Quick answer: The impeller type sets what a pump can pass, and the impeller material sets how long it lasts in that water. Clean water pumps use closed or open impellers because they are the most efficient. Puddle and site drainage pumps use semi-vortex or vortex impellers in urethane or Hytrel, because those materials shrug off sand. Sewage pumps use vortex or channel impellers in cast iron, so soft solids swirl through without touching the blades. Slurry and abrasive dewatering pumps step up to chromium iron, the hardest material we sell, and anything in seawater or chemicals goes to 316 stainless steel.
Once you know the pattern, you can read almost any pump specification.
Our guide to how submersible pumps work explains what an impeller does: it spins, builds a pressure difference and moves water. What it does not cover is why one pump has a plastic impeller shaped like a shallow dish, another has an iron one with two deep channels, and a third has a rubbery disc that looks almost too soft to pump anything. Those differences are not cosmetic. They are the reason a puddle pump leaves a floor dry, a sewage pump does not block, and a slurry pump survives a week in bentonite.
We have been selling and specifying pumps since 2012, and the same question can often arise: why does this one cost more than that one when the flow and head look the same? Very often the answer is the impeller. This guide sets out the types you will see across our range, the materials they are made from, and how to use that knowledge to choose the right pump first time.
Why the impeller decides what a pump can handle
Every impeller is a compromise between three things:
- Efficiency. The closer the blades fit inside the casing, the more of the motor's energy ends up as flow and head.
- Free passage. The bigger the gaps, the larger the solid that can pass without jamming. Our guide to pump solids handling covers what those numbers mean.
- Wear. Sand, grit and stones hit the blades thousands of times a minute. Some materials erode, some deform and spring back, and some are simply too hard to mark.
You cannot have all three. A tight, efficient impeller blocks on solids. A wide-open one wastes energy. A hard one is heavy and expensive. So the manufacturer picks the type for the job, then picks the material for the water, and the result is the pattern below.
The impeller types you will see on a specification
Closed (shrouded) impellers
The blades are sandwiched between two plates, so water can only travel through the channels between them. This is the most efficient design and gives the highest head for a given motor, which is why it is used wherever the water is clean and the lift is high.
Our borehole pumps from Umbra Pompe stack a column of small closed impellers inside a 4 or 5 inch body to lift water from depth, the DAB Divertron and APP MVH-10A do the same in a smaller package for wells, tanks and water butts, and the Speroni multistage surface pumps (the RS, RSX and RV ranges) use the same idea to boost pressure. The trade off is free passage: the channels are narrow, so these pumps want clean or lightly dirty water. A borehole pump will tolerate a trace of fine sand and no more.
Open and semi-open impellers
Take one plate away and you have a semi-open impeller; take both away and it is fully open. The blades now run close to the suction cover, and the gap between them decides the solids passage.
Open designs are a good middle ground: more efficient than a vortex, but able to pass grit and small stones. We sell them on small clean water pumps such as the EGO 500, the APP RS32EA and the APP BPS range, on the Speroni GA(M) and WX(M) surface pumps for slightly dirty water, and at the other end of the scale on the APP HD-15, the high flow site pump whose iron impeller is in the photo above. Its blades stand proud of a single back plate, which is what semi-open means in practice. The Tsurumi 80SFQ seawater pump is semi-open too, passing 15 mm solids on the larger model.
Semi-vortex impellers
This is the signature of the Tsurumi site drainage pumps we have sold for over a decade. The impeller sits partly recessed into the casing, so some water is pumped directly by the blades and some is spun in a vortex above them. It is less efficient than an open impeller but far more tolerant of sand and silt, and it runs with a wear plate and a rubber or urethane casing that absorb impact instead of eroding. Every pump in the LSC, LB, HS and NK ranges uses one, and the HSD2.55S pairs one with an agitator. If you have ever wondered why the LB480 impeller is an orange rubbery disc with shallow vanes, this is why.
Vortex impellers
Here the impeller is pulled right back out of the flow path. It spins a vortex in the casing and the vortex does the pumping, so most solids never touch the blades at all. That is what makes vortex pumps the default for sewage and dirty water: the EVAK Hippo range, the APP BCV and APP SV, the Speroni SXS and the stainless FEKA pump in our Iguazu FEKA station all use one, and so do the smaller Tsurumi OM5, POMA and 50PNI2 sump pumps. It is also the design behind the EVAK Residox and Stream SPK530 puddle pumps and the EVAK Drainox site pumps, which is why they cope so well with gritty flood water.
The trade-off is efficiency. A vortex pump needs a bigger motor for the same flow and head, which is why you will not see one on a borehole pump.
Channel impellers
A channel impeller has one or two wide, curved passages instead of several thin blades. It recovers most of the efficiency a vortex gives away while still passing large solids, so it is the choice where running cost matters and the pump runs for hours a day. The BBC Semisom 262 and 635 use a double channel impeller that passes 32 mm solids, and the APP DSPK cutter pump is built around a single channel impeller. Channel impellers handle solids by size, not by shredding, so stringy material can still wrap around them. That is where the next group comes in.
Cutter and grinder impellers
These are ordinary impellers with a cutting edge added. On the APP DSPK, a tungsten carbide tipped single channel impeller rotates against a serrated suction cover and chops rags and fibrous material before they enter. On the Speroni Cutty, a stainless steel cutting disc at the inlet works against a matching cutter on an open impeller, shredding waste so it can be pushed a long way through small bore pipe. Our guide to sewage, cutter and grinder pumps covers when each is the right call.
Peripheral (regenerative) impellers
One more type turns up on small booster pumps. A peripheral impeller is a disc with many tiny vanes around its rim, and it carries water around the pump body several times before discharging, adding pressure with each pass. It gives a high head from a small motor and a very stable pressure curve, which is why the Speroni KP(M), KF(M) and AKM60 use one, with the impeller in brass. The clearances are tight, so these are strictly clean water pumps.
Agitators (an add-on, not an impeller)
An agitator is a separate rotating element below the impeller that stirs settled sand, silt and bentonite back into suspension so the pump can carry it away. The Tsurumi HSD2.55S has one in chromium iron, the EVAK Agivort pairs a 316 stainless agitator with a vortex impeller, and the EVAK Trenchman uses a sintered agitator under a Hytrel coated steel impeller. Notice that the impeller behind the agitator is still a semi-vortex or vortex design; the agitator changes what reaches it, not how it pumps. We cover the choice in agitator pumps for silty water and trenches.
| Impeller type | How it works | Good at | Trade-off | Examples from our range |
|---|---|---|---|---|
| Closed | Blades enclosed between two plates | Highest head and efficiency | Blocks on solids | Umbra Pompe Acuasub, Acuatec, Acuabig and Eurojet; DAB Divertron; APP MVH-10A; Speroni RS, RSX, RV, C(M), CMX |
| Open / semi-open | Blades run close to the suction cover | Efficiency plus grit tolerance | Clearance wears over time | APP HD-15, RS32EA, BPS; EGO 500; Tsurumi 80SFQ; Speroni GA(M), WX(M) |
| Semi-vortex | Partly recessed; blades and vortex share the work | Sandy site water, long life, pumping low | Less efficient than open | Tsurumi LSC, LB, HS, NK, HSD2.55S |
| Vortex | Fully recessed; the vortex pumps, not the blades | Large soft solids, fibrous waste, no clogging | Lowest efficiency, bigger motor | EVAK Hippo, Residox, Drainox, Agivort; APP BCV, SV, KHL-20; Stream SPK530; Tsurumi OM5, POMA, 50PNI2, SQ2-2; Speroni SXS; Arven Mizar VOX, Arvex/S |
| Channel | One or two wide curved passages | Large solids at good efficiency | Stringy material can wrap | BBC Semisom 262 and 635; APP DSPK (with cutter) |
| Cutter / grinder | Impeller plus cutting ring or serrated cover | Shredding waste for long, small bore runs | Cutting edges need checking | APP DSPK; Speroni Cutty |
| Peripheral | Rim vanes carry water round the body several times | High pressure from a small motor | Clean water only | Speroni KP(M), KF(M), AKM60 |
Materials: why the same impeller comes in four different metals
Pick two site pumps with the same semi-vortex impeller and the same outlet size, and one may have a urethane impeller while the other has chromium iron. The shape is the same. The material has been chosen for the water, and for how hard the impeller will be hit.
Glass fibre plastics: GFRP, nylon, PPS, Noryl, technopolymer
Light, cheap to mould, completely rust-proof and easy on the motor, because a light impeller takes less energy to spin up. Plastics are used wherever the water is clean or the solids are soft: the Tsurumi OM5, POMA and 50PNI2 sump pumps (glass fibre reinforced polymer), the EGO 500 (PPS thermoplastic body and impeller), the APP MH pond pump, the Umbra Pompe borehole pumps and the APP MVH-10A, and the smaller APP SV-150 and SV-250 sewage pumps, which are rated for soft and fibrous solids up to 27 mm. Plastic is fine until something hard hits it. That is why the larger SV-400 and SV-750 switch to cast iron impellers and are rated for harder solids.
Urethane, polyurethane and Hytrel
These are the clever ones. An elastic impeller is softer than sand, which sounds like a weakness, but it means a grain of sand dents the surface and the surface springs back instead of being cut. Combined with a rubber or plastic pump casing and a steel and urethane wear plate, it is what gives the Tsurumi LSC, LB480, LB800 and HS ranges their reputation for lasting years on building sites and in hire fleets. The LSC1.4S, for example, has a urethane rubber impeller running against a steel and urethane suction cover inside a polypropylene casing, which is how it pumps sandy water down to 1 mm without eating itself. EVAK use the same thinking with Hytrel, a flexible abrasion resistant polymer, on the Residox puddle pumps and Drainox site pumps, and Stream fit a polyurethane vortex impeller to the SPK530. Elastic impellers are also light, which matters on a pump that gets carried up a ladder. Their limit is heat and heavy impact: you will not find one on a big three phase slurry pump.
Grey cast iron
The workhorse material for sewage. It is hard enough to take hard solids, cheap to cast in the wide shapes a vortex or channel impeller needs, and heavy enough that nobody minds on a fixed installation. The EVAK Hippo range is built in cast iron and stainless steel around its recessed vortex impeller, the APP BCV (apart from the smallest BCV-400), the SV-400 and SV-750, the BBC Semisom and the Speroni Cutty all have cast iron impellers, and the APP DSPK goes one step further with a cast iron impeller carrying tungsten carbide on the cutting edge. It is also the standard impeller and volute material on the Honda WB20 and WB30 and the larger Tsurumi TE, TEF/TEW and TDS petrol pumps, where silt and sand in the suction water would wear an aluminium impeller away.
Ductile iron
Ductile (spheroidal graphite) iron is grey iron with its carbon formed into spheres instead of flakes, which makes it much tougher under shock. It is used where an impeller is both large and fast: the APP HD-15 has a ductile iron semi-open impeller, casing and suction cover, and the Tsurumi NK4-22 high head drainage pump has a ductile iron impeller inside a natural rubber casing. The HSD2.55S uses ductile iron for its pump casing for the same reason, with something harder still for the impeller.
Chromium iron
Chromium iron, often called high chrome white iron, is the hardest material in our range. Add chromium to iron and the carbon forms chromium carbides, which are harder than quartz, so sand cannot scratch them. It is brittle and expensive to cast and machine, so it only appears where abrasion is the main enemy: the Tsurumi HSD2.55S, where both the agitator and the impeller are chromium iron for sand carrying water, sludge and bentonite; the LB1500, the heavy duty model in the LB range; and the NK3-22L, the high head version of the NK.
Stainless steel
Stainless is not chosen for wear, it is chosen for corrosion. 304 stainless covers general clean and dirty water and damp, corrosive chambers: the Arven Mizar and Regal have 304 stainless impellers, the Speroni SXS is 304 throughout, and the stainless FEKA pump in our Iguazu FEKA station has a microcast steel vortex impeller for installations that sit unused between uses, where a cast iron pump is more likely to seize. 316, with added molybdenum, is the grade for seawater, saltwater and stronger chemicals: the Tsurumi 80SFQ is cast in 316 from impeller to motor frame, the Speroni SA(M) is 316 on every wetted part, and the Arven Mizar 60/S and Arvex/S bring 316 to smaller duties. Even 316 is not corrosion proof in seawater, so rinse the pump through with fresh water after use where you can.
Brass
Brass turns up on small surface pumps where the water is clean and a precise, non-rusting casting is more important than hardness. The peripheral impellers on the Speroni KP(M), KF(M) and AKM60 are brass, as is the open impeller on the GA(M) 100 and the BR versions of the CF(M).
| Material | Why it is chosen | Typical water | Examples from our range |
|---|---|---|---|
| GFRP / nylon / PPS / Noryl | Light, rust-proof, low cost, easy on the motor | Clean water, soft solids | Tsurumi OM5, POMA, 50PNI2; EGO 500; APP MH, MVH-10A, SV-150, SV-250; Umbra Pompe borehole pumps |
| Urethane / Hytrel / polyurethane | Elastic surface resists sand abrasion; light | Site and flood water with sand and silt | Tsurumi LSC, LB480, LB800, HS; EVAK Residox, Drainox, Trenchman; Stream SPK530 |
| Grey cast iron | Takes hard solids, cheap in wide shapes | Sewage, dirty water, engine pumps | EVAK Hippo; APP BCV, SV-400, SV-750, DSPK; BBC Semisom; Speroni Cutty; Honda WB20, WB30 |
| Ductile iron | Tougher under shock; suits big, fast impellers | Sandy water at high flow or head | APP HD-15; Tsurumi NK4-22 |
| Chromium iron | Hardest; chromium carbides resist sand and grit | Slurry, bentonite, abrasive dewatering | Tsurumi HSD2.55S, LB1500, NK3-22L |
| 304 stainless | Corrosion resistance in damp and dirty water | Chambers, basements, light effluent | Arven Mizar, Regal; Speroni SXS; Iguazu FEKA station |
| 316 stainless | Resists chlorides and chemicals | Seawater, saltwater, chemicals | Tsurumi 80SFQ; Speroni SA(M); Arven Mizar 60/S, Arvex/S |
| Brass | Precise, non-rusting castings | Clean water boosting and transfer | Speroni KP(M), KF(M), AKM60, GA(M) 100 |
The pattern inside a range: bigger pump, harder impeller
The most useful thing to notice is what happens when you move up a range. The impeller type stays the same, and the material steps up:
- The Tsurumi LB480 and LB800 have urethane impellers. The LB1500 switches to chromium iron.
- The Tsurumi NK4-22 has a ductile iron impeller. The NK3-22L, the high head version, uses chromium iron.
- The APP SV-150 and SV-250 use plastic impellers rated for soft solids. The SV-400 and SV-750 use grey cast iron and are rated for hard solids.
- The Arven Mizar 30 and 60 pass 10 mm solids. The Mizar VOX swaps to a vortex impeller and passes 20 mm.
The reason is physics. A bigger impeller turning at the same motor speed has a faster blade tip, so every grain of sand hits it harder. A higher head means more energy in the water and more energy in each impact. Past a certain size, urethane and plastic would wear or deform too quickly, so the material moves to iron, and past a certain head it moves to chromium iron. If you are comparing two models in the same range and one costs noticeably more for a modest gain in flow, the impeller material is often where the difference is.
What this means when you choose a pump
Work out what is in the water first, then let the impeller follow.
- Clean water, pumped high or far. Closed impeller, plastic or stainless. Borehole pumps and multistage surface pumps.
- Clean water, a floor or sump to empty. Open or vortex impeller in plastic. The Tsurumi OM5, POMA and 50PNI2, or the EGO 500 and APP RS32EA.
- Flood water on a floor. A puddle pump with a semi-vortex urethane impeller (Tsurumi LSC) or a vortex Hytrel or polyurethane impeller (EVAK Residox, Stream SPK530). See our puddle pumps.
- Site water with sand and silt. Semi-vortex impeller in urethane (Tsurumi LB and HS) or a Hytrel vortex (EVAK Drainox). If the silt has settled and needs stirring, add an agitator: the HSD2.55S, Trenchman or Agivort.
- Sewage and foul water with soft solids. Vortex impeller in cast iron, or channel for big flows. See our sewage pumps.
- Sewage with rags and wipes, pumped a long way. A cutter or grinder: the APP DSPK or Speroni Cutty.
- Slurry, bentonite, heavy grit. Chromium iron impeller with an agitator.
- Seawater or chemicals. 316 stainless, whatever the impeller type. See our saltwater pumps.
How to tell an impeller is wearing out
If you are not sure which type or material your water needs, use the Pump Finder, or browse all pumps. Every product page lists the impeller type and material under Construction and Durability.
Order online, or by phone. Order by 2pm for next working day dispatch.
