Pumps and turbines

Pumps add energy to water; turbines take it out. Both are rotodynamic machines in civil works: centrifugal pumps lift water for supply and drainage, and turbines in hydropower stations convert the head of a reservoir into electricity. The same basic relations describe both, with energy flowing in opposite directions.

A pump is chosen by matching its characteristic curve, head against discharge at a given speed, to the system curve, the static lift plus friction losses that rise roughly with the square of the flow. The intersection is the operating point, which should lie near the best efficiency point. The power needed equals the weight of water lifted per second times the head, divided by efficiency. Pumps in parallel add flow; pumps in series add head.

Affinity laws scale a pump's performance with speed: discharge varies with speed, head with its square and power with its cube. That is why variable-speed drives save so much energy in systems with varying demand. Specific speed, a combination of speed, discharge and head, classifies impellers into radial, mixed and axial flow types and, for turbines, separates Pelton wheels for high head, Francis turbines for medium head and Kaplan turbines for low head.

Cavitation sets the limit. If pressure at the impeller eye falls to the vapour pressure, bubbles form and collapse violently, pitting the metal and cutting performance. The net positive suction head available at the site must exceed that required by the pump with a margin, which controls how high above the water a pump can sit.