Closed impellers are used to generate higher pressures by limiting flow, and are only effective at higher velocites. They can be noisy and need to be designed carefully. They are also very expensive to mold and fluid sensitve.
We use a close impeller on one of our OEM units, more do to the higher viscosity of the fluid this pump runs then for pressure.
Pressure equals rpm x diameter, and curved impellers REDUCE max possible pressure. You curve impellers to help match motor load / flow numbers.
It is a waste to pump fluid once you hit your flow requirement, so you angle blades to reduce motor load.
I have been doing impeller / load matching for about 6 months now, so I have some idea how it works
I think the senior engineers have me do it because they do not want to do all that math.
The process is Anolok, and there is no exposed surfaces now, except the one bearing chamber which needs the strengh of AL
Dave