General Characteristics

Sigma As in pumps, the problem of cavitation also exists in turbines. Sigma is defined as follows:

where Ha = feet (meters) of atmospheric pressure minus vapor pressure

Hs = distance in feet (meters) the centerline of the blades is above tailwater for vertical units or distance in feet (meters) the highest point of the blade is above tailwater for horizontal units

H = feet (meters) of elevation between inlet head water surface and tailwater surface elevation

Critical sigma is defined as that point at which cavitation begins to affect the performance of the turbine. Figure 10 shows the relationship between critical sigma and specific speed and also shows which type of runner is best for a given specific speed. The impulse

*Universal specific speed Hs (defined in Chapter 1 and Section 2.1) is found from these turbine Ns-values as follows:

' 4.344Vht X sp. gr. 16.5642% X sp. gr. where ht is the turbine efficiency in percent.

TABLE 1 Turbine statistics of existing units

Speed, rpm

Power, hp"

Head, ft'

N, USCS"

Typed

Use6

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