Several impellers in series on one shaft, each adding its share of head to the same flow. That produces pressures a single impeller could only reach with extreme diameter or speed. Flow is set by the impeller size and speed; head is multiplied by the number of stages.

The casing parts along the shaft centreline, so the whole rotating assembly can be lifted out for inspection or overhaul without disturbing the suction and discharge pipework. On a large pump that turns a major pipework job into a lifting job, and it is the main reason split case pumps are specified for critical duty.

Mostly on space and access. Vertical inline pumps sit in the pipe run and take almost no floor area, which suits plant rooms. Horizontal pumps need a footprint but are easier to reach, align and maintain. Vertical turbine pumps are chosen for a different reason entirely - suction conditions.

Net positive suction head - the margin between the pressure available at the pump suction and the vapour pressure of the liquid. If available NPSH falls below what the pump requires, the liquid boils at the impeller eye and cavitation follows, which erodes impellers rapidly. It is the most common cause of premature failure in this family.

Because running far from it costs energy and life. Well to the right of best efficiency the pump cavitates and overloads; well to the left it recirculates internally, which produces vibration and bearing and seal damage. A pump meeting its duty at the end of the curve is a pump that will not last.

Yes, and variable speed drives are common because the affinity laws make it very effective - power falls roughly with the cube of speed, so a modest speed reduction saves a large proportion of the energy. It also lets one pump cover a range of duties rather than throttling against a valve.

Clean and lightly contaminated liquids, principally water, treated water, condensate, and compatible process fluids. They are not solids-handling pumps: the close running clearances that make them efficient are exactly what abrasive particles destroy, and grit shows up quickly as lost head.

As many as the required head divides into per-stage head at the chosen speed and impeller size - it is an output of the selection, not an input. More stages mean a longer machine with more shaft deflection to manage, so a design achieving the head in fewer stages is generally preferable if it meets efficiency and NPSH.

Wear ring clearance, almost always. The rings form the seal between the high-pressure discharge side and the low-pressure suction eye, and as they wear an increasing proportion of the flow recirculates internally rather than leaving the pump - so the pump works as hard as ever and delivers less.

Generally no. Standard centrifugal designs must be filled and vented before starting, and running one dry damages the seal within moments. Where suction lift is unavoidable, either a self-priming design, a foot valve and priming arrangement, or a vertical turbine pump is the correct answer.