Recessed Impeller Pump

Recessed impeller pumps are specialized centrifugal pumps designed to handle raw wastewater and sludge containing high concentrations of solids, stringy materials, and debris without clogging. The impeller is recessed within the volute casing, creating a large, unobstructed flow path that allows solids up to 4 inches in diameter to pass through without contacting the impeller blades. Instead of direct mechanical action, the pump relies on hydraulic forces and vortex flow patterns to move fluids, making it ideal for raw sewage lift stations, primary sludge transfer, and scum handling applications in municipal plants from 0.5 to 50 MGD. These pumps typically achieve efficiencies of 60-75%, which is 10-15% lower than conventional centrifugal pumps. The primary trade-off is reduced hydraulic efficiency and higher energy consumption in exchange for exceptional clog resistance and minimal maintenance requirements when handling difficult municipal wastewater streams.

• Raw Sewage Lift Stations: Handle raw sewage from gravity collection systems, lifting wastewater 10-40 feet to treatment plant headworks. Typical municipal sizes: 200-2,000 GPM per pump at 15-50 feet TDH
• Primary Sludge Pumping: Transfer thickened primary sludge (2-6% solids) from primary clarifiers to digesters or dewatering equipment. Typical range: 50-500 GPM at 20-80 feet TDH
• Storm Water Pumping: Handle combined sewer overflows and storm water containing high debris loads during wet weather events. Municipal storm pumps: 1,000-8,000 GPM at 10-30 feet TDH
• Scum and Foam Pumping: Remove floating scum and foam from clarifiers to scum thickening systems, handling 15-25% air content typical in clarifier skimmings

Municipal operators report that recessed impeller pumps require significantly less maintenance intervention compared to conventional centrifugal pumps when handling raw sewage. The primary operational challenge is determining optimal pump cycling to prevent solids settlement in wet wells while minimizing energy consumption. Operators note that these pumps handle seasonal variations in debris loading well, particularly during storm events when leaf and trash content increases substantially.

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Maintenance staff appreciate the reduced clogging incidents but emphasize the importance of proper crane access for the infrequent but necessary impeller inspections. The trade-off in energy efficiency is generally acceptable given the labor savings from reduced emergency call-outs for pump failures.

• Recessed Impeller: Located within volute casing, creates unobstructed flow path for solids passage
• Volute Casing: Houses recessed impeller, designed for vortex flow patterns and hydraulic forces
• Shaft and Bearing Assembly: Supports impeller rotation, typically heavy-duty design for municipal applications
• Mechanical Seal System: Prevents leakage, must be compatible with pumped media chemistry
• Motor: 5-500 HP range for municipal applications, service factor 1.15 minimum
• Suction and Discharge Connections: Sized for minimum 3-inch clear waterway

• Flow Rate: 50-8,000 GPM (0.1-18 MGD equivalent), standard municipal sizes: 100, 200, 500, 1,000, 2,000, 4,000 GPM
• Total Dynamic Head: 15-200 feet range, typical municipal applications: 25-100 feet
• Solids Handling: Maximum spherical solids 2-6 inches diameter, minimum 3-inch clear waterway for 4-inch discharge
• Efficiency: Peak efficiency 70-85% for municipal sizes, operating within 10% of BEP for optimal performance
• NPSH Requirements: NPSH Required 8-25 feet, NPSH Available must exceed NPSHR by 2-4 feet minimum
• Motor Specifications: 5-500 HP for municipal applications, standard voltages: 460V, 2,300V, 4,160V

• Maximum solids size and concentration: Threshold >3-inch solids or >500 mg/L suspended solids. Wrong decision leads to frequent clogging and increased maintenance
• Variable speed operation requirement: Threshold >30% flow variation or energy optimization needed. Wrong decision causes excessive energy costs and poor process control
• Suction conditions and available NPSH: Threshold NPSH Available <15 feet or suction lift >10 feet. Wrong decision results in cavitation damage and performance degradation
• Wet pit vs dry pit configuration: Threshold wet pit depth >12 feet or limited maintenance access. Wrong decision leads to installation complications and higher lifecycle costs

• Primary: 40 05 13 - Centrifugal Pumps: Covers horizontal and vertical centrifugal pumps for water/wastewater applications, includes performance requirements, materials, testing, and installation
• Secondary: 40 05 23 - Submersible Pumps: Applicable when recessed impeller pumps are submersible configuration

• Material Verification: Verify impeller material specification (316SS standard, CD4MCu for seawater), confirm shaft seal compatibility with pumped media chemistry
• Installation Requirements: Adequate crane access for pump removal (15-25 ft clearance), electrical disconnects within sight per NEC, minimum 10 pipe diameters straight run upstream
• Field Challenges: Impeller inspection requires complete pump removal, alignment criticality increases with larger frame sizes (>75 HP), seal water systems require separate commissioning
• Coordination Issues: Electrical rough-in for VFD integration, HVAC sizing for motor heat rejection, control system integration for pump alternation
• Lead Times: Currently 16-24 weeks for standard municipal sizes, 26-32 weeks for custom configurations

• Xylem (Flygt): Representative Model N 3153 MT (15-150 HP range), strong municipal presence with N-technology impeller design
• KSB: Representative Model Sewatec (10-200 HP range), European engineering with North American manufacturing support
• Grundfos: Representative Model SEG series (5-75 HP), extensive municipal distributor network with strong aftermarket support
• Cornell Pump Company: Representative Model NH series (25-500 HP), established municipal presence with custom impeller geometries

• Conventional centrifugal pumps with chopper systems
• Progressive cavity pumps for high-solids applications
• Submersible grinder pumps
• Air-operated diaphragm pumps
• Peristaltic pumps for smaller flows

Performance Optimization: From a 12 MGD water treatment plant retrofit in Ohio, operating recessed impeller pumps at 85-95% of design flow provides optimal efficiency while maintaining solids-handling capability. Regular impeller inspection every 6 months prevents performance degradation from wear.