How to choose an industrial pump: centrifugal, submersible and positive displacement

Pumps move water, chemicals, oil, slurry and food products in almost every factory, building and farm. Pick the right one and it runs quietly for years. Pick the wrong one and it wastes energy, wears out early, or simply can't deliver the flow you need.

Pump selection starts with three questions: what liquid, how much flow, and against how much head. This guide explains each, and the main pump types.

The main pump families

Centrifugal pumps

A spinning impeller throws liquid outward, creating flow. Centrifugal pumps are the most common type in industry.

• Best for clean or slightly dirty, low-viscosity liquids such as water and thin chemicals.
• Handle large flows smoothly.
• Flow falls as the head (pressure) they must overcome rises.
• Types include end-suction, split-case (for large flows), multistage (for high pressure, such as boiler feed and high-rise water supply) and inline pumps.

Submersible pumps

The motor and pump are sealed together and run underwater.

• Borewell submersibles lift water from deep wells.
• Dewatering and drainage pumps remove water from pits, basements and construction sites.
• Sewage and effluent submersibles handle solids with special impellers.
• They don't need priming and are quiet, since they work below the surface.

Positive displacement pumps

These trap a fixed volume of liquid and push it through, so flow stays almost constant regardless of pressure.

• Gear pumps for oils and viscous liquids.
• Screw pumps and progressive cavity pumps for thick liquids and sludges.
• Diaphragm pumps (including air-operated double diaphragm pumps) for chemicals, slurries and transferring from drums.
• Plunger and piston pumps for very high pressure.
• Dosing (metering) pumps for precise chemical dosing in water treatment.

Use positive displacement pumps for viscous liquids, precise dosing or high pressure at low flow. Always fit a pressure relief valve, because they can build dangerous pressure if a valve is closed.

Step 1: define the liquid

Tell the supplier:

• what the liquid is and its concentration
• temperature range
• viscosity (thicker liquids may need a positive displacement pump)
• specific gravity (density relative to water)
• solids content and maximum particle size
• whether it is corrosive, abrasive, flammable or food-grade

Step 2: calculate flow

Flow is how much liquid you need to move, usually in cubic metres per hour (m³/h) or litres per minute (LPM). Base it on your process need, and note the minimum and maximum flow, not only the average.

Step 3: calculate total head

Head is the energy the pump must provide, expressed as a height of liquid in metres. Total head is made up of:

• Static head: the vertical height from the liquid surface on the suction side to the discharge point.
• Friction losses: pressure lost in pipes, bends, valves and filters. Longer, narrower pipes and more fittings add more loss.
• Pressure head: any pressure required at the discharge point, such as a pressurised tank or spray nozzles.

Underestimating friction is a common mistake. If you can, send the supplier a simple sketch with pipe sizes, lengths, fittings and heights.

Step 4: check suction conditions (NPSH)

Pumps need enough pressure at their inlet to stop the liquid boiling inside them, which causes cavitation: noise, vibration and rapid damage to the impeller. The supplier compares the pump's NPSH required with the NPSH available from your installation. Hot liquids, high altitude, long suction lines and lifting from below the pump all reduce NPSH available.

Step 5: choose materials and seals

• Cast iron: economical for clean water.
• Bronze: for seawater and some mild corrosives.
• Stainless steel (SS 304, SS 316): for food, pharma and many chemicals.
• Plastic (PP, PVDF): for many acids and alkalis.
• Hard metal or rubber-lined: for abrasive slurries.

Seals matter too: gland packing is simple and cheap but drips; mechanical seals are cleaner and more efficient; seal-less magnetic drive pumps eliminate leaks for hazardous chemicals.

Step 6: energy efficiency

Pumps can consume a large share of a plant's electricity. To keep costs down:

• choose a pump whose best efficiency point (BEP) is close to your normal duty
• use an energy-efficient motor
• consider a variable frequency drive (VFD) where flow varies, instead of throttling with a valve
• avoid oversizing, which forces the pump to run far from its BEP

What to send suppliers

1. Liquid, temperature, viscosity, specific gravity and solids.
2. Required flow (normal, minimum, maximum).
3. Total head, or a sketch of the system.
4. Suction conditions (flooded or suction lift).
5. Material preference and seal type.
6. Power supply: voltage, phase, frequency.
7. Operating hours per day.
8. Any standards or certifications required.

Frequently asked questions

What is total head?

Total head is the energy the pump must provide, expressed in metres of liquid. It includes the vertical lift, friction losses in pipes and fittings, and any pressure required at the outlet.

What causes cavitation?

Cavitation happens when pressure at the pump inlet is too low and the liquid boils inside the pump, forming bubbles that collapse violently. It is caused by insufficient NPSH available, often from long suction lines, high suction lift or hot liquids.

Which pump is best for viscous liquids?

Positive displacement pumps such as gear, screw and progressive cavity pumps handle viscous liquids much better than centrifugal pumps, whose performance drops sharply as viscosity rises.

Do VFDs really save energy on pumps?

Where flow varies, yes. Reducing pump speed to match demand uses much less energy than running at full speed and throttling with a valve.

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