P2

NlDl ND2

Most designs used are tested as exact homologous models, and performance is stepped up from the model by the normal affinity laws given above. Because of difficulties in measuring large flows at the field installation, only approximate or relative flow metering is normally done.

FIGURE 11 Specific speed versus Specific speed N, (SI) = 3.814n, (USCS)

Speed Characteristic This is defined as the peripheral speed of the runner divided by the spouting or free discharge velocity of the water:

where f = speed characteristic u = peripheral speed of runner, ft/s (m/s) H = head, ft (m) g = 32.2 ft/s2 (9.807 m/s2)

A plot of f versus speed can be seen in Figure 11, which shows that f becomes constant at after the specific speed has dropped into the impulse-runner region. Theoretically, for maximum energy conversion, f should equal 0.5 for impulse runners. However, because of small losses in the runner, this value is set at approximately 0.46. Control of the hydraulic turbine can be accomplished by means of a governor or a flow or load control system.

Torque All hydraulic turbines have maximum torque at zero speed; therefore, they have ideal starting characteristics. The pump can be accelerated to design speed by gradually opening the turbine gates or inlet valve while keeping within the limitations of hydraulic transients.

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Survival Treasure

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