Pneumatic Conveying Systems

Pneumatic conveying systems transport dry bulk materials like lime, activated carbon, polymer, and sludge cake using pressurized air through enclosed pipelines in municipal treatment plants. These systems use positive pressure (dense phase) or vacuum (dilute phase) to move materials from storage to feed points, eliminating manual handling and reducing contamination risks. Typical conveying velocities range from 15-25 ft/sec for dilute phase systems, with transport distances up to 1,000 feet horizontally. The primary trade-off is higher energy consumption compared to mechanical conveyors, with blower power requirements often exceeding 50 HP for larger installations, making them cost-effective primarily where flexibility and cleanliness outweigh energy costs.

• Biosolids Handling (5-50 MGD plants): Transports dewatered cake (18-25% solids) from belt filter presses or centrifuges to storage silos or truck loading stations. Selected for enclosed transport that eliminates odors and spillage concerns. Connects downstream from dewatering equipment to 200-500 cubic yard storage silos with dust collection systems
• Lime Feed Systems (10-100 MGD plants): Conveys quicklime or hydrated lime from bulk storage silos to slaking equipment or dry chemical feeders for pH adjustment and phosphorus removal. Chosen for consistent, automated delivery without manual handling. Upstream from 50-200 ton storage silos to downstream slakers or volumetric feeders
• Carbon/Chemical Addition (2-25 MGD plants): Moves powdered activated carbon (PAC) or polymer from storage to feed points for taste/odor control or enhanced coagulation. Preferred for precise dosing control and dust containment. Connects storage hoppers to injection points at rapid mix or filter influent locations

Daily Operations: Monitor conveying pressure (typically 8-12 psig), material flow rates, and filter differential pressure (<4 inches WC). Adjust rotary valve speed based on downstream demand and check for proper material discharge at receiving points. Verify blower operation and system air leaks through pressure trending.

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Maintenance: Weekly lubrication of rotary valves and blower bearings. Monthly filter bag inspection and pulse cleaning system checks. Quarterly pipeline inspection for wear, especially at elbows. Annual rotary valve rotor replacement for abrasive materials. Requires confined space training for silo entry and lockout/tagout procedures.

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Troubleshooting: Material bridging in hoppers indicates moisture content >8% or poor flow characteristics. Excessive conveying pressure suggests pipeline plugging or undersized blower. Dust emissions point to torn filter bags or loose connections. Typical rotary valve life: 2-5 years depending on material abrasiveness. Pipeline elbows require replacement every 5-10 years in lime service.

• Rotary Airlock Valves: Control material discharge rate from hoppers while maintaining system pressure. Cast iron or stainless steel construction with 6-12 inch rotor diameters for municipal applications. Selection based on material abrasiveness and required feed rates of 50-2,000 lbs/hr
• Blower Package: Provides motive air at 5-15 psig for dilute phase systems or 15-60 psig for dense phase. Positive displacement or centrifugal types sized 10-100 HP for typical municipal flows. Selected based on conveying distance (typically 100-500 feet) and material characteristics
• Conveying Pipeline: Carbon steel or stainless steel tubing, 2-8 inch diameter with long-radius elbows to minimize pressure drop. Wall thickness 0.25-0.5 inches for abrasive materials. Routing includes cleanout ports every 50-100 feet and expansion joints for thermal movement
• Receiving Equipment: Includes baghouse filters, cyclone separators, and storage vessels. Filter bags rated for 99.9% collection efficiency on particles >0.5 microns. Storage hoppers sized for 2-8 hours material capacity with level indicators and emergency venting

• Conveying Rate: 50-2,000 lbs/hr for typical municipal plants; lime systems up to 5,000 lbs/hr for 50 MGD facilities
• Conveying Distance: Horizontal 50-500 ft, vertical lift 10-100 ft maximum for reliable operation
• Air Velocity: Dense phase 8-15 ft/sec, dilute phase 3,000-6,000 ft/sec in conveying lines
• Pressure Requirements: Dense phase 15-45 psig, dilute phase 8-15 psig positive pressure systems
• Loading Ratio: Dense phase 15-50 lbs material/lb air, dilute phase 0.5-15 lbs material/lb air
• Particle Size Range: -20 mesh to +200 mesh for most municipal chemicals (lime, carbon, polymer)
• Moisture Content: <2% for free-flowing operation; >5% requires special handling
• Bulk Density: 20-80 lbs/ft³ typical range for municipal treatment chemicals
• Air Requirements: 15-100 SCFM per 100 lbs/hr conveying rate depending on system type
• Power Consumption: 3-15 HP per 1,000 lbs/hr conveying capacity including blowers and controls

• Dense Phase vs. Dilute Phase Selection: Should the system operate above 15 lbs material/lb air (dense phase) or below 15 lbs material/lb air (dilute phase)? Dense phase reduces product degradation and air requirements but requires higher pressures and specialized equipment. Wrong choice results in excessive maintenance costs or poor conveying performance. Need: material friability testing, conveying distance, and degradation sensitivity analysis
• Pressure vs. Vacuum System Configuration: Will the system operate under positive pressure (8-45 psig) or vacuum (-5 to -15 psig)? Pressure systems handle longer distances and higher capacities but create dust containment challenges. Vacuum systems provide better dust control but limit conveying distance to <300 ft. Wrong choice affects plant safety and maintenance access. Need: plant layout, environmental requirements, and operator safety protocols
• Batch vs. Continuous Conveying Mode: Should the system convey material continuously or in discrete batches every 2-15 minutes? Continuous systems provide steady chemical feed but require more complex controls. Batch systems are simpler but create feed rate variations. Wrong choice impacts process control stability and equipment complexity. Need: downstream storage capacity, feed rate requirements, and control system sophistication

• MasterFormat 40 05 23 - Chemical Feed Pneumatic Conveying Systems: Primary specification covers pneumatic conveyors for water treatment chemicals. May reference 23 34 00 for HVAC air handling components and 40 05 13 for chemical storage integration requirements

• Material/Equipment Verification: Verify material compatibility with chemicals (316SS for hypochlorite areas), confirm explosion-proof ratings for enclosed spaces, validate capacity at actual operating conditions vs. test conditions
• Installation Requirements: Dedicated electrical service for blowers (typically 480V, 3-phase), structural support for receivers and hoppers (design loads 150% of operating weight), compressed air supply for controls and cleaning systems
• Field Challenges: Routing through existing pipe racks and cable trays, maintaining minimum bend radii (typically 10-15 pipe diameters)
• Coordination Issues: Interface with existing chemical feed systems, HVAC coordination for dust collection ventilation, lead times: 12-16 weeks for standard systems, 20+ weeks for custom designs

• Schenck Process: AccuRate volumetric feeders widely used in lime/polymer feed applications at 10-500 lb/hr capacities
• Flexicon: Tube-Vey pneumatic conveyors popular for activated carbon systems
• Vac-U-Max: Industrial vacuum systems adapted for biosolids handling
• Mac Process: Specializes in water treatment powder handling with sanitary designs meeting FDA standards

• Mechanical Screw Conveyors: Lower cost ($15K vs $35K for pneumatic), preferred for short distances (<50 ft) with dry materials like lime
• Belt Conveyors: Cost-effective for high-capacity applications (>2000 lb/hr), though require more maintenance
• Manual Bag Handling: Still common for smaller plants (<5 MGD) using polymer or activated carbon - lowest capital cost but higher labor requirements and safety concerns

Oversizing Considerations: Size blowers 20% above calculated requirements - municipal plants often exceed design chemical usage during upset conditions. Manufacturer Support: Establish service agreements upfront; pneumatic systems require specialized troubleshooting expertise not typically available in-house. Cost Savings: Specify standard pipe sizes and fittings rather than proprietary connections to reduce long-term maintenance costs and enable competitive sourcing.