil/site work integration)

• Material/Equipment Verification: AWWA D100 compliance certification, Mill test certificates for steel plate (A36/A572), Coating system specifications and application records

• Installation Requirements: Concrete ring foundation with proper anchor bolt placement, 12-18 month lead times for custom tanks >2 MG, Crane access for lifting shell courses

• Field Challenges: Weather delays during coating application, Foundation settlement monitoring during hydrostatic testing

• Coordination Issues: Piping/electrical penetration conflicts with structural members, Access platform integration with adjacent equipment

• CST Industries: Aquastore glass-fused-to-steel tanks (municipal standard for 0.1-10 MG)

• DN Tanks: Welded steel clearwells and process tanks (0.5-50 MG range)

• Caldwell Tanks: Elevated and ground storage (municipal water systems)

• Tank Connection: Bolted RTP and welded steel for biogas/digester applications. All maintain AWWA D100 certification for municipal projects

• Prestressed Concrete Tanks: 15-25% higher initial cost but 40+ year life, preferred for large clearwells (>5 MG)

• Bolted Steel (RTP): 10-20% cost premium over welded, faster installation, ideal for remote sites with crane limitations

• HDPE/Fiberglass: Limited to <1 MG, 30% lower cost for smaller applications like chemical storage or equalization basins

Foundation Coordination: Work directly with tank manufacturer's structural engineer early - standard ring foundations often require modifications for local soil conditions. Cost Savings: Specify standard diameter increments (20', 30', 40' steps) to leverage manufacturer's standard tooling. Relationship Management: Establish preferred coating applicator relationships; quality coating work significantly impacts 20-year lifecycle costs and warranty coverage.