PROTOTYPE OBSERVATION TEST REPORT ON WATER DISCHARGING THROUGH NO.1 ORIFICE TUNNEL OF XIAOLANGDI PROJECT

Wang Jiangtao, Cao Zhengqi and Wang Xianru

Yellow river water & hydropower development corporation

xiaolangdi Dam Site ji Yuan,Henan,China  454681

tel: (86) 379-3905237

fax: (86) 379-3905247

E-mail: jiangtao_wang@sina.com.cn

Abstract: This report summarizes the necessity of taking the prototype observation test on water discharging in No.1 Orifice Tunnel, the main tests and conclusions, and describes the reflection characteristics of the adoption of energy dissipaters in large underground cave at hydraulics, structural mechanics, vibration properties etc. Thus, provides us with valuable lessons for the adoption and popularization of energy dissipaters in large underground caves.

Keywords: orifice ring, energy dissipating, orifice tunnel, pulse pressure, time average pressure, cavitation, and vibration

1    INTRODUCTION

The Orifice Tunnels (OT) of Xiaolangdi Multipurpose Dam Project were converted from the Diversion Tunnels with a diameter of 14.5 m at normal section, and of 10~10.5 m at orifice rings. The Middle Gate Chambers (MGC) are located at the middle of the tunnels with 2 eccentric sector gates. Installed. Through the contraction and expansion of the flow section, turbulence is produced and the water flow energy is dissipated. Considering the scale effects of modelling, the actual energy dissipating effect of the Orifice Tunnels and the discharge safety remained a concern for the specialists. Therefore, it was important to conduct flow discharge prototype observation tests before No.1 OT was put into service, and to analyze and study thoroughly such characteristics as hydraulics, structural mechanics, induced vibration, etc, of important locations like the orifice sections and MGC.

2    MAIN CONTENTS OF THE PROTOTYPE TEST IN NO.1 O.T

2.1    Design of the tests in the orifice tunnel

As a key area, the 3 OTs are equipped with the same instruments for the sake of comparing readings, verifying design and test results, as well as monitoring the safe operation of the OTs. The instrument receiving devices are installed in the MGC. The layout of the observation points is as shown at Figure 1.

2.2    Layout and contents of the main observation points

The contents of the observation points shown in Figure 1 are shown in Table 1: 

2.3    Duration of water discharging through No.1 OT

The actual duration of water discharging in No.1 OT is shown in Table 2:

3    MAIN MONITORING RESULTS OF THE TEST

3.1    Time-average/pulse pressure at orifice ring sections

Table 3: Average of the Time-average Pressure under Each Gate Expanse (Unit: KPa)

Table 4:The Distribution of the Mean Square Root Of the Maximum Pulse Pressure under Each Gate Expanse

3.2    Pressure and energy dissipation coefficient

As for the observation results, see Table 5 for the details.

3.3    Results of cavitation at orifice ring sections

See Table 6: Discharge Volume and Flow Cavitation under Different Gate Open Extent

3.4    Test results of aerification concentration in the m.g.c

See conclusions item (5).

3.5    Results of the flow induced vibration of the gate

See conclusions item (7) and (8)

3.6    Results of the left bank mountain vibration

3.6.1    The layout of the observation points

See Table 7: Locations and Elevations of the Vibration Observation Points of Xiaolangdi Mountain

3.6.2    Observation results

(1) The most usual flow mode in No1 OT is when the gate is fully opened, under which also the strongest vibration appears. The maximum vibration acceleration of the mountain is 4.91cm/s², and the mean square root is 2.59 cm/s².

(2) The duration to open or close the gate is very short, normally taking a little more than 10 minutes, during which the induced vibration is weaker than that under full expanse condition.

(3) The mountain vibrations induced by the flow through the tunnel under about 0.9, 0.8 and 0.7 gate expanses are all weaker than those under full expanse, among which, the induced vibration under about 0.9 gate expanse is relatively stronger, under 0.7 second and under 0.8 the weakest.

(4) When the gate is fully closed, the vibration represents the background vibration. The maximum acceleration is 3.77 cm/s², the mean square root is 2.406 cm/s².

(5) The maximum velocity is 180.55μm/s and the mean square root velocity is 60.86μm/s.  

4    CONCLUSIONS

(1) It can be seen from the past model tests and the interim tests conducted at Bikou Power Station that the adoption of the orifice rings for energy dissipation is very effective. It is further proved by the prototype observation tests that the cascade orifice rings is reliable in terms of energy dissipation. During the test, the total energy dissipation rate is 40.2% under full expanse condition (42.9% for the model test), the energy dissipation coefficients of the 3 levels of orifice rings are 1.19, 0.57 and 0.63 respectively. The energy dissipation of the first level of orifice rings is much more effective than that of the last two levels of orifice rings.

(2) The results of the prototype observation test and the model test conducted at the pressure section for pulse pressure are identical. The mean square root of the maximum pulse pressure on the sidewalls is 33.94kPa, which equals to 5.2% of hydrostatic head (10% was considered for design), the maximum pulse pressure amplitude at orifice ring sections is 28.22kPa, which equals to 4.3% of hydrostatic head. The range of the dominant frequency of most observation points is below 2 Hz, which falls into the low frequency scope.

(3) Under full gate expanse, cavitation is found at Orifice Ring Nos. 2 and 3, but is weak, the flow cavitation values are 4.49 and 3.393 respectively. When the gate expanse is reduced to about 0.87, with an water flow area of 45.1m2, the initial cavitation appears. At this moment, the flow cavitation value and the initial cavitation value of 2 levels of orifice rings are 6.78, 6.2 and 6.17, 5.56, which are all higher than the initial cavitation values of the model test under reduced pressure. This indicates that the scale effect of the initial cavitation values of the orifice rings is very obvious. Because the cavitation of the rings occurs mainly inside the water body, there is only a small chance for the cavitation to disappear at the concrete sidewalls, which will not create serious cavitation damage to the structure. When operating under high water level, the flow cavitation value will not be reduced by a big margin, but the initial cavitation value will probably be increased by a small amount. Therefore, it is necessary to monitor the cavitation noise when operating under high water level.

(4) During the test, the maximum amplitude of the stress on the Orifice Rings and Orifice Tunnels is 0.57 Mpa, the maximum stress amplitude of the reinforcing steel is 5.71 Mpa. This indicates that the structure stress is small, with a high safety margin, and therefore the operation is carried out under a safe condition. Judging from the developing curve of the readings taken from concrete strain meters, there was no tensile stress in the concrete structure.

(5) Under full gate expanse, the highest velocity of the wind inside the ventilation shaft is 28.6 m/s, which is within the value limitation as indicated in the Specification. The average air volume through the ventilation shaft is 147.9 m3/s, the air distributes evenly, which indicates the design of the ventilation system is reasonable. During the gate lifting, the wind velocity reaches the highest when the relative expanse is about 0.35. Two types of air concentration meters are used in the prototype observation test. The measured air concentration at bottom is 0.6% and 1.2% according to the pin type and the electric conductance type respectively, and at sidewalls is 0.1% and 1.8% respectively. It indicates that the aerification is insufficient by a small margin.

(6) Using pin type to measure bottom velocity would be very useful at determining the velocity distribution at and measuring actual velocities of free flow sections.

(7) During continuous open-close period, the strongest vibration happens at 0.05 and 0.6~0.82 expanse. The maximum dynamic stress is 8.67 Mpa, which is much smaller than 20% of the allowed stress for the steel gate structure. The maximum mean square root of displacement is 104.6μm, appearing during partial expanse. Considering the vibration displacement and stress response of the sector gate, it is fair to say that the gate vibration is weak and the vibration displacement and the acceleration of the gate fall into the ranges of allowance respectively.

(8) At full expanse, the gate vibration reaches the strongest with a maximum acceleration of 4.91m/s². The induced vibration frequency induced by the flow is 0.5 ~1 HZ, and the predominant frequency of the left bank mountain is 8~12 HZ, therefore, no resonance would be induced. When the 3 Orifice Tunnels are discharging simultaneously, vibration amplitude of the mountain would be getting bigger by a value which follows the regular extracting pattern of the sum of current value squares. But the vibration amplitude would not be greater than that of 2 times of the current value. Even operating at high water level, the induced mountain vibration will not create any harmful damages.

(9) As seen from the checking after the test, there was no cavitation trace found at either the pressure section or the MGC section. It was found during the checking that there are cavitation pinholes in the stainless steel structure close to the side-rail of the MGC. According to the preliminary judgment, it was caused by gap leaking during partial expanse of the gate, which will have no major impact on the operation of the gate.

(10) It can be determined from the prototype observation test results that if a 500 year design flood was encountered during the flood season of the year 2000, the reservoir water level would reach 235.8m, and the flow cavitation value of each level of the orifice rings in No.1 OT would decrease by a small margin. So seeing from cavitation, mountain vibration, structure stress and gate vibration, there will be no big changes. Therefore, the 3 OTs can be operated under normal conditions and put into operation during flood seasons, and the theory basis is established for the normal utilization of the Orifice Tunnels under high water elevations.

References

[1]    Wang Xian Ru,Lin Xiushan; Orifice Dissipation Test Report on Sediment Tunnel of Bikou Water Power Station; March 1988

[2]    IWHR&AWHR&XECC, Prototype Observation Test Report on Water Discharging Through No.1 Orifice Tunnel of XiaoLangdi Multipurpose Dam Project; June 2000

Table 1    The corresponding observation contents of the observation points as shown in figure 1

Table 2    Filling process during the test  (april 26)            EL210.22m 

Table 3    Average of the time-average pressure under each gate expanse    (unit: KPa)  Water Level:210.22m

Table 4    The distribution of the mean square root of the maximum pulse pressure under each gate expanse

Table 5   Energy dissipation head and energy dissipation coefficient of each level of orifice rings under full gate expanse (the water discharging prototype test, 26 april and 27 april, 2000)

Table 6    Discharge volume and flow cavitation under different gate open extent

Table 7    Locations and elevations of the vibration observation points of xiaolangdi mountain

Fig. 1    Instrumentation layout of no.1 orific tunnel for xiao lang di