Progressing Cavity Metering Pumps

Progressing cavity metering pumps deliver precise chemical dosing in municipal water and wastewater treatment through a helical rotor rotating within a double-helix stator, creating sealed cavities that advance fluid from suction to discharge. The positive displacement design enables accurate flow control from 0.1 to 500 GPH with typical turndown ratios of 100:1, making them ideal for polymer feed, coagulant dosing, and pH adjustment applications. However, the elastomeric stator requires periodic replacement and limits maximum operating temperatures to 180°F, creating ongoing maintenance costs.

• Polymer Feed Systems: PC metering pumps excel at feeding viscous polymer solutions (0.1-2.0% active) for dewatering operations. They handle shear-sensitive polymers without degradation, delivering 0.5-50 GPH at 50-150 PSI. Connected upstream to polymer makeup systems and downstream to injection points ahead of centrifuges or belt filter presses.

• Sodium Hypochlorite Injection: These pumps reliably dose 12.5% hypochlorite solutions despite their corrosive nature and tendency to off-gas. Typical flows of 1-25 GPH at 25-100 PSI for disinfection or odor control. Upstream connections include day tanks with level controls; downstream connects to injection quills in pipelines or contact chambers.

• Ferric Chloride Dosing: PC pumps handle concentrated ferric solutions (35-40%) for phosphorus removal or enhanced coagulation. Flow ranges of 2-40 GPH at 40-120 PSI are common. The pumps' positive displacement action ensures accurate dosing despite ferric's high specific gravity (1.4) and crystallization tendencies at low temperatures.

Daily Operations: Operators monitor pump flow rates via calibration columns or flow meters, checking for consistent discharge pressure (±5 PSI variance acceptable). VFD displays show motor current, which increases with wear or blockages. Suction piping requires daily inspection for air leaks that cause erratic pumping. Chemical feed rates are adjusted based on process demands and residual measurements.

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Maintenance: Stator replacement every 6-18 months depending on chemical abrasiveness and operating hours. Rotor inspection quarterly for wear patterns or coating damage. Mechanical seal replacement annually or when leakage exceeds manufacturer specifications. Maintenance requires confined space training for tank entry and chemical handling PPE including supplied air systems for chlorine compounds.

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Troubleshooting: Erratic flow indicates worn stator or rotor components requiring inspection and potential replacement.

• Rotor/Stator Assembly: The heart of PC pumps - a helical metal rotor rotates within an elastomer stator. Rotor materials include 316SS or Hastelloy for chemical compatibility. Stator elastomers range from Buna-N for mild chemicals to Viton for aggressive solutions. Municipal sizes typically feature 1:2 to 1:6 pitch ratios with rotors 0.75-3 inches in diameter.

• Drive System: Variable frequency drives (VFDs) provide precise flow control from 10-100% capacity. Gear reducers (typically 10:1 to 50:1 ratios) reduce motor speed to optimal rotor RPM (50-500). Standard motors are TEFC, 0.5-5 HP for municipal applications.

• Suction/Discharge Manifolds: Ductile iron or stainless steel construction with flanged or threaded connections (1/2" to 4" typical). Include pulsation dampeners, pressure relief valves, and calibration columns. Suction manifolds often incorporate priming systems for reliable startup.

• Seal System: Mechanical seals or packing glands prevent chemical leakage. Single or double mechanical seals with barrier fluid systems for hazardous chemicals. Seal materials must match pumped fluid - silicon carbide faces for abrasive slurries, tungsten carbide for corrosives.

• Flow Rate: 0.1-500 GPH (0.4-1,900 L/h) typical municipal range. Chemical feed applications commonly 1-50 GPH for 0.5-10 MGD plants, up to 200 GPH for 50 MGD facilities.

• Discharge Pressure: 50-300 PSI standard range. Municipal applications typically require 75-150 PSI for polymer feed, 100-250 PSI for chemical injection against system pressure.

• Accuracy: ±2% of set point for quality units (Seepex, Moyno, NOV Mono). Critical for chemical dosing where overdosing wastes chemicals and underdosing compromises treatment.

• Turndown Ratio: 20:1 to 100:1 depending on manufacturer. Higher ratios enable precise control during varying plant loads without pump changeover.

• Suction Lift: Up to 25 feet dry suction capability. Important for chemical storage tank installations where NPSH may be limited.

• Viscosity Range: 1-100,000 cP. Municipal chemicals typically 1-1,000 cP (polymer solutions, ferric chloride, sodium hypochlorite).

• Temperature Rating: -10°F to 200°F standard. Municipal applications rarely exceed 100°F but freeze protection critical for outdoor installations.

• Materials: 316SS wetted parts standard. Hastelloy C-276 for sodium hypochlorite above 1% concentration.

• What flow rate accuracy is required versus chemical cost impact? ±2% accuracy costs 40-60% more than ±5% units but saves $10,000-50,000 annually in chemical costs for 10+ MGD plants. Need chemical unit cost, dosing rates, and annual volumes to justify precision pumps.

• Can the pump handle minimum and maximum flow requirements within one unit? Single pump with 50:1 turndown eliminates dual-pump systems but costs 25-30% more initially. Wrong decision requires expensive pump station modifications. Need 5-year flow projections and peak/minimum ratios.

• What discharge pressure is needed including future system modifications? Undersizing by 50 PSI requires pump replacement ($8,000-15,000). Oversizing wastes 20-30% on initial cost but provides flexibility. Need accurate system curves including planned piping changes and injection point pressures.

• How will viscosity changes affect pump performance throughout the year? Polymer solutions can vary 10-fold seasonally. Wrong rotor/stator selection reduces flow 30-50% or causes premature wear. Need viscosity data at operating temperatures and seasonal chemical concentration ranges.

• Division 40 - Process Integration

• Section 40 05 23 - Chemical Feed Pumps

• Primary specification location for progressing cavity metering pumps in municipal water/wastewater treatment applications. May also reference Division 43 (Process Gas and Liquid Handling) for specialized chemical handling requirements.

• Material/Equipment Verification: Verify elastomer compatibility with specific chemicals, Confirm wetted materials meet NSF-61 requirements, Check motor enclosure ratings for installation environment

• Installation Requirements: Minimum 5-pipe diameters straight run upstream, Pulsation dampeners often required on discharge, Adequate clearance for rotor/stator replacement

• Field Challenges: Alignment critical during installation, Suction piping must be air-tight, VFD compatibility verification essential

• Coordination Issues: 12-16 week lead times typical, Coordinate rotor/stator material selection with chemical supplier

• Moyno (NOV): 2000 Series for municipal chemical feed (0.1-50 GPM)

• Seepex: BN Series for polymer/coagulant dosing (0.05-100 GPM)

• NETZSCH: TORNADO Series for lime slurry applications (1-200 GPM)

• Waukesha Cherry-Burrell: Universal I Series for general chemical metering

• All maintain strong municipal references with 24/7 service networks supporting water/wastewater facilities.

• Diaphragm Metering Pumps: Better for precise dosing, lower maintenance, but limited to clean chemicals. Cost: 20-30% less than PC pumps.

• Peristaltic Pumps: Excellent for corrosive chemicals, easy maintenance, but limited flow rates (<10 GPM typically). Cost: Similar to PC pumps.

• Centrifugal with VFD: Lower cost for high-flow applications (>50 GPM), but poor accuracy at low flows. Cost: 40-50% less than PC pumps.

Maintenance Planning: Stock spare rotors/stators - they're 60-70% of pump cost but critical for uptime. Many plants keep complete pump assemblies as spares rather than individual components. Manufacturer Relationships: Establish service contracts early; most offer predictive maintenance programs using vibration analysis. Cost Savings: Consider standardizing on one manufacturer across facility to reduce spare parts inventory and training requirements.