This page takes you through a hydro facility from the catchment to the tailrace, following the path that the water takes.
On the way, you will learn something of the components that make up a hydro installation. There are also numerous diversions where you can branch off to learn about the key Mechanical and Electrical components.
Table of Contents
If you want to jump to a particular section, you can use this menu:
- Catchment
- Reservoir
- Dam
- Spillway
- Intake
- Headrace and Surge Tank
- Penstocks and Pressure Tunnels
- Bifurcations
- Powerhouse
- Tailrace
Catchment
The most upstream component of a hydro scheme is the water catchment. You can use a contour map of the area upstream of your water intake to find which areas will drain down towards your intake and which will not. The line separating the two areas is called the watershed. The area that drains towards the intake is the catchment area.
A hydrologist or hydrotechnical engineer can characterise the catchment in terms of steepness, stream length and vegetal cover, and use historical records from flow gauging stations and meteorological records to work out the flows that will arrive at your hydro intake, as well as the size of the expected floods.
In general terms:
- The larger the catchment area, the larger the flow available for hydropower;
- The more vegetation on the catchment, the less sediment in the water;
- The steeper the catchment and the more paving, the higher the flood peak.
Reservoir
The reservoir of a hydro facility is the component used to store water.
Flows from catchments vary with the seasons; they are greater in the wet season and lower in the dry season. A reservoir is used to store up excess water in the wet season so that it can be used in the dry season. In this way the reservoir increases the Firm Flow available for power production. (Drag the red dot on the widget below to increase the reservoir storage and show how this increases the Firm Flow – the top of the dark blue area.)
By increasing the firm flow, a reservoir increases the firm power available from the hydro facility.
Not all hydro facilities have a reservoir. Some have a small head pond on a river from which water is diverted to the hydro system.
Climate Change
Climate change is creating more extreme weather, including higher floods in the wet season and drier dry seasons. Reservoir storage is therefore increasingly important for hydro production. Reservoirs designed for historical flows may not regulate the inflows sufficiently in the future.
Booms
Logs and other debris from the catchment can accumulate in reservoirs and move downstream towards the dam due to the flow through the reservoir and wind action. If not managed, it can interfere with the functioning of the power intake and spillway.
The primary debris management tool is a debris boom. Traditionally, these booms were constructed out of logs and chains. Most modern booms are plastic. They are equipped with hanging debris skirts to prevent material from pushing under the boom. They may also incorporate floating walkways for personnel to move the debris to shore.
Two of the debris boom suppliers are Tuffboom and Worthington waterway barriers.
The booms are anchored at each end, often with one end on shore and one end on the dam, in front of the spillway and in front of the power intake. Intermediate anchors may be placed underwater to shape the boom so that logs tend to move towards the shore for easier removal.
Reservoir Range
A reservoir is operated over a range of water levels from:
- Minimum Reservoir Level (sometimes called Minimum Normal Operating Level or Minimum Pool) to
- Full Supply Level (also called Maximum Normal Operating Level.
If a reservoir is operated below Minimum Operating Level, there is a risk that air might be entrained into the hydro Intake. These are sometimes impacts on other uses of the reservoir, such as recreation.
- When a reservoir is operated above Full Supply Level, it enters the Freeboard range (see below), which may reduce Dam Safety.
- The volume of water stored between the minimum and maximum normal levels is called the Active Storage.
- The volume of water below the Minimum Operating Level is called the Dead Storage, because it is not available for use. (See Reservoir Sedimentation below.)
Freeboard
The dam and valley sides are the water barriers that enclose the reservoir (see Reservoir Rim below). They must be higher than the Maximum Normal Water Level by a margin known as freeboard. Without freeboard, water would spill out of the reservoir frequently and cause erosion that could eventually result in a breach.
Calculation of the required freeboard takes account of many factors, such as:
- Flood surcharge (how much the reservoir will rise when passing floods through the spillway (see Spillways below);
- The run-up of waves, that are generated by wind blowing over the reservoir surface and then run up the valley sides and the face of the dam; and
- The set-up of the water surface caused by wind blowing over the reservoir and causing the reservoir to slope in the down-wind direction.
- The ability of the dam structure to tolerate overtopping
In some cases, local dam safety regulations specify a Minimum Freeboard. In this case the freeboard is the greater of the calculated freeboard and the specified minimum freeboard.
The elevation of the reservoir water barriers must be at least as high as the
Normal Maximum Reservoir Level plus the freeboard.
In the case of a concrete gravity dam, the dam body is the water barrier and the dam can be designed for some overtopping, so the dam crest is set at this level.
In the case of a fill dam, the water barrier is often an impervious core and an access road is built above this level. The dam crest is therefore higher than the top of the water barrier by an amount equal to the thickness of the road works, and the freeboard on the top of the core is higher than in the case of a concrete dam because even a small amount of overtopping can cause erosion that leads to a dam breach.
Dam Safety
Water stored in a reservoir has an enormous potential for destruction. Dam safety regulations and procedures keep the public and downstream areas safe from the damage that would result from an uncontrolled release of the reservoir. Engineers take this responsibility extremely seriously and every aspect of dam design is carried out with especial care.
Before a new reservoir is impounded, emergency procedures and Operation and Maintenance procedures are drawn up and approved by the dam owner and dam regulation authorities.
The emergency procedures are shared with civil authorities and police, so that everyone know what to do in the event of an emergency at the dam.
The operation and maintenance procedures cover the operating parameters for safe operation of the dam, such and the safe operating range of the reservoir. They also cover the routine inspection, monitoring and maintenance required for all key components, including cofferdams and the reservoir rim (see below). They also set out the maximum interval between dam safety reviews.
Dam safety reviews are carried out by independent engineers. They look at the original design criteria and compare them to current regulations and guidelines. They examine the data gathered from the dam monitoring instrumentation and reports on maintenance and repair work. Then they carry out a site inspection and examine the facilities thoroughly.
Reservoir Rim
The valley sides abound the reservoir also need to be higher than the Maximum Normal Operating Level by an amount equal to the freeboard. When there are low points (saddles) in the natural valley sides, saddle dams need to be constructed to raise the ground level and prevent spilling. Because the saddle dams are water-retaining structures, they also need to be maintained and inspected regularly.
The valley sides of a reservoir need to be watertight and stable. If the regional geology contains karstic limestone, for example, there might be natural water passages in the rocks around the reservoir that will allow the impounded reservoir water to leak past the dam or into an adjacent valley.
If the valley sides are steep, they might be destabilized and fail when the reservoir is filled. Sudden landslides into the reservoir can cause displacement waves which can travel to the dam and cause a breach.
Reservoir Sedimentation
The rivers or streams entering the reservoir will contain sediment from the catchment. When the catchment area is poorly vegetated and the area has a high rainfall, the quantity of sediment can be very large.
When the sediment enters the reservoir, it tends to settle out. Usually the coarse sediments are deposited at the point where the stream enters the reservoir. Finer sediments can travel further into the reservoir. Some sediment might even pass right through the reservoir.
Depending on where the sediment settles out, it might reduce the dead storage or the live storage.. Reduction in the live storage affects the usefulness of the reservoir for storing water.
Once sediment has settled in a reservoir, it is very costly to remove. Sometimes the reservoir is drained and large earth-moving equipment is used to excavate the sediment. Hydraulic flushing and hydraulic dredging have also been used.
The volume of incoming sediment can be greatly reduced by erosion-control measures in the catchment.this might include planting forests and construction upstream sediment traps.
Then turbid flows enter a reservoir as density currents, it has been possible to route them to an outlet at the dam with limited deposition in the reservoir. However, “sluice valves” at dams only produce a very localized depression in the reservoir sediments.
In some cases, it is possible to construct a diversion weir at the upstream end of the reservoir to divert the inflow when it is carrying a heavy sediment load so that it does not enter the reservoir. When the topography is suitable, the diverted flow via a bypass canal or tunnel to a point immediately downstream of the dam. One example is Nagle Dam on the Mgeni River, where the reservoir has a U shape and the diversion weir and dam are at the top of the arms of the U.
In some cases, the flow is diverted to an off-stream reservoir, or to an adjacent valley.
Dam
A dam is the component that holds (impounds) the reservoir. It is usually built across a stream or river, blocking the flow. This causes the water level behind the dam to rise.
It must:
- remain stable under the applied loading; and
- prevent water from leaking out.
There are numerous dam types, as illustrated on the graphic below. [This graphic comes from an animated widget that is currently being rewritten as it does not presently run on some browses]
Concrete Gravity Dam
An ICOLD publication on dam failures showed that the safest type of dam is the concrete gravity dam. The water loads exert only modest stresses on the dam body and foundations, and this type of dam can be designed for safe overtopping during floods.
The main design consideration is often the tensile stresses that develop in the dam body due to the heating of the concrete as it cures. To counter this, low-heat cement is used in the concrete mix, a pozzolan such as fly ash or blast furnace slag is used to replace part of the cement, and part of the mix water is replaced by ice. If this is insufficient, a system of cooling water pipes is incorporated in each concrete lift to limit the maximum temperature of the dam body as the concrete cures.
Since the 1980s, the majority of concrete gravity dams and some arch dams have been constructed using RCC instead of conventional vibrated concrete (CVC). RCC is “roller-compacted concrete”. It is more economical than conventional concrete because it uses mass-production techniques, usually incorporating one or more of the following:
- Mixing of RCC in a continuous process as it flows through a rug mill, rather than in the batch process used to produce CVC;
- Transporting the RCC to site in dump trucks or by a conveyor system, rather than in transit mixer trucks;
- Spreading the RCC on the lift surface using small conventional bulldozers;
- Compacting the RCC in lift thickness of about 300 mm using normal vibrating road rollers and normal plate vibrators near the formwork;
- Creating contraction joints using crack-initiators instead of conventional formwork.
The animated sketch above shows a cross section of a typical RCC gravity dam. Within this dam there is a longitudinal gallery for drainage and inspection, connected to a access gallery from the downstream face of the dam.
Drain holes are drilled upwards from the gallery to ensure that seepage pressure cannot build up between the layers of RCC. Drain holes are also drilled down from the gallery into the dam foundation to reduce the uplift that seepage water can exert on the bottom of the dam.
At the heel of the dam (the bottom of the upstream face) there is a reinforced concrete grouting plinth, connected to the foundation rock with rock anchors. One or more rows of boreholes are drilled deep into the dam foundation from this plinth, and grout (cement paste) is injected into the holes to form a grout curtain to reduce seepage flow beneath the dam foundation.
RCC is usually porous. to prevent water seeping through the dam, the upstream face may be lined with a patented dam membrane supplied and installed by Carpi Tech. S.A. Alternatively, each layer of RCC near the upstream face of the dam may be enriched by vibrating additional cement grout into the RCC as it is compacted. This technique is known and GERCC (grout-enriched RCC).
Concrete Buttress Dam
A concrete buttress dam has an inclined face slab to contain the reservoir. The water loads on this slab are transferred to the dam foundation through buttresses (supporting walls).
Sometimes the upstream face is flat; sometimes it is curved like the downstream face.
The illustration below shows the Daniel-Johnson dam in Canada, which is a thick buttress dam.
The following illustration shows a downstream view of a thin buttress dam. In seismic areas, the thin buttresses need to be supported laterally to prevent excessive flexing under cross-valley earthquake motions. In this illustration, a system of beams is used to provide this support.
Concrete Arch Dam
In a narrow valley with competent rock sides, a more structurally efficient dam is possible – the arch dam. The relatively thin arch carries the water loads to the valley sides and bottom. The interactive view below shows Victoria Dam in Sri Lanka, designed by Sir Alexander Gibb & Partners. (The author led one of the SAGP design team working on this dam.)
River Diversion
To build a dam, you need to be able to inspect the foundation rock or soil and carry out any required excavation, strengthening and preparatory treatment. Since there is usually a river flowing over the foundation, you need to divert the flow so that you can do this work and build the dam “in the dry”.
In a wide river valley, cofferdams (temporary fill dams) are usually constructed to move the river to one side of the water channel while part of the dam is built – including gated sluices that can pass the river flow. Then the cofferdams are rebuilt to divert the water through the partial dam while remainder of the foundation is dewatered and the remainder of the dam is built. When the dam is complete, the cofferdams are removed and the sluices are closed to impound (fill) the reservoir.
In a narrow river valley, it is more usual to excavate one or more diversion tunnels to carry the river flow past the dam site. The tunnel inlet, upstream of the dam site, is equipped with a diversion gate. The outlet is downstream of the dam site. When the tunnel is complete and the diversion gate has been tested, cofferdams are constructed upstream and downstream of the dam foundation to divert the river through the tunnel while the foundati9n is prepared and the dam is built.
Example of a River Diversion
The diversion works are designed for smaller floods than the dam itself, because:
- The diversion works only need to be in operation for two or three years, whereas a dam typically has a service life of 100 years.
- The consequence of failure of the diversion works is lower than for a fully-impounded dam because the volume of stored water is usually small.
- The partly complete dam structure is usually extremely stable, and therefore highly unlikely to fail and release a flood wave downstream.
- The cost of repairing the partly completed dam is usually manageable and insurable.
The diversion flood is typically selected using a hydro-economic analysis, in which the likelihood of failure is weighed against the cost of repairs.
The design flood usually has a Return Period, T of 25 to 50 years. The probability that this flow will be exceeded during a construction period of “n” years is:
For example, the probability that a flood with 50-year return period will be exceeded in a 3-year construction period is 5.88%.
Spillway
When there is a flood in the catchment of a reservoir, it needs to be discharged safely into the river downstream without raising the reservoir level above its maximum flood level. The spillway is the facility that manages this discharge.
The discharge through a spillway is governed by the weir equation as follows:
Discharge, Q = Cd x L’ x He^1.5
where
- Cd is a discharge coefficient;
- L’ is the effective length of the crest;
- He is the hydraulic head on the crest; and
- He^1.5 means that the hydraulic head is raised to the power 1.5.
The discharge coefficient varies with the head (He), the shape of the crest (usually broad-crested or ogee shape), and the approach channel depth (P). The discharge can also be reduced by flow conditions downstream (especially if the spillway tailwater is higher than the spillway crest and partly submerges the crest.
If you are using a design standard to calculate the discharge coefficient, please be aware that it is dimensional. That means that the value of the discharge coefficient depends on whether the flow, crest length and hydraulic head are measured in Imperial (feet) units or Metric (metre) units.
The effective length is equal to the total crest length reduced by the width of any piers and further reduced by allowances for flow contractions at each pier and each abutment. (The US Corps of Engineers and the US Bureau of Reclamation have formulas for these contraction allowances.
The hydraulic head is the total head immediately upstream of the spillway crest and is the sum of the water depth at this point and the velocity head, v²/2g.
Spillways fall into three basic categories:
- ungated spillways (also called uncontrolled crests);
- gated spillways; and
- fuse spillways.
Ungated spillways have crests set at the Maximum Normal Reservoir Level. If the reservoir rises above this level, it simply flows over the crest. Because ungated spillways don’t rely on any mechanism to release the water, they are preferred from the dam safety point of view.
Spillway Profile
Channel Spillways
Channel spillways are a type of ungated spillway. They comprise a simple discharge channel, remote from the dam, with a “grade control sill” and scour protection. The sill is a concrete curb or roadway, which defines the precise reservoir level at which the spillway will start to discharge water. The ends of the sill are protected with concrete or rip rap, otherwise scour could widen and scour deep channels that bypass the sill and led to an uncontrolled release of the reservoir.
The flow through channel spillways is governed by channel friction and minor losses, so the discharge capacity is developed from backwater calculations. In general terms, they are normally the least efficient type of spillway, so they must be wider than other types of spillway to pass the reservoir design flood.
Uncontrolled Crests
If the approach channel is lower than the spillway crest, the profile shape of the crest affects the discharge coefficient. For example, the discharge coefficient of an ogee spillway profile is 41% higher than the discharge coefficient of a broad crested weir (overtopping dam crest road.
Labyrinth Spillways
Labyrinth spillways are a special type of uncontrolled crest. The crest follows a zig-zag arrangement to increase the effective crest length, L’. However, as the water depth over the depth increases, the benefit of the increased length diminishes.
A 3D model of a labyrinth spillway is shown at the following link:
https://sketchfab.com/3d-models/labyrinth-spillway-v3-567409aad7324591aef9a7d4dd36a590
Piano Key Spillways
The piano key spillway is an evolution of the labyrinth spillway, as shown in the 3D model at the following link:
https://sketchfab.com/3d-models/piano-keiy-weir-v4-a58440f079c049c98c68f24ef03ed040
Morning Glory Spillways
A special application of an ungated spillway is the morning glory spillway. This is a vertical tower built in the reservoir and topped by a circular ogee spillway. When the reservoir rises above the ogee crest, water spills into the hollow core of the tower. At the base of the tower the water flows into a conduit or tunnel that carries it to the river downstream of the dam.
Gated Spillways
The crest on gated spillways is lower than Maximum Normal Reservoir Level. At reservoir elevations higher than spillway crest level, the gates keep the water from spilling out of the reservoir. This has several advantages:
- When the reservoir rises above Maximum Normal Reservoir Level, the head on the spillway crest is (Reservoir Level – Spillway Crest Elevation), whereas the head on an uncontrolled crest is only (Reservoir Level – Maximum Normal Reservoir Level). The discharge capacity of a spillway depends on He to the power 1.5, so the discharge capacity of a gated spillway is much larger than that of an ungated crest, allowing the spillway crest length to be reduced considerably.
- The gates can be opened to lower the reservoir reasonably rapidly, for example to carry out a safety inspection after an earthquake.
- Reservoirs equipped with flood warning systems can be lowered in advance of incoming floods, to reduce the maximum flood outflows from the spillway to the river downstream of the dam.
The chief drawbacks of gated spillways are as follows:
- the gate bays are narrower than uncontrolled crests, which makes them more susceptible to blockage by trees carried down by the flood.
- the safety of the dam depends on the ability of the gates to open when a flood enters the reservoir. If they fail to open, the dam can be overtopped, leading to a dam break.
- gates therefore require regular maintenance and testing to confirm that they are in working order.
- some dams are equipped with automatic spillway gates, but most spillway gates are opened by the dam operator. This means that operators must be on duty at all times time, night and day, to manage discharges and reservoir levels in the event of a flood.
- the power supply to the gates is also mission-critical. Spillways on dams with a high incremental consequence of failure typically have several redundant power supplies to the gates.
- gated spillways are usually more expensive than ungated spillways.
Spillway Gates
There are numerous types of spillway gates. The most common types are described on the hydro-mechanical page.
Fuse Plugs and Fuse Gates
Sometimes a reservoir is equipped with a (gated or ungated) spillway that is designed for a relatively low flood frequency, such a 1:1000 in any year of operation. Larger floods are possible, but they have a low probability of occurring during the lifetime of the project. To cater for these rare events, such reservoirs are sometimes equipped with a fuse plug spillway or fuse gate spillway.
- A fuse plug spillway has a crest below Maximum Normal Reservoir Level, topped with a specially-designed erodible earth fill. When a flood causes the reservoir level to approach the Maximum Flood Level of the reservoir, the fill breaches and washes away, allowing the crest to pass the excess flood waters.
- A fuse gate spillway is similar, but the crest is fitted with a system of tipping elements that wash off the crest when required. The reservoir level at which each gate tips can be set accurately, and tipping one gate does not tip the rest. The patent for fuse gates is held by HydroPlus.
Spillway Chute
The water flowing downstream of a gated or ungated spillway crest is usually carried in a chute to the river downstream of the dam. This is typically an open channel, although tunnels are used where there is insufficient space at the site for a channel.
The flow in spillway chutes is usually rapid and turbulent. A short distance downstream of the crest, the flow becomes fully aerated, causing the water to bulk up. Small irregularities at the head of the chute (like the piers that support the spillway gates and any narrowing of the spillway chute) gives rise to cross waves in the chute.
The walls of spillway chutes need to be high enough to prevent overtopping by the bulked up flow and the cross waves. Models of large spillways are normally tested in a hydraulic laboratory to ensure that the prototype spillway will perform safely.
The velocity of flow on a spillway chute can be high. When the calculated velocity approaches 20 m/s, the chute needs to be checked for cavitation. (Cavitation occurs when a small irregularity on a flow surface creates a region in the adjacent high-velocity flow that is below the vapour pressure. Bubbles of low-temperature water vapour then form in the water. The flow carries these bubbles downstream where the water pressure is higher than the vapour pressure, causing the bubbles collapse explosively. each collapse emits a shock wave that can damage the adjacent concrete surface, even causing large holes to develop.)
The solution to cavitation is to aerate the flow, usually by including a specially designed offset into the chute surface so that air can be drawn into the flow.
Stepped spillways and baffled apron spillways are designed to dissipate excess energy in the flow as it passes down the chute.
Energy Dissipators
When spillway flow arrives at the foot of a conventional spillway chute, it usually has high velocities and a large amount of turbulent energy. If it is released directly into the river, it will lead to extremely large scour of the river bed and river banks, and the scoured material will pollute the river.
The answer is to dissipate the energy in a terminal structure. The most typical types of terminal structure are as follows:
- plunge pools (most common downstream of arch dams with spillways on the dam crest)
- roller buckets, which are designed to hold a rotating roller at the toe of the dam;
- flip buckets, which eject the flow into the air, where is travels in a trajectory and lands downstream of the dam. This dissipates some of the energy and spreads it out over a wider area on the river bed so there is less scour.
- stilling basins, which create a hydraulic jump (standing wave) where the energy is dissipated within the reinforced concrete boundaries of the basin before it flows out onto the river downstream.
Intake
Hydro intakes are used to lead water from a head pond or reservoir to the water conveyance system.
The illustration above shows an intake leading to a headrace tunnel. The red and blue components are described on the hydro-mechanical page.
On the face of the intake is a trash rack, which stops large logs and other debris from entering the intake.
Downstream of the trash rack is a smooth transition, which is designed to reduce hydraulic entrance losses.
At the end of the transition is the intake gate, which is shown closed in the illustration. It is often called a control gate, but it is NOT used to control the flow through the turbine. It is used to shut off the flow in an emergency, so it is designed to operate even when there is a high rate of flow through the intake. It is also used to isolate the water conveyance system.
The type of intake shown in the illustration is an upstream sealing gate, which has several advantages over downstream sealing intake gates. One of these is that the gate well (the vertical shaft immediately downstream of the gate) is dry when the gate is closed, so it is usually equipped with a stairway for access to the top of the intake gate. There is often a ladder on the back of the intake gate for access to the invert (floor) of the water conveyance.
The gate well also acts as an air shaft when the intake gate is being closed. A large volume of air rushes into the intake as the flow reduces and the water conveyance empties. The gate shaft is sized so that the air velocity is less than 30 m/s.
At the top deck of the intake is a hydraulic cylinder connected to the top of the gate by a stainless steel shaft. The cylinder is operated by a control system and HPU (high pressure unit), and together this equipment is called the gate hoist.
Immediately upstream of the intake gate is another gate slot where a bulkhead gate or stop logs can be installed using a mobile crane. This extends the isolation zone upstream when we need to inspect of maintain the operating gate. The flow must be reduced to zero before stop logs or a bulkhead gate can be installed or removed.
Immediately downstream of the intake gate, there is a transition. This is where the cross section of the water passage changes shape. In this case, the flow cross section at the location of the gate has a rectangular shape, while the cross section at the downstream end of the transition has a circular cross section to match the shape of the headrace tunnel. (The cross sectional shape of a tunnel is usually selected to suit the proposed excavation method.)
While intakes are generally connected to a water conveyance system, in low head hydro facilities it is common for the intake to be connected directly to the turbine inlet.
Headrace
When the reservoir or head pond location more than a kilometre (approximately) from the best location for the powerhouse, transients in the water conveyance system can make the generating units hard to regulate. the inertia of the water in the long water conveyance regulation of the turbine difficult.
This problem can be minimized by reducing the waterway length from the turbine to a free water surface.
In the illustration at the top of this section, a headrace tunnel has been introduced with a surge tank at the downstream end. The free water surface in the surge tank is much closer to the turbine than the reservoir.
A headrace might also be a large-diameter pipe or a power canal. The surge tank might be a surge shaft (if it is nearly all underground), or it might be a head pond constructed at the downstream end of a power canal.
The headrace is itself subject to transients, but the transient waves travel slowly and they are referred to as surges. The water level in the surge tank rises and falls in response to these surges. The maximum and minimum water levels in the surge tank is calculated from transient analyses, which are carried out for:
- The full range of reservoir levels;
- The starting and stopping of different numbers of generating units; and
- Unusual cases such as a unit trip at the worse time interval after a unit start.
The top elevation of the surge shaft is higher than the maximum upsurge. The bottom elevation of the surge shaft is lower than the lowest calculated downsurge.
The range of surge levels in the tank can be adjusted by the choice of surge tank design. The most common designs are:
- Simple surge tank;
- Throttled surge tank; and
- Compound surge tank.
Penstocks and Pressure Tunnels
The part of the water conveyance system that connects to the turbine is called the penstock. It carries water at high pressure, so it needs to be designed and fabricated with specialist knowledge and great care since it would otherwise be a hazard to life and a dam safety concern.
Penstock welding is carried out in accordance with the Pressure Vessel Code, and all welds are checked by non-destructive testing (NDT), preferably by X-ray.
When a penstock is supported above the ground on concrete or steel saddles, it is heated by the sun and cooled by the water inside. The thermal expansion and contraction creates length changes and axial forces that must be considered in the design. Usually expansion couplings are used between selected lengths of the penstock to allow these movements to occur.
When a penstock changes direction, the bend experiences similar thermal forces as well as static and dynamic hydraulic forces, so it needs to be restrained or it could move, leading to a penstock rupture. The restraints may take the form of concrete anchor blocks, or the penstock might be buried; in that case it is called a “soil-restrained” penstock.
It is sometimes economical to carry the high pressure water underground. In this case, the vertical or steeply-inclined waterways are called pressure shafts and the horizontal or gently-sloping waterways are called pressure tunnels. If the rock around these shafts and tunnel is reasonably impermeable, much of their length can be unlined. Where the tunnel or shaft passes through a permeable zone, or where the rock has voids (common in karstic limestone rock) a liner is used. Tunnel liners are usually concrete or steel, although reinforced PVC liners by CarpiTech S.A. have been used.
A liner is smoother than a rock tunnel or shaft (even tunnels excavated with a tunnel boring machine), so lined pressure tunnels and shafts have a lower friction coefficient.
On the other hand, the liner occupies some of the cross sectional area available for flow, so for the same excavated size of tunnel liner, the flow velocity is higher. Since the hydraulic head loss proportional to the square of the velocity, the head loss in a lined tunnel is sometimes larger than if the tunnel had been unlined.
When a steel tunnel liner is used, the internal water pressure is carried by the steel and the surrounding rock. When the tunnel is dewatered for inspection and maintenance, the steel liner must be able to withstand the external pressure from the groundwater without buckling.
When a concrete tunnel liner is used, the dilation of the tunnel under internal water pressure is generally enough to cause the liner to crack. Even if reinforcement is used to distribute the cracks, a concrete liner is not considered waterproof.
This is not serious except when the stress in the surrounding rock is too low to prevent cracks from opening in the rock. [Technically speaking, when the minor principal in-situ rock stress is less than the internal water pressure multiplied by a Load Factor.] In this case, water enters joints in the rock and propagates uncontrollably, opening joints far and wide and leading to massive water loss and failure of the tunnel lining. This is known as hydro-jacking.
Such low rock stress occurs where a pressure tunnel is near the ground surface or near the upstream side of a powerhouse excavation. In these locations, the tunnel needs to be steel lined to prevent hydro-jacking. Steel tunnel liners near penstocks are usually called “penstocks”.
Bifurcations
As you can see from the box above, a surface or underground penstock sometimes needs to supply water to several generating units in a powerhouse. In this case, the penstock must branch into several smaller penstocks, called “unit penstocks”, each leading to one generating unit.
These branches are designed to suit the geometry, so they come in many different shapes and layouts, as shown in the illustration above and as follows:
- The simplest branch is a bifurcation, in which the penstock branches into two unit penstocks.
- A more complex branch is a trifurcation, in which the penstock branches into three unit penstocks.
- When a penstock or pressure tunnel supplies four units, an upstream bifurcation divides the tunnel in two, and then downstream bifurcations further subdivide each branch.
- Sometimes the penstock runs parallel to the centreline of the powerhouse, and individual unit penstocks branch off to each generating unit.
Powerhouse
The powerhouse is where the generating equipment, ancillary equipment and the powerhouse auxiliary equipment are housed. These terms are further explained in the pages on generating equipment and mechanical and electrical equipment.
the location of a powerhouse must be selected to suit the topography and overall site layout. It might be located:
- At the ground surface (Surface powerhouse);
- Underground (Underground powerhouse);
- Excavated in a pit so that the roof of the powerhouse is level with the ground surface (Semi-underground powerhouse); or
- In a deep shaft (pit or shaft powerhouse).
Each type of powerhouse is uniquely adapted to its location.
Powerhouse “Design”
When people talk about the “design” of a powerhouse, they mean the many design functions that are involved:
- One of the first design functions is to locate where the powerhouse will be situated. It must be as low as possible in elevation to maximize the hydraulic head available for power production, and at a suitable elevation relative to the tailrace. (Pelton turbines need to be above tailwater level, since the runners must rotate in free air; while Francis and Kaplan turbines generally need to be set below tailwater elevation to prevent formation of cavitation in the turbine.
- At this point the topography will generally dictate what type of powerhouse is appropriate: (i) above ground, (ii) underground, or (iii) semi-underground.
- The orientation of the powerhouse is then set. In the case of surface and semi-underground powerhouses, it is usually most economical to place the longest dimension of the powerhouse parallel to the ground contours. Underground powerhouses in massive rock are oriented to minimize rock stresses around the excavated powerhouse cavern. In foliated or highly-jointed rock, the powerhouse is oriented to reduce the possibility of encountering large unstable wedges in the cavern walls.
- The layout of the unit penstocks can then be defined. The powerhouse layout is simpler if the penstocks enter the powerhouse at right angles to the upstream wall, but if this requires longer penstocks or sharper penstock bends (both of which increase head loss), it is usual to turn the spiral case and incoming unit penstock up to 30° to the perpendicular. (This does not affect the orientation of the turbine draft tubes, which are normally at right angles to the downstream wall of the powerhouse.)
- The next step is the layout of equipment on each floor of the powerhouse. This requires input from the turbine-generator supplier, and the mechanical, electrical, civil, structural, hydrotechnical and geotechnical engineers. The layouts then need to be reviewed to ensure that the powerhouse will be safe to construct, operate, maintain and clean.
- Finally, each engineering discipline needs to size and lay out the many powerhouse systems, which often requires revisiting the overall layout to accommodate the detailed design requirements. For example, in this step, the structural team receive the loads that the turbine and generator, powerhouse crane, turbine inlet valve and other equipment will impose on the supporting concrete, and design the structure to carry them safely down to the powerhouse foundations; often the sizes of the structural components needs to be adjusted.. The Heating, Ventilation and Cooling (HVAC) designer must design the air intakes, cleaners, coolers, heaters and fans, and accommodate them and the ventilation duct system in the limited confines of the powerhouse; sometimes the powerhouse width or length needs to be adjusted.
Some Layout Guidelines
Generally, the powerhouse equipment is laid out to reduce the length of cables, oil lines, etc. For example:
- The governor HPU is located near the turbine servomotors.
- The exciter transformer is located along the main electrical bus close to the generator terminals.
- The generator neutral cubicle is placed adjacent to the outer wall of the generator enclosure.
At the same time, it is advisable to locate all electrical cabinets above normal tailwater level because of the risk of powerhouse flooding.
Tailrace
The water leaving the turbine runner passes through a draft tube where the flow velocity is reduced and head is recovered. It then needs to be conveyed to the receiving water body, which might be a river, canal, reservoir, lake, etc.
The water conveyance that does this is the Tailrace.
The form that the tailrace takes depends on the layout of the generating station, but typically it is either a channel or tunnel.
A surface powerhouse typically uses a surface channel as the tailrace. As shown on the following illustration, gates or stop logs are usually located near the draft tube, which isolate the draft tube from the tailrace water and allow the turbine water passages to be dewatered.
The water leaving the draft tube is turbulent and there are often recirculating eddies in the flow immediately downstream of the draft tube gate structure. These eddies can sweep rocks back into the draft tube, where they can prevent proper closure of the draft tube gates, so a rock trap is usually included immediately downstream of the structure to prevent this from happening.
The invert of the draft tube channel then rises to the invert of the tailrace channel. The depth and width of the channel is selected so that the flow velocity will be moderate even when the tailrace is operating at minimum elevation, because high velocities lead to head loss and less power production in the powerhouse.
An underground powerhouse must use a tailrace tunnel to carry the turbine outflow to the receiving water body. If the powerhouse has impulse turbines, the tailrace tunnel typically flows partly full (with an air space above the flowing water).
If the powerhouse is equipped with Francis or Kaplan turbines, extended draft tubes are often used to connect the powerhouse outlet to a surge chamber (which provides a free water surface close to the turbine, and improves regulation of the turbine). The draft tube gates are then often located in the surge chamber.
Downstream of the surge chamber, a tailrace tunnel carries the flow to an outlet structure, which is also equipped with gates or stop logs so that the tailrace tunnel can be dewatered for inspection and maintenance.
In this application, the tailrace tunnel flows full, so the full cross sectional area of the tunnel is used by the flow, which reduces the flow velocity and head loss and makes it more economical than a tunnel with free-surface flow.
