Small Decentralized Hydropower Program National ... - Cd3wd.com

damente por la siguiente formula:
H, = 0.72 VT (1)
donde V es la velocidad de entrada de1 can0 (metros
por Segundo), D es la altura de la compuerta
(metros). La sumersidn H, (metros) se mide desde la
superficie de1 agua hasta la corona de1 Cairo.
Las caiierias de presidn se construyen nor-
malmente de acero, pero para facilidades pequenas se
pueden considerar varillas de madera, fibras de
vidrio o caCos de pohvinil-cloruro. El espesor de las
paredes y el diametro de las canen’as de presion
pueden hacerse optimos teoricamente si se considera
el costo de1 material y el costo de la energia perdida
por la resistencia al fhujo. En realidad, si se quiere
ahorrar en la manufactura por lo general se resulta
en una medida uniforme e igual densidad de las
paredes para una facilidad pequena. Se deben
suministrar bloques de soporte en todos 10s lugares
en que las caiierias de presion cambien de ilireccion
fuere lateral o verticalmente, para contrarrestar la
presion desigual y las fuerzas de cambio de1 impulso
(normalmente las fuerzas debidas a la presidn son
mayores). Las carietias de presion se deben disenar
corn0 para resistir las presiones estiticas totales m&s
las presiones transitorias que se deban a cambios
subitos de la carga. Las presiones transitorias max-
imas en aumento o disminucion han de resuitar por
causa de subito cierre o apertura respectivamente de
las compuertas de mariposa de la turbina o de las
valvulas de control. En la practica las velocidades
maximas en las carierias de presion son de 2.5 a 3.5
metros por segundo. La presion maxima de cambio
se da aproximadamente por la siguiente fomula:
AP = PAV, (2)
donde P es la densidad de1 agua, AV es el cambio en
velocidad producido en la caneria de presion, y c es
la velocidad de1 sonido en la caneria de presion (nor-
malmente 120011500 metros por Segundo). AP sera
el sign0 algebraic0 asociado con AV. Para las
canen’as de presidn m&s largas y m&s grandes, setia
an tieconomico utilizar paredes de espesor suficiente-
mente grande coma para soportar dicha suba de la
presion. Un tanque de pulsation, colocado tan cerca a
la turbina coma sea posible, ha de suministrar
liberacidn de la presibn de manera economica, si la
pared de1 canon es suficientemente empinada. Si
dicha pared de1 canon tiene un declive pequeno, el
costo de un tanque de pulsation seria prohibitivo y se
debera colocar una vjlvula de destio simultineo cer-
ca de la turbina. La Figura 3 ilustra un instalacion
tipica.
173
facility. Thrust blocks must be provided wherever
the penstock changes direction eit?,er laterally or
vertically in order to counteract unbalanced
pressure and momentum-change forces (normally
pressure fcrces are much the larger). The
penstock must be designed to resist total static
pressures plus transient pressures arising due to
sudden load changes. The maximum transient.
pressure increase or decrease will result from
sudden closure or opening respectively of turbine
wicket gates or control valves. In practice, max-
imum velocities in the penstock are between 2.5
and 3.5 meters/second. The maximum pressure
change is given approximately by:
AP = eAVc (2)
where e is the density of water, AV is the change
in velocity produced in the penstock, and c is the
speed of sound in the penstock (normally
1200-1500 meters/second). AP will have the
algebraic sign associated with AV. For longer and
larger penstocks, it will be uneconomical to
utilize a penstock wall thickness sufficiently
large enough to withstand such a pressure rise.
A surge tank, located as near the turbine as
possible, will provide pressure relief economically
if the canyon wall is steep. If the canyon wall has
a flat slope, cost of a surge tank may be pro-
hibitive and a sirnultaneous bypass valve must be
located near the turbine. Figure 3 shows a typical
installation.
In theory, the bypass valve must opened slowly
before the turbine is started. Once flow has been
established in the penstock, the turbine is started
with the valve being closed simultaneously main-
taining a constant rate of flow In the penstock.
For load-reduction operation, flow through the
turbine is reduced while the valve is
simultaneously closed maintaining a constant
flow rate in the penstock. Mechanical coupling of
the valve and the turbine control is simplest, but
electronic controls provide more advantageous
operation. To conserve water, valve controls can
close the valve slowly after equilibrium operation
conditions have been reached. If water wasting is
acceptable, the valve setting can :emain cons-
tant until another load change is required. Pro-
perly designed Howell Bunger valves3 provide the
most economical bypass valves for larger plants.
Any quick-acting valve can be used for smaller
plants.
Air vents should be installed at the upstream
end of the penstock to limit negative pressures
produced by accidental closure of the upstream