Pump Impeller Types: Open, Semi-Open and Closed Explained

When pumps are compared, the talk is of flow, head, power or material. But the part that really decides how a pump behaves — what efficiency it reaches, what NPSH it needs, whether it handles solids or clogs with them, how quickly it wears — is a single component: the impeller.

Choosing the right pump impeller type is one of the most underrated specification decisions. A closed impeller handling a liquid laden with crystals will clog; an open impeller on a clean acid transfer duty wastes energy for years. And in thermoplastic pumps for corrosive fluids there is a further point that is almost never explained: the clearances of a plastic impeller do not behave like those of a metal one.

In this guide we explain what the impeller is and how it works, the real differences between an open impeller, a semi-open impeller and a closed impeller, the role of clearance, and how to choose the right geometry according to the suspended solids and the type of fluid — in other words, how to get solids handling right from the specification stage.

The impeller is the hydraulic heart of the centrifugal pump. It is mounted on the shaft and turns with the motor. The liquid enters at its centre, known as the impeller eye, and the vanes force it to rotate and move outwards towards the periphery. Along that path the liquid gains velocity and pressure thanks to centrifugal force.

On leaving the impeller, the liquid enters the volute, the spiral casing that surrounds it, where much of that velocity is converted into pressure before it reaches the discharge flange. The division of labour is clear: the impeller supplies the energy and the volute puts it in order. That is why impeller and casing are designed together, and why the clearance between them matters so much.

Everything the pump can do — the head it develops at a given flow, the efficiency with which it does so, the suction pressure it needs to avoid cavitation — depends on the geometry of the impeller: its diameter, the number and angle of the vanes, the size of the eye and the presence or absence of shrouds.

Although there are many variants, almost all centrifugal impellers share the same elements:

The presence or absence of these two shrouds gives rise to the classic classification we set out below: with two shrouds the impeller is closed; with one, semi-open; with none, open.

Impeller diameter is the variable a manufacturer uses to adapt the same casing to different duty points. Reducing the diameter — known as impeller trimming — shifts the curve downwards in a predictable way, according to the affinity laws. As an approximation, valid for moderate trims:

Trimming is an excellent way to match the pump to the actual duty point of the installation instead of wasting energy across a partly closed discharge valve, but it has limits: each design allows a minimum trim, below which efficiency drops and recirculation and vibration increase.

The impeller also sets the NPSH required. The geometry of the eye and of the vane leading edge determines how much pressure the liquid needs in order not to vaporise as it enters. If you want to go deeper into that calculation, we explain it in our guide NPSH Explained: NPSHa vs NPSHr and How to Calculate Both. And when the impeller runs far from its design point, recirculation and cavitation appear; we describe the damage they cause — pitting on the vanes, loss of profile at the leading edge — in our article on pump cavitation.

In a closed impeller, the vanes are enclosed between the back shroud and the front shroud, forming closed channels through which the liquid flows. This construction prevents the liquid from ‘slipping’ over the tips of the vanes and gives the highest hydraulic efficiency of all the types.

Sealing between the high-pressure zone and the suction is not provided by contact between the vanes and the casing, but by a small radial clearance at the impeller inlet, usually through wear rings. As long as that clearance is maintained, efficiency remains stable over time; as it wears, internal recirculation increases and the pump loses capacity, but the ring can be replaced without changing the impeller.

Its advantages are clear: better efficiency, high heads, smooth running and lower axial thrust. So is its main limitation: the closed channels have a limited free passage and clog with large solids, fibres or sludge. It is the reference impeller for transferring acids, alkalis, brines, demineralised water and, in general, clean liquids or liquids with very fine particles in low concentration.

The semi-open impeller keeps the back shroud but dispenses with the front one. The vanes are exposed on the suction side and rotate very close to the casing wall or to a wear plate. Sealing is now provided by a very small axial clearance between the edge of the vanes and that wall.

This geometry has two consequences. On the one hand, the passage between the vanes is more accessible and the impeller copes much better with suspended solids, crystals and light slurries, with less risk of blockage. On the other, efficiency depends directly on the axial clearance: as wear opens it up, more liquid recirculates over the vanes and the pump loses head and efficiency. Many process pumps with semi-open impellers allow that clearance to be readjusted externally to restore performance.

The open impeller goes one step further: it has no shroud at all, or only a small reinforcing web, and the vanes are attached directly to the hub. It is the type most tolerant of solids, fibres and liquids with entrained gas, and the easiest to clean and inspect. In return, it has the lowest efficiency, is the most sensitive to clearance wear and is the least robust mechanically, so it is reserved for small pumps or for duties where blockage is the main risk.

There is a fourth option for extreme cases: the vortex (recessed) impeller, housed in a recess in the casing out of the main flow path. The impeller creates a vortex that carries the fluid along without most of the solids ever touching it. Its free passage is the largest possible, ideal for large, fibrous or fragile solids, but its efficiency is clearly lower than that of the other types.

The following table summarises the practical differences between the four impeller types:

Clearance deserves a separate comment. In metal process pumps with semi-open impellers, the running axial clearance is typically of the order of a few tenths of a millimetre, and every tenth that wear opens up translates into lost head and efficiency. In thermoplastic pumps an additional factor comes into play: plastics expand with temperature between 4 and 12 times more than stainless steel.

Indicative values: the exact coefficient depends on the material grade and the temperature range. In an all-thermoplastic pump, impeller and casing expand in a similar way, but the shaft and the bearing bracket are metal and axial expansion is not the same. That is why the clearances of a plastic pump are designed around the operating temperature, and why you should always state the actual maximum temperature when ordering a pump, not just the nominal one.

Impeller selection always starts with the fluid. First rule out what will not work, then optimise efficiency among the options that remain. This table is a starting point:

Three further criteria complete the decision. The maximum solid size sets the minimum free passage the impeller must have. The concentration and hardness of the particles determine the rate of wear and, with it, the material: with abrasive solids, the winner is not the most chemically inert plastic but the toughest. And the rotational speed: with abrasive fluids, a larger pump running more slowly usually lasts much longer than a small pump at high speed, because wear increases very rapidly with the impeller tip speed.

In thermoplastic pumps, the impeller must also be chemically resistant to the fluid and transmit torque without deforming. Across the Tecnium Pumps range of plastic pumps, torque is transmitted to the impeller by a key and a large-surface steel core, and the shaft sleeve is removable and independent of the impeller. Depending on the application:

These applications are common in the mining industry and in waste water treatment, where the fluid is rarely clean and impeller geometry determines the service life of the whole unit.

Not sure which impeller your application needs? Send our technical team the fluid and its concentration, the maximum temperature, the flow and head, and details of the solids: maximum size, concentration and whether they are abrasive or fibrous. With that information we will propose the right pump, material and hydraulic configuration. Request your free quote.