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Water Supply Methods and Valve Components on Turbine Pipes

Lowest water level

min in tank appears in unstable working mode

When increasing the load of the TTĐ, when (Z - 1) units are working, increase the last unit to full

load. If the water level symbol in the tank when (Z - 1) is working at full load is z 1 , and the pressure drop (figure 12-13,a) is the largest when the last unit is fully loaded:

h is wave

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min z 1 h

(12-20)

+ For self-regulating channels, z 1 when (Z - 1) the generator operates stably with the largest capacity that can be determined from the water level in the channel, and for non-self-regulating channels, the water level on the spillway is taken as the water level elevation, corresponding to the overflow layer when discharging.

maximum flow of a unit (Q tm ) through the spillway;

+ Pressure drop wave height h is determined on the basis of flow calculation

instability in the channel when increasing the maximum flow of a unit. To calculate correctly

We have to calculate the unsteady flow. In the preliminary design stage, we can use the method

The water balance process during the load increase period T S according to (Figure 12-13,a) is as follows:

Q tm

2 T s

c T sh b 2

, orhtm ; pressure drop wave propagation velocity c =

yes​

Q

cb k

k

The top of the pressure tank wall and the pressure wall are placed higher than the highest water level.

ensure safety against overflow, determined by the following formula:

dmaxh sand (0.3 - 1) m (12-21) In which: + h S is the wave height calculated according to wave calculation standards;

+ is the safe height according to the construction level, taken as follows:

= 0.2 - 0.25 m when the flow channel Q < 30 m 3 /s;

max to

= 0.3 - 0.4 m when the flow channel Q = 30 - 100 m 3 /s;

= 0.4 - 0.5 m when the flow channel Q > 100 m 3 /s.

+ max - is the highest water level in the tank, calculated for the case of a total load cut. For self-regulating channels, it is determined when calculating the unstable flow with the initial water level being the normal water level and the load cut with

Q max of the station. We have:MNBT'

max h

h

can be preliminarily determined '

according to equilibrium (figure 12-13,b) and has:

Q

yes​

'tm , with the pressure wave propagation velocity c 'V.​

h c ' b k k

For non-self-regulating channels,max is taken as the water level before the spillway when the entire TTD flow is discharged through the spillway:maxMNBTh tran​

b. Front dam and excess water discharge structure (spillway):

The spillway has dimensions B T x H T to discharge the maximum flow of the spillway when suddenly cutting off the entire load. Usually the depth of the overflow layer above the spillway threshold H T is taken from experience H T = (0.2 - 0.6) m, thus calculating the spillway width B T for a practical spillway according to the formula (Figure 12-13):

m

2g H T 3/ 2

Q

B T o

(12-22)


Overflow threshold elevation

ng.tran is taken higher than the normal water level (NTL) at

The end of the channel and tank is (3 - 5) cm to avoid water overflow when there is water surface fluctuation.

The front compartment of the pressure tank is the section connecting the end of the channel to the pressure wall (there may be a separate transition section as shown in Figure 12-13, or the transition section can also be combined with the front compartment as shown in Figure 12-10). The size and structure of the front compartment must ensure sufficient arrangement of the water intake and must be in line with the flow to reduce hydraulic loss in it. The cross-section of the flow of the compartment increases gradually from the channel to the pressure wall. The bottom of the compartment is lower than the water intake threshold ( dov ) of the pressure wall by (0.5 - 1) m so that mud and sand can settle and not be pulled into the water intake when discharging sand. Therefore, the depth at the end of the front compartment will be equal to:

Hddov( 0.51) m

The preliminary pre-compartment length is determined by the formula:

(12-23)

L = 5 (H - h) + 1 m (12-24)

In which h is the depth at the end of the channel. If the pressure tank functions as a sand settling tank, the length L must also be calculated to satisfy the sand settling conditions.


2. Calculation of pressure tank stability

The characteristic of the pressure tank of the non-pressurized pipeline is that it is usually located on a steep hillside, so the issue of sliding stability and seepage effects need to be carefully calculated. Figure (12-14) is a diagram of the arrangement of the tank on the slope and the possibility of instability of the tank under the effects of sliding layers and seepage.



Figure 12-14. Pressure tank stability calculation diagram

1- pressure tank retaining wall; 2- side wall; 3- channel bank; 4- seepage stream; 5- soil layers


- Diagrams I and II show seepage flow 4 flowing out of the slope causing slope failure. Calculating stability in this case requires calculating seepage stability including calculating seepage flow, determining seepage saturation line, seepage pressure distribution, seepage flow velocity at the bottom and around the construction;

- Diagram III shows the case where the pressure tank is placed on many soil layers 5 with different physical and mechanical properties and the slope direction of the layers coincides with the slope direction of the terrain, then the possibility of mixed sliding along the contact surfaces is very likely to occur. In this case, it is necessary to conduct a stability check for each soil layer along with the construction.

Calculation of the overall stability of the pressure tank must be carried out with the diagrams of the flat sliding of the foundation bottom, mixed sliding, settlement calculation and stress check calculation under the foundation bottom. For structures on rock foundation, the purpose of permeability calculation is to determine the pressure on the underground part of the structure.


Chapter XIII. TURBINE PIPELINES


Chapter XII has presented the pressurized water pipeline of the power plant. The turbine pipeline is the pressurized water pipeline, which is responsible for carrying pressurized water from the Water Intake (in the post-dam type power plant) or from the Pressure Tank (in the non-pressurized pipeline power plant), or from the Pressure Chamber (in the pressure pipeline power plant, there is a pressure chamber) into the hydraulic turbine.


XIII. 1. OVERVIEW OF TURBINE PIPELINE

XIII. 1. 1. Turbine pipeline classification

Turbine pipes come in many different shapes and sizes, working with different water columns... Turbine pipes can be classified according to the following signs:

1. Classification by pipe material

According to the material used to make the pipe, there are: steel pipe, wooden pipe, reinforced concrete pipe, plastic pipe... Among them, steel pipe and reinforced concrete pipe are the most commonly used.

- Steel pipes are used for all water columns from low to high (steel pipes of Bogota Hydropower Plant in Colombia with H = 2000 m) due to the high load-bearing capacity of steel, compact structure, small roughness, so the loss is small. Steel pipes are usually placed exposed on the ground, not buried directly underground without anti-rust protection and a concrete jacket to withstand soil pressure;

- Reinforced concrete pipes, often used with water column H < (30 - 50) m due to the low load-bearing and water-permeability of concrete. However, reinforced concrete pipes have a large thickness so they can be buried underground, because they do not rust, so they do not need maintenance when buried underground, on the other hand, reinforced concrete is also used with large flow. However, the disadvantage of this type of pipe is that in addition to low load-bearing capacity and difficulty in waterproofing, it also has a heavy structure;

- Wooden pipes, used in places where wood is available, the climate is mild and transportation is inconvenient, it is difficult to preserve the pipes against rot. In fact, they are no longer used today;

- Plastic pipes are now also starting to be used in the power industry, but not much.

2. Classification by pipe location

According to the location of the pipe, there are: exposed pipes on the ground, buried pipes in the ground (such as under the dam, buried around the dam), pipes placed in concrete dams, underground pipes (underground pipes).

- Open pipe: pipe is placed on the ground or placed in a trench or corridor in an earth and rock dam. Open pipe is easy to check and repair, however it is affected by the environment (temperature changes, landslides, rainwater threats...);

- Underground pipes: when the pipe diameter is small (usually less than 2 - 2.5 m), people place the pipe in a trench and cover it with a layer of soft soil. This type directly bears the pressure of the soil and rocks above and on the sides, so it must be thick enough to withstand the external force. It is best to use reinforced concrete pipes;

- Pipes placed in concrete dams: cast in concrete and placed with load-bearing steel;

- Underground pipes: used with underground power plants.

XIII. 1. 2. Selecting the route and arranging the turbine pipeline

1. Select turbine pipeline route

The pipeline route is selected based on the overall layout of the power plant. The reasonable arrangement of the pipeline route has a great influence on the cost of the project and the safety and reliability in the operation of the hydroelectric station. Propose some calculation options and choose which option must be through economic and technical comparison. The route selection must meet the following requirements:

- Choose a short, straight route. Choosing such a route not only reduces costs, reduces hydraulic loss and the value of water pressure in the pipe is beneficial for stable operation of the TTĐ. Usually, the pipeline is placed perpendicular to the contour line to shorten the length of the pipe. However, if the route is straight and the excavation volume is large, the direction of the pipe center can be changed and abutments can be built there to hold the pipeline tightly;

- The pipe should not be placed at too steep a slope to avoid difficulty in construction and to avoid instability of the pipe. The slope of the pipe should not exceed 40 0. In which the slope of reinforced concrete pipes and wooden pipes is smaller than that of steel pipes;

- The slope of the ground where the pipe is placed must be stable to avoid landslides. The pipe should be placed along the positive slope of the mountainside to easily drain rainwater along the pipe and avoid the threat of water flow from the mountain crevices. Do not place the pipe in places where water collects or landslides occur. The pipe supports and anchors must be placed in stable places, preferably on bedrock;

- Where the pipeline must curve, the required radius of curvature of the pipeline must be three times larger than the pipe diameter and a pipe anchor must be placed there. The top of the pipe cross-section must be 2 - 3 m lower than the corresponding negative water pressure to avoid vacuum in the pipe.

2. Water supply methods and valve gate components on turbine pipes

a- Methods of supplying water to turbines



Figure 13-1. Methods of water supply to the generator set.


Water introduction into the turbine can be divided into three following methods (Figure 13-1 above):

- Separate water supply: in this method, each unit has a separate water supply pipe (diagrams I, II and III, figure 13-1). Water supply according to these diagrams is safe, when a separate pipe has a problem, only that unit stops, the other units still generate electricity, the pipeline structure is simple. The disadvantage of this method is the cost of pipe volume as well as the works on the pipe such as the number of abutments, temperature joints must be large, the construction volume of the pipe laying line must be large. Therefore, this method is beneficial when the turbine pipeline is short, such as used for post-dam type power plants or when the length from the pressure tank to the turbine is short.

- Combined water supply method (diagram V, VI, figure 13-1), meaning that the whole factory has only one common water supply pipe. This method is the opposite of the separate supply method, it is cheaper but less safe because when the common pipe has a problem, the whole factory must stop working. The structure of the pipeline at the branch section will be more complicated and must be equipped with additional valves at each branch. Therefore, the combined water supply method is beneficial when the power plant has a small flow, large water column and very long pipeline.

- Group water supply method (diagram IV), this method is each pipeline supplies water to a number of generators. This is an intermediate water supply method between the two methods above. This method is used when the pipeline is relatively long, the flow is relatively large and the number of generators is large.

In addition, in dam-type power plants with large unit capacity, if using one turbine tube to supply the unit, there is difficulty in manufacturing technology for tubes with too large diameters or using turbines with two spiral chambers, then the solution is to use two tubes for one unit (diagram VII, figure 13-1).

In the common water supply methods of bringing water from the source to the factory, the choice of water direction into the factory can have the following forms:

+ The diagram (Figure 13-1,a,e) leads water into the generators through separate pipes perpendicular to the factory axis. This diagram is in line with the flow, but the factory is threatened when the pipe breaks or the anchorage is pushed and slid. To protect the factory, it is necessary to build a solid retaining wall to direct the flow out of the factory through the drainage channel.

+The diagram (Figure 13-1,d,c) uses a common pipeline or a group pipe entering the factory from the slope. This diagram is safer for the factory but increases hydraulic loss and increases the volume of excavation for pipe placement along the factory.

b - Valve placement diagram on turbine pipe

To ensure the operating conditions, incidents and repairs of pipelines and turbines, there are valve gates on the pipeline. However, whether or not to install the valve and where to install it depends on the length of the pipe, the water column acting on the pipe and the water supply method of each specific pipe. The valve gate on the turbine pipeline is usually a flat valve (with small hydraulic loss, simple structure, small diameter), disc valve gate (small operating force, large hydraulic loss when fully opened, used for large diameter pipes), globe valve (heavy structure, little leakage, small operating force, used at high water column, small pipe diameter). Figure (13-2) presents an overview of some types of valve placement diagrams, they need to be specifically considered for appropriate usage conditions.

- Diagram I (Figure 13-2): only arrange repair valve 1, working valve 2 at the water intake at the beginning of the pipeline, do not place the valve on the turbine pipe. This diagram is used when one pipe supplies water to one turbine, the pipe length is small (no more than 150 m) and the water column H 150 m. When there is an incident, close valve 2, when repairing, close valve 1, the amount of water in the pipe is not large so the energy loss is small and the time to drain water from the pipe is short, the cage rotation time is short so there is no danger of rotating the cage of the generator set;



Figure 13-2. Valve arrangement diagrams on pipelines.


- In case of a water supply pipe for a turbine as above, but the pipe length is large (over 150 m) and the pipe withstands a large water column (over 200 - 300 m) (diagram

II) In addition to valves 1 and 2 placed at the beginning of the pipeline, valve 5 must also be placed at the end of the pipeline to close when the turbine needs to be repaired without having to drain all the water in the pipeline;

- In case the pipeline has branches (one pipe supplies many generators), in addition to valve gates 1 and 2 at the beginning of the pipeline, there must also be an additional valve gate 5 at the end of the pipeline on the branch pipes (diagram III). Use valve gate 5 in case a generator has a problem or needs repair, then close its own valve gate, while the other valves remain open and operate normally;

- In case of long pipes with high water pressure, if a pressure chamber is needed on the pipe to reduce the water pressure, there are still two valves 1 and 2 placed at the beginning of the pipe. In addition, it is necessary to arrange the valve gates as follows (diagrams IV, V, VI in figure 13-2):

+ If after pressure chamber 13 there is a branch to the generators, but if the

For short branch pipes, it is enough to arrange valve 5 on the branch pipes;

+ If after the pressure chamber 13 the branch pipes are long and the water column is up to 400 m high, then the turbine pipe head (right after the pressure chamber) must have valve 9 and the end of the turbine pipe must have a valve before turbine 6. Valve 9 is used to close the turbine pipe of the branch that needs repair or has an accident. Valve 6 is used to close the turbine for repair or accident;

+ Sometimes, on the turbine pipe, there are two valves 8, 9 arranged at the beginning and at the end of the pipe, there are also two valves before turbine 6, 7 (diagram VI, figure 13-2). This diagram is used when the pipeline has a high water column (from 800 m or more) and the pipe is long.


XIII. 2. STEEL PIPELINES

In Hydropower construction steel pipelines are widely used by

Steel roads have the following advantages:

- Withstand high pressure, withstand water column from several meters to thousands of meters;

- Because the metal surface is smooth, the small roughness leads to small hydraulic loss;

- Easy to manufacture, process and assemble conveniently. Adaptable layout to all changes in terrain and geology, easy to branch. Simple pipe construction.

- Easy to install thermal joints at the contact ends of two connected pipe sections to eliminate stress due to temperature changes...

The disadvantage of steel pipes is that due to the small wall thickness, they do not directly bear the pressure of soil and rock pressing on the pipe. Therefore, steel pipes are widely used in the form of exposed laying on the ground. If buried in the ground, there must be concrete or a protective surrounding corridor.

XIII. 2. 1. Structure of steel pipelines and its equipment and construction components

The pipeline consists of the pipe wall and the anchors, supports, temperature joints, observation holes, drain pipes, and air valves placed together with the pipe. Figure (13-3) shows the forms of pipe laying and their names. There are three following forms of pipe laying:

- The pipe has a straight core, with no temperature joints between the two anchors, called a continuous pipe (Figure 13-3,a). This type will generate thermal stress in the pipe wall when the temperature changes, and is used when the environment has little temperature change;

Figure 13-3. Forms of steel pipe laying.

- The straight pipe heart form, with a temperature joint placed between the two anchors (Figure 13-3,b), is called a segmented pipe . This form, when the temperature changes, the joint between the two pipes in the temperature joint will move freely, thereby eliminating thermal stress on the pipe wall. This form is widely used in hydropower construction;

- The shape of the curved pipe follows the terrain, there is no temperature joint on the pipe, this is also a continuous form. Because the pipe is curved, when the temperature changes, the pipe will expand and contract, causing thermal stress in the pipe wall. However, because the pipe is curved, some parts can move freely, so the thermal stress is limited. This form is difficult to determine the pipe center, so it is rarely used.

Next we move on to study the parts of the tube.

1. Types of steel pipes

a - Prefabricated steel pipes

This type of pipe is pre-cast in the factory into sections with lengths from 4 to 12 m and small diameters not exceeding 0.6 m according to standardization for ease of use. This type of pipe is of high quality, easy to transport and assemble on site by welding or bolting. However, it has the limitation of only casting small diameter pipes.

b - Plain steel pipes are manufactured by welding or riveting


Figure 13-4. Types of steel pipes and pipe joints.


Unlike precast pipes, these pipes are made by bending steel plates to a predetermined radius and then using welding or riveting to connect the plates together. Therefore, any size of pipe can be made in the factory or on site. Using welding is better than riveting because it consumes less steel and the smooth inner surface of the pipe reduces the loss of water column in the pipe. Of the two types of longitudinal and transverse welds, the longitudinal weld is subject to high water pressure, so the longitudinal weld is more important and is arranged staggered along the pipe to avoid stress concentration (Figure 13-4,a).

Figure (13-4,c) shows some forms of welding steel plates when creating pipes, or using flange connection between two pipe sections.

c. Pipe with hard belt welding and pressure-resistant belt welding

- Welded pipe with hard rings (Figure 13-4, ): the outer surface of the pipe wall is welded with hard rings to make the pipe wall stiff enough to resist instability (distortion of the pipe) of the pipe wall under the effect of vacuum in the pipe or pipe deformation during transportation. The hard rings do not participate in withstanding the water pressure inside the pipe;

- Welded pipe with bearing belt (Figure 13-4,b): this is the type of pipe with the outer ring installed on the pipe wall (used for hot or cold installation). When the ring shrinks (hot installation) or the fluid is blown into the pipe, the pipe wall becomes corrugated (cold installation), creating pre-stress for the pipe wall, increasing the bearing capacity of the pipe wall. The type of pipe with bearing belt has the advantage of

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