Open vs Closed Impellers

An impeller is either shrouded on both sides, on one side, or on neither. That single geometric decision changes efficiency, solids handling and how the pump wears.

The Flowsteer range uses all three: a full open impeller on the ANSI CN series, closed impellers on the ISO CI and VNP series, and a choice of closed or semi-open on the KCP.

Closed Impellers

A closed impeller has shrouds on both the front and back of the vanes, forming enclosed passages.

That geometry gives the highest hydraulic efficiency because leakage from discharge back to suction is limited by wear rings rather than by a running clearance.

The cost is solids handling. Anything fibrous entering the impeller has a shroud to catch on, and once a passage starts to block it does not clear itself.

Closed impellers are the right choice for clean and treated water where efficiency matters over years of continuous running.

Open Impellers

A full open impeller has no shrouds; the vanes run against the casing wall with a set clearance.

There is nothing for rag and fibre to wrap behind, which is why open impellers are chosen for waste and water treatment duties. The Flowsteer ANSI CN series uses a full open impeller for exactly this reason.

Efficiency is lower because the running clearance leaks, and that leakage grows as the pump wears. The compensation is that the clearance can be reset from outside the casing, restoring head without a strip-down.

Semi-Open Impellers

A semi-open impeller carries a shroud on the back only. It sits between the two in both efficiency and solids passage.

The Flowsteer KCP series offers closed or semi-open impellers so the selection can trade one against the other for a given effluent duty.

Choosing For Your Duty

Clean or treated water, continuous running, efficiency is the priority — closed.

Solids-bearing water, rag and fibre present, availability is the priority — open.

Effluent with moderate solids where efficiency still matters — semi-open.

Abrasive water — expect wear whichever you choose, and prefer a design whose clearance can be reset.

Maintaining an Open Impeller

Record the clearance set at commissioning. It is the reference for every later adjustment.

When delivered head falls without a process change, the clearance is the first thing to check.

Resetting the clearance is a far cheaper way to recover performance than replacing an impeller, and it can usually be done without removing the pump.