Impeller

An impeller adds energy to the liquid, and the casing converts it into head.

Liquid enters at the impeller eye, in the centre. The vanes fling it outward as they rotate, so it leaves the vane tips fast. The volute, the spiral chamber wrapped around the impeller, widens as it goes, and that widening slows the liquid down. Slowing it down is what raises its pressure. A multistage pump does the same trick several times over, feeding one impeller's outlet into the next impeller's eye.

Two things about the impeller set the curve. Diameter decides how much head the pump can make at a given speed, and the vane passages decide how much flow it can pass. Speed moves both, which is why a variable speed drive shifts a centrifugal pump along its curve rather than replacing the curve.

The impeller eye is also where the pressure in the pump is lowest, and that is why it is where trouble starts. If the pressure there falls below the liquid's vapour pressure, vapour bubbles form and then collapse as the pressure recovers across the vane. That is cavitation, and the pitting it leaves shows up on the impeller before anywhere else.

Closed, open and vortex are the three shapes that cover most of what you will meet. That is also their order, falling in efficiency and rising in tolerance for solids.

The vortex is the one worth understanding properly, because it is the choice a wastewater specifier actually has to defend. A free-flow impeller sits back in a recess in the casing, out of the main path through the volute, so most of what you pump never touches a vane. Nothing in the path means nothing for a length of fibre to catch on and gather more behind it. You buy tolerance of the medium, and you pay for it in efficiency for the life of the installation.

Ebara publishes what that trade costs. The DW and DW VOX are the same pump on the same frames, and both pass 50 mm solids. The single-channel DW reaches 54 m³/h and 20 m of head. The vortex DW VOX, on the same range, stops at 48 m³/h and 15.7 m. Same motors, same passage, less pump.

So specify the vortex for the medium, not for the duty. It earns its place where the flow carries wipes, laundry fibre or gassy sludge, because a blocked pump costs a site visit and a lifted pump every time it happens, and that arithmetic beats the efficiency loss quickly. On grey water, seepage or a clean excavation, the closed or channel impeller is the right selection and the cheaper one to run.

Channel and screw centrifugal impellers are the solids-handling variations, and they take opposite routes to the same wet well.

A channel impeller is a proper hydraulic passage cast through the vane, usually one or two channels rather than the several vanes of a clean-water impeller. The medium is worked by the vane, so the pump holds a real duty point, and the passage itself is the published limit on what will fit. Calpeda's GM 50 range is a clean illustration of how the two approaches differ inside one pump family. The single-channel GMC passes 45 mm solids; the free-flow GMV takes 50 mm.

A screw centrifugal impeller is a single helical vane that stretches the passage out along the shaft, so the solid travels through in one continuous path rather than being flung across a chamber. Hidrostal built its range around it. The company specifies these pumps for wastewater and effluent with a high solids content, for large amounts of fibrous material, and for media that need handling gently. That last one is the part people miss. A Hidrostal submersible is the selection when the solids need to arrive intact rather than mashed, which matters upstream of a digester and matters again on a food product that must not be sheared.

Both are solids-handling pumps, and neither is a macerator. If a station is putting through more rag than any impeller was going to clear, the answer is screening or maceration upstream, not another impeller. The enquiry we decline most often is a like-for-like swap on a pump that already had the right impeller in it.

Trimming means machining the impeller down to a smaller outside diameter so the pump meets a fixed duty point instead of overshooting it. It works, and it costs you efficiency.

The wording is not ours. Commission Regulation (EU) No 547/2012, the ecodesign rule for water pumps, requires manufacturers to publish this with the pump. The efficiency of a pump with a trimmed impeller is usually lower than that of a pump with the full impeller diameter. Trimming adapts the pump to a fixed duty point, which reduces energy consumption, and the minimum efficiency index is based on the full impeller diameter. Because the regulation makes that wording mandatory in the product information, it turns up verbatim in data booklets across the trade, including for multistage ranges like the Grundfos CR.

Read it the way it is written. Trimming an oversized pump saves energy against running that oversized pump whole. It does not get you back to where you would have been had somebody specified the right pump in the first place. An oversized pump trimmed back is not the same machine as the right pump run at full diameter, and the gap is paid for every hour for twenty years.

This is why oversizing is the mistake worth catching at specification. Flow gets stated, the consultant adds margin for future duty, the contractor adds margin, and the installer adds a bit more. Nobody in that chain is careless, and the pump that comes out of it is far too big. Trim is the sticking plaster that gets applied afterwards.

Pick the impeller against the medium, then size the diameter against the duty point.

Clean water, boosting and multistage transfer take a closed impeller, and there is nothing to gain by paying for solids handling the medium will never test. Effluent and light solids take an open or semi-open impeller. Raw sewage and anything carrying rag takes a channel or vortex impeller in a submersible pump, sized on free passage as well as on flow and head. Where the solids have to arrive whole, or the product must not be sheared, that is the screw centrifugal. Above roughly 3–4% solids you have left centrifugal territory altogether and want a positive displacement pump, which has no impeller in it at all.

Whatever the shape, check the free passage against the largest solid the pump will actually see, not the one on the drawing, and check NPSHa against the pump's NPSHr at the duty point before anyone orders. The sewage and solids-handling pumps we supply cover the range from closed impeller to screw centrifugal. Send us the flow, the head, what is in the medium and the largest thing in it, and we will tell you which impeller the duty needs.