Supposing you have an impeller and you want to design the casing for the impeller how do you go about it? How do you get the size? what parameters determine the dimensions?
Assuming you have some total dynamic discharge head figure in mind, you must first accelerate the fluid from a low static and low velocity head condition to a rapid enough velocity within and exiting the impeller (low static + high velocity head) such that, as you slow the fluid down as it enters the volute and finally exits at the discharge, the excess velocity head given up during the deceleration is converted to a total dynamic discharge head with a high static and a low velocity head components that equal the initial figure you had in mind.
"If everything seems under control, you're just not moving fast enough."
- Mario Andretti- When asked about transient hydraulics