What is pump priming and why is it necessary?

Priming is filling the pump casing and suction line with liquid before starting. Centrifugal pumps cannot pump air (air is too compressible) - they need liquid present to develop pressure. Self-priming pumps have special casing design to retain liquid for re-priming. Priming methods: foot valve in suction line (maintains liquid), vacuum priming systems, flooded suction (liquid level above pump), and manual filling. Failure to prime causes no flow, overheating, and potential seal damage.