hydraulic structures.ppt - HYDRAULIC STRUCTURES...

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Unformatted text preview: HYDRAULIC STRUCTURES Introduction ►A hydropower development includes a number of structures, the design of which will be dependant upon the type of scheme, local conditions, access to construction material and also local building traditions in the country or region. Cont’d The following structures are common in a hydro scheme: ► Diversion structure ► Dam Spillway Energy dissipation arrangement Fish pass Residual flow arrangements Water conveyance system Intake Canals Tunnels Penstocks Power house Design aspects Dams Dams and weirs are primarily intended to divert the river flow into the water conveyance system leading to the powerhouse. It produces additional head and provide storage capacity. Its choice depends largely on local topographical and geotechnical conditions. For instance if sound rock is not available within reasonable excavation depth, rigid structures such, as concrete dams are difficult. Conversely, for narrow valleys, it can be difficult to find space for separate spillways, and concrete dams can be the natural choice with their inherent possibilities to integrate spillways etc in the dam body. 1. Cont’d World wide, embankment dams are the more common partly due to the following characteristics: ► Can be adapted to a wide range of foundation conditions. ► Construction uses natural materials, which can often be found locally, limiting needs for long transportation. ► The construction process can be continuous and highly mechanized. ► The design is extremely flexible in accommodating different fill materials. Disadvantages with embankment dams are that they are sensitive to overtopping and leakage, and erosion in the dam body and its foundation. There is a higher mortality rate among embankment dams as compared to concrete dams. Cont’d Concrete dams on the other hand have drawbacks that correspond to the pros of the embankment dams: ► Require certain conditions with respect to the foundations. ► Require processing of natural materials for aggregate at the site, hauling of large quantities of cement and has a labor intensive and discontinuous construction process, leading to large unit costs. Cont’d Advantages of Concrete Dams ► They are suitable for most ranges of topography that is for wide and narrow valleys, provided that foundation conditions are right. ► They are not very sensitive to overtopping. ► A spillway can be placed at the crest, and if required over the entire length of the dam. ► Chambers or galleries for drainage, tubing and ancillary works can readily be housed within the dam body. ► Powerhouses can be placed right at the toe of the dam. Cont’d ► The development of the Concrete Faced Rock fill Dam (CFRD) neutralizes many of the drawbacks with core-type embankments. In particular, sensitivity to leakage and erosion is reduced, and dependence of good core material is removed. ► The development of the Roller Compacted Concrete Dams (RCC-dams) introduces a continuous, highly mechanized construction process and low unit costs. ► New large dams are almost always CFRD and RCC designs. Embankment Dams 1. Homogeneous dams: These dams are used for low embankments (H<4m) and often as secondary dams. For dam safety reasons, some type of drainage is almost always provided. 2. Zoned embankment dams: These are used for dam heights for 4m and above. Constructions are extremely sensitive to the engineering design and construction, and it is therefore vital to engage highly skilled consultants/Engineers and contractors. It requires experienced site-supervision engineers. Critical components of these dams are the core, the transition zones (filters) surrounding the core and drainage capacity of the dam toe. Cont’d 3. Embankments dams with membrane: The membranes can be of different types and be located either at the upstream front of the embankment or vertically in the centre of the embankment. Membranes can be made from concrete (as in the CFRD), asphalt (Norwegian type) or in the form of a geomembrane on the upstream slope. Concrete Dams Generally, concrete dams are categorized according to how they function statically Cont’d 1. Gravity dams: These are dependent on their own mass for stability. Their cross-section is basically triangular in order to provide adequate stability and stress distribution across the foundation plane and their upper part is normally rectangular in order to provide adequate crest width for installation and transportation. Design issues include stability analysis (sliding and overturning), stress control, temperature control during construction to avoid cracking, control of uplift pressures under the dam, etc. Cont’d. 2. Buttress dams: These dams consist of a continuous upstream face that is supported by buttresses at regular intervals. The upstream face is normally divided into vertical sections by dilatation joints, each section being supported by a buttress. Cross-sections are similar to those of gravitation dams. Cont’d 3. Arch and Cupola dams: These dams function structurally as horizontally laid out arches that transfer the water pressure on the upstream face into the abutments rather than into the foundation. Arch dams can be designed with a constant radius over the dam height, or with varying radii (Cupola dams). Arch dams with a constant radius have a vertical and “straight” cross-section. These dams will be subject to considerable vertical strain forces since the deformation of the dam will tend to be greatest in the vertical centre of the dam. This requires that the dam be heavily reinforced to avoid cracking with accompanying leakage. Cont’d ► The Cupola dam is designed to have only compression forces for all directions and at all sections. This requires the radius of the curvature to vary over the dam height, which produces a curved vertical crosssection. ► The arch and cupola dams are structurally efficient and greatly reduce the required volume of concrete. They require, however, a narrow valley topography and strong foundation rock in the abutments. Weirs and spillways ► The large majority of small hydro schemes are of the run-of-river type, where electricity is generated from discharges larger than the minimum required to operate the turbine. ► In these schemes a low diversion structure is built on the streambed to divert the required flow whilst the rest of the water continues to flow over it. Such a structure is commonly known as a weir, whose role is not to store the water but to increase the level of the water surface so the flow can enter into the intake. Cont’d ► Weirs and spillways can be subdivided into fixed and mobile structures. Smaller fixed structures are generally referred to as weirs, whereas larger structures are often referred to as spillways. Spillways are often divided into un-gated and gated spillways, corresponding to fixed and mobile structures, the un-gated spillway in fact being a large-scale weir. Cont’d ► Fixed storage structures, such as weirs and ungated spillways have the advantage of security, simplicity, easy maintenance, and are cost effective. However, they cannot regulate the water level and thus both the water level and energy production fluctuates as a function of discharge. Cont’d ► Mobile storage structures such as gated spillways can regulate the water level such that it stays more or less constant for most incoming flow conditions. Depending on gate configuration and discharge capacity they may also be able to flush accumulated sediment downstream. These structures are generally more expensive than fixed structures, for both construction and maintenance, and their functioning is more complicated Weir Types The sharp-crested weir: 1. • It is easy to construct and relatively costeffective. Special attention has to be paid to the shape of the downstream face of the upper part of the weir in order to obtain sufficient aeration between the lower nappe of the jet and the structure. If the lower nappe of the jet sticks to the structure, vibrations may be transferred from the flow to the structure. Cont’d. 2. The broad-crested weir: • It is often applied for temporary structures or for structures of secondary importance, such as in case of temporary flow diversion. Its design is simple and cost-effective. The hydraulic conditions are far from optimal, expressed by a low discharge coefficient and the presence of under-pressures along the weir crest and downstream face. The discharge depends on the form of the structure. Cont’d 3. Ogee weir: • It is hydraulically the most ideal solution giving the highest discharge coefficient. Its curved shape is defined by the jet trajectory that would appear for the design discharge HD. For lower or higher discharges, over-or under-pressures will appear along the downstream face. For discharges much higher than the design discharge, these underpressures may lead to cavitation and damage to the downstream concrete face. Recent work suggests fortunately that separation will not occur until H > 3HD. Weir configurations Gated Spillways The installation of mobile elements on dams or weirs allows control of the flow conditions without changing the water level. This is performed by means of gates, which are designed such that, when the gate is fully open (and the structure functions as if it where fixed) the discharge has to pass the structure without noticeable water level increase upstream. Gate operation needs permanent maintenance and an external energy source. As a result, there is a risk that the gate remains blocked during floods. Other spillway types ► Flashboards Cont’d ► Inflatable weirs Cont’d ► Fuse gates Cont’d ► Siphon spillways Cont’d. ► Shaft (or Morning glory) spillways Cont’d ► Labyrinth weir Energy dissipating structures ► Stilling basin ► Baffled apron drop ► Plunge pool ► Chute cascades Intake structures ►A water intake must be able to; Divert the required amount of water into a power canal or into a penstock without producing a negative impact on the local environment and with the minimum possible head losses. Handle a major challenge of debris and sediment transport. Points to be considered in the design of intake structure ► Hydraulic and structural criteria common to all kind of intakes. ► Operational criteria (e.g. percentage of diverted flow, trash handling, sediment exclusion, etc.) that vary from intake to intake. ► Environmental criteria characteristics of each project (eg requiring fish diversion systems, fish passes, etc). Intake types The first thing for the designer to do is to decide what kind of intake the scheme needs. These can be classified according to the following criteria. Power intake: The intake supplies water directly to the turbine via a penstock. These intakes are often encountered in lakes and reservoirs and transfer the water as pressurized flow. Intake types Conveyance intake: The intake supplies water to other waterways (power canal, flume, tunnel, etc.) that usually end in a power intake. These are most frequently encountered along rivers and waterways and generally transfer the water as free surface flow. Conveyance intakes along rivers can be classified into lateral, frontal and drop intakes. Intake characteristics Cont’d ► The lateral intake functions by using a river bend or by using a gravel deposition channel ► This intake favorably applies the presence of a strong secondary current along the outer bend of the curved river. This secondary current prevents bed load from entering the intake. The installed discharge Qep has to be smaller than 50 % of the critical river discharge Qcr, where the latter is defined as the discharge for which the bedload transport starts. Typical layout of lateral intake Frontal Intakes The frontal intake is always equipped with a gravel deposition tunnel and is well adapted for rectilinear river reaches. The deposition tunnel has to be flushed in a continuous manner and the maximum river width is 50 m. A major advantage of this type of intake is its ability to handle large quantities of both bed and suspended load. However, this needs continuous flushing and thus large losses of water. The frontal intake is largely applied in regions with very large bed and suspended loads. Cont’d. The drop intake is generally used in steep sloped rivers, such as torrents, and for rectilinear reaches Drop Intake Tyrolean intake Longitudinal view Cont’d. Lateral view Head losses For small hydro plants, head losses can be of huge importance to the feasibility of the project and should thus be minimized as much as possible. Approach walls to the trashrack designed to minimize flow separation and head losses Piers to support mechanical equipment including trashracks, and service gates Guide vanes to distribute flow uniformly. Vortex suppression devices. Appropriate trashrack design. Cont’d The velocity profile decisively influences the trashrack efficiency. The velocity along the intake may vary from 0.8 - 1 m/s through the trashrack to 3 - 5 m/s in the penstock. A good profile will achieve a uniform acceleration of the flow, minimizing head losses. A sudden acceleration or deceleration of the flow generates additional turbulence with flow separation and increases the head losses. Cont’d A constant acceleration with low head losses requires a complex and lengthy intake, which is expensive. A trade-off between cost and efficiency should be achieved. The maximum acceptable velocity dictates the penstock diameter; the need for a reasonable velocity of the flow approaching the trashrack dictates the dimensions of the rectangular section. Cont’d The research department of "Energy, Mines and Resources" of Canada commissioned a study of entrance loss coefficients for small, low-head intake structures to establish guidelines for selecting optimum intake geometry. The results showed that economic benefits increase with progressively smoother intake geometry having multiplane roof transition planes prepared from flat form work. In addition, it was found that cost savings from shorter and more compact intakes were significantly higher than the corresponding disadvantages from increased head losses Cont’d Analyses of cost/benefits suggests that the best design is that of a compact intake with a sloping roof and converging walls, whilst the length of the intake is unlikely to be the major factor contributing to the overall loss coefficient. The K coefficient of this transition profile was 0.19. The head loss (m) in the intake is given by: ΔH = 0.19 V2/2g Cont’d Where V is the velocity in the penstock (m/s). Head losses due to the trashrack depend on spacing and shape of the bars, orientation of the trashrack compared to the flow and eventual obstruction due to debris and are discussed in more detail below. Trashracks One of the major functions of the intake is to minimize the amount of debris and sediment carried by the incoming water, so trashracks are placed at the entrance to the intake to prevent the ingress of floating debris and large stones. A trashrack is made up of one or more panels, fabricated from a series of evenly spaced parallel metal bars. If the watercourse, in the flood season, entrains large debris, it is convenient to install, in front of the ordinary grill, a special one, with removable and widely spaced bars (from 100 mm to 300 mm between bars) to reduce the work of the automatic trashrack cleaning equipment. Cont’d Trashracks are fabricated with stainless steel or plastic bars. Since the plastic bars can be made in airfoil sections, less turbulence and lower head losses result The bar spacing varies from a clear width of 12 mm for small high head Pelton turbines to a maximum of 150 mm for large propeller turbines. The trashrack should have a net area (the total area less the bars frontal area) so that the water velocity does not exceed 0.75 m/s on small intakes, or 1.5 m/s on larger intakes to avoid attracting floating debris to the trashrack. Cont’d Trashracks can be either be bolted to the support frame with stainless steel bolts or slid into vertical slots, to be removed and replaced by stoplogs when closure for maintenance or repair is needed. In large trashracks it must be assumed that the grill may be clogged and the supporting structure must be designed to resist the total water pressure exerted over the whole area without excessive deformation. Cont’d Prefabricated Booms Cont’d The trashrack is designed in such a way that the approach velocity (V0) remains between 0.60 m/s and 1.50 m/s. The maximum possible spacing between the bars is generally specified by the turbine manufacturers. Typical values are 20-30 mm for Pelton turbines, 40-50 mm for Francis turbines and 80-100 mm for Kaplan turbines. trash rack head losses Cont’d Cont’d Vorticity A well-designed intake should not only minimize head losses but also preclude vorticity. Vorticity can appear for low-head pressurized intakes (power intakes) and should be avoided because it interferes with the good performance of turbines especially bulb and pit turbines. Cont’d. Vortices may effectively Produce non-uniform flow conditions. Introduce air into the flow, with unfavorable results on the turbines: vibration, cavitation, unbalanced loads, etc. Increase head losses and decrease efficiency. Draw trash into the intake. Cont’d The criteria to avoid vorticity are not well defined, and there is not a single formula that adequately takes into consideration the possible factors affecting it. According to the ASCE Committee on Hydropower Intakes, disturbances, which introduce non-uniform velocity, can initiate vortices. Cont’d Vortices can be initiated by: Asymmetrical approach conditions; Inadequate submergence; Flow separation and eddy formation; Approach velocities greater than 0.65 m/sec; Abrupt changes in flow direction; Lack of sufficient submergence and asymmetrical approach seem to be the most common causes of vortex formation. An asymmetric approach is more prone to vortex formation than a symmetrical one. When the inlet to the penstock is deep enough and the flow is undisturbed, vortex formation is unlikely. Cont’d Empirical formulas exist that express the minimum degree of submergence of the intake in order to avoid severe vortex formation. Nevertheless, no theory actually exists that fully accounts for all relevant parameters. Cont’d ► The submersion is defined as ht Cont’d ► The following formulas express the minimum values for Submergence (ht) Cont’d It is important to highlight that V is the velocity inside the downstream conduit in m/s and D is the hydraulic diameter of the downstream conduit in m. Sediment traps Conveyance intakes are designed on rivers in order to eliminate possible floating debris and bedload transport. However, they cannot prevent the entrance of suspended sediment transport. For this, a sediment trap is projected downstream of an intake. The main objective of such a trap is to avoid sedimentation on downstream structures (canals, shafts, etc.) as well as to limit the possible damage of sediments on the hydro mechanical equipment. Cont’d A sediment trap is based on the principle of diminishing the flow velocities and turbulence. This results in a decantation of suspended sediments in the trap This diminishing is obtained by an enlargement of the canal, controlled by a downstream weir Cont’d Cont’d Efficiency of a sediment trap It is a measure of the grain diameter that deposits in the trap. The choice of efficiency depends on the type of hydro mechanical equipment and on the gross head difference of the power plant Cont’d Reparation intervals of Francis turbines Around 6-7 years for a sediment trap efficiency of 0.2 mm, 3-4 years for an efficiency of 0.3 mm and 1-2 years for an efficiency of only 0.5 mm. Experience has shown that the most economical solution is around 0.2 mm efficiency for severe conditions (significant gross head, quartz particles) and around 0.3 mm for normal conditions. Cont’d Design The necessary length of a sediment...
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