Screw Pump

Screw pumps are positive displacement pumps that move water or wastewater by trapping fluid between rotating helical screws and a stationary housing. Twin or triple screw configurations create sealed chambers that advance fluid axially from suction to discharge with each rotation. These pumps typically achieve flows from 50 to 5,000 GPM with efficiencies ranging 70-85% in municipal applications. The primary trade-off is higher capital cost compared to centrifugal pumps, though they excel in applications requiring steady flow regardless of head variations or when handling fluids with entrained solids.

• Raw Water Intake Pumping (0.5-15 MGD): Screw pumps handle debris-laden source water from rivers or lakes, connecting intake screens to clarifier headworks. Selected for ability to pass 6-8 inch solids without clogging, eliminating need for expensive screening equipment.

• Primary Sludge Transfer (2-50 MGD): Pumps thickened primary sludge (3-6% solids) from clarifier hoppers to digesters or dewatering equipment. Chosen over centrifugal pumps because gentle pumping action preserves sludge structure and prevents pipe plugging in 6-12 inch force mains.

• Stormwater Lift Stations: Handles combined sewer overflow with high debris content during wet weather events. Screw pumps pass rags, bottles, and organic matter that would jam conventional pumps, maintaining flow capacity when most critical.

• Return/Waste Activated Sludge: Transfers mixed liquor (2,000-8,000 mg/L MLSS) from secondary clarifiers back to aeration basins, handling typical municipal flows of 25-200% of plant influent without shearing biological flocs.

Daily Operations: Operators monitor amperage draw (indicates wear or blockages), vibration levels, and flow rates via ultrasonic meters. Speed adjustments through VFD maintain target flow rates as system head changes. Visual inspection for unusual noise, leakage at packing glands, and proper lubrication levels required during rounds.

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Maintenance: Stator replacement every 3-7 years requires confined space entry and rigging equipment. Weekly bearing lubrication, monthly coupling alignment checks, and quarterly vibration analysis prevent catastrophic failures. Maintenance requires millwright skills for precision alignment and confined space certification for trough entry.

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Troubleshooting: Declining flow with constant speed indicates stator wear or rotor-to-stator clearance increase. Excessive vibration suggests bearing wear or shaft misalignment. Cavitation damage occurs when suction conditions deteriorate. Typical service life 15-20 years for rotors, 3-7 years for stators in municipal service with proper maintenance.

• Screw Rotor Assembly: Single or twin helical rotors (12-96 inches diameter) manufactured from ductile iron or stainless steel. Municipal sizes typically 18-48 inches for 1-25 MGD capacity. Rotor pitch and clearances determine pumping rate and efficiency.

• Stator/Trough: Cast iron or concrete housing containing rotor, with rubber or polyurethane stator elements creating sealed chambers. Stator wear directly impacts pump efficiency and requires replacement every 3-7 years depending on abrasives.

• Drive System: Variable frequency drives (5-200 HP) with gear reducers providing 20-60 RPM operation. Torque requirements increase significantly with solids content - 150% design factor typical for municipal sludge applications.

• Inlet/Discharge Flanges: ANSI 150# flanged connections, typically 8-24 inches for municipal applications. Inlet design critical for preventing air entrainment that reduces pumping efficiency.

• Flow Rate: 50-15,000 GPM per pump (municipal plants typically use 2-4 pumps for redundancy)

• Small plants (0.5-5 MGD): 200-2,000 GPM per pump
• Medium plants (5-25 MGD): 1,500-8,000 GPM per pump
• Large plants (25-50 MGD): 5,000-15,000 GPM per pump

• Total Dynamic Head (TDH): 8-40 feet typical for municipal lift stations

• Raw water intake: 15-35 feet
• Intermediate pumping: 10-25 feet
• High service: 20-40 feet

• Pump Efficiency: 75-85% at best efficiency point (BEP)

• Speed Range: 60-180 RPM (variable frequency drive controlled)

• Solids Handling: Up to 4-inch spherical solids without clogging

• Inlet Submergence: Minimum 1.5 times pump diameter plus 2 feet

• Wet Well Dimensions: Width = 2.5 × pump diameter minimum; Length accommodates pump removal

• Power Requirements: 10-500 HP per pump depending on capacity and head

• NPSH Available: Must exceed NPSH Required by 2-3 feet minimum

• Pump Configuration: Dry pit vs. wet pit installation?

• Wet pit: Lower initial cost, easier installation, limited to <25 feet TDH
• Dry pit: Higher maintenance access, suitable for all heads, 30-40% higher installed cost
• Decision threshold: TDH >25 feet typically requires dry pit

• Number of pumps and redundancy level?

• Two pumps: Each sized for 100% peak flow (small plants <2 MGD)
• Three pumps: Each sized for 50% peak flow (most common 2-25 MGD)
• Four+ pumps: Each sized for 33-40% peak flow (>25 MGD plants)
• Wrong decision consequences: Inadequate redundancy during maintenance or emergency conditions

• Speed selection: Fixed vs. variable speed drive?

• VFD justified when flow varies >30% from average
• Energy savings: 15-25% typical with proper control
• Added cost: $8,000-15,000 per drive plus controls
• Payback threshold: >1,000 operating hours annually at reduced speed

• Pump curve selection: Steep vs. flat characteristic?

• Steep curve (>0.02 ft/GPM): Better for constant head applications
• Flat curve (<0.01 ft/GPM): Better for variable system conditions
• Wrong selection: Poor efficiency, potential instability, premature wear

• Division 40-48: Process Integration

• Primary: 46 13 16 - Wastewater Pumps (wet pit screw pumps)

• Secondary: 46 12 16 - Water Pumps (potable water applications)

• Related: 40 06 00

• Material/Equipment Verification:

• Verify 316SS construction for wastewater applications
• Confirm trough liner material (typically HDPE or 316SS)
• Check motor enclosure ratings (NEMA 4X minimum)

• Installation Requirements:

• Crane access for screw assembly (units typically 20-40 feet long)
• Precise elevation control (±0.25" typical)
• Adequate electrical service routing to upper bearing

• Field Challenges:

• Alignment during concrete pour critical
• Weather protection during installation
• Access for maintenance platforms

• Coordination Issues:

• Interface with existing wet well modifications
• Electrical coordination for VFD integration

• Lead Times: 16-20 weeks typical, 24+ weeks for custom configurations

• Lakeside Equipment: Type 141 Raptor Screw Pump series, dominant in North American municipal market

• Evoqua (formerly Envirex): Spiralift screw pumps, strong in retrofit applications

• Huber Technology: ROTAMAT Screw Lift, growing presence in larger installations

• Kuhn North America: Spirajoule series, established in Canadian market

• All offer standard municipal sizes from 18" to 144" diameter with capacities up to 100 MGD

• Submersible Pumps: Lower capital cost, easier installation, preferred for smaller flows (<5 MGD) and retrofit applications. Typically 30-40% less expensive initially.

• Vertical Turbine Pumps: Better for high-head applications (>30 feet), more energy efficient at higher heads.

• Progressive Cavity Pumps: Superior for high-solids content, but higher maintenance costs. Screw pumps preferred for municipal raw sewage due to clog resistance and gentler solids handling compared to centrifugal alternatives.

Bearing Maintenance: Specify easily accessible upper bearings - many operators report difficulty reaching standard locations. Consider bearing monitoring systems on units over 72" diameter. Manufacturer Relationships: Lakeside and Evoqua offer comprehensive service agreements including predictive maintenance. Cost Savings: Bundling multiple screw pumps in single procurement reduces unit costs by 15-20%. Standard diameters (36", 48", 60") have shorter lead times and lower costs than custom sizing.