Pump jargon buster

Pump terminology.

Every term on our product pages, data sheets and spec tables, explained in a sentence or two. Use it when comparing pumps, reading a data plate or working out what a spec actually means.

9 categories From pump types to data sheets

129 terms Each with its own link

Plain English No engineering degree needed

What's inside

Jump to what you're after.

Each category covers the terms you'll see on our product pages and spec sheets. Click any to skip straight there, or link to a single term with its anchor.

01 Pump typesSubmersible, surface, puddle, sewage, engine and the rest.
02 Flow, head and performanceFlow, head, friction loss, the pump curve and suction lift.
03 Impellers and solidsImpeller types, free passage and the wearing parts.
04 Motors and electricskW, amps, voltage, RCDs, thermal protection and IP ratings.
05 Level controlManual and automatic, floats, probes, cut-in levels and short cycling.
06 Seals, bearings and materialsMechanical seals, oil chambers, bearings and what the pump is made of.
07 Connections, hose and fittingsInlet and outlet sizes, BSP threads, hose types, valves and strainers.
08 Engine pumpsPetrol and diesel pumps, oil alert, fuel and priming.
09 Reading our data sheetsDimensions, weights, cable, temperature limits and immersion depth.

Pump types

Nearly every pump we sell is a centrifugal pump: a spinning impeller flings water outwards and the pressure difference draws more in behind it. The names below describe where the pump sits, what it is built to swallow and how it is driven.

Submersible pump: A pump that sits in the water with a sealed motor, so it needs no priming and only a discharge hose. The water cools the motor. Most flood, sump, drainage and sewage pumps on this site are submersibles.
Surface pump: A pump that sits beside or above the water and draws it up through a suction hose. It needs an inlet and an outlet, must be primed, and cannot lift much more than 6 m. Used for boosting, transfer and irrigation.
Puddle pump: A submersible built to pump a flat surface down to a millimetre or two, with the inlet at floor level instead of a raised strainer. Examples on this site: Tsurumi LSC1.4S, Stream SPK530, EVAK Residox.
Sump pump: A submersible that sits in a chamber (the sump) and empties it automatically when water collects, usually on a float switch. Sized by the volume of the sump and the lift to the discharge point.
Sewage pump: A submersible designed to pass soft solids and fibrous waste: no strainer, legs or a stand to keep the inlet open, and a vortex, single-channel or cutter impeller. Free passage is the figure that matters.
Cutter pump: A sewage pump with hardened cutting edges on the impeller or suction cover that chop solids before they enter the volute, so the waste passes through smaller pipework. The APP DSPK is a cutter pump.
Grinder pump: A sewage pump that mills waste into a slurry rather than cutting it, so it can be pushed long distances through small-bore pipe. The Speroni Cutty is this type.
Drainage pump: A general name for submersibles that move clean or lightly dirty water: cellar drainage, site dewatering, emptying tanks. A strainer with small holes keeps grit out of the impeller.
Site drainage pump: A heavy-duty submersible for sandy, silty water on construction sites, with wear-resistant impeller and casing and a large strainer. Tsurumi LB and HS, EVAK Drainox and the APP HD-15 are examples.
Agitator pump: A site pump with a paddle below the impeller that stirs settled silt back into suspension so it is pumped away instead of burying the pump. The EVAK Trenchman is an agitator pump.
Trash pump: A high-flow pump, usually engine-driven, built to pass leaves, stones and debris. The TD and DryMax diaphragm pumps and the Tsurumi TDS are trash pumps.
Engine pump: A surface pump driven by a petrol or diesel engine instead of a motor. Outdoors only, because of exhaust fumes. Centrifugal types need priming; diaphragm types prime themselves and can run dry.
Diaphragm pump: A pump that moves water with a flexing rubber diaphragm and two check valves rather than an impeller. It self-primes, runs dry without harm and passes large solids, at the cost of a pulsing, lower flow.
Self-priming pump: A surface pump whose casing holds enough water to clear air from the suction line and start lifting on its own, once it has been filled for the first time. Not the same as being able to run dry.
Booster pump: A surface pump that raises the pressure of water already flowing, for example from a tank to taps or sprinklers. Usually paired with a pressure switch so it starts when a tap opens.
Demand pump: A small diaphragm pressure pump that starts when it senses a drop in pressure, such as a tap or spray gun opening, and stops when it closes. The FL on-demand pump is this type.
Multistage pump: A pump with several impellers in series, each adding pressure to the last. Gives high head from a modest motor, so it suits boosting and borehole duty rather than flood work.
Borehole pump: A tall, narrow submersible multistage pump built to fit down a 4 or 5 inch well casing and lift water from depth. Pressure rather than volume. The Umbra Pompe Eurojet and Acuasub are examples.
Pond pump: A continuously rated submersible for water features and filtration, with a large strainer and low power draw. Built to run all day, not to clear floods.
Pool pump: A self-priming surface pump with a built-in strainer basket that circulates pool water through the filter. Must never run dry and usually needs its own circuit.
Packaged pump station: A chamber supplied with the pump, pipework, non-return valve and float already fitted, ready to sink into the ground and connect. The FPS Iguazu range and the Microforce station are examples.

Flow, head and performance

Flow figures on our product pages are litres per minute pumping clean, cold water through the stated outlet. Head is given in metres. The two always trade off against each other, which is what the pump curve shows.

Flow rate: How much water the pump moves, in litres per minute (L/min) or cubic metres per hour. 1,000 litres is one cubic metre. The figure on the box is the maximum, at zero head.
Head: Pressure expressed as the height of a column of water, in metres. A pump rated at 10 m head could push water up a 10 m vertical pipe and no further. Head and flow trade off against each other.
Static head: The plain vertical distance from the water level to the highest point of the discharge, before any friction is added. Measure from the water surface, not the pump.
Friction loss: The head lost pushing water along hose or pipe and through fittings. It rises steeply with flow and hose length and falls with bore. Long runs of small hose can halve a pump's output.
Total head: Static head plus friction loss, sometimes called total dynamic head. This is the figure to read against the pump curve, not the vertical lift on its own.
Pump curve: The graph of flow against head for a given pump. The right-hand end is maximum flow at no head; the left-hand end is shut-off head, where flow stops. Your installation sits somewhere between.
Duty point: The single point on the curve where the pump actually runs once head and friction for your installation are added up. Where it falls decides the real flow you will see.
Middle third: The central portion of the pump curve where a pump is efficient and lightly loaded. We size pumps to run here. Running at either extreme wears the pump and is excluded from warranty.
Shut-off head: The maximum head on the curve, at which flow falls to zero. A pump run near shut-off heats its own water, wears the seal and achieves nothing.
Maximum flow: The flow at zero head with no hose, as printed on the box. Add lift, hose and fittings and the working flow is often half this or less. Nothing is wrong; that is the curve.
Suction lift: The vertical distance a surface pump can draw water up its inlet. The theoretical limit is just over 10 m; in practice plan on 6 m for water, 3 m for fuels and 2 m for oils.
Cavitation: Bubbles forming and collapsing inside a pump that is starved of water on the suction side. It sounds like gravel in the casing and pits the impeller. Cure it by shortening or enlarging the suction line, never by throttling it.
Bar: A unit of pressure. One bar is roughly 10 m of water head, so a pump rated at 3 bar can lift about 30 m.
Litres per minute: The flow unit used across this site. Divide by 16.7 for litres per second, or multiply by 0.06 for cubic metres per hour.
Hose length tables: The flow-by-hose-length figures on many of our product pages, showing how much flow a pump delivers through 10, 20, 50 or 100 m of layflat. Use them before blaming the pump for low flow.

Impellers and solids

The impeller is the spinning part that moves the water. Its shape decides what the pump can swallow and how much pressure it makes. Free passage, the size of solid the pump will pass, follows from the impeller type and the strainer in front of it.

Impeller: The rotating vaned disc inside the pump that throws water outwards to create flow. Its design fixes the balance between pressure, flow and solids handling.
Closed impeller: Vanes enclosed between two shrouds. Most efficient and gives the most head, but only for clean water with small solids. Found in surface, booster and borehole pumps.
Semi-open impeller: Vanes with a shroud on one side only, running close to a wear plate. Tolerates grit and sand, which is why site drainage pumps such as the Tsurumi LB and APP HD-15 use it.
Vortex impeller: A recessed impeller that spins above the flow path, creating a whirlpool that carries solids through without touching the vanes. Passes large soft solids at some cost in efficiency. Common in sewage and dirty water pumps.
Single-channel impeller: An impeller with one wide passage so a solid up to the free passage size travels straight through. Used in sewage pumps where blockages matter more than efficiency.
Free passage: The largest solid the manufacturer says will pass through the pump, in millimetres. Anything bigger than this, or than the strainer holes, will block it. Sewage pumps quote 50 mm or more; puddle pumps a few millimetres.
Solids handling: A general term for how much and what kind of debris a pump will pass. Depends on free passage, impeller type and whether fibrous material is involved. Rags defeat most pumps that are not cutters.
Strainer: The perforated guard around a submersible's inlet. Its holes set the practical free passage and keep gravel off the impeller. Clear it regularly; a blocked strainer is the commonest cause of no flow.
Wear plate: A replaceable plate the impeller runs against. As it wears the gap grows and flow falls. Replacing it restores performance on site pumps that have handled sand.
Suction cover: The plate under the impeller that forms the inlet side of the pump, often combined with the wear plate. On cutter pumps it carries the serrated cutting edge.
Clogging: The impeller stopping because debris has jammed it. Usually announced by a sudden fall in flow, a hum from the motor, or the thermal protector tripping. Clear it with the power off.
Agitator: A paddle fitted below the impeller on some site pumps that stirs settled silt into suspension so the pump can carry it away rather than being buried by it.

Motors and electrics

The plate on a pump gives its motor power, voltage, phase and running current. The numbers that catch people out are starting current, which is several times the running figure, and the difference between output and input power.

kW (motor power): The power on the data plate is the output at the shaft. The electrical input is higher, typically by 20 to 40 per cent, which is the figure a generator or supply has to provide.
Amps (full-load current): The running current at full load, as shown on the rating plate. Use it to size cable and the breaker, but remember it is not the starting current.
Starting current: The surge a motor draws for a fraction of a second as it starts, four to six times the running current. It is what trips an undersized fuse and what a generator must be able to supply.
Single phase: A motor run from a standard two-wire supply, 230 V domestic or 110 V site. Needs a capacitor to start and is limited to about 2.2 kW on a 13 A plug.
Three phase: A motor run from a 400 V three-wire industrial supply. Starts without a capacitor and suits larger pumps. Swapping two phases reverses the rotation, which shows as low flow and odd noise.
110 V: The reduced-voltage site supply used on UK construction sites through a centre-tapped transformer, yellow plugs and sockets. A 110 V pump needs a transformer sized for its starting current.
230 V: The standard UK domestic and commercial supply. A 230 V pump with a fitted plug runs from any socket on a circuit protected by an RCD.
400 V: The UK three-phase supply. Pumps at this voltage are industrial units and are supplied without a plug.
Capacitor: The component that gives a single-phase motor the push to start and keeps it running. A failed capacitor leaves the motor humming without turning.
RCD: Residual current device. A safety switch that cuts the supply within milliseconds if current leaks to earth through a damaged cable, a wet connection or water in the motor. Outdoor use must be RCD protected. A 30 mA RCD is the norm.
Thermal overload: A protector inside the motor that opens the circuit when the windings overheat, from dry running, a jammed impeller or low voltage, and resets as it cools. Repeated trips damage the windings and are not a way to run a pump.
Duty (S1): The duty rating on the plate. S1 means continuously rated: the pump can run all day. Some small or DC pumps are intermittently rated and need rest periods.
IP rating: Ingress protection, two digits. The first is protection against solids and touch (0 to 6), the second against water (0 to 8). IP68 is fully submersible; IP44 to IP55 is a surface pump that can be rained on but not immersed.
Insulation resistance: A measure of how well the motor windings are isolated from earth, tested with a megger. A new pump reads 20 megohms or more; a reading falling towards 1 megohm means water is getting in.
Frequency (50 Hz): UK mains runs at 50 Hz. A pump built for 60 Hz runs slow on it and delivers less. Generators must hold 50 Hz under load or the pump will trip or overheat.
Generator sizing: Size a generator for the pump's starting current, not its running current. Small puddle pumps need about 2 kVA; a large single-phase site pump such as the HD-15 needs around 6 kVA.
Extension lead: Any added cable drops the voltage at the pump. Use the heaviest core and shortest length you can; a long thin lead makes a pump run hot and slow.
Control panel: A separate enclosure holding the starter, overloads and level-control logic for a fixed installation, used with probe systems such as SwitcH2O or with three-phase pumps.
Variable speed drive: An inverter that changes the motor speed to match demand, saving energy and removing the need for pressure tanks and check valves on boosting duties.

Level control

A manual pump runs as soon as it has power and needs someone to switch it off. An automatic pump starts and stops on a level control. Which control, and how much room it has to move, decides whether the pump looks after itself or short-cycles.

Manual: A pump with no float or sensor. It runs when switched on and keeps running until switched off, so the operator controls the level and must not let it run dry for long.
Automatic: A pump fitted with a float switch or sensor that starts it when water rises and stops it when it falls. The sump must give the control room to work; residual water after it stops is normal.
Float switch: A sealed float containing a switch that tilts or rises with the water to start and stop the pump. The commonest level control. It needs free movement: anything that stops the float moving stops the pump working.
Pendant float: A float on a length of cable that swings up as the water rises. Simple and cheap, but it needs a sump wide enough to swing in, typically 300 mm or more.
Tube float: A float that slides up and down a vertical tube fixed to the pump, so it works in a narrow sump. Debris in the tube stops it; the tube pulls off for cleaning. The Tsurumi POMA uses one.
Compact float: A small float on a short arm or pivot that needs only a little space to operate and switches at low levels. Used on the APP BPS100EA and RS32EA for narrow pits.
Sensor switch: Electronic level control using two stainless probes instead of a float. The pump starts when water touches the probes and runs for about a minute after it clears, so it can pump down to a few millimetres. Fitted to the Tsurumi LSC automatic.
Extension probes: Longer probes fitted to a sensor switch so it detects water, and starts the pump, at a lower level. They do not change how low the pump can pump.
SwitcH2O: A three-probe level system with a control panel for fixed installations. Start, stop and common probes are positioned to set the levels exactly, with at least 10 mm between on and off.
Cut-in level: The water level at which an automatic pump starts, measured from the floor. On the data sheet as switch-on or start level.
Cut-out level: The level at which the pump stops, leaving residual water. Usually set so the impeller is still covered; a non-return valve near the pump keeps hose water from refilling the sump.
Starts per hour: How often a motor can start without overheating. Most makers allow about 25 starts an hour. A sump too small for the float's travel exceeds this and the pump short-cycles.
Short cycling: A pump starting and stopping every few seconds or minutes. Caused by a small sump, a float adjusted into a narrow band, or discharge water flowing back after each stop. It wears the motor out quickly.
High level alarm: A separate float and sounder that warns when water rises above the pump's start level, meaning the pump has failed or cannot keep up. Fitted to any installation where flooding would do damage.
Pressure switch: A control on a booster or demand pump that starts it when pressure falls (a tap opens) and stops it when pressure recovers. Often combined with dry-run protection.
Dry-run protection: A control that stops a pump when there is no water to pump, before the seal and motor overheat. Built into some pressure controllers; on submersibles the float or sensor does the job.

Seals, bearings and materials

The seal is what keeps water out of the motor, and the oil chamber is what keeps the seal alive. These are the parts that fail when a pump runs dry, runs on a closed tap or runs outside its curve.

Mechanical seal: The rotating seal on the shaft between the wet end and the motor. Two lapped faces, one spinning and one still, kept apart by a film of liquid. It needs liquid to survive, which is why dry running kills pumps.
Double mechanical seal: Two seals in series with an oil chamber between them. If the lower seal weeps, the oil takes the water and the upper seal protects the motor. Standard on good submersibles.
Lip seal: A simple rubber seal pressed on the shaft. Fine for clean liquids and cheaper pumps, but it wears quickly in grit and gives no warning before it leaks.
Oil chamber: The oil-filled space between the seals of a submersible. Milky oil on inspection means water has passed the lower seal and the pump needs a service before the motor floods.
Oil lifter: A device in the oil chamber that keeps the seal faces lubricated whatever angle the pump lies at. Lets a puddle pump run on its side or at very low levels without starving the seal.
Bearing: The bearings carry the shaft. A worn bearing makes a pump rumble or vibrate and will take the seal with it. Replacement is a workshop job.
Cast iron: The usual body material for site and sewage pumps: heavy, tough and cheap to cast. Rusts on the surface but lasts for years in water.
Stainless steel (304 and 316): Corrosion-resistant steels used for casings, shafts and strainers. 304 suits clean and grey water; 316 resists chlorides, so it is used for seawater, pool and chemical pumps.
Thermoplastic: The moulded plastic body of many domestic and puddle pumps. Light and rust-proof, with wear parts in rubber or urethane for abrasive water.
Wetted parts: Every part of the pump the liquid touches. These decide whether a pump can handle seawater, chemicals or hot water, and they are what to check on a chemical-resistant pump.
Anti-vibration feet: Rubber mounts under a surface pump that stop it walking across the floor and transmitting noise into pipework.
ATEX: The European marking for equipment rated for explosive atmospheres. Only pumps carrying it may be used where fuel vapour or methane can collect.

Connections, hose and fittings

If an inlet or outlet size is given in inches, such as 1¼", it is threaded and needs a hose tail to take a flexible hose. If it is given in millimetres, such as 32 mm, a hose tail is supplied. Outlet is the bore of the discharge; inlet is the bore of the suction, and the inlet should never be reduced.

Outlet: The discharge connection, given as a bore in millimetres or a thread in inches. Reducing it adds friction, cuts flow and raises the risk of blockage on solids pumps. Increasing it helps long runs.
Inlet: The suction connection on a surface pump. Never reduce it: a starved inlet causes cavitation, erratic running and early seal failure. The suction hose must be rigid enough not to collapse.
BSP thread: British Standard Pipe, the thread on pump connections and fittings. A 1" BSP thread is about 33 mm across, not 25 mm, so match fittings by BSP size rather than by measuring the outside.
Male and female: A male thread is on the outside of the fitting and screws into a female thread on the inside. A pump with a male outlet needs a female hose tail, and the other way round.
Hose tail: A barbed fitting that screws onto the pump's thread and takes a flexible hose, held on with a hose clip. Swivel versions can be tightened without twisting the hose.
Layflat hose: Flat, flexible discharge hose that rolls up small and lies flat on the ground. Delivery only: it collapses under suction. Our standard hose for flood water.
Suction and delivery hose: Rigid-walled, spiral-reinforced hose that holds its shape under suction as well as pressure. Required on the inlet side of any surface or engine pump.
Hose clip: The worm-drive band that secures a hose to its tail. Size it to the hose's outside diameter; a loose clip leaks air on the suction side and water on the delivery side.
Non-return valve: A one-way valve that stops water in the discharge running back through the pump when it stops. Fit it close to the pump, the right way round, to stop a sump refilling and the pump short-cycling.
Foot valve: A non-return valve with a strainer fitted at the bottom of a suction hose. It keeps the suction line full between runs so a surface pump does not need re-priming.
Suction strainer: A perforated basket on the end of a suction hose that keeps leaves and stones out of a surface pump. Keep it off the bottom and clear of silt.
Gate valve: A valve that throttles or isolates a pipe. Use it on the discharge to control flow; never on the suction side of a surface pump.
Camlock coupling: A quick-connect fitting with two cam arms that lock onto a mating adaptor. Standard for joining layflat and suction hose quickly on site.
Quick release coupling: A smaller push-fit hose connector for garden and small flood pumps, used in our pump kits so hose can be joined without tools.
Reducing bush: A fitting that steps a thread down to a smaller size. Useful on the discharge; avoid on an inlet.
Priming: Filling a surface pump's casing and suction line with water before starting, so the impeller has liquid to work on. A pump started empty will not lift and will damage its seal.
Guide rail: A permanent installation where the sewage pump slides down two rails onto a fixed discharge elbow in the chamber, so it can be lifted out for service without entering the pit.
Free-standing: A pump installed simply standing on the chamber floor with a flexible discharge, rather than on guide rails. The usual arrangement for portable and domestic pumps.

Engine pumps

Engine pumps are for outdoors and for sites without power. Petrol engines run on unleaded; diesel engines on red or white diesel but not biodiesel. Centrifugal engine pumps need priming and must not run without water; diaphragm types prime themselves and can run dry.

Oil alert: A sensor on petrol engines that stops the engine if the oil level is low or the pump is tipped at an angle. If an engine pump will not start on a slope, this is usually why.
Centrifugal engine pump: The common site pump with an impeller: high flow, must be primed through the filler plug, and will damage its seal if run without water. Honda WB and Tsurumi TE pumps are this type.
Diaphragm engine pump: A slower, pulsing pump that lifts itself, handles mud and leaves, and can run dry safely. The Tsurumi TD and DryMax are diaphragm pumps.
Priming time: How long a primed engine pump takes to start delivering: about 30 seconds at 2 to 3 m of lift and up to a minute at 6 m. Longer than that means an air leak on the suction side.
Suction lift (engine pumps): Plan on 6 m maximum for any engine-driven pump, and 2 to 3 m for comfortable priming. Beyond that, lower the pump or use a submersible.
Carbon monoxide: Engine exhaust is lethal in enclosed spaces. Engine pumps run outdoors only, never in a cellar, garage or tent.
Running hours: Engine pumps are rated by hours per tank. Flow falls as the engine labours; black smoke means it is overloaded and the speed should be backed off.

Reading our data sheets

Dimensions and weights on our pages come from the manufacturer or our own measurements and should be treated as a guide. If an installation depends on an exact figure, ask us for written confirmation before ordering.

W x L x H: Width, length and height of the pump, or diameter and height for a round pump, without hose or cable. Check it against the sump before buying an automatic pump.
Dry weight: The weight of the pump without cable, hose or packaging. For Tsurumi free-standing sewage pumps it includes the flanged elbow.
Packing weight: The weight as delivered, including packaging. The figure that decides the delivery service.
Cable length: The power cable supplied, usually 10 m on professional pumps. Extending it is fine with the right core size; joining it under water is not.
Maximum liquid temperature: The hottest water the pump can handle continuously, typically 35 to 40 °C for submersibles. Hot tub and process water often exceeds it; check before pumping.
Immersion depth: How far under water a submersible may be installed, set by the seal and cable entry. Most domestic and site pumps are rated to 5 or 10 m.
Minimum water level: The depth of water needed for the pump to keep pumping and cooling. Puddle pumps run down to a few millimetres; standard submersibles need the strainer covered.
Warranty: The manufacturer's guarantee period, from the date of purchase. Tsurumi submersibles carry 3 years, APP and Speroni 2 years, most other products 1 year unless the product page says otherwise. Wear, misuse and running outside the middle third of the curve are excluded.
Data plate: The metal or printed label on the pump giving model, voltage, phase, power, current, flow, head and free passage. A clear photo of it is the quickest way to get a replacement matched.