within the range of lifts anticipated at
the emptying valves were fully
regarding the cavitation index value
opened (actual b/B of 97 percent as
the project.
that is associated with incipient cavi-
explained previously) in 480 sec.
tation in unvented systems. A value
These simulations showed that
The field experiments found the
of 0.61 has been used by many and
although the steady-state pressures
83-sec valve time to be the project's
this value is substantiated by the pro-
downstream of the filling and empty-
normal valve opening time and the
totype study of Bay Springs Lock
ing valves were acceptable, pres-
480-sec valve time was taken from
(McGee 1989). The air vents down-
sures downstream of the emptying
the recommendations of the physical
stream of the valves at Lock 1 are in
valves were quite low during valve
model study. As the chamber water
questionable condition. Therefore,
operation. The lowest pressures
surface fell, the flow evolved to a
this study adopted maintaining a cav-
occurred when the valve was
steady state with flow in the intakes
itation index above 0.61.
between 50 percent and 70 percent
and out the discharge laterals.
open. These low pressures could
Valve Operation Optimization
Steady state was determined by a
lead to cavitation, the potential of
continuity check of the lock system.
The optimization software was
which is quantified using the cavita-
When the difference in discharge in
tion index, σ, expressed as:
programmed to run the model from
and out of the system was insignifi-
initial conditions of an empty lock
P + (Pa - Pv )
(2)
cant, the model was considered at
σ=
chamber with filling and emptying
V
steady state. The model results at
2
valves closed, to the steady-state
2g
steady state were then used to
condition with the valves fully open.
where, P is the gage pressure head
determine a discharge coefficient for
The operation constraints were the
at the top of the vena contracta of the
the lock system. A discharge-rating
maximization of the minimum cavita-
jet emerging from the partially open
curve for the lock system is provided
tion index below both the filling and
valve, Pa is the atmospheric pressure
in Figure 4. The discharge coeffi-
emptying valves during the estab-
head, Pv is the vapor pressure head
cient for the lock system is constant
lishment of steady flow. A rule was
of water, and V is the velocity in the
for the heads evaluated. The lock
imposed that eliminated the consid-
vena contracta of the jet emerging
discharge coefficient, which was
eration of any valve operations that
from the partially open valve. A value
determined to be 0.55, can be used
produced minimum cavitation
of 10.15 m was used for the term Pa -
to compute other head-discharge
indexes lower than 0.61. Operation
Pv. There has been much discussion
relations for flow through the lock,
rates of both the filling and the
Figure 4. Lock discharge rating curve, all tainter valves fully open and miter gates closed
41
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