Lobe Metering Pump

Lobe metering pumps deliver precise chemical dosing in municipal water and wastewater treatment through positive displacement action using two intermeshing rotors. The lobes create sealed chambers that capture and transfer fluid without pulsation, making them ideal for polymer, coagulant, and disinfectant feed applications. These pumps typically achieve flow rates from 0.1 to 50 GPM with accuracy within ±1% of setpoint across turndown ratios up to 100:1. The primary trade-off is higher capital cost compared to centrifugal chemical feed pumps, though operational precision often justifies the investment for critical dosing applications.

• Chemical Feed Systems: Lobe metering pumps deliver polymer, coagulant, and disinfectant solutions at 0.1-50 GPM with ±2% accuracy. Connected downstream from chemical storage tanks through suction strainers, they feed directly into process streams or rapid mix chambers. Selected for precise dosing control and ability to handle viscous polymers up to 10,000 cP without shearing.
• Biosolids Processing: Used for transferring thickened sludge (4-8% solids) and feeding lime or polymer at 5-100 GPM. Positioned between thickeners and digesters or dewatering equipment. Chosen for gentle handling of fragile floc and ability to maintain consistent flow against varying discharge pressures up to 150 PSI.
• Process Water Transfer: Moves clarified water, backwash water, or plant service water at 10-200 GPM between treatment units. Connected through flanged piping systems with minimal suction lift requirements. Selected for self-priming capability, low maintenance, and ability to run dry briefly without damage.

Daily Operations: Operators monitor flow rates via local indicators or SCADA, checking for consistent displacement and proper speed control. Routine checks include bearing temperature, seal condition, and timing gear oil level. Flow adjustments made through VFD speed control with typical response time under 10 seconds.

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Maintenance: Quarterly timing gear oil changes and annual mechanical seal replacement are standard. Requires confined space training for chemical feed applications and proper PPE for polymer handling. Rotor timing adjustment requires millwright-level skills, typically performed during annual overhauls with 8-16 hour downtime.

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Troubleshooting: Flow reduction indicates worn rotors or seals after 3-5 years typical service life. Excessive noise suggests timing gear wear or rotor contact. Seal leakage provides 2-4 week warning before failure. Cavitation from inadequate NPSH causes erratic flow and accelerated wear, requiring suction line modifications.

• Rotor Assembly: Twin three-lobe rotors manufactured from 316SS or Hastelloy, sized 2"-8" diameter for municipal flows. Rotors maintain 0.003"-0.008" clearances and operate at 50-400 RPM. Material selection based on chemical compatibility and abrasion resistance requirements.
• Timing Gears: Hardened steel gears maintain precise rotor synchronization, isolated from pumped fluid in oil-filled housing. Gear ratios typically 1:1 with bronze or steel construction. Critical for preventing rotor contact and ensuring consistent displacement.
• Mechanical Seals: Single or double cartridge seals rated for pressures up to 250 PSI. Materials range from carbon/ceramic for water service to silicon carbide for chemical applications. Seal selection determines chemical compatibility and maintenance intervals.
• Variable Speed Drive: AC motors with VFDs provide 10:1 turndown ratios for flow control. Standard motors 1-25 HP for municipal applications. Drive selection based on required flow range and control precision requirements.

• Flow Rate Requirements: Capacity range: 0.1-500 GPM (typical municipal range 1-100 GPM), Turndown ratio: 10:1 minimum, 20:1 preferred for chemical feed applications, Accuracy: ±1% at design flow, ±2% across operating range
• Pressure Parameters: Discharge pressure capability: 150-300 PSI standard, up to 500 PSI for specialty applications, Suction lift: 10-15 feet maximum practical lift, Net Positive Suction Head Required (NPSHr): 3-8 feet typical
• Chemical Compatibility: Wetted materials: 316SS, Hastelloy C-276, or PVDF depending on chemical, Seal materials: EPDM, Viton, or PTFE based on application, Temperature range: 32-180°F for standard applications
• Performance Specifications: Repeatability: ±0.5% for polymer feed, ±1% for coagulants, Viscosity handling: Up to 10,000 cP (alum solutions typically 5-50 cP), Solids handling: Generally limited to dissolved chemicals; some models handle up to 5% suspended solids
• Control Integration: 4-20mA input standard, Modbus/Ethernet options available, Variable frequency drive compatibility for flow modulation

• What is the required flow accuracy and chemical compatibility? Polymer feed requires ±1% accuracy with PTFE seals, while alum feed accepts ±2% with EPDM seals. Wrong selection leads to overdosing (increased costs) or underdosing (poor treatment). Need: chemical properties, dosing tolerance requirements, annual chemical costs.
• What discharge pressure and piping layout constraints exist? Static head plus friction losses determine required discharge pressure. Systems over 150 PSI require higher-grade pumps (+30-50% cost). Inadequate pressure sizing causes flow reduction and poor control. Need: elevation profile, pipe sizing, injection point pressures.
• How will the pump integrate with existing SCADA and control systems? Basic 4-20mA control costs $8,000-12,000 while networked systems with diagnostics cost $15,000-25,000. Wrong choice limits operational flexibility or creates integration issues. Need: existing control architecture, required data points, future expansion plans.
• What redundancy and maintenance access requirements apply? Single pump systems risk treatment interruption; duplex systems add 60-80% cost but ensure continuity. Poor access increases maintenance time 3-5x. Need: criticality assessment, maintenance staff capabilities, space constraints.

• Primary: Division 40 - Process Integration, Section 40 05 23 - Chemical Feed Equipment
• Secondary: Division 46 - Water and Wastewater Equipment, Section 46 21 13 - Metering Pumps (when part of larger treatment system specification)

• Material/Equipment Verification: Verify wetted materials compatibility with specific chemicals (316SS minimum for most applications), Confirm ANSI/NSF 61 certification for potable water contact
• Installation Requirements: Dedicated concrete pad with vibration isolation, Minimum 3-foot clearance for maintenance access, Local disconnect and NEMA 4X control panel
• Field Challenges: Precise alignment critical - field welding often causes misalignment issues, Suction piping must be completely air-tight
• Coordination Issues: 12-16 week lead times typical, Coordinate electrical rough-in with pump control requirements

• Grundfos - Hilge HYGIA series for chemical feed applications up to 50 GPM
• Xylem/Lowara - e-SV vertical multistage pumps adapted for metering service
• Watson-Marlow - Qdos peristaltic pumps (technically different technology but similar applications)
• Wanner Engineering - Hydra-Cell positive displacement pumps for municipal chemical feed systems up to 25 GPM

• Diaphragm metering pumps - Lower cost ($3-8K vs $8-15K for lobes), better for corrosive chemicals, but limited flow capacity
• Progressive cavity pumps - Handle higher viscosities, 20-30% lower cost, but require more frequent stator replacement
• Centrifugal with VFD - Significantly cheaper ($2-5K total), suitable for clean water applications where precise metering isn't critical

Establish relationships with local manufacturer reps early - they provide free sizing assistance and troubleshooting support. Specify spare rotor sets during initial purchase (30% cost savings vs. later procurement). For chemical feed applications, oversizing by 25% provides operational flexibility without significant cost penalty. Always request factory testing documentation and witnessed testing for critical applications.