Lobe Metering Pumps

Lobe metering pumps provide precise volumetric dosing of chemicals and additives in municipal water and wastewater treatment applications. These positive displacement pumps use two or more intermeshing rotors (lobes) that create sealed chambers to move fluid from suction to discharge without pulsation. Typical municipal installations achieve flow rates from 0.1 to 500 GPM with accuracy within ±1% of set point. The primary trade-off is higher capital cost and more complex maintenance requirements compared to centrifugal chemical feed pumps, particularly for abrasive slurries like lime or polymer solutions.

• Chemical Feed Systems: Lobe metering pumps excel in precise polymer feed for clarification and thickening processes, typically handling 0.1-5 GPM flows of 0.25-1.0% polymer solutions. Selected for their ability to maintain consistent feed rates despite viscosity changes and downstream pressure fluctuations. Connected upstream to polymer makeup systems and downstream to flash mixers or flocculation basins.
• Sludge Conditioning: Used for polymer addition to waste activated sludge prior to dewatering, handling flows of 0.5-10 GPM. Chosen for their gentle handling of shear-sensitive polymers and ability to maintain accuracy at low flows. Connected between polymer storage and sludge feed lines to belt presses or centrifuges.
• pH Adjustment: Applied for caustic soda or acid feed in 2-15 MGD plants, typically 0.2-3 GPM flows. Selected for their ability to handle corrosive chemicals with appropriate wetted materials and provide precise dosing for regulatory compliance. Connected downstream of chemical storage tanks to rapid mix chambers.

Daily Operations: Operators monitor flow rates through integral meters or external flowmeters, checking for consistent output against SCADA setpoints. Routine checks include seal leakage inspection, drive oil levels, and chemical feed line pressures. Flow adjustments made through VFD speed control or stroke length adjustment, with typical accuracy maintained within ±2% of setpoint.

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Maintenance: Quarterly seal inspection and annual replacement typical for continuous service. Oil changes required every 6 months using manufacturer-specified lubricants. PPE includes chemical-resistant gloves, face shields, and appropriate respiratory protection when handling seal areas. Maintenance requires basic mechanical skills for seal replacement and electrical knowledge for VFD troubleshooting.

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Troubleshooting: Common failures include seal leakage (chemical attack or dry running), erratic flow output (air entrainment or worn rotors), and excessive noise (bearing wear or misalignment). Warning signs include increasing seal flush consumption, flow meter discrepancies, and unusual vibration. Typical service life ranges 5-8 years for rotors and 1-2 years for seals in continuous chemical service.

• Rotor Assembly: Twin helical rotors create positive displacement chambers, manufactured from 316SS or Hastelloy for chemical compatibility. Rotor clearances of 0.003-0.005 inches ensure minimal slip. Size selection based on flow requirements from 0.1-50 GPM for municipal applications.
• Timing Gears: External timing gears maintain rotor synchronization without metal-to-metal contact between rotors. Typically hardened steel with oil bath lubrication, requiring 6-month oil changes. Gear ratios determine pump speed and flow output.
• Mechanical Seals: Single or double mechanical seals prevent chemical leakage, with materials selected based on pumped media (silicon carbide for abrasives, PTFE for acids). Double seals with barrier fluid recommended for hazardous chemicals.
• Variable Speed Drive: VFD systems provide flow control from 10-100% capacity with 4-20mA input signals from plant SCADA. Enables precise chemical dosing based on process demands while maintaining consistent accuracy across flow range.

• Flow Rate Capacity: 0.1-500 GPM typical municipal range, with accuracies of ±0.5% at design conditions. Most applications fall within 1-100 GPM for chemical feed systems.
• Discharge Pressure: Standard units handle 50-150 PSI; high-pressure models reach 300-500 PSI for injection applications or long distribution runs.
• Viscosity Range: 1-10,000 cP capability, though municipal chemicals typically 1-100 cP (polymer solutions, sodium hypochlorite, ferric chloride).
• Turndown Ratio: Minimum 10:1 for variable speed drives; 20:1 achievable with premium units. Critical for seasonal demand variations.
• Materials: 316SS standard for municipal chemicals; Hastelloy C-276 for aggressive applications like ferric chloride at high concentrations.
• Temperature Limits: -20°F to 200°F operating range covers municipal applications including heated polymer makeup systems.
• Suction Requirements: Self-priming to 15 feet typical; NPSH requirements 2-8 feet depending on capacity and speed.
• Speed Range: 10-350 RPM standard; slower speeds improve accuracy and reduce shear on polymer solutions.
• Connection Sizes: 1/2" to 6" flanged or threaded connections for municipal flow ranges.

• What chemical flow accuracy is required versus system pressure? Municipal polymer feed requires ±1% accuracy at 50-100 PSI, while chlorine injection may accept ±2% at 200+ PSI. Higher accuracy demands slower speeds and larger displacement chambers, increasing cost 20-40%. Need: chemical type, injection pressure, process control requirements.
• How will turndown requirements affect pump selection and control strategy? Seasonal flow variations of 3:1 are manageable with standard VFDs, but 10:1+ turndown requires premium units with specialized controls. Undersized pumps lose accuracy at low flows; oversized units waste energy. Wrong choice impacts chemical dosing precision. Need: minimum/maximum flow demands, seasonal variations.
• What viscosity and shear sensitivity constraints exist for the pumped chemical? Polymer solutions >100 cP require slow speeds (<100 RPM) to prevent chain scission, while low-viscosity chemicals allow higher speeds for smaller pump sizing. Excessive shear reduces polymer effectiveness 30-50%. Need: chemical specifications, makeup concentration, shear sensitivity data.
• How do maintenance access and spare parts availability influence manufacturer selection? Municipal plants require 24/7 reliability with local service support. Premium manufacturers offer better parts availability but cost 25-50% more than generic units. Need: plant maintenance capabilities, service territory coverage, budget constraints.

• Primary: Division 40 - Process Integration, Section 40 23 00 - Chemical Feed Equipment
• Secondary: Division 46 - Water and Wastewater Equipment, Section 46 71 13 - Liquid Chemical Metering Pumps
• Use Division 40 for integrated chemical feed systems; Division 46 for standalone pump specifications in treatment facilities.

• Material/Equipment Verification: Verify wetted materials match specified corrosion resistance, Confirm motor efficiency ratings meet project specifications, Check impeller trim and curve selection against design point
• Installation Requirements: Adequate NPSH available - critical for suction lift applications, Proper foundation sizing for vibration control, Electrical coordination with VFD compatibility
• Field Challenges: Pipe strain on pump casings during tie-ins, Alignment issues with close-coupled configurations, Access limitations for maintenance in tight pump rooms
• Coordination Issues: VFD programming often requires manufacturer technician, Control system integration timing with SCADA contractor
• Lead times typically 12-16 weeks for standard municipal models, 20+ weeks for custom configurations.

• Grundfos - CR/CRI vertical multistage series, widely used in municipal booster stations
• Xylem (Bell & Gossett) - e-1510 series for smaller plants
• Flowserve - SIHI series for larger installations
• KSB - Multitec series popular in Canadian markets
• All maintain strong municipal service networks and stock common repair parts locally

• Centrifugal pumps - Lower cost, simpler maintenance, preferred for constant-speed applications above 100 GPM
• Diaphragm metering pumps - Better for chemical feed applications requiring precise dosing, roughly 20% higher initial cost
• Progressive cavity pumps - Superior for high-viscosity fluids or solids handling, 30-40% premium over centrifugal but lower lifecycle costs in difficult applications

Establish relationships with local manufacturer reps early - they often provide free sizing assistance and can expedite warranty repairs. Stock common wear parts (mechanical seals, impellers) for critical applications. Consider purchasing identical backup units for 24/7 operations rather than relying on rental pumps. Many municipalities negotiate bulk pricing for standardizing on single manufacturer across multiple facilities, reducing training and inventory costs.