1Location: At the header of water conveyance system
Function: To let the water that meet certain criteria enter canals or pressure conduits under well-controlled conditions.
The intake structure should satisfy the following essential requirements:
(1) It is capable of to admit plenty of water at any working reservoir level;
(2) Water quality must be satisfactory;
(3) The head loss should be as small as possible;
(4) The flow should be controllable and intake shall be equipped with necessary gates
(5) Suction vortex and reservoir area circulations should be avoided. Set anti-vortex beam if necessary
(6) Intake should satisfy the common requirements imposed on all hydraulic structures.
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Inlet section +(transition section)+ gate section + transition section
(1) Submerged intake —— Below the min. water level of the reservoir, mainly to introduce deep water into pressurized water conveyance pipeline.
(2) Exposed intake —— Mainly to introduce the surface water into non-pressure water conveyance pipeline and the water flow is channel flow.
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Propertieswith a rock-bored gate shaft and an excavated inlet section.
Application: better local geological conditions and moderate terrain slope.
Advantage make full use of the role of rock with less reinforced concrete.
(1) Intake with a rock-bored gate shaft
(Tunnel Type Intake)
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(2) Tower type intake
Properties the inlet section and gate section form a tower-structure in the reservoir, and connect with the bank by a bridge.
Application: poor local geological conditions and mild terrain slope.
Disadvantagesbear the wave pressure, ice thrust and forces due to earthquake force.
AdvantagesAllow to draw water from one side or radially.
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* Tower type intake leaned against the rock mass (Wall type intake)
Properties lean against the rock mass.
Applications poor geological conditions at the entrance zone or steep terrain slope.
Disadvantages bear the hydraulic pressure and sometimes rock pressurewhich requires sufficient strength and stability.
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Inclined horizontal intake
Inclined horizontal intake
reduce or eliminate the rock pressurehydraulic pressure is transferred to the rock mass partially or totally.
(3) Intake with inclined gate slots towards the mountain slope
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(4) Dam type intake
Propertiesset directly on the upstream side of the dam. The inlet section and gate section are often combined into one section. The transition section is compactly connected together to shorten the length of intake.
Applicationthe only choice when penstocks are embedded in the dam.
Requirementsits arrangement should be coordinated with the damand its shape varies with the dam type.
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(5) Intake of run-of-river
a. The intake and powerhouse are connected togetherand it also plays the role of flood control and water retaining
b. low head and large dischargethe issues of sediment flushing and float cleaning are more prominent
c. Large unit’s dimensionunit’s capacity, the intake are often divided into two or three channels by isolated piers.
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(6) Multi-level intake
For the ecological demands downstream, the intake can admit the surface water by used of stoplog gate under high reservoir’s water level. It is suitable for large and medium hydropower stations with relative wide head variation and the head loss is slightly larger.
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Well type intake of the pumped storage power station
(6) Intake/outlet of the upper reservoir and lower reservoir of the pumped storage power station
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1.dwg
2.dwg
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(1) Location of submerged intake
the inflow is smooth and symmetricalto avoid backflow and vortex. No sediment and pollutants accumulation happens in front of the intake. It can be normal operation during flood season
The intake arrangement is coordinated with the tunnel line
suitable topographic conditiongeological condition and water flow condition.
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a. Bottom elevation of the intake
should be higher than the design sediment accumulation elevationand designed with a reverse slope connected with the reservoir’s bottom and intake.
or be assisted with the flushing sluice.
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b. Top elevation of the intake
Arrangement requirements Its top elevation is lower than the min. operating water level, and has a certain submergence to avoid the funnel-shaped suction vortex (controlling condition) and negative pressure in pressurized diversion pipeline.
Common forms of surface vortex and underwater vortex
Enough submergence
—no vortex
—superficial cruising vortex
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The critical submergence Scr (m) for the possible suction vortex can be estimated by Gordon (J. L. Gordon) Empirical Formula:
in which c is an empirical coefficientc=0.55-0.73small values for symmetrical inflowlarge values for side inflow.
*Fr meet
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Engineering measuresto improve inflow conditions, to set floating mats or anti-vortex beam, to upwarp the upper part of the entrance section.
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Intake/outlet of the pumped storage power station
The intake and outlet of the pumped storage power station are a hydraulic structure. For the upper reservoir, it is an intake during power generationand it is an outlet during pumping operationand on the contrary for the lower reservoir. Therefore, it is named as the intake /outlet.
Properties
During inflow, it should gradually shrinkduring outflow, it should gradually spread to realize a velocity distribution on the full section as evenly as possibleand the backflow and flow separation should not occur. Generally, the diffusion section is relatively long.
(1) Be applicable to the two-way flow
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(2) The head loss should be smaller
During power generation and pumping operation, for inflow and outflow, the head loss should be as small as possible, otherwise the total efficiency of the hydraulic system will reduce.
(3) The operating depth of the reservoir is relatively large, and then the submergence of the intake/outlet is small. In order to avoid suction vortex, we need to set anti-vortex beam
(4) the flow velocity passing through the trash rack is relatively large, so the trash rack is easy to have vibrate problems
(5) the outflow is large while the reservoir capacity is small, maybe the erosion to reservoir’s bottom and side slope is serious, and it is easy to result in holistic circulation or vortex.
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(1) Side intake/outlet
The diversion pipeline is nearly horizontal to be connected with the intake/outlet. The intake/outlet is built close to reservoir’s side slope with relatively large structure and excavation.
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(2) Well-type intake/outlet (for the upper reservoir)
The intake/outlet is connecting with the vertical diversion pipeline, which is named well-type intake/outlet or shaft intake/outlet. This intake/outlet locates in the upper reservoir and has orifices both on the top of shaft and around the tower. The intake/outlet can be open-type, or has a head cover.
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It should meet:
(1) not air admission during the inflow, and to prevent or weaken the formation of vortex
(2) the water flow should spread evenly during inflow
(3) the head loss should be as small as possible
(4) the allowed velocity passing through the trash;
(5) to save engineering quantities and reduce the engineering cost.
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The shapes and related indexes of side intake/outlet
(1) It should have planar diffusion segments. If the diffusion angle is too largemaybe the flow will be separated away from the side wall (flow separation).
(2) It should also have vertical diffusion. Generally the bottom of intake is horizontal and the roof is tilted up.
(3) The reasonable shape and arrangement of diversion piers.
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Statistical graph of the horizontal diffusion angle for side intake/outlet
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Case analysis:
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Local head loss coefficient of the intake/outlet under turbine and pump mode:
(1) General description
The intake/outlet of the pumped storage power station is generally made up of entrance section, trash rack, diffusion section, diversion pier, transition section and connecting section, and some of the intake/outlets also include gate shaft section. Generally, the pipe length between diffusion section and gate shaft section is very short or they are connected directly without connection section. Therefore, the head loss of intake/outlet under inflow or outflow condition is defined as the total head loss of all the sections of the intake/outlet.
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In order to meet the layout design, the plane bending pipe or vertical bending pipe is often applied close to the intake/outlet. When bending pipes exitfor the water flow leaving the bending pipes, because the velocity outside of bending pipe is fast and that inside is slow, which leads to pressure difference and centrifugal force difference at the downstream section. Then two secondary flow occurs and forms a complex water flowwhich has a certain impact on the flow pattern of intake/outlet and its head loss.
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The influence of plane bending pipe: if it is relatively close to the intake/outlet, it will lead to the occurrence of bias flow for the outflow of each flow channel, which directly causes uneven velocity and discharge distributions for the whole intake/outlet flow section, increases its head loss. Therefore, if possible, a straight connection section with length 30 ~ 40D is needed to connect the intake/outlet and the bending pipe.
5.unknown
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The influence of vertical bending pipe: if it is close to the intake/outlet, the high flow velocity region locates at its top in the outflow section. In practical engineering, this phenomenon does not appear and the flow velocity distribution is relatively uniform, mainly because two symmetrical vertical bending pipes are designed and thus mostly eliminate the influences of bias flow caused by a vertical bending pipe.
6.unknown
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(2) Head loss of intake/outlet
Setting diversion piers in diffusion section of intake/outlet is mainly to make the water flow shrink or diffuse evenly, and the velocity distribution is uniform and stable.
The head loss of intake/outlet depends on inflow and outflow conditions. It mainly includes diffusion shock, local separation and local impact. in the case that the intake/outlet is divided into three or four channels by the diversion piers, the uniformity of flow distribution of all the channels is a more important factor on the head loss of intake/outlet.
Generally, the head loss is smaller for inflow in the intake/outlet, and the head loss coefficient varies between 0.2 and 0.3; in the case of outflow, the head loss is larger and its coefficient is about 0.4 to 0.8.
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Comparison of the head loss coefficients for the intake/outlet of the pumped storage power stations in China.
Pumped-storage power stations
(1) The flow field in reservoir area
Under low reservoir level, the velocity field of the reservoir bottom in the case of outflow
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Under low reservoir level, the velocity field of the reservoir bottom in the case of inflow
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Under low reservoir level, the velocity field of the reservoir bottom in the case of outflow (with higher reservoir bottom)
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Under low reservoir level, the velocity field of the reservoir bottom in the case of inflow (with higher reservoir bottom)
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Flow pattern design and anti-vortex design for inflow
The min. submergence of the intake/outlet is very small when the reservoir is in dead water level. It is necessary to use different structures such as anti-vortex beams and plates, to eliminate the vibration caused by suction vortex and trash rack. In addition, the intake/outlet has complex two-way water flow. In order to make the velocity and discharge distribution as uniform as possible, without any backflow and flow separation, and with smaller head loss, it is important to have a best design for the intake/outlet.
Due to various, frequent and complex conditions switch in the pumped storage power station, and wide variation of reservoir’s water level, it is more important to avoid the harmful vortex near the intake/outlet.
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Suction vortex with air admission is more harmful and must be avoided. There are many factors that influence the vortex's occurrence and development, such as the flow direction and velocity distribution (or inflow Fr.), the near velocity distribution, the layout of the transition section and diversion pier, the terrain of reservoir nearby, the designed submergence of the intake/outlet and so on.
In order to discriminate whether harmful vortex occurs in the intake/outlet, J. L. Gordon obtained the critical submergence formula for the intakes based on many field tests and further numerical analysis.
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In order to guarantee the safe and reliable operation of the pumped-storage hydropower stations, to set anti-vortex beams above the intake/outlet is commonly used to prevent the inflow vortex. From the effectiveness of anti-vortex beam, to set the floating mats is the best choice, however, because the floating mats are easy to be affected by floating debris and its structure is relatively complex, the anti-vortex beams are often used instead of the floating mats.
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Height of the anti-vortex beamsoriginal 1.5mfinal 1.0m
Spacing of anti-vortex beamsoriginal 0.8mfinal 0.5m, 5 anti-vortex beams.
Intake/outlet height original 12.2mfinal 9.9m
Intake/outlet width original 42.6mfinal 28.5m
Horizontal diffusion angle original 39.6final 44.9
Length of diffusion section original 44.5mfinal 22.9m
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Trash rack design
(1) Operating conditions
(3) Vibration sources
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The requirement of minimum velocity passing through the trash rack
No.
Velocity (m/s)
Velocity (m/s)
0.69
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Working conditions for trash rack in intake/outlet of the pumped storage power station
(1)Two-way flow in the flow channels
(2)The rotation modes of the water flow at runner outlet under turbine condition varies greatly under different operating modes, and the forces on the trash rack varied simultaneously
(3)Flow patterns in the transient conditions of the pumped-storage hydropower stations are complex, and then the forces on trash rack increase accordingly.
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If the trash rack of the pumped-storage power station’s intake/outlet is poorly designedthe damages are easy to occur. There are two main destructions
Lateral vibration damage to single rack bar of trash rack, or vibration damage along the water flow direction. There may also have torsion vibration
Vibration damages to the whole trash rack perpendicular or parallel to water flow direction. Generally, the main reason for damages of the trash rack is the excited vibration of water flow.
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There are two vibration sources of excited vibration
The disturbance produced by the vortex shedding separated from the tail of the cascade. It is unstable
The disturbances generated by the turbine and its disturbance frequency is relatively stable. The disturbance frequencies of these excited vibrations may be coupling with the natural frequency of the cascade in the lateral or along the water flow direction, or maybe coupling with that of the whole trash rack perpendicular or parallel to water flow direction. These may cause resonance and make the trash rack damaged.
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In the design of trash rackin order to avoid resonance and further destructions, the natural frequencies of the trash rack and its rack bars should be far greater than the disturbance frequency as much as possible. The main measures are
Reduce the velocity passing through the trash rack or thicken its cascades, which can reduce the disturbance frequency of vortex;
Strengthen the connection of the trash rack with well welding to improve its rigidness. Shorten the span of single rack bar, namely increase fixed points in the middle of the rack bar, which can effectively improve the natural frequency of the rack bar;
If reservoir’s water is clear without floating debris, we can consider removing the trash rack, or lift up the trash rack during outflow.
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