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A medida que aumenta la velocidad especifica, 10s
valores ctiticos de o T aumentan espectacularmente.
Para minimizar 10s problemas de la cavitation, la
planta o T ha de exceder la (3 T criitica denotada en
el cuadro. Esto puede tener repercusiones impor-
tantes para la instalacion de la turbina y la cantidad
de excavation que habra que efectuar. Esto puede
verse facilmente refiriendose a la figura 14. Se puede
formular una ecuaeion de energia entre la salida de
la turbina y el agua de descarga
(51)
Pl V12 Pam vc2
y +ZB+29=v+-
29
+ HL
donde HL son las perdidas de la caida entre la en-
trada y la descarga de la galaetia de suction. Esto
puede reformularse de la siguiente manera:
(52)
Pl V12 pv
y + z-7 =
Patm Pv Vc2
-+
Y + 29
{Dv =
HL-ZB
donde pv es la presion de1 vapor. El valor ctitico de
HD, ocurre cuando P1 = Pvo HJ-J~ = V12/2g o cuan-
do toda la caid;a disponible en el lado de la suction
est6 presente en forma de &da de velocidad. HD, es
aproximadamente
(53)
Patm pv
--ZB
HDv Y--
Y
asi, pues, el reglaje maxim0 de la turbina por encima
de1 agua de descarga es
Patm Pv
(54) ZBmax = y-y - *Tc H
donde 0~~ se obtiene a partir de la figura 15. ~~~~
puede ser un valor negativo, lo que significa que la
turbina ha de instalarse por debajo de la elevation
de1 agua de descarga.
A titulo de ejemplo, consideren una maquina de 500
kW que opera a 500 rpm bajo una &da de 10
metros. La velocidad especifica es de 3,8. Esta ser&
una turbina de flujo axial que tiene unit UT critica de
0,9 aproximadamente. Al nivel de1 mar, el reglaje
maxim0 de la turbina setia
ZBm
= 10 - 0.09 - (0.9 x 10) = 1 meter
Si se instalara la misma turbina en el lugar de esta
conferencia (Quito, Ecuador, elevacidn m 3000 m), el
The specific speed is 3.8. This will be an axial
flow turbine having a critical or of about 0.9. At
sea level, the maximum turbine setting would be
ZBm
= 10 - 0.09 - (0.9 x 10) = 1 meter
If the same turbine was installed at the site of
this conference (Quito, Ecuador, elevation m
3000 mj, the maximum turbine setting would be
zB = 6.7 - 0.09 - (0.9 x 10) = -2.3 meters
Considerable excavation would be necessary.
Thus, cavitation can be an important considera-
tion.
Speed Regulation
The speed regulation of a turbine is an impor-
tant and complicated problem. The magnitude of
the problem varies with size, type of machine and
installation, type of electrical load, and whether
or not the plant is tied into an electrical grid. It
should also be kept in mind that runaway or no-
load speed can be higher than the design speed
by factors as high as 2.5. This is an important
design consideration for all rotating parts, in-
cluding the generator.
It is beyond the scope of this paper to discuss
the question of speed regulation in detail.
However, some mention of this should be made
since much of the technology is derived from
large units. The cost of standard governors is
thus disproportionately high in the smaller sizes.
Regulation of speed is normally accomplished
through flow control. Adequate control requires
sufficient rotational inertia of the rotating parts.
When load is rejected, power is absorbed, ac-
celerating the flywheel and, when load is applied,
some additional power is available from deac-
celeration of the flywheel. Response time of the
governor must be carefully selected since rapid
closing time can lead to excessive pressures in
the penstock.
A Francis turbine is controlled by opening and
closing the guide vanes which vary the flow of
water according to the load. A powerful governor
is required to overcome the hydraulic and fric-
tional forces and to maintain the guide vanes in
fixed position under steady load. On the other
hand, impulse turbines are more easily controll-
ed. This is due to the fact that the jet can be
deflected or an auxiliary jet can bypass flow from
the power producing jet without changing the
flow rate in the penstock. This permits long delay
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