Diaphragm-type Metering Pumps

Diaphragm-type metering pumps deliver precise chemical dosing in municipal water and wastewater treatment by using a flexible diaphragm to displace fluid in measured volumes. A motor-driven eccentric mechanism moves the diaphragm back and forth, creating suction and discharge strokes that draw chemical from the supply tank and inject it into the process stream. These pumps typically achieve ±2% accuracy across flow ranges from 0.1 to 500 GPH, making them ideal for chlorine, polymer, and pH adjustment applications. The primary trade-off is diaphragm replacement frequency, typically every 6-18 months depending on chemical compatibility and operating conditions.

• Chemical Feed Systems (Chlorination/Dechlorination): Diaphragm pumps meter sodium hypochlorite (0.5-15 gpm) or sodium bisulfite (0.1-5 gpm) with precise flow control. Connected downstream of chemical storage tanks with suction strainers, feeding directly into process piping or contact chambers
• Polymer Feed (Sludge Dewatering): Delivers diluted polymer solutions (0.2-8 gpm) to belt presses or centrifuges. Fed from polymer preparation systems through static mixers into sludge streams
• pH Adjustment: Meters caustic soda or acid solutions (0.1-3 gpm) for pH control in clarifiers or lime softening. Connects upstream of rapid mix chambers with pH feedback control
• Coagulant Feed: Doses ferric chloride or alum (1-20 gpm) into raw water streams. Fed from bulk storage through day tanks into rapid mix basins

Daily Operations: Operators monitor stroke rate settings (10-180 spm typical), verify prime and check valve operation through sight glasses, and confirm chemical feed rates against process demands. Flow verification through calibration columns or totalizer readings ensures proper dosing. Suction pressure gauges indicate potential supply issues or valve problems.

​

Maintenance: Monthly diaphragm inspection and quarterly check valve cleaning prevent failures. Annual diaphragm replacement recommended for aggressive chemicals, biennial for mild service. Operators need basic mechanical skills for head disassembly and chemical handling PPE (face shields, chemical-resistant gloves, aprons). Drive oil changes every 2,000 hours on hydraulic units.

• Diaphragm Assembly: PTFE or EPDM diaphragms (4"-12" diameter) create pumping action through reciprocating motion. Material selection based on chemical compatibility - PTFE for aggressive chemicals, EPDM for general service
• Drive Mechanism: Mechanical or hydraulic drives convert motor rotation to linear diaphragm motion. Mechanical drives use connecting rods and eccentric cams for smaller pumps (<10 gph), while hydraulic drives handle larger capacities (10-100+ gph)
• Check Valves: Ball, poppet, or diaphragm-type valves ensure unidirectional flow. Hastelloy or PVC construction for corrosive service, with 0.5"-3" port sizes
• Pump Head: Wetted components in polypropylene, PVDF, or 316SS construction. Includes suction/discharge ports, valve seats, and diaphragm mounting surfaces

• Flow Rate: 0.1-100 GPH for small plants; 1-500 GPH for medium facilities; up to 2,000 GPH for large plants (50+ MGD). Size based on maximum chemical demand plus 25% safety factor
• Discharge Pressure: 50-300 PSI typical municipal range. Must overcome system head including static lift, friction losses, and injection pressure
• Turndown Ratio: Minimum 10:1 for effective process control; premium units achieve 100:1. Critical for seasonal demand variations and process optimization
• Accuracy: ±1% of set point for hydraulically-actuated units; ±2% for mechanically-actuated pumps. Repeatability should be ±0.5% for critical applications
• Suction Lift: Maximum 10 feet for most chemicals; 6 feet for high-viscosity solutions. Flooded suction preferred for reliability
• Chemical Compatibility: Wetted materials must resist specific chemicals - PTFE/PVDF for aggressive chemicals, 316SS for chlorine solutions, Hastelloy-C for chlorine dioxide
• Stroke Rate: 30-180 SPM typical range. Lower rates provide better accuracy and longer diaphragm life

• What flow range and accuracy requirements drive pump selection? Municipal plants need 10:1 minimum turndown for seasonal variations. Hydraulic actuation required for ±1% accuracy on critical feeds like fluoride. Wrong choice results in poor process control, regulatory violations, or chemical waste
• Does system pressure require duplex configuration or pressure relief? Single pumps limited to ~300 PSI; higher pressures need duplex pumps or pressure-reducing systems. Undersizing causes cavitation and premature failure
• What chemical compatibility and wetted materials are required? Sodium hypochlorite requires 316SS or Hastelloy-C wetted parts; polymer feeds need abrasion-resistant ceramics; acids demand PTFE/PVDF construction. Material incompatibility causes rapid seal failure
• Is redundancy required for critical chemical feeds? Primary disinfection and pH control typically need duplex pumps with automatic switchover. Single-pump failure on critical processes causes permit violations and potential public health issues

• Division 40-05-23 - Chemical Feed Pumps (Primary)
• Also applicable: 40-05-13 Chemical Storage and Feed Equipment for complete chemical feed systems including storage tanks and controls

• Material/Equipment Verification: Verify wetted materials compatibility with specific chemicals (316SS vs Hastelloy C-276), Confirm diaphragm material rating for temperature/pressure conditions
• Installation Requirements: Pulsation dampeners typically required for runs over 50 feet, Calibration columns mandatory for most municipal applications, 120V single-phase power standard
• Field Challenges: Access for diaphragm replacement often overlooked in design, Priming issues common with suction lifts over 10 feet
• Coordination Issues: Chemical feed system integration with SCADA requires early coordination, Lead times typically 8-12 weeks for standard municipal configurations

• Milton Roy - Milroyal Series for municipal chemical feed, proven in 1-50 MGD plants
• Grundfos - DME and DDA series dominate smaller municipal facilities under 10 MGD
• LMI (Liquid Metronics) - P series pumps widely specified for chlorination systems
• IWAKI - Magnet drive pumps gaining traction for corrosive chemical applications in larger treatment facilities

• Peristaltic pumps - Better for abrasive slurries like polymer, 20-30% higher capital cost but lower maintenance
• Progressive cavity pumps - Preferred for high-viscosity chemicals, roughly equivalent costs
• Centrifugal with VFD - Cost-effective for high-flow applications over 50 gpm, 40-60% lower equipment costs but requires flow measurement feedback for accurate dosing control

Establish relationships with local manufacturer reps early - they provide invaluable troubleshooting support and spare parts inventory. Specify redundant pumps for critical applications (chlorination, coagulation) rather than oversizing single units. Consider pump skid packages from established integrators like Evoqua or Hach for turnkey installations - often more cost-effective than field assembly for systems under 5 MGD capacity.