Open, semi-open and closed
Shrouds on both sides, one side, or none — and why dirty water changes everything.
⏱ 5 min
- Everyday comparisonStep 1 of 6
The sandwich test
Think of the vanes as a sandwich filling. A closed impeller has bread on both sides. A semi-open one has only the bottom slice. An open one has no bread at all — just the filling on a stick.
The “bread” is the . How much bread you use decides how clean the water must be.
- Try itStep 2 of 6
Switch between the three
Change the Type and spin the impeller around. Look at how the vanes are covered — or not.
Challenge: On the closed impeller, turn on the see-through shroud. Could a long rag get caught inside those channels?
- ExplainedStep 3 of 6
Closed: most efficient
In a the water flows through sealed channels, so very little slips past the vanes. It is the most efficient type and the most common in clean-water pumps.
Downside: rags, hair and stringy things can wrap around the vanes inside the channels, where you can’t reach them. That is called .
- ExplainedStep 4 of 6
Open: tough and easy to clean
An has nothing covering its vanes, so solids pass through and it is easy to clean. It is also cheap to make.
Downside: water leaks over the tops of the vanes through the gap (the ) between the vanes and the casing. As that gap wears bigger, the pump gets weaker.
- ExplainedStep 5 of 6
Semi-open: the compromise
A keeps the back shroud for strength but leaves the front open. It handles some solids and fibres and is easy to inspect.
Many semi-open pumps let a mechanic adjust the gap as it wears, getting the performance back without new parts.
- The impeller connectionStep 6 of 6
Where each one lives
Closed: water supply, heating, fire fighting — clean water. Semi-open: wastewater, paper pulp, food. Open: small drain pumps, slurries, and many compressors and fans.
📌 Remember this
Closed impellers (shrouds on both sides) are the most efficient but can clog. Open impellers handle solids and are easy to clean but leak more. Semi-open sits in between.