Glossary

Every technical word in the course, in plain language.

Absolute velocity
The water’s speed as seen from the casing that stands still. Written c.
Affinity laws
Rules for how a pump changes with speed: flow grows with the speed, head with the speed squared, and power with the speed cubed.
Area
How much surface something covers, like the size of a floor. Measured in m² or cm².
Axial flow
The fluid moves along the direction of the shaft, straight through the impeller like air through a desk fan. Moves huge amounts but with little pressure.
Axial thrust
The push along the shaft that an impeller feels, because the pressure on its two sides is not equal. Bearings have to hold it.
Backward-curved vanes
Vanes that bend away from the direction of rotation. Used by almost every pump because they are efficient and stable.
Baffle
A flat strip fixed to the wall of a tank. Baffles stop the liquid from just spinning round and turn the swirl into up-and-down mixing.
Balancing
Measuring where a spinning part is heavy and removing a little material there (or adding weight opposite) until it spins smoothly.
Like: The small lead weights a tyre shop clips onto a car wheel.
Bar
A unit of pressure. 1 bar is roughly the pressure of the air around you at sea level, or of a 10 m tall column of water.
Bearing
A part that holds a spinning shaft in place and lets it turn smoothly with very little friction, usually with small steel balls or rollers.
Bernoulli’s principle
In a flowing fluid, speed and pressure trade places: where the fluid speeds up, its pressure drops; where it slows down, its pressure rises.
Like: Money in two pockets — move coins from one pocket to the other and the total stays the same.
Best efficiency point (BEP)
The flow rate at which a pump wastes the least energy. Pumps also run quietest and last longest near it.
Booster pump
A pump that raises the pressure in a water system that is too weak, for example to reach the upper floors of a tall building.
Casing
The housing around the impeller. It collects the fast fluid leaving the impeller and guides it to the outlet.
Cavitation
Bubbles of vapour that form where the pressure drops too low, then collapse violently when the pressure rises again. It sounds like gravel in the pump and eats away the metal. Module 5 covers it in detail.
Centrifugal
Latin for “fleeing the centre”. In a spinning system, things get flung outward, away from the middle.
Like: Water leaving the lettuce in a salad spinner.
Channel impeller
An impeller with only one or two vanes and very wide passages, so large solids can pass through. Common in sewage pumps.
Circulator
A small pump that moves water round a closed loop, such as a central heating system. It only needs to overcome friction, not lift water up.
Clearance
The small gap between a spinning part and a part that stands still. Smaller gaps leak less, but the parts must never touch.
Clogging
When rags, hair, fibres or solids get stuck in the impeller and block the flow.
Closed impeller
An impeller with shrouds on both sides of the vanes, so the water flows through closed channels. The most efficient type, used in most clean-water pumps.
CNC machining
Cutting a part out of a solid block with a computer-controlled milling machine. Used for high-speed compressor wheels and prototypes.
Compressor
A machine that squeezes a gas into a smaller space, raising its pressure. A centrifugal compressor uses an impeller spinning very fast.
Continuity
What goes into a pipe must come out. So where a pipe gets narrower, the fluid has to go faster.
Like: A crowd leaving a stadium walks faster through a narrow gate to keep the same number of people per minute.
Corrosion
Metal being eaten away chemically — like rust, or sea water attacking iron.
Cutwater
The sharp tongue in a volute casing where the narrowest part of the spiral meets the outlet. It splits the flow between “go out” and “go round again”.
DC motor
A small electric motor that runs on low-voltage direct current, like from a battery or a 12 V power supply. Safe to use for water experiments.
Density
How heavy a fixed volume of something is. One litre of water weighs 1 kg; one litre of petrol only about 0.74 kg.
Diameter
The distance straight across a circle through its centre. Twice the radius.
Diffuser
A ring of fixed (non-spinning) vanes around an impeller, or a widening passage, that slows the fluid down gently so its speed becomes pressure.
Disc friction
Power lost because the outer sides of the shrouds spin in the water trapped between impeller and casing, like a paddle stirring.
Discharge
The outlet side of a pump, where the fluid leaves with more pressure than it came in with.
Double suction
An impeller that takes in water from both sides at once — like two impellers back to back. It handles twice the flow and balances its own sideways push.
Efficiency
How much of the motor’s power really ends up in the fluid. The rest turns into heat, noise and swirl. Written as a percentage or the Greek letter η (eta).
Energy
The ability to do work — to push, lift, heat or move something. A pump’s whole job is to give energy to a fluid.
Erosion
Metal being worn away mechanically by sand and grit carried in the liquid, like sandpaper.
Euler’s pump equation
The basic law of every impeller: the ideal head equals blade speed × swirl speed at the outlet ÷ g (H = u₂·cᵤ₂ / g).
Eye
The opening in the centre of an impeller where the fluid enters.
Flexible impeller
A rubber impeller whose vanes bend as they pass an off-centre part of the casing, squeezing liquid out like a sponge. Self-priming, but it must never run dry.
Flow rate
How much fluid passes by per unit of time — for example litres per minute (L/min) or cubic metres per hour (m³/h). Engineers write it as Q.
Like: How fast a bucket fills up.
Fluid
Anything that can flow and take the shape of its container — liquids like water and oil, and gases like air.
Like: A crowd of tiny marbles that can roll past each other.
Force
A push or a pull. Measured in newtons (N). About 10 N is the weight of a 1 kg bag of sugar.
Forward-curved vanes
Vanes whose tips lean into the direction of rotation. They move a lot of air from a small wheel, as in squirrel-cage fans, but waste more energy.
Friction loss
Head lost as the liquid rubs against pipe walls, bends and valves. Double the flow and the friction loss roughly quadruples.
Grinder pump
A pump with cutting blades at its inlet that chop solids into small bits before the impeller moves them.
Hub
The solid centre of an impeller that clamps onto the shaft. The vanes and shrouds are all attached to it, directly or through the back shroud.
Hydraulic losses
Head lost inside the pump to friction on the walls, water hitting the vane edges at the wrong angle, and mixing of fast and slow streams.
Hydraulic power
The power a pump actually gives to the fluid: density × g × flow × head. In kW, roughly flow (m³/h) × head (m) ÷ 367 for water.
Impeller
A spinning wheel with curved blades (vanes) that gives energy to a fluid, making it move faster and push harder.
Like: A salad spinner that throws water outward — on purpose, and non-stop.
Incompressible
Cannot be squeezed into a smaller space. Water is almost perfectly incompressible; air is easy to compress.
Inertia
Things keep doing what they are doing — a moving object wants to keep going in a straight line unless something pushes it.
Like: Why you slide sideways in your seat when a car turns sharply.
Investment casting
Also called lost-wax casting: a wax copy is coated in ceramic, the wax is melted out and metal poured in. Gives smooth, precise parts.
Kilowatt (kW)
A unit of power: 1000 watts. A kettle uses about 2 kW; a small car engine can make about 70 kW.
Lip seal
A rubber ring with a soft lip that presses lightly on a spinning shaft to keep liquid in. Cheap and simple, but it must stay wet.
Magnetic drive
A way to spin an impeller through a sealed wall using magnets, so the liquid can never leak out along the shaft.
Mechanical seal
Two very flat rings, one spinning and one still, pressed together to stop liquid leaking out where the shaft leaves the casing.
Misalignment
When the motor shaft and the pump shaft are not perfectly in line. It shakes the coupling and wears out bearings and seals.
Mixed flow
The fluid leaves at a slant — partly outward, partly along the shaft. A middle ground between radial and axial.
Mixer
A machine that stirs a liquid in a tank. Its impeller does not need to build pressure — it just needs to keep everything moving.
Molecule
The tiniest piece of a substance that is still that substance. A single drop of water holds more than a billion trillion of them.
Multimeter
A handheld meter that measures voltage (volts), current (amps) and resistance. Volts × amps = watts.
Multistage pump
A pump with several impellers in a row on one shaft. Each stage adds more pressure, like climbing stairs one step at a time.
NPSH
Net Positive Suction Head: how far the pressure at the pump inlet stays above the boiling point (vapour pressure), measured in metres of liquid.
NPSH available
The NPSH the installation offers at the pump inlet. It rises if the tank is higher or the liquid colder, and falls with long suction pipes and hot liquid.
NPSH required
The NPSH the pump needs to avoid harmful cavitation, measured by the maker. It grows as the flow increases.
O-ring
A small rubber ring that seals a joint when it is squeezed in a groove.
Open impeller
An impeller whose vanes stand free, with no shroud on either side (or only a small one at the hub). Easy to clean and good with dirty liquids, but less efficient.
Operating point
Where the pump curve crosses the system curve: the flow and head the pump will actually run at in that system.
PETG
A tough, slightly flexible 3D-printing plastic that copes well with water. A good first choice for printed pump parts.
Pressure
How hard a fluid pushes on each bit of surface around it. Measured in bar, psi or pascal (Pa).
Like: How hard toothpaste pushes on the inside of the tube when you squeeze it.
Priming
Filling a pump with liquid before starting it. An impeller full of air cannot create enough suction to pull water up.
Propeller
Spinning blades in open water or air that push the fluid backward to create thrust and move a boat or plane forward.
Prototype
A first working version of a design, built to test the idea and learn what to improve.
psi
Pounds per square inch — a pressure unit used mostly in the USA. 1 bar ≈ 14.5 psi.
Pump
A machine that moves a fluid by giving it energy. Most pumps in the world use an impeller — they are called centrifugal pumps.
Pump curve
A chart from the pump maker that shows how much head the pump gives at each flow rate. More flow always means less head.
Radial flow
The fluid leaves the impeller straight outward from the centre, at right angles to the shaft. Gives the most pressure per stage.
Radius
The distance from the centre of a circle to its edge. Half of the diameter.
Recirculation
Swirling backflow at the eye or the outlet of an impeller when a pump runs at far too little flow. It causes noise, vibration and wear.
Relative velocity
The water’s speed as seen by someone riding on the spinning impeller. Written w. It points along the vane.
Rotor
The spinning part of any machine. An impeller is the rotor of a pump.
RPM
Revolutions per minute — how many full turns something makes in one minute. Many pump motors in Europe spin at about 1450 or 2900 RPM.
Sand casting
Pouring molten metal into a mould made of packed sand, with sand cores shaping the channels inside. The most common way to make metal impellers.
Self-priming
A pump that can clear the air out of its own suction pipe and start pumping without being filled by hand first (after the very first fill).
Semi-open impeller
An impeller with a back shroud but no front shroud. A popular middle ground for liquids with some solids in them.
Shaft
The steel rod that connects the motor to the impeller and makes it spin.
Shear
When layers of fluid slide past each other at different speeds. High shear tears bubbles and droplets into tiny pieces — useful for some jobs, harmful for delicate things like cells.
Shroud
A disc that covers the side of the vanes. The back shroud carries the vanes; the front shroud (if there is one) closes the channels on the inlet side.
Like: The two slices of bread around a sandwich filling.
Shut-off head
The head a pump makes when the outlet valve is completely closed and nothing flows. It is the highest point of the pump curve.
Slicer
A program (such as Cura or PrusaSlicer) that cuts a 3D model into thin layers and plans every move of the 3D printer.
Slip
Water between the vanes does not follow them perfectly; it lags behind, so it leaves with less swirl than the vane angle promises. Fewer vanes, more slip.
Slip factor
A number (usually 0.7–0.9) that says how much of the ideal swirl the water really gets because of slip.
Slurry
A liquid carrying lots of solid particles, like muddy water, sand or crushed ore. It wears out pumps quickly.
Specific speed
One number, made from speed, flow and head, that tells which impeller shape fits a job: low for radial, middle for mixed flow, high for axial.
Speed of sound
How fast sound travels: about 343 m/s in air at room temperature. Gas moving near this speed starts to behave differently, which limits compressor designs.
Static head
The height difference the liquid must be lifted, from the surface it comes from to where it is delivered. It does not depend on flow.
STL file
A common 3D-model file made of many tiny triangles. Slicer programs turn it into instructions for a 3D printer.
Suction
The inlet side of a pump, where the fluid comes in. The pressure there is the lowest anywhere in the pump.
Swirl
The part of the water’s motion that goes round in the direction of rotation. Engineers write its speed as cᵤ.
System curve
A chart of how much head the pipes need at each flow rate: the height to lift plus the friction, which grows fast as the flow increases.
Tangent
A straight line that just touches a circle at one point. A drop that leaves a spinning wheel flies off along the tangent.
Throttling
Reducing flow by partly closing a valve. Simple, but the pump keeps working hard and the extra pressure is burned off in the valve.
Thrust
The push a propeller, jet or rocket makes by throwing fluid backward. Push the fluid back, and it pushes you forward.
Tip speed
How fast the outer edge of a spinning part is moving. It grows with both the size of the part and its RPM.
Torque converter
The fluid coupling in a car’s automatic gearbox: a pump impeller spun by the engine throws oil at a turbine connected to the wheels.
Like: Two fans facing each other — switch one on and the other starts turning.
Turbine
A spinning machine driven by a moving fluid. It takes energy out of the fluid — for example to make electricity. The opposite job of an impeller.
Turbocharger
A small turbine spun by an engine’s exhaust gas, which drives a compressor impeller on the same shaft to push extra air into the engine.
Turbopump
A pump driven by its own turbine. Rocket engines use turbopumps to push huge amounts of fuel into the combustion chamber at very high pressure.
Unbalance
When the mass of a spinning part is not evenly spread around its centre, so it pulls the shaft round in a circle and shakes the machine.
Vane
One of the curved blades of an impeller. The vanes grab the fluid and push it outward as the impeller spins.
Vapour pressure
The pressure below which a liquid boils. For water it is tiny when cold (2.3 kPa at 20 °C) and equals normal air pressure at 100 °C.
Variable speed drive (VSD)
An electronic box that changes a motor’s speed. It lets a pump slow down instead of wasting energy against a half-closed valve.
Velocity
How fast something moves, and in which direction. Measured in metres per second (m/s).
Velocity triangle
A little diagram of three speeds at a vane’s edge: how fast the vane moves (u), how fast the water slides along the vane (w), and the water’s real speed (c = u + w).
Like: Walking on an airport’s moving walkway: your ground speed is the walkway’s speed plus your walking speed.
Viscosity
How “thick” a fluid is — how much it resists flowing. Honey has high viscosity, water has low viscosity.
Like: Walking through a crowd that holds hands (thick) versus one that doesn’t (thin).
Volute
A snail-shaped casing that gets wider and wider around the impeller. It collects the fast water and slows it down, turning speed into pressure.
Like: A snail shell wrapped around a spinning wheel.
Vortex
A whirlpool: fluid spinning around a centre. In an unbaffled tank it makes a funnel-shaped dip in the surface.
Vortex impeller
An impeller tucked back into the casing so it spins up a whirlpool. The whirlpool moves the liquid, so rags and solids pass through without touching the vanes.
Waterjet
A boat drive with an impeller inside a tube: it sucks water in under the hull and shoots it out the back as a fast jet. Used by jet skis and fast ferries.
Wear ring
A cheap, replaceable ring that keeps the gap between the spinning impeller and the casing very small, so little water leaks back to the eye. It is designed to wear out instead of the expensive parts.