Dosing Pumps: Metering Pump Types, Accuracy and Chemical Injection Sizing

Quick Answer

A dosing pump, or metering pump, is a small positive displacement pump that delivers a set volume of chemical per stroke, so its output can be set, repeated and held against changing line pressure. The main types are solenoid diaphragm pumps for small water-treatment duties, motor-driven mechanical diaphragm pumps, hydraulically actuated diaphragm pumps, which stay sealless at high pressure, packed plunger pumps for the highest pressures, and valveless peristaltic pumps for gassing liquids and slurries. For petroleum, chemical and gas services, API 675 asks for steady-state accuracy within ±1% over a turndown of at least 10:1, and repeatability and linearity within ±3% of rated flow. Flow is adjusted by stroke length, stroke frequency or both. Size from the dose rate: chemical flow (l/h) = water flow (m³/h) × dose (mg/l) ÷ product strength (g/l); a common guideline puts the maximum duty at about 85–90% of rated capacity and keeps the minimum above 10%. Then complete the system: compatible wetted materials (PVDF or PVC, not 316 stainless, for hypochlorite), a back-pressure valve, a relief valve, a calibration column and an injection quill.

A common problem on water treatment plants: a hypochlorite pump that held the chlorine residual at commissioning starts losing prime within weeks, and at night the residual drifts although the pump sits at its lowest setting. The pump is rarely faulty. Usually it was chosen on maximum flow alone, in a head material picked on price, with no back-pressure valve and no allowance for the gas the chemical gives off.

A dosing pump is a precise instrument inside a small hydraulic system. This guide covers the types, what "accurate" means, and how to size the pump and the system around it.

What a Dosing Pump Does

A dosing pump draws a measured volume into a chamber through a suction check valve and pushes it out through a discharge check valve, so:

Flow = volume per stroke × strokes per minute

Line pressure affects it far less than it affects a centrifugal pump, though diaphragm pumps deliver somewhat less as discharge pressure rises. That is also why a closed valve downstream is dangerous: pressure rises until something gives. See our guide to positive displacement pumps.

API 675 calls these machines controlled-volume pumps. Its scope covers packed plunger and hydraulically actuated diaphragm pumps for petroleum, chemical and gas industry services. It excludes diaphragm pumps with direct mechanical actuation, and rotary pumps, which come under API 676. Many water-treatment dosing pumps are therefore outside API 675, though makers often quote performance in its terms.

What "accurate" means: the API 675 terms

Two points follow. The ±3% limits are percentages of rated flow, so at the bottom of the range they are a large share of the dose: ±3% of a 12 l/h rating is ±0.36 l/h, 30% of a 1.2 l/h dose. And the limits hold only under fixed conditions, so installed pumps are checked with a calibration column. API 675 also asks for a rated capacity of at least 110% of the maximum specified, and tests flow at settings down to 10% of rating; check the current edition for details.

Dosing Pump Types

Why a hydraulic diaphragm reaches higher pressure. In a mechanical diaphragm pump the diaphragm alone resists the discharge pressure, and its fatigue life sets the limit. In a hydraulic diaphragm pump, oil supports the whole diaphragm at almost the process pressure, so the diaphragm only separates two liquids at nearly equal pressure.

Diaphragm or plunger? The Pump Industry Australia technical handbook notes that plunger designs have a static seal that is subject to leakage and must not run dry, while diaphragm designs are sealless, which matters with hazardous or expensive liquids. Air-operated double-diaphragm pumps are transfer pumps, not metering pumps; see our guide to AODD diaphragm pumps.

Capacity Control

• Stroke length: set by a micrometer knob or an electric positioner, typically over 10–100% of capacity
• Stroke frequency: set by a variable-frequency drive or by solenoid electronics; one trade guide advises keeping motor-driven pumps above about 15 strokes per minute, while solenoid pumps can run below one
• Both together: 10:1 on stroke length with 5:1 on speed gives 50:1 in principle; ask the maker how accuracy holds at short stroke and low speed

Automatic control usually paces stroke frequency from a flow meter's pulse or 4–20 mA signal, so the dose follows the water flow. Stroke length is set for the dose, or trimmed by a chlorine residual or pH analyser. Avoid the highest stroking speeds with viscous or abrasive chemicals.

Sizing: Dose Rate, Turndown and Pressure

The dose-rate formula

One mg/l is one g/m³, so water flow in m³/h times dose in mg/l gives grams of active chemical per hour. Divide by the product strength:

Chemical flow (l/h) = water flow (m³/h) × dose (mg/l) ÷ product strength (g/l)

Where the supplier states strength as percent by weight, convert it with the density:

Strength (g/l) = % w/w × 10 × density (kg/l)

Worked example: hypochlorite and caustic for a 250 m³/h plant

A plant runs at 250 m³/h (about 1,100 US gpm) normally, 400 m³/h at peak and 100 m³/h at night. The chlorine dose is 1–3 mg/l, normally 2 mg/l. Sodium hypochlorite arrives at 150 g/l available chlorine and loses strength in storage, so the example assumes 120 g/l at the end of a delivery; the supplier's certificate governs.

1. Turndown needed: 10.0 ÷ 0.67 = 15:1, beyond the 10:1 over which API 675 states its accuracy.
2. Rated capacity: 110% of the maximum gives 11.0 l/h; the 85–90% guideline gives 11.1–11.8 l/h. Choose 12 l/h (3.2 US gal/h) at the maximum injection pressure.
3. Range check: the maximum duty is 83% of rating, but the minimum is 5.6%, below the 10% floor of 1.2 l/h. Pace stroke frequency from the plant flow meter, which covers the 4:1 flow range, with stroke length set for the dose; or split the duty between a large and a small pump.
4. Caustic for pH: 6 mg/l as NaOH at 250 m³/h, using 50% caustic soda. One producer's data sheet gives a density of 1.525 kg/l at 20 °C, so the strength is 50 × 10 × 1.525 = 762.5 g/l, and the flow is 1,500 g/h ÷ 762.5 = 1.97 l/h (0.52 US gal/h). The same sheet gives a viscosity of about 79 mPa·s at 20 °C and solidification from about 10–12 °C, so protect outdoor tanks and lines from cold.
5. Discharge pressure at peak flow: a crank-driven simplex pump delivers on half of each cycle with a sinusoidal plunger velocity, so its peak flow is π, about 3.14, times the average. In 20 m of 6 mm bore tubing the caustic loses about 0.27 bar at the average 1.97 l/h, but 0.85 bar at the 6.2 l/h peak (laminar flow). Rate the pump for the highest injection pressure plus line, quill and valve losses at peak flow, and set the relief valve clear of the peaks, or fit a pulsation dampener.

Materials and Chemical Compatibility

Wetted parts are the head, valve balls and seats, diaphragm or plunger, and seals. Ratings depend on concentration and temperature; the maker's chart and the chemical supplier's data govern. Typical makers' ratings at about 21 °C (70 °F):

Gassing chemicals. Hypochlorite and hydrogen peroxide release gas in the pump head. Gas compresses instead of being expelled, so the pump air-binds and loses prime; sunlight and heat make hypochlorite worse. Use a degassing head, flooded suction without high points, cool shaded storage and the lowest practical concentration, or a peristaltic pump.

Accessories That Make Dosing Work

Most dosing problems come from the system, not the pump.

Relief settings. In API 675, a hydraulic diaphragm pump carries an integral hydraulic relief valve set at least 10% or 175 kPa (25 psi) above the rated discharge pressure, whichever is greater, and a packed plunger pump needs an external relief valve in the purchaser's piping. The pump and driver must also run at maximum speed and relief setting together, and the motor must suit the relief valve's accumulation pressure, so size them above the set pressure. See pressure relief and safety valves.

Suction side. The pump refills on every stroke, so the liquid in the suction line must accelerate each time, and long, narrow or lifting lines starve it. One trade guide suggests no more than about 1.2 m (4 ft) of suction lift, short flooded lines, and pipe one size larger than the pump connection. Hydraulic diaphragm pumps need flooded suction.

Applications

• Water and wastewater: hypochlorite, ferric chloride or aluminium sulfate, caustic or acid, and polymer; see wastewater treatment equipment
• Desalination: antiscalant, acid and dechlorination chemicals ahead of reverse osmosis; see desalination plant equipment
• Boiler and cooling water: oxygen scavengers, phosphates, amines, inhibitors and biocides; see boiler and cooling water treatment
• Oil and gas: corrosion and scale inhibitors, demulsifiers, biocides, and methanol or glycol against hydrates, usually by hydraulic diaphragm or plunger pumps at wellhead pressures and often to API 675; see oil and gas pipes, valves and fittings
• Food and agriculture: flavourings, preservatives and fertiliser injection

Common Mistakes

1. Sizing on maximum flow only. The pump that covers the peak may spend every night below 10% of its range, where the dose drifts.

   Prevention: Calculate minimum, normal and maximum flows; stay within 10:1, or add frequency pacing or a second pump.

2. Choosing wetted materials on price. 316 stainless steel corrodes in hypochlorite, ferric chloride and hydrochloric acid; PVDF is not recommended for warm 50% caustic.

   Prevention: Check every wetted part, including balls, seats and seals, at the real concentration and temperature.

3. No back-pressure valve. Where the injection point is below the tank or at low pressure, chemical siphons through both check valves and the pump overfeeds.

   Prevention: Fit a back-pressure or anti-siphon valve set to the differential the maker requires.

4. Relying on the pump's internal relief valve. A closed valve or blocked quill overpressures the tubing and fittings; an internal valve protects only the pump.

   Prevention: Fit an external relief valve in the discharge line, with no isolating valve between it and the pump.

5. Ignoring off-gassing. Hypochlorite and hydrogen peroxide air-bind ordinary diaphragm heads.

   Prevention: Use a degassing head, flooded suction and cool storage, or a peristaltic pump.

6. Designing the discharge side on average flow. Peak flow from a crank-driven simplex pump is about three times the average, and from a solenoid pump at a low stroke rate it is much higher, so line losses and relief margins come out too low.

   Prevention: Design on peak flow, π × average for a crank-driven simplex pump, or fit a pulsation dampener.

7. Never calibrating. The dial reflects test-stand conditions; real suction and discharge pressures move the flow.

   Prevention: Check the flow with a calibration column at commissioning, after maintenance and after any change of chemical.

What to Send for a Quotation

• Chemical: name, concentration (% w/w or g/l), density, viscosity at the lowest temperature, solids, tendency to gas, safety data sheet
• Flows: minimum, normal and maximum chemical flow in l/h, or the water flow and dose ranges
• Pressures and temperatures: minimum and maximum at the injection point; tank position and suction arrangement; chemical and ambient temperatures
• Control: manual, positioner, flow-paced pulse or 4–20 mA signal, analyser loop
• Construction and site: pump type; head, valve, diaphragm and seal materials; API 675 if specified; power supply and hazardous-area classification
• Accessories and configuration: back-pressure and relief valves, dampener, calibration column, quill; duty and standby pumps
• Documents: data sheets, test records and material certificates; see our guide to material test certificates

Supply from Kasko Makine

Kasko Makine supplies dosing and plunger pumps:

• Dosing pumps: positive displacement pumps that inject precise, controlled quantities of chemicals or additives into water, gas or process streams. The product page lists diaphragm, piston and peristaltic types; motor-driven, solenoid or pneumatic drives; flows from 0.1 ml/h to 1,000 l/h depending on model; pressure ratings up to 20 bar as standard, higher on request; and accuracy of ±1% of set flow rate. Listed applications include water and wastewater treatment (chlorine, coagulant and pH adjustment), chemical processing, food and beverage, agriculture and fertilisation, medical and laboratory use, and petrochemical corrosion inhibitors and demulsifiers.
• Plunger pumps: a smooth cylindrical plunger moving through a stationary high-pressure seal, for higher pressures than piston pumps; listed for high-pressure cleaning, oil and gas, water jetting and injection systems, chemical processing and industrial fluid transfer.

Engineering support: Send the chemical and its concentration, the flow or dose range, the injection pressure and the suction arrangement. We will review pump type, wetted materials and the accessories the installation needs against your data, and reply with pricing, lead time and engineering details.

Send the chemical, its concentration, the dosing range and the injection pressure for a dosing pump proposal and quotation — we reply within 24 hours. Reach us at info@kaskomakine.com or WhatsApp +90 (537) 521 1399.

Continue Reading

• Positive Displacement Pumps: Types, vs Centrifugal & Selection Guide
• Diaphragm Pumps (AODD): How They Work, Types & Selection Guide
• Boiler and Cooling Water Treatment: Chemistry, Equipment and Control Limits
• Wastewater Treatment Equipment: Screens, Clarifiers, Aeration and Dewatering
• Desalination Plant Equipment: SWRO vs Thermal, Materials & Selection
• Gear Pumps: External vs Internal Types, Slip, Sizing and Selection

Frequently Asked Questions

Q: What is the difference between a dosing pump and a metering pump?
A: In practice there is none. Both terms describe a small positive displacement pump that delivers a set volume of liquid per stroke or per revolution and can be adjusted precisely. Metering pump is the usual term in process industries, and API 675 calls them controlled-volume pumps; dosing pump is common in water treatment. Reciprocating types are adjusted by stroke length, stroke frequency or both; peristaltic pumps by rotor speed.

Q: What accuracy does API 675 require of a metering pump?
A: API 675 requires steady-state flow accuracy within plus or minus 1 percent over a turndown of at least 10 to 1, and repeatability and linearity within plus or minus 3 percent of rated flow over the specified turndown. These limits apply under fixed conditions, so installed pumps are still calibrated in place. API 675 covers packed plunger and hydraulically actuated diaphragm pumps, not mechanically actuated diaphragm pumps.

Q: How do you calculate the flow rate of a dosing pump?
A: Multiply the water flow in cubic metres per hour by the dose in milligrams per litre to get grams of active chemical per hour, then divide by the product strength in grams per litre. For a product sold by weight percent, strength in grams per litre equals the weight percent times 10 times the density in kilograms per litre. Work out minimum, normal and maximum cases before choosing the pump.

Q: Which type of dosing pump should I choose?
A: Solenoid diaphragm pumps suit small water-treatment duties at modest pressure. Motor-driven mechanical diaphragm pumps handle larger continuous duties. Hydraulically actuated diaphragm pumps stay sealless at high pressure and suit hazardous refinery and gas-plant chemicals. Packed plunger pumps reach the highest pressures, but their packing leaks by design. Peristaltic pumps have no valves, so they handle off-gassing chemicals, slurries and viscous liquids, at lower pressures.

Q: Why does a dosing pump need a back-pressure valve?
A: A metering pump's check valves need a steady pressure difference to seat properly. Where the injection point is at low or negative pressure, or below the chemical tank, liquid can siphon through both check valves and the pump overfeeds. A back-pressure valve in the discharge line holds a minimum pressure on the pump. Follow the pump maker's required pressure differential.

Q: What materials are suitable for sodium hypochlorite dosing?
A: PVC and PVDF pump heads with PTFE diaphragm faces are the usual choice, and makers' resistance charts rate 316 stainless steel as not recommended for hypochlorite. Hypochlorite also gives off gas, which air-binds ordinary diaphragm heads, so use a degassing head, flooded suction and cool, shaded storage, or a peristaltic pump. Confirm compatibility at the actual concentration and temperature with the pump maker.

Q: What is turndown and why does it matter for a dosing pump?
A: Turndown is the ratio of rated capacity to the lowest flow at which the pump meets its accuracy, commonly 10:1 on stroke length. A 12 litre per hour pump therefore meets its figures down to about 1.2 litres per hour, but those limits are percentages of rated flow, so the error is a large share of a small dose. For lower doses, combine stroke frequency with stroke length, add a smaller second pump, or choose a wider control range.