What is the Differences Between Open Impeller, Semi-open Impeller and Closed Impeller?

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Impellers are generally divided into two categories: open and closed. The open impeller has a series of vanes attached to a central unit. This is done for the mounting on the shaft. However, this design is more sensitive to wear and tear of the blades. The closed impeller has a sidewall. An open impeller is less likely to get clogged and even if it does, it is easier to clean, whereas in a closed impeller, if stringy material or solids are pumped, the impeller can clog and it becomes really difficult to clean them. on the either sides (top and bottom) of the vanes. Also, the internal parts of a closed impeller are hidden hence it is difficult to cast and inspect for flaws whereas in an open impeller, all the parts are visible. It thus becomes easy to inspect for damage. 
An open impeller has vanes that are attached to a center hub and mounted directly onto a shaft. There is no wall surrounding the vanes which makes open impellers weaker than closed or semi-closed valves. Open impellers are normally faster and very easy to clean and repair. Normally small pumps are idle for open impellers. Besides, open impellers are the best at handling suspended solids and hence are idle for pumps that deal with such tasks.
 
Semi-open impellers have a back-wall shroud that adds mechanical strength to the vanes, whilst remaining open on the other side. They are somewhat of a middle ground between open and closed impellers in terms of efficiency and NPSHr, making them suited to medium sized pumps with a small amount of soft solids. It’s important to note with semi-open impellers that the clearance between the vanes and the pump casing needs to be small, as if too large slippage and recirculation will occur.
 
These consist of a back and front wall that surrounds the vanes, which increases their strength. These are specifically used in larger pumps and they are idle in applications that involve liquids and fluids with suspended solids.These are normally used when dealing with a clear liquid. They are not good when it comes to solids as they are so difficult to clean when they become clogged.