Dry-Pit Sump Pump

Dry-pit sump pumps are centrifugal pumps installed in dedicated dry chambers adjacent to wet wells, designed to lift wastewater, stormwater, or process water in municipal treatment facilities. The pump operates in an accessible dry environment while drawing liquid through suction piping from the adjacent wet well, allowing for easier maintenance and inspection compared to submersible alternatives. These pumps typically handle flows from 50 GPM to 5,000 GPM with heads up to 200 feet, making them suitable for lift stations, influent pumping, and process water applications in 0.5-50 MGD plants. The primary trade-off is higher initial installation cost due to the required dry chamber construction and more complex piping compared to submersible pumps.

• Lift Station Installations: Dry-pit sumps handle raw wastewater and return activated sludge (RAS) in 2-25 MGD plants. Selected over wet-pit designs for easier maintenance access and reduced H2S exposure. Upstream: gravity collection or clarifier underflow. Downstream: treatment headworks or aeration basins. Flow ranges 500-5,000 GPM.
• Plant Drainage Systems: Collect equipment wash-down water, filter backwash, and facility drainage in 1-50 MGD facilities. Chosen for reliable operation with intermittent flows and debris handling. Upstream: floor drains and equipment sumps. Downstream: plant headworks or equalization basins.
• Solids Handling: Transfer thickened sludge (4-8% solids) from gravity thickeners to digesters or dewatering equipment. Selected for positive displacement capabilities and reduced clogging versus submersible pumps. Typical flows 50-800 GPM with 15-100 HP motors.
• Chemical Feed Systems: Pump polymer solutions and liquid chemicals requiring precise flow control. Dry-pit access enables easy calibration and maintenance of metering pumps and variable speed drives.

Daily Operations: Operators monitor pump run times, flow rates, and amperage draw through SCADA systems. Check oil levels in gear-driven units and verify proper rotation. Adjust VFD setpoints based on plant loading. Record wet well levels and pump cycling frequency. Visual inspection for leaks, unusual noise, or vibration during routine rounds.

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Maintenance: Monthly lubrication of bearings and coupling alignment checks. Quarterly vibration analysis and motor megger testing. Annual pump curve verification and impeller inspection. Confined space entry procedures required with gas monitoring, ventilation, and attendant. Full PPE including respirators for sludge applications. Requires millwright skills for alignment and mechanical repair capabilities.

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Troubleshooting: Common failures include bearing wear (2-5 year service life), seal leakage, and impeller plugging. Warning signs: increased vibration, rising motor amps, or reduced flow rates. Cavitation damage from improper suction conditions. Control valve failures cause flow fluctuations. Typical pump service life 15-20 years with proper maintenance. Emergency bypass pumping capabilities essential during major repairs.

• Pump Unit: Centrifugal or progressive cavity design with 316SS or cast iron volute. Municipal sizes 3-12 inch discharge, 10-200 HP motors. Selection based on flow (100-5,000 GPM), head (10-150 feet), and solids content.
• Dry Pit Structure: Reinforced concrete chamber with waterproof membrane, typically 8x12 to 20x30 feet. Houses pumps above flood level with crane access. Includes ventilation systems for safety and equipment protection.
• Level Controls: Float switches, ultrasonic sensors, or bubbler systems control pump staging. Redundant level detection prevents overflow and dry running. Typical control range 2-8 feet between start/stop elevations.
• Piping Systems: Suction piping with eccentric reducers, discharge check valves, and isolation valves. 6-24 inch diameter with flanged connections. Materials: ductile iron or stainless steel based on application.
• Drive Systems: Direct-coupled motors with VFDs for flow control. Soft starters for fixed-speed operation. Emergency backup power connections standard.

• Flow Capacity: 50-5,000 GPM per pump (municipal plants typically use 2-4 pumps for redundancy)
• Total Dynamic Head (TDH): 15-150 feet typical, with most municipal lift stations operating 25-75 feet
• NPSH Required: 3-15 feet depending on pump size and speed; must maintain 3-foot minimum margin above available NPSH
• Motor Power: 5-200 HP for municipal applications, with 15-75 HP most common for 0.5-50 MGD plants
• Pump Speed: 1,170-3,500 RPM; slower speeds (1,170-1,750 RPM) preferred for longer service life
• Solids Handling: 2-6 inch spherical solids passage for raw wastewater; 1-3 inch for treated effluent applications
• Efficiency: 70-85% at best efficiency point (BEP) for properly sized units
• Wet Well Dimensions: Minimum 8-foot diameter for maintenance access; depth varies 10-30 feet based on hydraulic grade line
• Control Range: Pumps should operate between 20-120% of BEP flow to avoid cavitation and excessive wear
• Materials: Cast iron volute standard; 316SS impeller for corrosive applications; bronze fitted for saltwater exposure

• What is the required firm pumping capacity and how many pumps achieve this? Threshold: Largest single pump out of service, remaining pumps handle peak flow. Typical municipal: 2-pump stations handle 1.5x average flow; 3+ pump stations handle peak with largest pump down. Wrong decision causes overflows during maintenance or failures. Need: 20-year flow projections, peaking factors, maintenance philosophy.
• What total dynamic head accounts for all system losses and future conditions? Threshold: Include static lift + friction losses + 10-15% safety factor. Municipal range: 25-75 feet typical. Under-sizing causes insufficient flow; over-sizing wastes energy and reduces pump life operating left of curve. Need: Accurate wet well invert elevation, discharge point elevation, pipe sizing/routing analysis.
• Should pumps be constant speed or variable frequency drive (VFD) controlled? Threshold: VFDs justified when flow varies >3:1 ratio or energy costs >$0.08/kWh. Municipal break-even typically 25+ HP pumps with significant flow variation. Wrong choice: VFDs on small/constant-flow applications waste capital; missing VFDs on large variable-flow systems waste energy. Need: Flow variation analysis, energy cost evaluation, maintenance capabilities.
• What level of redundancy and backup power is required? Threshold: Critical facilities need n+1 pumping plus emergency generator; standard plants may accept brief overflows during extended outages. Consequences: Under-designed systems cause environmental violations; over-designed waste capital. Need: Regulatory requirements, downstream impact assessment, utility reliability history.

• MasterFormat 40 05 23 - Wastewater Pumps
• Primary section for municipal sump pumps

• Material/Equipment Verification: Verify cast iron/stainless construction per specifications, Confirm motor efficiency ratings (IE3 minimum), Check mechanical seal materials for sewage compatibility
• Installation Requirements: Crane access for pump removal (typically 10-ton capacity), Electrical coordination with VFD manufacturers, Grouting procedures for pump pedestals
• Field Challenges: Alignment issues with discharge piping, Inadequate ventilation in pump rooms, Power quality problems affecting VFD operation
• Coordination Issues: HVAC systems for pump room temperature control, SCADA integration timing, Lead times: 16-20 weeks for custom configurations

• Flygt (Xylem) - NP 3085 series for 100-2,000 GPM municipal applications
• KSB - Etanorm series, widely specified for lift stations
• Grundfos - CR/CRN vertical multistage pumps popular in smaller municipalities
• Sulzer - ABS AFP series for larger installations 500+ GPM
• All maintain strong municipal references and local service networks across North America

• Submersible Pumps: 15-25% lower installed cost, eliminate dry-pit construction but require specialized lifting equipment. Preferred for smaller stations <500 GPM.
• Horizontal Split-Case: 20-30% higher efficiency for large installations >1,000 GPM, require more floor space.
• Vertical Turbine: Best for deep sumps >20 feet, 10-15% premium cost but excellent NPSH characteristics. Consider site constraints and O&M capabilities when selecting.

Manufacturer Relationships: Establish preferred vendor agreements with local distributors - they provide faster service response and stock common wear parts. Cost Savings: Standardize on one manufacturer across facilities to reduce spare parts inventory by 40-60%. Consider factory startup services ($2,000-4,000) to avoid warranty issues and ensure proper commissioning documentation for operators.