~10
.
Jin Feng Jiang Bole Chen Jian
Yangtze River Scientific Research Institute(YRSRI), Wuhan 430010, China
Tel. & Fax: 027 82633828, E-mail: Jinfeng-8@163.net
Abstract: Practice shows that when working head is over 80m, common valve sealing cannot function normally. For this reason, tainter valve with eccentric hinge or hydraulic sealing is adopted. In this case, the both sidewalls of valve stand laterally broaden, and the bottom vertically drops. By means of physical modeling, numerical modeling and prototype survey, cavitation characteristics of valve stand with drop step and lateral offsets was studied. Physical models consisted of a 1:30 vacuum tank model of the bottom outlet at the Longyangxia Power Station, a 1:30 vacuum tank model of the deep outlet at the Three Gorges Project (TGP), and a 1:20 open flume model of the deep outlet of TGP. Weakly compressible method and large vortex modeling were applied in three dimensional numerical modeling. The equations of conservation of mass and conservation of momentum were dispersed in finite-volume method on the bound grid of hexahedron, so as to satisfy the computation in complicated flow channel.
The research indicates that, for the valve stand with drop step and lateral offsets, the characteristics of areation and pressure at bottom of chute is similar to that for areator only with drop step, but broadening water jet produces high pressure gradient and intensive pressure fluctuation on the sidewalls, where areation concentration is low, so that cavitation is liable to happen.
To eliminate cavitation on the sidewalls, application of side deflectors should be avoided . When width of lateral offsets is not large enough, it is recommended to reduce the top slope at the outlet and to shorten the horizontal length of lateral offsets.
Cavitation,
produced by the lateral offsets, can be eliminated through configuration
optimization. Nevertheless, as areation concentration is low and velocity of
water jet is high, it is necessary to control the irregularity of wall surface
strictly within 20m downstream of the valve stand, to prevent irregularity
cavitation damage.
Keywords: valve stand, lateral offsets, drop step, side deflectors, top slope, small bucket, cavitation damage, areation concentration, underwater noise, irregularities
Along with the development of high dam construction, the problem of valve sealing attracts more and more attention from designers, operation mangers and researchers. Tainter valve with eccentric hinge or hydraulic sealing has been proved to be a good way to solve the problem. There are more than 20 projects where these sealing measurements have been employed. Most of the projects operate very well, but damages occurred in a few of them. There were different opinions of the cause.
For both tainter valve with eccentric hinge and hydraulic sealing, it is required that both sidewalls of valve stand laterally broaden and the bottom vertically drop (as shown in Fig.1). Immediately downstream of the valve stand, not only a bottom cavity but also two side cavities form. With reasonable design, the demands both on valve sealing and on chute areation can be meet. If not, cavitation damages, vibration of valve structures and other hydraulic problems will be induced.
With collected operation material from constructed projects, general experiences in design were summed up, and project damages were primarily analyzed. Based on this, scale model tests in vacuum tank and in open flume, and three dimensional numerical analysis were carried out, aimed at the cavitation characteristics downstream of the valve stand and measurement to eliminate the cavitation.
Since end nineteen fifties, sealing by tainter valve with eccentric hinge or by hydraulic system has been adopted and popularized. According to project operation data collected, most projects with drop step and lateral offsets form of valve stand have operated very well, but in a few projects, erosion was found out downstream of the valve stand. Most damages occurred at the area with some kinds of irregularities, such as residual concrete mortar, step of construction joint, end of reinforcing bar and etc. (e.g the flood release outlet of the Tabeila Power Station of Pakistan[1] and the bottom outlet of the Longyangxia Power Station of China[2]). Some damages happened at sidewalls near the bottom edge of water jet immediately downstream of the valve stand (e.g the Kelasloyalske Power Station of Soviet Union).
Primary analysis shows that in addition to the irregularities of walls, main reason of the damages mentioned above is lack of areation, which leads to shortening or blocking up of bottom cavity, so as to induce shearing cavitation between the main flow and bottom return flow and supply little air for the chute downstream.
According to the engineering practice, the essential condition for safe operation of the valve stand is forming a flow pattern with free jet (i.e in operation condition, both the bottom cavity and the side cavities are complete and through). To get the flow pattern, configuration of the valve stand should be suitably defined, or, when no alternative is available, the operation head should be limited. To choose the dimension of the valve stand reasonably, effects on hydraulic characteristics of different parts of the configuration must be analyzed at first.
Dimension of the drop step should fulfill the requirement for areation in addition to that for valve sealing structure. Choice of height of the step must be combined with that of the chute slope downstream. With steep slope, the step can be small, or else the step should be relatively larger. The height d of the step is generally within 0.6m~2.0m, with d/h at 0.09~0.29.
Size of lateral offsets should meet need of sealing installation and ensure that flow detaches itself from both sidewalls immediately downstream of the valve stand, forming two side cavities linking up the bottom cavity. In engineering practice, there exist two kinds of lateral offsets: large lateral offsets and small lateral offsets, with relative width b/B=0.3~0.4 and b/B=0.05~0.17 respectively [3]. For large lateral offsets, side cavities are always same as or even larger than the bottom cavity, and after falling on the chute bottom, water jet will widen violently, inducing very high water wings on both sidewalls of the chute and water crown downstream, leading to a very confused flow pattern. In contrast to this, by small lateral offsets, water jet links up the sidewalls smoothly, with much better flow pattern in the chute downstream.
By setting small bucket on the step, excavation
can be reduced in the same condition of areation. Specially when choice of the
height of step and the slope of chute is limited, it is convenient to form a
stable bottom cavity with the small bucket. But, with small bucket, lateral
broadening of water jet will be intensified, which leads to shortening of side
cavities and rising of the water wings, so a comprehensive consideration should
be given. The height of the small bucket is generally limited within 0.1m~0.2m
and the elevation angle within 5
~10
.
Slope i of the chute bottom is most important in aerator configuration. Increasing of the slope will lead to enlarging of the bottom cavity and dropping of the critical head (i.e. minimum head with flow pattern of free jet). But too steep slope should be avoided to prevent intensive shock wave forming in the chute.
For a few constructed projects with the valve stand with drop step and lateral offsets, although the essential condition mentioned above had been satisfied through model tests at design stage, yet damages occurred in the chute. Because there have been ripe experiences at drop step areator, the lateral offsets are suspected possibly to be source of the damages.
Aimed at suspicion that lateral offsets can be the source of the damages, a 1:30 vacuum model test of bottom outlet at the Longyangxia Power Station was at first carried out (The model is shown in Fig.2). The exit of the bottom outlet is 5m in width and 7m in height. An eccentric tainter valve has been adopted, with lateral offsets of 0.6m in width and drop step of 2m in height. It was reported that the construction quality at flow boundary of the chute was not good and there existed irregularities of more than 5cm in dimension on. After first passage of flood in 1987 with head of 54.5m and lasting of operation of 5417h, inspection revealed that tiny damages occurred in different parts of the chute. Damages were distributed mainly at the construction joints and downstream of them. With no recovery processing, flood was released through the bottom outlet again in 1988. The passage of the flood lasted 137h with maximum head of 46.7. In 1989, flood was released for 1583h with maximum head of 89m and maximum flow velocity of 36m/s. A survey showed that the damages occurred before had been seriously extended.
The model tests indicated that in operation head of 45m~90m, side cavities could keep steady and through, with minimum length of 6m~7m and little areation effect. On sidewalls in the scope of about 50m downstream of the valve stand, areation concentration reached about 1%. Tests also showed that side cavities were large enough to meet the demand of areation to the bottom cavity even without areation pipes downstream of the valve stand.
In order to detect cavitation, measurements of underwater noise were conducted. The survey revealed that (1) for the configuration of construction design, no cavitation was discovered at the places with damages in prototype (downstream of Do+85), but with modeling of concave slot of 1.6cm in width or rising step of 5cm in height at the places with damages, cavitation occurred obviously, (2) in the dashing area on sidewalls within the scope of 10m from the valve stand (where no damage occurred with steel lining in prototype), cavitation happened, which intensified a little along with rising of head, and was in the stage of cavitation inception with head of 90m, (3) cavitation did not varied at the dashing aera and downstream of the irregularities, when the areation pipes at both sides of the chute were elliminated.
It can be concluded that (1) the damages in prototype were reduced by irregularities , (2) in operation condition, stable side cavities could form immediately downstream of the valve stand, but cavitation happened at the dashing area of sidewalls in a small scope, (3) for cavitation with slight extend at the dashing area, damages could be prevented by controlling irregularity on the flow boungary.
For the sake of finding out the cavitation characteristics and optimization measurements of the valve stand with drop step and lateral offsets, a 1:30 model test in vacuum tank was completed in combination with the configuration optimization of deep outlet of TGP.
Analysis based on the researches so far reveals that in addition to the outlet dimension and the width of lateral offsets, which are always restricted, main factors affecting the hydraulic characteristics downstream of the valve stand are as follows: horizontal length of lateral offsets, top and bottom shrinkage ratio of the outlet, configuration of side deflectors at the outlet and so on.
In the vacuum model test, average pressure, fluctuating pressure, underwater noise and areation concentration were surveyed and analyzed, showing that (1) the maximum average pressure on the dashing area of sidewalls were about equal to that on the bottom of chute, but pressure gradient and fluctuation in the dashing area were much larger, inferring that in flow near the dashing area of sidewalls there existed intense shearing and turbulence, where cavitation was liable to happen than the bottom of chute, (2) areation concentration on sidewalls near the dashing area was generally below 1%, in contrast to this, areation concentration in the dashing area of chute bottom was over 2% in common, so that it was difficult for areation protection to form in the dashing area of sidewalls as on the chute bottom, (3) in a certain condition of configuration, cavitation would happen downstream near the edge of side cavities, but no intense cavitation occurred in the test scope, (4) with side deflectors, larger side cavities were liable to form, but pressure fluctuation was intensified on the sidewalls, inducing cavitation to different extend (Fig.3 (a) and (b) show the noise spectra with and without side deflectors respectively), (5) suitable shortening of horizontal length of lateral offsets would enlarge the side cavities, and be of benefit to cavitation ellimination, (6) decreasing top and bottom shringage of the outlet would weaken the lateral widenning of the flow, so as to benefit enlargement of side cavities. A finalized configuration with no cavitation for deep outlet of TGP is shown in Fig.4.
In view of that areation ability of water jet relates to flow velocity, surface tension and other physical properties, to reduce the scale effect, a 1:20 model test in open flume was carried out. In test condition, flow velocity at the valve stand was up to 5.3m/s~7.6m/s, with We=600~1100. Results showed that in the dashing area of chute bottom and downstream of it , the areation concentration near the walls could keep up to 2~3%, but in the dashing area of sidewalls , areation concentration was generally below 1%. It means that downstream of the valve stand, areation protection can be available on the chute bottom, but not on the sidewalls. Cavitation at the dashing area of sidewalls should be eliminated by configuration optimization.
To approach the flow pattern downstream of the drop step and lateral offsets and its influence factors, and to optimize the configuration, three-dimensional numerical analysis was carried out. With weakly compressible method and large vortex modeling, equations of conservation of mass and conservation of momentum can be described as follows:
(1)
(2)
Where T
- shearing stress tensor;
and
p – velocity vector and pressure respectively; ρ
– density of water; g –
acceleration of gravity; c0
– velocity of sound wave; t –
time; z – coordinates of space. With large vortex modeling,
is regarded as average velocity of
vortex, and influence of fluctuating velocity of vortex is included in stress
tensor T.
The equation (1) & (2) were dispersed in finite-volume method on bound grid of hexahedron, so as to satisfying computation in complicated flow channel. With MacCormark method of prediction and correction, computation was conducted step by step till stabilized solution occurred. In addition, a tagging method was adopted to track the motion of free surface.
Comparison between the results of numerical modeling and physical modeling indicated that the numerical model built in the research was reasonable. The computation and theory on three dimensional complicated free surface were successful.
The computation revealed that high pressure gradient formed at the dashing area of sidewalls and reflecting negative pressure occurred dowstream. With shortening of horizontal length of lateral offsets, the side cavities enlarged, the dashing pressure decreased and the water curtains weakened at sidewalls. Along with the decrease of width of side deflectors, the negative pressure on sidewalls reduced obviously and side cavities shortened. By reducing top slope of the outlet, the reflecting negative pressure and maximum impacting pressure could be cut down, and side cavities could be enlarged. In brief, relatively short horizontal length of lateral offsets, gentle top scope of outlet and thin side deflectors are suitable for forming complete and through side cavities and eliminating cavitation at the dashing area on sidewalls.
(1) In valve stand with drop step and lateral offsets, valve sealing and areator are combined together. Its Characteristics of areation at chute bottom is similar to that for areator only with drop step, but owing to the lateral offsets, in water dashing area on sidewalls, high pressure gradient occurs and reflecting negative pressure forms downstream. Broadening water jet impacts sidewalls in large angel within a small scope, with a little areation by shearing and involving. The jet forms water wings upward and water curtain downward, where the edges of water jet are too smooth to form shearing needed for areation. Due to lack of areation, suitable configuration should be employed to eliminate cavitation on sidewalls.
(2) In order to ensure the safety of project, first of all, complete and through side cavities and bottom cavity should form in operation condition. Then, to decrease the pressure gradient, the pressure fluctuation and reflecting negative pressure in the dashing area or downstream, application of side deflector at the outlet should be avoided. When the side cavities are not large enough and width of lateral offsets is limited, relative gentle top slope at the outlet and shorter horizontal length of the lateral offsets should be adopted.
(3) By configuration optimization, cavitation induced by the lateral offsets can be eliminated or decreased to slight extent. But, because of the high velocity and low areation concentration of water jet impacting sidewalls, irregularities of sidewalls should be controlled strictly within 20m from the valve stand.
References
Bangash H.D., Chao P.C., Some Experiences with High Velocity Flow at Tarbela Dam Project , ICOD 13th. Q. 50, R. 13.
Fu Jianchen, Erosion and Repair Treatment of
Spillway of Longyangxia Power Station, The Third Collected Works of Concrete
Repair Technique of Hydro-structure.
Zhang Chunrong, Primary Test Research on the Configuration
Choices of Valve Stand with drop step and lateral offsets, North West Scientific
Research Institute of Water Conservancy.
Fig. 4 Co nfiguration with no cavitation for deep outlet of TGP (unit: m)