Welded Steel Gas Storage Sphere

Welded steel gas storage spheres provide pressurized storage for process gases in municipal water and wastewater treatment plants, primarily storing chlorine gas, ammonia, or biogas from anaerobic digesters. These pressure vessels utilize spherical geometry to uniformly distribute internal stress across the steel shell, enabling efficient gas storage at operating pressures typically ranging from 150-300 psig. Typical municipal installations range from 10,000 to 100,000 gallon capacity, with storage efficiency of 85-90% usable volume. The primary trade-off involves high capital cost and complex foundation requirements versus superior pressure rating and minimal footprint compared to cylindrical alternatives.

• Biogas Storage at Anaerobic Digesters (5-50 MGD plants): Spheres store biogas from primary and waste activated sludge digesters at 2-6 inches WC pressure. Connected downstream of digester gas piping and upstream of gas conditioning equipment. Selected for consistent pressure maintenance and corrosion resistance to H2S. Typical capacity 10,000-50,000 cubic feet.
• Methane Collection at Lagoon Systems (0.5-10 MGD): Spheres collect methane from covered anaerobic lagoons, maintaining 1-4 inches WC storage pressure. Connected between lagoon gas headers and flare/generator systems. Chosen for weather resistance and ability to handle variable gas production rates while providing surge capacity.
• Digester Gas Surge Storage (10-100 MGD): Large spheres provide surge capacity between digesters and cogeneration engines, storing excess gas during low demand periods. Connected via automated pressure control valves to maintain optimal engine feed pressure of 15-25 psig after compression.

Daily Operations: Operators monitor sphere pressure via SCADA, typically maintaining 2-8 psig for biogas applications. Visual inspection of external surfaces, foundation settlement, and relief valve discharge piping. Pressure readings logged hourly with automatic data trending. Gas quality sampling performed weekly through dedicated sample ports.

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Maintenance: Annual internal inspection requires confined space entry with gas-free certification and continuous air monitoring. External coating inspection every 2-3 years with touch-up painting as needed. Relief valve testing annually per ASME requirements. Foundation and support structure inspection quarterly. Requires certified welders for any shell repairs and pressure vessel inspector for major work.

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Troubleshooting: Pressure fluctuations indicate downstream blockages or upstream production changes. Coating failure shows as rust staining or pitting, requiring immediate attention in biogas service due to H2S corrosion. Relief valve weeping suggests overpressure conditions or valve seat damage. Typical service life 25-30 years with proper maintenance, with coating renewal every 10-15 years in harsh environments.

• Welded Steel Shell: ASTM A516 Grade 70 carbon steel construction with 1/4" to 3/8" wall thickness for municipal applications. Sized 20-80 feet diameter based on required storage volume. Selection considers internal pressure rating (typically 15-50 psig) and local wind/seismic loads.
• Pressure Relief System: Primary relief valve set at maximum allowable working pressure plus secondary flame arrester for biogas applications. Spring-loaded valves sized per ASME Section VIII requirements with 1"-4" discharge connections.
• Foundation and Supports: Concrete ring foundation with steel support columns or full concrete pad. Foundation designed for sphere weight plus wind uplift forces. Requires engineered design for seismic zones.
• Instrumentation Package: Pressure transmitters (0-25 psig range), temperature sensors, and level indication if applicable. Connected to plant SCADA system with high/low pressure alarms and automatic isolation capabilities.

• Storage Volume: 100-10,000 cubic feet typical for municipal applications (0.5-50 MGD). Sizing based on 2-4 hours peak demand storage plus emergency reserve.
• Operating Pressure: 5-150 psig standard range. Low pressure (5-15 psig) for chlorine gas; medium pressure (50-100 psig) for compressed air systems; higher pressures for specialty gases.
• Design Pressure: 1.5x operating pressure minimum per ASME Section VIII. Temperature range typically -20°F to 120°F for outdoor installations.
• Material Specifications: ASTM A516 Grade 70 carbon steel standard for non-corrosive gases. ASTM A240 Type 316L stainless steel for chlorine or corrosive service. Wall thickness 1/4" to 2" depending on diameter and pressure.
• Diameter Range: 6-20 feet typical for municipal applications. Larger diameters (>12 feet) require field welding and specialized transport considerations.
• Foundation Requirements: Concrete ring foundation with anchor bolts. Load calculations include wind (90-120 mph design), seismic (per local codes), and thermal expansion. Minimum 3-foot clearance from property lines for pressure vessels >15 psig.
• Safety Systems: Pressure relief valves, pressure gauges, isolation valves, and emergency venting per NFPA standards.

• What storage capacity balances supply security with capital cost? Calculate peak hourly demand × storage hours (2-4 typical) plus 25% emergency reserve. Under-sizing risks supply interruption during peak demand or equipment maintenance. Over-sizing increases capital cost by $50-150/cubic foot and requires larger foundation/site area.
• Should the sphere be designed for low pressure (<15 psig) or medium pressure (50-100 psig) operation? Low pressure reduces wall thickness and cost but requires larger diameter for same gas mass. Medium pressure enables compact design but increases material costs 30-50% and requires more robust safety systems. Decision depends on available site area and downstream pressure requirements.
• What material specification meets corrosion resistance requirements within budget? Carbon steel costs $15-25/cubic foot but limited to non-corrosive gases. Stainless steel adds 200-300% material cost but handles chlorine, ozone, or other corrosive gases. Coating systems can extend carbon steel life but require maintenance access and periodic renewal.
• How does site accessibility affect fabrication approach? Shop-fabricated spheres (≤12 feet diameter) reduce field labor but require specialized transport. Field-welded units accommodate larger sizes and difficult access but increase quality control complexity and weather exposure during construction.

• Division 40 - Process Integration
• Section 40 05 00 - Common Work Results for Process Integration
• Secondary: Division 23 (HVAC) for compressed air systems, Division 22 (Plumbing) for water system integration

• Material/Equipment Verification: Mill test certificates for all steel plates, Welding procedure specifications (WPS) and welder qualifications, Third-party inspection certifications
• Installation Requirements: Concrete foundation design for seismic loads, Crane access for lifting pre-assembled sections, Nitrogen purging systems for commissioning
• Field Challenges: Weather delays during welding operations, Access limitations in existing plant sites, Coordination with ongoing plant operations
• Coordination Issues: Utility relocations around foundation area, Lead times: 16-24 weeks for custom spheres

• Chicago Bridge & Iron (CB&I/McDermott) - Horton Sphere series, 50-15,000 barrel capacity
• Matrix PDM Engineering - Standard welded spheres, 100-10,000 barrel range
• Tank Connection - Aquastore glass-fused steel spheres, municipal chlorine/ammonia service
• Caldwell Tanks - Pedesphere elevated storage, dual water/gas storage capability for smaller municipalities

• Horizontal cylindrical tanks - 20-30% lower cost, easier maintenance access, preferred for <500 barrel capacity or space-constrained sites
• Underground storage - Eliminates visual impact concerns, better security, but 40-50% higher installation cost due to excavation
• Composite overwrapped pressure vessels (COPV) - Lighter weight, corrosion resistant, but limited to smaller capacities and 2x material cost compared to steel spheres

Establish manufacturer relationships early - CB&I and Matrix PDM offer field service packages that significantly reduce commissioning time. Specify factory hydrostatic testing when possible; field testing adds 2-3 weeks. Bundle multiple gas storage projects for better pricing - manufacturers offer 15-20% discounts on multiple units. Consider standardizing on one manufacturer's connection details to simplify maintenance and spare parts inventory across facilities.