Lobe Pump

Lobe pumps are positive displacement pumps that move fluids by trapping liquid between rotating lobes and the pump casing, creating a continuous, pulsation-free flow. Two or three intermeshing lobes rotate without contact, driven by external timing gears that maintain precise clearances of 0.004-0.008 inches. These pumps excel in municipal applications handling viscous sludges and sensitive biological solids, typically achieving volumetric efficiencies of 85-95% across flow ranges from 5-2,000 GPM. The primary trade-off is higher capital cost compared to centrifugal pumps, with limited ability to handle abrasive solids that can damage the tight clearances essential for proper operation.

• Waste Activated Sludge (WAS) Transfer: Lobe pumps handle 50-500 GPM WAS from secondary clarifiers to thickeners or digesters. Selected for gentle handling that maintains floc structure and prevents shearing. Typically connects downstream from clarifier underflow and upstream to dissolved air flotation thickeners or gravity belt thickeners
• Thickened Sludge Pumping: Moving 2-6% solids thickened sludge at 25-200 GPM from gravity thickeners to digesters or dewatering equipment. Chosen for ability to handle varying consistency without pulsation. Connects between thickener underflow and centrifuge or belt press feed systems
• Polymer Feed Systems: Delivering diluted polymer solutions at 5-50 GPM for flocculation processes. Selected for accurate, pulse-free delivery that prevents polymer degradation. Connects from polymer preparation tanks to injection points at clarifiers or thickeners
• Scum Pumping: Handling floating scum and grease at 10-100 GPM from primary clarifiers. Chosen for ability to handle stringy, viscous materials without clogging. Connects from clarifier scum boxes to scum thickeners or waste handling systems

Daily Operations: Operators monitor discharge pressure, motor amperage, and flow rates via plant SCADA. Lobe pumps require minimal adjustment once flow setpoints established through VFD control. Key monitoring includes bearing temperature, seal flush pressure (if equipped), and unusual noise or vibration. Typical municipal units operate continuously with automated start/stop based on level controls.

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Maintenance: Quarterly bearing lubrication and annual mechanical seal inspection standard. Rotor timing check required every 2-3 years or after seal replacement. Operators need basic mechanical skills for routine maintenance, millwright support for rotor removal. Full PPE required when handling sludge applications. Lock-out/tag-out critical due to positive displacement operation creating high pressures.

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Troubleshooting: Loss of prime indicates suction line air leaks or low sump levels. Excessive noise typically means worn timing gears or rotor contact from debris damage. Mechanical seal failure shows as stuffing box leakage - replace

• Rotor Lobes: Twin three-lobe rotors create pumping chambers through synchronized rotation. Cast iron or 316SS construction for municipal service. Lobe timing maintained by external gears - critical for preventing metal-to-metal contact. Size ranges from 2-inch to 12-inch for municipal flows
• Timing Gears: External helical gears maintain precise rotor synchronization and handle all torque loads. Hardened steel construction with splash lubrication. Gear ratios typically 1:1 with anti-backlash design. Protected by separate gearbox housing from pumped fluid
• Mechanical Seals: Single or double mechanical seals prevent leakage between rotating shafts and stationary housing. Silicon carbide faces with Viton elastomers standard for municipal service. Flush plans available for abrasive applications. Seal selection critical for MTBF
• Pump Housing: Split-case design allows rotor removal without disconnecting piping. Cast iron standard, 316SS for corrosive service. Internal clearances typically 0.003-0.008 inches - critical for efficiency and wear life. Includes suction/discharge flanges sized for low NPSH requirements

• Flow Rate: 5-5,000 GPM typical municipal range. Size based on peak hourly flow plus 25% safety factor for sludge applications, 15% for clear liquids
• Discharge Pressure: 15-150 PSI operating range. Consider system head, pipe friction losses, and elevation changes. Allow 20% margin above calculated total dynamic head
• Suction Conditions: Maximum 15 ft suction lift for standard units. NPSH required typically 3-8 ft depending on pump size and speed
• Solids Handling: Up to 3-inch spherical solids passage for wastewater applications. Verify with actual waste stream characteristics including rag content
• Viscosity Range: 1-10,000 cP effectively handled. Derating factors apply above 1,000 cP - consult manufacturer curves
• Speed: 100-600 RPM typical operating range. Lower speeds extend seal life and reduce shear for sensitive applications like polymer feed
• Temperature: Standard seals handle 32-180°F. Higher temperatures require special elastomers and cooling considerations
• Efficiency: 70-85% overall efficiency typical at BEP. Expect 5-10% lower efficiency at minimum flows

• What is the required flow turndown ratio and accuracy? Standard lobe pumps achieve 10:1 turndown with ±2% accuracy. VFD operation extends to 20:1 but reduces efficiency below 30% speed. Wrong sizing leads to poor control at low flows or excessive energy consumption at high flows. Need: actual minimum/maximum flow requirements and control precision needs
• What solids concentration and size require handling? Clearances of 0.006-0.012 inches limit abrasive solids. Wastewater applications typically handle 2-4% solids by weight with 2-inch maximum particle size. Oversizing clearances reduces efficiency; undersizing causes premature wear. Need: solids analysis including hardness, concentration, and size distribution
• What system pressure and NPSH are available? Lobe pumps are positive displacement - discharge pressure directly affects power requirements. Available NPSH must exceed 3-8 ft depending on size. Insufficient NPSH causes cavitation damage; excessive discharge pressure wastes energy. Need: complete system curve and suction tank conditions
• Is the application continuous or intermittent duty? Continuous operation requires premium seals and bearings, adding 15-25% to cost. Intermittent duty allows standard components but limits starts per hour. Need: actual operating schedule and maintenance access requirements

• Division 40-48: 40 31 33 - Rotary Lobe Pumps
• Primary specification under process mechanical equipment. May also reference 40 05 00 for common motor and control requirements

• Material/Equipment Verification: Verify elastomer compatibility with actual chemicals and temperatures; Confirm mechanical seal materials for specific applications; Check motor enclosure ratings for installation environment
• Installation Requirements: Rigid foundation/skid mounting to prevent misalignment; Proper piping support to avoid pump stress; Adequate clearance for rotor removal
• Field Challenges: Precise shaft alignment critical - field verification required; Temperature cycling affects clearances; Bypass piping often needed for startup
• Coordination Issues: VFD compatibility verification essential; 12-16 week lead times typical for engineered units

• Waukesha (SPX Flow) - Model 060 for 50-500 GPM municipal applications
• Viking Pump (IDEX) - HL4724 series for biosolids and chemical feed
• Grundfos - Hilge CONTRA series for smaller plants (10-200 GPM)
• Flowserve - Durco Mark 3 for aggressive chemical applications
• All maintain strong municipal references and local service networks

• Progressive Cavity Pumps: Better for high-solids applications, 20-30% lower cost, but higher maintenance
• Peristaltic Pumps: Ideal for small chemical feeds under 50 GPM, minimal maintenance but higher energy costs
• Diaphragm Pumps: Preferred for abrasive slurries, self-priming capability, but pulsating flow requires dampening. Lobe pumps typically cost 40-60% more than centrifugal but offer superior metering accuracy

Establish service relationships early - lobe pumps require specialized knowledge for timing adjustment and clearance setting. Many plants stock complete rotor assemblies rather than individual parts for faster turnaround. Negotiate service training as part of initial purchase. Consider standardizing on one manufacturer across multiple pumps to reduce spare parts inventory and training requirements.