External gears hold the two screws in phase so they never touch each other. That removes the requirement for the fluid to lubricate screw-to-screw contact, which is what lets a timed twin-screw pump handle poor-lubricity fluids, some entrained solids and higher speeds.

Fluid enters at both ends of the screw set and travels inward to a central discharge, so the axial thrust generated by each half opposes the other and cancels. This removes the need for a large thrust bearing and is why twin-screw pumps handle high differential pressure reliably.

Yes - this is one of their distinguishing capabilities. Twin-screw pumps handle high gas fractions without vapour-locking, which is why they are used for multiphase crude, cargo stripping and any duty where the pump must keep working as the liquid runs out and gas is drawn in.

Twin-screw handles a wider viscosity range, poorer lubricity, some solids and gas, and runs faster - at higher cost and complexity because of the timing gears. Triple-screw is simpler, quieter and cheaper but relies on the fluid for lubrication, so it needs clean, lubricating fluid.

A wide one, including fluids whose viscosity changes substantially with temperature, which is part of why they suit heavy fuel oil and bitumen. Very thin fluids reduce volumetric efficiency through slip, but the timed design tolerates them far better than an untimed pump would.

Better than triple-screw pumps but still not a slurry pump. Some entrained solids are tolerable, particularly in timed designs specified with generous clearances, but sustained abrasive duty opens those clearances and costs both capacity and pressure. Filtration remains worthwhile wherever it is practical, and where the fluid is genuinely abrasive a peristaltic or progressive cavity pump will outlast a screw pump considerably.

Crude oil and multiphase transfer, heavy fuel oil, bitumen and asphalt, polymers, and marine cargo and tank stripping. The common factor is a difficult fluid - viscous, gassy, variable or poorly lubricating - combined with a need for high pressure and reliable continuous operation.