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Las razones para la velocidad especifica optima en local pressure falls below the vapor pressure of
la turbina de reaction son m&s complejas ya que hay the liquid at bulk temperature.67 In its initial
m&s variables. Para una mhquina de flujo radial (les stages cavitation is in the form of individual bubremitimos
a la figura 3): puede obtenerse una ex- bles which move with the flow into regions of
presion algo m&s simple. El influjo radial es higher pressure and collapse. The collapse
(44) VR = Cl sin al ‘/2gH
pressures are so intense that noise, vibration,
and erosion of metal surfaces ensues. When
Analogamente,
cia son
la tasa de flujo, velocidad y potencavitation
occurs over a large portion of the turbine
runner, reduced performance can occur.
(45) Q = VPA, = Cq sin al G fbnD2 E
r I
2$ Gg
(46) v, = D
Cavitation is not normally a problem with impulse
wheels. However, by the very nature of their
operation, cavitation is an important consideration
for reaction turbine installations.
The susceptibility for cavitation to occur is a
function of the installation and the turbine
design. This can be expressed conveniently in
terms of Thomas sigma
(47) P = qcgQH
(50) HDv
donde fb es el porcentaje de espacio abierto en 10s
OT = -
pasajes de entrada. Combinando las ecuaciones (45) a
H
(4’7), obtenemos la expresion para la velocidad wherein HDv is the net positive draft head and H
especifica
is the head available. The critical value of OT is a
function of specific speed as illustrated in Fig.
(48) N, = 25’4 (2TqfbCl sin 41
15.
As the specific speed increases, the critical
o, anroximadamente,
Vahe of OT increases dramatically. For minimization
of cavitation problems, the plant OT must be
(49) N, = 5.5 (Cl sin a1 : )A’2 +
in excess of the critical
This can have important
aTdenoted on the chart.
implications for turbine
Utilizando cuadros de diseno normalizados para las
settings and amount of excavation. This is easily
seen by referring to Fig. 14. An energy equation
turbinas Francis,4 N, cal normalmente en la gama de
between the turbine outlet and the tailwater can
0,3 a 2,5 (58 a 480 en unidades metricas).
be written
Cavitacih
La cavitation puede definirse coma la formation de (51) Pl V12 patm vc2
- + HL
la fase de vapor en un flujo cuando la presion local -r +zBc29=-+ Y 29
desciende por debajo de la presion de1 vapor de1 wherein HL is the head losses between the inlet
liquid0 a temperatura de volumen ,6. 7. En sus etapas and discharge of the draft tube. This can be
iniciales, la cavitation se produce en forma de rewritten as
burbujas individuales que se rnueven con el flujo a las
regiones de mas altas presion y se desploman. Las
presiones de desplome son tan intensas que se pro-
(52)
Pl
y
V12 pv
+ 29 - -=
Y
HD,, =
duce ruido, vibration y erosion de las superficies de1
metal. Cuando ocurre la cavitacidn a traves de una
portion grande de1 rotor de la turbina, puede ocurrir
P atm-pv
Y +
vc2
29
+ HL-ZB
una merma en el desempeiio. Normalmente, la
cavitation no representa un problema con las ruedas
de impulsos. Sin embargo, debido a la naturaleza inwherein
pv is the vapor pressure. The critical
value of HDv occurs when p1 = pv or HDv=
vl2/2g or when all the head available on the suctn’nseca
de su opera&n, la cavitacidn es una impor- tion side is in the form of velocity head. HDv is
tante consideration
de reaction.
para las instalaciones de turbinas approximately
La susceptibilidad de que ocurra cavitation es una
fun&n de la instalacion y el disefio de la turbina.
(53)
HDv
Patm
--Y---pv
--ZZg
Y
140