Cast Iron Pressure Tanks

Cast iron pressure tanks maintain system pressure and provide surge protection in municipal water distribution systems by storing pressurized water in a steel shell with an internal bladder or diaphragm. When demand drops, the compressed air cushion forces stored water into the distribution system, maintaining pressure without pump cycling. Typical municipal installations range from 120-1,000 gallon capacity with operating pressures of 30-150 psi, providing 20-40% usable water storage. The primary trade-off is the heavy weight and corrosion susceptibility of cast iron construction, requiring substantial foundations and regular maintenance compared to modern composite alternatives.

• Hydropneumatic Storage Systems (0.5-5 MGD plants): Cast iron pressure tanks serve as pressurized water storage in smaller municipal systems, maintaining 40-80 psi system pressure. Connected downstream of high-service pumps and upstream of distribution mains, they reduce pump cycling frequency and provide emergency storage during power outages.
• Chlorine Contact Chambers (2-25 MGD plants): Large cast iron vessels function as pressurized chlorine contact tanks, providing 10-30 minutes contact time at operating pressures of 20-60 psi. Positioned between chemical feed points and clearwell storage.
• Chemical Feed Day Tanks (All sizes): Cast iron pressure vessels store sodium hypochlorite, ferric chloride, or polymer solutions under slight pressure (5-15 psi) for consistent chemical feed. Located near chemical feed pumps, they prevent cavitation and ensure steady chemical delivery rates of 1-50 GPM.

Daily Operations: Operators monitor system pressure via gauges, checking for proper cut-in/cut-out cycling (typically 20 psi differential). Log pressure readings twice daily and verify pump run times haven't increased, indicating potential bladder failure or air loss. Adjust air pre-charge seasonally to compensate for temperature variations affecting air volume.

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Maintenance: Quarterly inspection of air valves and pressure relief operation. Annual bladder inspection through access ports, checking for cracking or water-logging. Replace bladders every 8-12 years or when frequent air charging required. Maintenance requires confined space entry procedures, gas monitoring equipment, and mechanical aptitude for valve replacement. Tank draining and system isolation necessary for major repairs.

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Troubleshooting: Short pump cycles indicate bladder failure or air loss - verify with pressure gauge readings and tank tapping for water level. Reduced system pressure suggests relief valve leakage or shell cracking. Warning signs include rusty water (internal corrosion), frequent air makeup requirements, and pump short-cycling. Properly maintained cast iron tanks achieve 40-50 year service life before shell replacement becomes necessary.

• Shell Construction: Heavy-duty cast iron shells with 1/4" to 3/8" wall thickness, designed for 150-250 psi working pressure. Municipal tanks typically range from 120-10,000 gallons.
• Internal Bladder/Diaphragm: Butyl rubber or EPDM membrane separates water from compressed air, preventing waterlogging. Bladder pre-charge pressure set at 2-5 psi below cut-in pressure.
• Air Valve Assembly: Automatic air release valves, pressure gauges (0-200 psi range), and air charging connections maintain proper air-to-water ratio. Includes pressure relief valve set 10% above maximum working pressure.
• Inlet/Outlet Connections: Flanged or threaded connections sized 2" to 12" diameter based on system flow requirements. Internal diffusers prevent direct impingement on bladder during filling cycles.

• Operating Pressure Range: 25-150 psi working pressure, with 200-250 psi maximum allowable working pressure (MAWP) for municipal distribution systems
• Tank Volume: 80-15,000 gallons capacity range. Municipal systems typically use 500-5,000 gallon tanks for 0.5-10 MGD plants
• Flow Rate Capacity: Size for 1.5-3x average demand flow to handle peak conditions. Typical municipal sizing: 100-2,500 GPM continuous flow capacity
• Pressure Differential: Maintain 20-40 psi operating band between cut-in and cut-out pressures
• Air-to-Water Ratio: Standard 1:3 ratio (25% air, 75% water) for conventional tanks. Bladder tanks operate at 2:3 ratio
• Wall Thickness: Minimum 0.25" for smaller tanks, up to 0.75" for large municipal units per ASME Section VIII requirements

• What system pressure range and acceptable pressure swing can the distribution system tolerate? Need: System hydraulic analysis, minimum service pressure requirements, and pump station head curves. Wrong decision consequences: Undersized tanks cause excessive pump cycling, while oversized tanks waste energy and increase capital costs.
• Should we specify conventional air-over-water or bladder/diaphragm tank design? Threshold: Bladder tanks justified for systems >2 MGD or where air dissolution is problematic. Need: Water quality analysis, maintenance capability assessment.
• What drawdown volume is required for acceptable pump run times? Threshold: Minimum 1-minute pump runtime between cycles, targeting 10-15 minute cycles during average demand. Need: Pump capacity, system demand curves, acceptable cycling frequency.
• What connection size and configuration matches the pump station hydraulics? Need: Pump discharge pipe sizing, system head loss calculations, velocity limitations (5-8 fps maximum).

• Division 40 - Process Integration
• Section 40 05 23 - Process Piping and Pumping Appurtenances
• Primary specification section covering pressure tanks as integral components of municipal water pumping systems, including mounting, connections, and controls integration with pump stations

• Material/Equipment Verification: ASME Section VIII certification required, Cast iron grade verification (ASTM A48 Class 30 minimum), Pressure rating confirmation for system conditions
• Installation Requirements: Concrete pad with vibration isolation, Access clearance for maintenance (minimum 3-foot radius), Proper piping support to prevent stress on connections
• Field Challenges: Weight requires crane access during installation, Foundation settling can crack cast iron bodies, Temperature cycling causes expansion joint failures
• Coordination Issues: 8-12 week lead times for custom sizes, Electrical coordination for level controls and alarms

• Smith Cast Iron Boilers - Model 315 series for municipal applications up to 15 MGD
• Wessels Company - FLEXcon series with ASME certification for water systems
• CEMLINE - Cast iron expansion tanks with municipal references in 50-500 gallon range
• Niles Steel Tank - Custom cast iron pressure vessels for larger municipal installations

• Steel Pressure Tanks: Lower cost ($2,000-8,000 vs $4,000-15,000 for cast iron) but require cathodic protection and more frequent repainting. Preferred for temporary installations.
• Concrete Pressure Vessels: Cost-effective for larger capacities (>1,000 gallons) at $15-25/gallon vs $30-45/gallon for cast iron. Better for permanent installations with space constraints.
• Composite Tanks: Lightweight alternative costing 20-30% more but eliminating corrosion concerns entirely.

Establish relationships with local Wessels or Smith representatives early - they provide free sizing verification and troubleshooting support. Specify removable internals for bladder replacement rather than welded assemblies. Consider factory hydrostatic testing to avoid field pressure testing complications. Group multiple tank orders to negotiate better pricing and coordinate single delivery, typically saving 10-15% on total project cost.