How do pumps behave when operated in parallel or series?

Parallel operation: flows add at same head - used to increase capacity or provide redundancy. Combined curve is horizontal sum of individual curves. Series operation: heads add at same flow - used for high head requirements. Combined curve is vertical sum. Parallel considerations: check valves prevent backflow through idle pump, pumps should have similar characteristics to share load, and operating point stability. Series: first pump must have adequate NPSH, second pump handles higher suction pressure.