Metering Pumps

Pumps ·

Metering pumps — also called dosing pumps — are motor-driven positive-displacement machines designed for the precise, repeatable injection of chemicals at defined volumetric flow rates. The pumping element is a flexible membrane actuated by a motor-driven crank, eccentric or hydraulic mechanism, combined with spring-loaded check valves on suction and discharge. Each stroke delivers a fixed volume that is independent of system back-pressure, making metering pumps the standard technology for chemical dosing in water treatment, oil and gas and industrial process applications wherever accuracy, high pressure and chemical compatibility are simultaneously required.

History

The modern motor-driven precision dosing pump was developed in the United States in the early 1930s to meet a new regulatory requirement: accurate, continuous chlorination of municipal drinking water. Milton Roy Company, founded in Philadelphia, Pennsylvania in 1936 by Roy Laucks, commercialised the first industrial metering pump designed specifically for this purpose. Laucks designed a variable-stroke piston-and-diaphragm mechanism that could be adjusted during operation without stopping the pump — a capability that became fundamental to automated water treatment plant operations across North America. The Milton Roy design set the technical reference standard for dosing accuracy, pressure capability and adjustability that the entire metering pump industry subsequently followed and continues to build upon.

Operating Principle

A motor drives a crank or eccentric mechanism that reciprocates a push rod. The push rod deflects a flat membrane — typically PTFE-reinforced — into and out of the fluid chamber. On the suction stroke the chamber volume expands and fluid is drawn in through the spring-loaded suction check valve. On the discharge stroke the membrane advances, pressurising the fluid and expelling it through the outlet check valve. In hydraulically actuated designs, an oil cushion between the push rod and membrane distributes force evenly and protects the membrane from stress concentrations, enabling discharge pressures above 100 bar.

Actuator Types

  • Mechanically actuated (MAM): Push rod connects directly to membrane — simplest design, no hydraulic oil, limited to lower pressures (up to 10 bar); suitable for water treatment chemical dosing
  • Hydraulically actuated (HAM): Hydraulic fluid between push rod and membrane — distributes force evenly across the membrane surface; achieves high pressure (up to 250 bar); preferred for high-pressure injection
  • Solenoid-actuated: Electromagnetic plunger drives the membrane at high frequency — very compact, no rotating parts, used in laboratory and small-scale dosing (0.001 to 30 litres per hour)
  • Pneumatically actuated: Controlled air pressure deflects the membrane — for ATEX zones where electrical actuation is restricted

Key Technical Parameters

  • Flow rate (Q): 0.001 to 10 m³/h — purpose-built for low to moderate precise dosing
  • Dosing accuracy: ±0.5 to ±2% of setpoint under stable conditions
  • Maximum discharge pressure: Up to 250 bar for hydraulically actuated designs
  • Turndown ratio: Typically 1:100 by stroke length adjustment or 1:1000 with VFD speed control
  • Stroke frequency: 6 to 180 strokes per minute — higher frequency gives smoother, less pulsating flow
  • Membrane material: PTFE-coated fabric, pure PTFE, EPDM or Viton depending on fluid chemistry

Applications

  • Water and wastewater treatment — dosing of chlorine, sodium hypochlorite, sodium hydroxide, flocculants and coagulants
  • Oil and gas — chemical injection at wellheads: corrosion inhibitors, scale inhibitors, methanol and biocides
  • Reverse osmosis — antiscalant and biocide dosing into high-pressure feed water
  • Swimming pools and spas — automated pH correction and disinfection
  • Pharmaceutical manufacturing — sterile fluid metering in batch production
  • Power generation — boiler water treatment chemical dosing
  • Food and beverage — flavour, preservative and acidulant addition at precise rates

Advantages

  • High dosing accuracy — repeatable delivery of precise volumes per stroke
  • No rotating parts in the fluid path — zero contamination risk
  • Membrane isolates fluid completely from actuator and oil — suitable for hazardous and corrosive chemicals
  • Achieves high discharge pressure in a compact, lightweight unit
  • Adjustable stroke length and speed allow flow rate adjustment over a wide range without changing hardware
  • PTFE membrane offers near-universal chemical resistance

Limitations

  • Maximum flow rate is low — not suitable for bulk transfer duties
  • Pulsating flow — pulsation dampener required for accurate inline flow measurement
  • Check valves must be kept clean and correctly oriented — fouling or incorrect installation causes dosing errors
  • Membrane has a finite fatigue life — regular inspection and scheduled replacement are essential
  • Hydraulic oil must be maintained in correct condition for hydraulically actuated designs
  • Not suitable for fluids containing abrasive particles above ~50 micron without upstream filtration

Selection Criteria

  • Define the required dose rate (litres per hour or milligrams per litre) and select pump capacity to operate at 50 to 80% of maximum stroke for best accuracy
  • Choose actuator type: mechanically actuated for low-pressure water treatment; hydraulically actuated for high-pressure injection; solenoid for very low flows in laboratory or analytical duty
  • Select membrane material for the chemical: PTFE for acids, oxidisers and solvents; EPDM for dilute alkalis and water-based reagents; Viton for hydrocarbons
  • Match check valve material to fluid: ceramic or stainless steel balls for corrosive chemicals, PTFE valve seats throughout
  • Install a pulsation dampener on the discharge line whenever inline flow metering or pressure-sensitive instruments are present downstream
  • Use a calibration column (cylinder with scale markings) to verify actual dose rate under operating back-pressure conditions after commissioning

Top 10 Manufacturers

  • 1. Milton Roy (Accudyne Industries) (USA) — ~17% global market share; the founding manufacturer and industry technical reference
  • 2. ProMinent GmbH (Germany) — ~15%; electromagnetic and motor-driven metering for water treatment
  • 3. LEWA GmbH (Germany) — ~11%; high-pressure hydraulically actuated metering for oil and gas
  • 4. Grundfos A/S (Denmark) — ~10%; electronic metering pumps for water treatment applications
  • 5. IDEX / Pulsafeeder (USA) — ~8%; peristaltic and mechanically actuated metering for chemical feed
  • 6. Seko SpA (Italy) — ~7%; compact electromagnetic and motor metering for industrial water treatment
  • 7. Sera GmbH (Germany) — ~6%; diaphragm metering pumps for process chemistry applications
  • 8. LMI / Roper Technologies (USA) — ~6%; electronic metering pumps for water and wastewater
  • 9. Tacmina Corporation (Japan) — ~5%; smooth-flow metering pumps for semiconductor and pharma
  • 10. Blue-White Industries (USA) — ~4%; peristaltic and diaphragm metering for water treatment