How Does a Metering Pump Work? A Practical Guide for Adelaide & South Australia

A metering pump, also known as a dosing pump, is designed to deliver a controlled and repeatable quantity of liquid into a process.

Across Adelaide and South Australia, metering pumps are commonly used in water and wastewater treatment, irrigation and fertigation, commercial swimming pools, food and beverage processing, industrial water treatment, chemical processing, and other applications where accurate chemical dosing is required.

Unlike a general-purpose transfer pump, a metering pump is selected primarily for controlled dosing. Its job isn’t simply to move liquid from one place to another. It needs to deliver a known amount at a required flow rate while overcoming the pressure of the receiving system.

For South Australian water-treatment applications, this distinction is particularly important. SA Water maintains technical standards covering liquid chemical dosing and materials in contact with drinking water, while SA Health provides guidance covering chemical dosing, monitoring, alarms and maintenance for drinking-water systems.

Understanding how a metering pump works is therefore only the starting point. Selecting the right pump also requires an understanding of the chemical, required dose, pressure, materials, controls and operating environment. Mayfair Plumbing provides Adelaide metering and dosing pump installation and repair for commercial and industrial systems across greater Adelaide.

What Is a Metering Pump?

Many conventional metering pumps are positive-displacement pumps.

Instead of relying primarily on velocity to create flow, as a centrifugal pump does, a positive-displacement metering pump repeatedly captures and displaces a defined volume of liquid.

In a typical reciprocating metering pump, the cycle works like this:

1. The pumping element moves backwards, increasing the volume of the pump chamber.
2. Pressure inside the chamber falls and liquid enters through the suction-side check valve.
3. The pumping element moves forwards, decreasing the chamber volume.
4. The suction valve closes and the discharge valve opens.
5. Liquid is displaced into the discharge line.

The cycle then repeats.

This operating principle allows controlled-volume pumps to provide repeatable chemical delivery when they are correctly selected, installed and operated within their specified flow, pressure and fluid-property limits.

Controlled-volume positive-displacement pumps are well-established industrial equipment. API Standard 675, for example, establishes requirements for certain reciprocating controlled-volume pumps used in petroleum, chemical and gas-industry services.

How Is a Metering Pump’s Flow Rate Controlled?

Depending on the pump design, chemical output can be adjusted by changing:

• the amount of liquid displaced during each stroke;
• the number of strokes completed over a given period; or
• a combination of both.

Stroke Length

Changing stroke length alters the effective displacement of the pumping mechanism.

A longer effective stroke generally produces greater displacement per cycle, while reducing the stroke decreases the amount delivered.

However, the usable adjustment range and accuracy at reduced stroke settings depend on the particular pump.

For this reason, you shouldn’t assume that every metering pump maintains the same accuracy throughout its entire adjustment range. The manufacturer’s performance data should always be checked for the specific model.

Stroke Frequency or Motor Speed

Another method is changing how frequently the pump cycles.

Solenoid metering pumps commonly regulate output by changing stroke frequency. Motor-driven pumps may use motor-speed control, mechanical stroke adjustment or another control arrangement depending on their design.

Modern electronic dosing pumps may also respond automatically to external control signals from flow meters, sensors, PLCs or other process equipment.

Why Are Check Valves Important?

Many diaphragm and plunger metering pumps use inlet and outlet check valves to maintain one-way flow through the pump head.

During the suction stroke, the inlet valve allows chemical into the chamber while the discharge side remains closed.

During the discharge stroke, the inlet valve closes and the outlet valve opens, allowing the chemical to enter the discharge line.

Although mechanically simple, these valves can have a significant effect on dosing performance.

Contamination, crystallisation, suspended solids, unsuitable viscosity, worn valve components or gas accumulation can interfere with valve seating and reduce dosing consistency.

This is one reason the characteristics of the chemical need to be understood before selecting a dosing pump.

For chemicals that tend to release gas, the pump and installation may require features specifically designed to manage that behaviour. The pump manufacturer’s recommendations for the particular chemical and concentration should be followed.

Types of Metering Pumps

There isn’t one metering-pump technology that is best for every Adelaide application.

The appropriate design depends on factors including:

• chemical being pumped;
• chemical concentration;
• required dosing rate;
• discharge pressure;
• fluid viscosity;
• presence of solids;
• required control method;
• operating environment;
• maintenance requirements; and
• consequences of equipment failure.

Here are the main technologies you’re likely to encounter.

Mechanically Actuated Diaphragm Pumps

A diaphragm pump uses a flexible diaphragm to change the volume of the liquid chamber.

The diaphragm separates the process liquid from much of the drive mechanism, making diaphragm designs useful for many corrosive and hazardous chemical applications. Mayfair Plumbing also services commercial diaphragm pump systems for chemical dosing where containment and chemical compatibility matter.

However, it is misleading to describe every diaphragm pump as completely leak-proof.

Diaphragms, check valves, connections, pipework and other wetted components still require appropriate materials, installation, inspection and maintenance.

Solenoid Diaphragm Metering Pumps

A solenoid metering pump uses an electromagnetic actuator to generate the pumping stroke.

These pumps are widely used where relatively small, controlled chemical doses are required.

Depending on the model, control options can include:

• manual adjustment;
• stroke-frequency adjustment;
• pulse input;
• 4–20 mA analogue control; and
• integration with external instrumentation.

Pressure and flow capabilities vary considerably between manufacturers and models.

A solenoid pump therefore shouldn’t be selected simply because an application is described as “low pressure.” The required duty should be compared with the actual performance data for the proposed pump.

Motor-driven Diaphragm Pumps

Motor-driven diaphragm pumps use an electric motor and mechanical drive to actuate the diaphragm.

They are available for a broad range of industrial duties and can be suitable for continuous operation when the pump is correctly rated for the required conditions.

For more demanding installations, hydraulic diaphragm designs may also be considered.

Hydraulic Diaphragm Metering Pumps

A hydraulic diaphragm pump uses hydraulic fluid between the mechanical drive piston and the process diaphragm.

This arrangement allows the drive mechanism to move the diaphragm without the process chemical directly contacting the drive components.

Hydraulic diaphragm pumps are commonly considered for demanding industrial applications involving higher pressures, hazardous chemicals or duties where containment and reliability are particularly important.

API Standard 675 covers certain hydraulic diaphragm and packed-plunger controlled-volume pumps used in petroleum, chemical and gas-industry services.

Plunger and Piston Metering Pumps

Plunger pumps use a reciprocating plunger to displace liquid from a closely controlled pumping chamber.

They can be suitable for high-pressure and controlled-volume applications, although actual pressure capability and metering performance are specific to the individual pump.

Because the plunger and sealing arrangement interact differently with the process liquid than a diaphragm design, chemical compatibility, seal maintenance and leakage management require careful consideration.

Where hazardous or environmentally sensitive chemicals are involved, the choice between plunger and diaphragm technology should be based on the complete system and consequences of failure rather than pressure capability alone.

Peristaltic Metering Pumps

A peristaltic pump works differently from a conventional check-valve metering pump.

Rotating rollers or shoes compress flexible tubing or hose. As the compression point travels along the tube, liquid is displaced towards the outlet.

The process liquid remains contained within the tube or hose.

This can be useful where conventional check valves would be problematic, including some applications involving suspended material, or where keeping the process liquid isolated inside a replaceable tube is desirable.

However, a peristaltic pump isn’t automatically the correct choice for every slurry or viscous liquid.

Tube material, chemical compatibility, pressure, suction conditions, solids characteristics and expected tube life all need to be checked against the manufacturer’s specifications.

How Does a Dosatron Dosing Pump Work?

A Dosatron is an example of a different dosing approach because the conventional system doesn’t require an electric motor to produce its dosing action.

Dosatron describes its technology as a water-powered volumetric proportional dosing system.

Installed in a flowing water line, the incoming water operates a hydraulic piston. This piston drives a separate dosing piston, which draws concentrate from a container and introduces it into the water stream.

Because the hydraulic mechanism operates in response to water passing through the unit, the quantity of concentrate injected is proportional to the volume of water passing through the injector.

The dosing ratio can then be mechanically adjusted within the operating range of the particular model.

This type of water-powered proportional injector can be useful in applications such as:

• irrigation;
• horticulture;
• animal production;
• cleaning systems; and
• selected water-treatment processes.

Actual dosing accuracy, allowable water-flow range, operating pressure, chemical compatibility and adjustment range are model-specific.

Always check the manufacturer’s specifications rather than assuming a proportional injector will maintain any selected ratio under unlimited operating conditions.

How to Choose a Metering Pump in Adelaide

Pump selection should begin with the actual process requirements rather than a preferred pump brand or technology.

For Adelaide and South Australian installations, these five questions provide a useful starting point.

1. What Chemical Are You Pumping?

Identify the exact chemical and its concentration.

Material compatibility needs to be checked for every component that contacts the chemical, potentially including:

• pump head;
• diaphragm or tube;
• valve balls and seats;
• O-rings;
• seals;
• suction tubing;
• discharge tubing;
• injection valves;
• fittings; and
• storage and containment components.

A statement that a pump is suitable for “acid,” “chlorine” or “caustic” generally isn’t enough.

Chemical concentration, temperature and operating conditions can significantly affect material compatibility.

Always check the current chemical Safety Data Sheet (SDS) and the pump manufacturer’s chemical-compatibility information.

2. What Flow Rate or Dose Is Required?

The required dosing rate should be calculated from the process requirement rather than estimated from the pump’s maximum output.

For a simple continuous dosing application, the required chemical flow is determined from the required chemical dose and process flow.

The exact calculation will depend on the units being used, chemical concentration, specific gravity and whether the specified dose refers to active ingredient or bulk chemical.

Once the minimum and maximum dosing requirements are known, a pump can be selected with an appropriate operating range.

Avoid relying on universal rules such as saying a metering pump should always operate at a particular percentage of maximum capacity.

Correct sizing depends on the pump technology, manufacturer’s specified operating range, required accuracy, expected future demand and actual process conditions.

3. What Pressure Must the Metering Pump Overcome?

A metering pump needs to deliver the required flow against the actual discharge pressure of the system.

This can include:

• pressure in the process pipe;
• static elevation;
• friction through pipework and fittings;
• injection-valve losses; and
• other restrictions between the pump and injection point.

The pump’s pressure rating must therefore be suitable for the maximum credible operating condition rather than only the pressure normally observed during operation.

Any required design margin should be based on the application and manufacturer recommendations rather than an arbitrary percentage or PSI allowance.

4. How Will the Pump Be Controlled?

The required control method can significantly affect pump selection.

A straightforward application may only require manual adjustment.

More sophisticated dosing systems may use:

• pulse signals from a water meter;
• 4–20 mA analogue signals;
• variable-speed control;
• PLC integration;
• SCADA integration;
• flow-paced dosing;
• pH instrumentation;
• ORP instrumentation;
• tank-level interlocks;
• low-flow shutdowns; and
• high or low dosing alarms.

For drinking-water chlorination, SA Health guidance highlights the importance of reliable dosing and monitoring equipment, including alarms for underdosing and overdosing and regular maintenance of dosing and monitoring equipment.

5. What Happens If Something Goes Wrong?

This question is particularly important when designing a chemical-dosing system.

Consider what happens if:

• the pump loses prime;
• the suction line leaks;
• a check valve becomes contaminated;
• the chemical tank runs empty;
• process flow stops while dosing continues;
• the discharge line becomes blocked;
• a diaphragm or tube fails;
• instrumentation sends an incorrect signal; or
• electrical power is lost and subsequently restored.

The appropriate safeguards depend on the chemical and application, but these failure scenarios should be considered during system design rather than after commissioning. Where a new unit is required, plan for correct metering pump installation and commissioning alongside controls and interlocks.

Metering Pumps for Adelaide Water-treatment Applications

South Australia’s water sector demonstrates why chemical dosing should be treated as a complete process rather than simply a pump-selection exercise.

SA Water publishes engineering standards including TS 0850 – Liquid Chemical Dosing and TS 0800 – Materials in Contact with Drinking Water.

SA Water also monitors South Australian drinking-water supplies against the Australian Drinking Water Guidelines and requirements associated with South Australia’s Safe Drinking Water Act 2011.

For chlorination applications, SA Health guidance discusses reliable dosing and monitoring equipment, maintenance, and alarms for over- or underdosing.

The important principle is straightforward:

A metering pump is only one component of a reliable chemical-dosing system.

Depending on the application, the complete system may also need to consider:

• chemical storage;
• secondary containment or bunding;
• ventilation;
• chemical compatibility;
• pipework;
• injection equipment;
• instrumentation;
• calibration;
• alarms;
• interlocks;
• maintenance access; and
• operating procedures.

Selecting a high-quality pump doesn’t compensate for deficiencies elsewhere in the dosing system.

Adelaide Operating Conditions to Consider

Local operating conditions can also affect equipment selection.

Outdoor equipment around Adelaide may experience substantial seasonal temperature variation and strong solar exposure.

Where a dosing pump, chemical tank or associated tubing is installed outdoors, check:

• permitted ambient temperature;
• permitted fluid temperature;
• UV resistance;
• enclosure requirements;
• ventilation requirements; and
• chemical storage recommendations.

For irrigation, horticultural and rural applications around Greater Adelaide and regional South Australia, source-water characteristics can also influence the dosing system.

Suspended material, mineral content and other water-quality characteristics can affect filters, valves, injectors and chemical reactions.

Where SA Water mains water is involved, local water-quality information can help identify characteristics such as hardness, pH and disinfectant levels.

The actual site conditions should therefore form part of the pump-selection process rather than assuming that one standard configuration will suit every South Australian installation.

Common Metering Pump Problems

Loss of Prime

Possible causes include:

• air or gas in the suction line;
• leaking fittings;
• an empty chemical tank;
• excessive suction lift; or
• check-valve problems.

The correct solution depends on the chemical and pump design.

For chemicals prone to releasing gas, a pump or pump head specifically designed for that service may be required.

Inconsistent Chemical Output

If a dosing pump isn’t delivering consistently, check:

• suction conditions;
• valve cleanliness;
• chemical viscosity;
• gas accumulation;
• calibration;
• discharge pressure;
• pump settings; and
• worn wetted components.

Don’t assume that the percentage shown on a pump controller equals the actual delivered chemical flow without verifying performance under operating conditions.

Check-valve Contamination

Solids, crystals and other deposits can prevent a valve element from seating correctly.

If check-valve contamination repeatedly occurs, cleaning the valve may only address the symptom.

The chemical condition, filtration, pipework design, pump technology and maintenance procedures should also be reviewed.

Premature Diaphragm, Seal or Tube Failure

If a wetted component fails earlier than expected, investigate:

• chemical compatibility;
• chemical concentration;
• fluid temperature;
• ambient temperature;
• operating pressure;
• installation conditions; and
• maintenance history.

Simply replacing a failed component with the same material without identifying the cause may result in the same failure occurring again. Scheduled preventative servicing for dosing and metering pumps helps catch wear before it becomes a process failure. See also when to book water pump servicing for broader pump-care intervals.

Why Metering Pump Calibration Matters

A metering pump should be commissioned and its output verified under conditions that reasonably represent the actual installation.

The manufacturer’s maximum flow rating isn’t a substitute for checking how much chemical is actually being delivered on site.

Depending on the system, calibration may involve measuring the change in volume in a calibration column over a known period while the pump operates against representative discharge conditions.

Calibration can be particularly important:

• during commissioning;
• after maintenance;
• after changing chemicals or concentration;
• after significant changes to operating conditions; and
• where dosing performance is critical to the process.

For critical applications, the appropriate calibration and verification interval should be determined from process risk, regulatory requirements, manufacturer recommendations and operating history.

Which Type of Metering Pump Is Best?

There is no universal best metering pump.

A diaphragm pump may be appropriate where chemical isolation and controlled dosing are priorities.

A plunger pump may suit certain higher-pressure applications.

A peristaltic design may be advantageous where containing the process liquid inside a replaceable tube is useful.

A water-powered proportional injector may suit an application where chemical addition needs to follow water consumption without using an electrically driven dosing pump.

The correct selection depends on the complete duty:

Chemical + Concentration + Flow + Pressure + Control + Environment + Maintenance + Consequences of Failure

A Practical Adelaide Metering Pump Selection Checklist

Consider an Adelaide business that needs to inject a treatment chemical into a pressurised water line.

Before selecting a pump, the following information should be documented:

1. Chemical name and concentration.
2. Current Safety Data Sheet.
3. Minimum, normal and maximum process-water flow.
4. Required treatment dose.
5. Calculated minimum and maximum chemical feed rate.
6. Normal and maximum system pressure.
7. Chemical temperature.
8. Ambient operating temperature.
9. Wetted-material compatibility.
10. Required control signal.
11. Alarm and shutdown requirements.
12. Chemical storage and containment requirements.
13. Ventilation requirements where applicable.
14. Calibration method.
15. Inspection and maintenance requirements.

Only after these conditions are understood does comparing individual pump models become meaningful.

This approach is considerably more reliable than starting with a catalogue flow rate and trying to make the process fit the pump.

Frequently Asked Questions

Why Is a Metering Pump Better for Chemical Dosing Than a Standard Centrifugal Pump?

A positive-displacement metering pump is specifically designed to provide controlled displacement and adjustable delivery.

A centrifugal pump’s operating point depends strongly on the interaction between the pump curve and system curve. It is therefore a fundamentally different machine for controlled chemical-feed applications.

The correct solution still depends on the process, and critical dosing systems should be designed around the actual operating requirements.

Does a Diaphragm Pump Guarantee There Will Never Be a Chemical Leak?

No.

A diaphragm separates the process liquid from much of the drive mechanism and can provide excellent chemical containment.

However, the complete dosing system still includes connections, valves, pipework and other components that can potentially fail.

Some metering-pump designs also incorporate diaphragm-failure detection or secondary containment. These features should be considered where the consequences of chemical leakage are significant.

Are Peristaltic Pumps Always Best for Slurries?

No.

Their valveless flow path can make peristaltic pumps useful for some liquids containing suspended solids, but suitability depends on particle characteristics, tube or hose material, pressure, viscosity, required dosing performance and expected service life.

Does a Dosatron Require Electricity?

Conventional Dosatron water-powered proportional dosing systems don’t require electricity to drive the dosing mechanism.

Water flow and pressure operate the hydraulic mechanism, which drives the dosing piston.

However, each model has specified operating ranges for flow, pressure, dosing ratio and chemical compatibility that must be observed.

How Accurate Is a Metering Pump?

There is no single accuracy figure that applies to every metering pump.

Accuracy and repeatability depend on factors including:

• pump design;
• operating point;
• calibration;
• fluid characteristics;
• suction conditions;
• discharge pressure;
• maintenance; and
• the manufacturer’s specified test conditions.

API Standard 675 establishes requirements for certain industrial controlled-volume pumps, while individual manufacturers publish performance specifications for their particular equipment.

Accuracy claims should therefore be compared on the same basis and against conditions representative of the actual installation.

Conclusion

A metering pump works by delivering liquid in a controlled and repeatable manner, commonly through a positive-displacement mechanism such as a diaphragm, plunger, flexible tube or dosing piston.

Understanding how the pump moves liquid is important, but reliable chemical dosing involves considerably more than the pump itself.

For an Adelaide or South Australian installation, pump selection should begin with the actual chemical, concentration, required dose, process flow, discharge pressure, material compatibility and control requirements.

The overall system should then consider calibration, instrumentation, alarms, containment, maintenance and what happens if equipment or instrumentation fails.

For water-treatment applications in particular, relevant South Australian requirements and guidance should be incorporated into the design rather than relying solely on generic pump-selection advice.

Ultimately, the best metering pump isn’t simply the model with the highest pressure rating, widest adjustment range or most sophisticated controller.

It’s the pump - and complete dosing system - that has been correctly matched to the chemical, process and operating conditions.

If you need help selecting, installing or repairing a dosing system, request a metering pump quote from Mayfair Plumbing across greater Adelaide and the Adelaide Hills.