PetroStreet

What are the basis to apply the type of agitators within the unit operations?
It is service based selection?
What are the major planned maintenance for installed ones? And what particular signs of failures should be observed?

My limited knowledge tells me that it depends on the main process objective & fluid viscosity.

You can consider the following factors contributing to the selection as far as the process objective is considered:
Blending: Mixing two miscible liquids (e.g., fuel additives). Requires high flow.
Solid Suspension: Keeping catalysts or reactants from settling. Requires high bottom velocities.
Gas Dispersion: Breaking gas bubbles into a liquid (e.g., hydrogenation). Requires high shear.
Heat Transfer: Moving fluid against internal coils or a jacketed wall.
Won't be able to comment on the maintenance part of your query.

For installed units in a continuous process environment, maintenance focuses on the mechanical integrity of the drive train and the containment of the process fluid.
We tend to perform the following 4 maintain activities:
Lubrication Management: Frequent oil analysis of the gearbox to check for metal shavings or moisture.
Mechanical Seal Servicing: In pressurized vessels, the seal is the most common failure point. Planned replacement of O-rings and faces is standard.
Alignment Checks: Verifying the coupling alignment between the motor and the gearbox, and the gearbox to the agitator shaft.
Steady Bearing Inspection: If the shaft is long and uses a bottom "steady bearing" (bush), this must be checked for wear to prevent shaft "whip."