Conduits,intakes,power house and Accessories

Chapter Six
Chapter Six
Conduits, Intake, Power House and Accessories
By Yimam Alemu
December 17, 2020
By Yimam Alemu Chapter Six December 17, 2020 1 / 52

Chapter Six
Outline
1 Intake structures
2 Components of a conduit system
3 Power house
4 Electrical and mechanical equipments in a hydro plant
5 Safety measures in hydroelectric power plants

Chapter Six
Learning Objectives
At the end of this chapter the students should be able to:
Understand intake structures of hydroelectric power plant.
Understand types of power house and equipment provided in the power house of hydro-
electric power plant.
Understand electrical and mechanical equipments in a hydropower plant.
Understand safety measures in hydro electric power plants.

Chapter Six
Intake structures
Intake structures
The water from the reservoir/forebay is let into the penstocks through intake structures.
Main components are trash rack and gate
Trash rack prevents entry of debris into the water passage of hydropower plant.
The gates are provided to control the entry of water into the penstocks.

Chapter Six
Intake structures
1.Trash rack
It is provided to preventing the debris from getting in to the intakes from dam or from
the forebay in order to avoid entering plant waterways and damaging electromechanical
equipment.
Trash rack is made up of steel bars and is fitted directly at the intake to prevent the
debris from going into the intake.
The spacing of bars depending up on the following factors
Type of turbine
Size of floating material
Velocity of flow through trash rack

Chapter Six
Intake structures
Cont....
Figure: Intake structures

Chapter Six
Intake structures
2. Control gates
The gates and valves are used for controlling the flow rate of water entering the intake.
Intake or control gates are the gates built on the inside of the dam. The water from
reservoir is released and controlled through these gates.
A gate is used to regulate or control the flow of water from the dam.
Interior gate or sluice valves are also used to regulate the flow rate. These valves are
generally placed at the downstream of the sluice.

Chapter Six
Intake structures
Cont....
Modern dams use the following crest gates to control the flow rate. Vertical lift gates,
Traitor (radial) gates, Wheeled gates (roller gates, drum or cylindrical gates and but-
terfly valves etc.
For moderate heads butterfly valves are used while for high head needle valves are used.
These valves are not only used as intake gates for penstocks but also they are often used
before the turbine to facilitate the inspection without dewatering the whole penstock
line.

Chapter Six
Components of a conduit system
Components of a Conduit system (Inlet waterways)
Inlet waterways are the passages through which the water is conveyed from the dam(reservo
to the turbines.
These includes canals, tunnels, flumes, penstocks, forebay and also surge tanks.
Canals and flumes are open conduit, while tunnels, pipeline and penstocks are closed
conduit.
The penstocks are the pressure conduit, while canals and flumes are non-pressure con-
duits.
A canal is an open waterway excavated in natural ground following its contour.
A hydropower canal conveys water from the reservoir to the turbine intake structure.
In some cases, the canal conveys water to a forebay upstream of the turbine intake.

Chapter Six
Tunnel
A tunnel is a closed channel excavated through an obstruction such as a ridge of higher
land where pipeline or canal cannot be used due to topography.
Tunnel is made by cutting the mountains where canal or pipe line cannot be used due
to topography.
Tunnel provides a direct and short route and is made by cutting mountain.

Chapter Six
Cont...
Figure: Tunnel

Chapter Six
Flume
It is an artificial open channel made of wood, metal, concrete or masonry.
A flume is used when it becomes difficult or expensive to construct a canal either in a
more rocky area or topographical terrain or depression.
It is erected on a surface above the ground supported on a trestle.
A trestle is a supporting structure consisting of frameworks of timber or steel.

Chapter Six
Cont....
Figure: Flume

Chapter Six
Forebay
Forebay is an enlarged body of water just above the intake to store water temporarily
to meet the hourly load ﬂuctuations.
The forebay serves as a regulating reservoir storing water temporarily during light load
period and providing the same for initial increase on account of increasing load during
which water in the canal is being accelerated.
A forebay is created by widening the power canal in the form of small basin or reservoir.
It acts as a regulating reservoir similar to surge tank temporarily.

Chapter Six
It is Provided in case of run oﬀ river plants and pondage plants when the power house
is located far away from the dam.
The main function of forebay is to store some water to act as a regulating reservoir for
the penstocks.
When the hydroelectric plants are located just at the base of the dam, no forebay is
required because the reservoir itself serves the purpose of the forebay.
However, where the plants are situated away from the storage reservoir a forebay is
provided.
Power house located closed to dam penstock directly take power from reservoir, reser-
voir acts as forebay.

Chapter Six
Cont....
The forebay has the following parts
Entrance bay
Spillway
Flushing sluice
Screens
Valve chamber
Penstock valve

Chapter Six
Cont....
Figure: Forebay

Chapter Six
Penstocks
It is a closed conduit which connects the forebay or surge tank to the scroll case(main
inlet valve) of the turbine.
In case of medium head power plants each unit is usually provided with its own pen-
stock.
In case of high head power plants a single penstock is frequently used, and branch
connections are provided at the lower end to supply two or more units.
Penstocks are made of steel, r.c.c, asbestos, cement, cast steel.
At the end of the penstock a drainage valve is provided which drains water from
penstock to the draft tube.

Chapter Six
Cont....
In case of penstock and high head, butterﬂy valve is provided just before the penstock.
It takes oﬀ from the surge shaft in addition to spherical valve at the end of the penstock
acting as the main inlet valve.
A penstock supplies water under pressure from the forebay to the turbine. When
the power plant is nearer to dam the penstock may be embedded in the dam itself.
Otherwise it connects forebay or surge tank to the turbine.
When the distance between the forebay and the power house is short, a separate pen-
stock for each turbine is preferred, while for moderate heads and long distance, a single
penstock is used to feed two or more turbines.
Valve is provided in the penstock for inspection of turbine without dewatering it.

Chapter Six
Penstock thickness
The thickness of penstock depend on water head and hoop stress allowed in the material
t =
(p × d)
2 × f × η
Where, t = penstock thickness, d = diameter of penstock
f = permissible stress, η = eﬃciency of penstock
p = pressure due to water including water hammer
The thickness of the penstock increases as working pressure or head of the water in-
creases.
A large diameter of penstock gives lesser friction loss.

Chapter Six
Number of penstocks
A hydro power plant uses a number of turbine which are to be supplied water through
penstock
To use a single penstock for the whole plant
To use one penstock for each turbine separately
To provide multiple penstock but each penstock
supplying water to at least two turbine
Factors for selecting number of penstocks
Economy
Operational safety
Transportation facilities

Chapter Six
Penstock protection valve
The penstock protection valves are provided after the surge shaft to facilitate mainte-
nance of the penstocks.
The valves are of butterﬂy(BF) type.
The BF valve are operated hydraulically with provision of pressure accumulators in
case of power failure.

Chapter Six
Surge tank
When the load on the generator is reduced the governor tends to closed the turbine
gates and water pressure is increased in the penstock. Similarly in case of larger load
,the gate opens to allow large quantity of water to ﬂow through the penstock.
The surge tank relieves water hammer eﬀect in the conduit or penstock and acts as a
storage reservoir which stores water during low load and quickly supplies extra water
during high load on turbine.
Additional storage for near to turbine, usually provided in high head plants
Located near the beginning of the penstock.
As the load on the turbine decreases or during load rejection by the turbine the surge
tank provides space for holding water.

Chapter Six
Cont....
Surge tank over comes the abnormal pressure in the conduit when load on the turbine
falls and acts as a reservoir during increase of load on the turbine.
Its purpose is to relieve the excessive pressures resulting from the manipulation of the
turbine gates as the load on the generating unit varies, and to supply initial water for
an increasing load while the water in the tunnel is being accelerated.
The forebay also fulﬁls the objectives similar to surge tank, and both are also known
as balancing reservoir.
In case of canal or open channel, forebay serve the purpose of surge tank, while for
long pipeline or penstock, surge tank is used.

Chapter Six
Cont....
Purpose of surge tank
1 They reduce the distance between the free water surface and turbine there by reducing
water hammer eﬀect on the penstock.
2 To serve as supply tank to the turbine in case of increased load conditions, and storage
tank in case of low load conditions.
In case of medium head and low head power plants where the power house is located
within the short distance of the head works, surge tank is not necessary. for run oﬀ river
type plants and medium head scheme no surge tank is needed.
Surge tanks are required for high and medium head plants where water is taken to the
power house through tunnels and long penstocks.
Their main function is to protect the low pressure conduit system from high internal
pressures.

Chapter Six
Cont....
Figure: Surge tank

Chapter Six
Cont....
Pressure Tunnel
It is a passage that carries water from the reservoir to the surge tank.
Pressure Shaft
When the water conduits in the surge shaft and main inlet valve are not exposed to
the atmosphere and buried in the ground/concrete due to its high pressure, these are
called pressure shaft.

Chapter Six
Main inlet valve
Main inlet valve works as the gate valve/isolating valve in the water conductor system.
It is located before turbine and allows water flow from penstock to turbine
MIV acts as closing valve and cuts the flow of water during an emergency trip
They are of following type
Butterfly valve(up to 200m head)
Spherical valve(more than 200m head)

Chapter Six
Draft tube
An air tight pipe of suitable diameter attached to the runner outlet and conducting
water down from the wheel and discharging it under the surface of the water in the
tailrace is known as draft tube.
The draft tube provides continuous column of water through the turbine to the tailrace.
The turbine runner can then be located above the tailrace (for safety) and permit full
use of the available water head.
It is designed to maximize head utilization by the turbine.
The draft tube is an essential part of reaction turbine installation.

Chapter Six
Cont....
The draft tube is a pipe of gradually increasing area which connects the outlet of the
runner with the tailrace.
It creates a negative head at the outlet of the runner there by increasing the net head
on the turbine. It converts a large proportion of rejected kinetic energy into useful
pressure or potential head.
A draft tube has two main functions:
1 To reduce the velocity head losses of the water
2 To allow the turbine to be set above the tailrace to facilitate inspection and maintenance

Chapter Six
Tail race
The water after having done its useful work in the turbine is discharged to the tailrace
which may lead it to the same stream or to another one.
The design and size of tail race should be such that water has a free exit and the jet
of water ,after it leaves the turbine, has unimpeded passage.
The tailrace is channel or canal that carries water away from the turbine. Also some-
times called an after bay.
Tail race is a water path to lead the water exiting from the turbine to the canal or
river.
In the tail race the water detained is called Tailrace water level.

Chapter Six
Power house
Power house
The power house is a building in which the turbines, alternators and the auxiliary plant
are housed. Some important items of equipment provided in the power house are as
follows:
Hydraulic Turbines
Electric Generators
Governors
Relief valve for penstock setting
Gate valve

Chapter Six
Power house
Cont....
Transformer
Switch board equipment and instruments
Oil circuit breaker
Storage batteries
Outgoing connections
Cranes
Shops and oﬃces

Chapter Six
Power house
Classiﬁcation of Power House
According to the location of the hydel power station, the power houses are classiﬁed
as
Surface power house
Semi Under Ground power house
Under Ground power house

Chapter Six
Power house
1. Surface power house
A surface power house is one which is founded on earth’s surface and its superstructure
rests on the foundation.
Surface power house has the advantage of pre determined topography, design and is
easy to construct. However, these have the disadvantage of limitation of head available
as per the topography.
In such projects the water inlet to the machines could be from a penstock or from a
tunnel terminating into a penstock. The water outlet goes into a tail race.
If the power house is located just adjacent to the dam then it is sometimes called a
Dam-toe power house.

Chapter Six
Power house
Cont....
The surface power house has been broadly divided into three subdivisions which is
separated from the intake as mentioned below.
I. Substructure:
The part which extends from the top of the draft tube to the soil or rock.
Its purpose is to support the hydraulic and electrical equipment
Considered as the foundation block of the power house
it is a building, housing an operating equipments.

Chapter Six
Power house
III. Intermediate Structure
Part of the power house which extends from the top of the draft tube to top of the
generator foundation.
This structure contains two important elements of the power house
I) The scroll case which feeds water to the turbine. The generator foundation rests on the
scroll-case which is embedded in the concrete.
II) Galleries, adits and chambers also rest on the same foundation. Scroll or spiral case is a
part of the turbine and it distributes water coming from penstock uniformly and smoothly
through guide vanes to the turbine.
◦ Scroll or spiral case is a part of the turbine and it distributes water coming from penstock
uniformly and smoothly through guide vanes to the turbine.
◦ The scroll case is required only in case of reaction turbines.

Chapter Six
Power house
Cont....
In case of impulse turbine the place of scroll case is taken by the manifold supplying
water to the jets.
III. Super structure
The part of the power house above the generator ﬂoor right up to the roof is known as
superstructure.
This part provides walls and roofs to power station and also provides an overhead
travelling crane for handling heavy machine parts.

Chapter Six
Power house
2. Semi underground power house
Some components of the power house are underground, while others are on surface.
The advantages of both surface & underground are clubbed together in a semi under-
ground powerhouse, provided topography & geology so permit.

Chapter Six
Power house
3. Underground power house
The underground power house is one which is built underground.
A cavity is excavated inside earth surface where the sound rock is available to house
the power station and the complete power house equipment are located inside Cavern.
In such power houses various tunnels such as Head race tunnel for the water inﬂow to
the turbine, Tail race tunnel for water out ﬂow of the turbine and various access tunnel
have to be provided inside the hill/mountain.
The considerations supporting the construction of underground power stations are:
Non-availability of a suitable site for a conventional surface station and good slope for
penstock.
Danger of falling rocks and snow avalanches particularly in narrow valleys.

Chapter Six
Power house
Cont....
Availability of underground sound rock and avoidance of a long pressure tunnel and
facility for a convenient tail-race outlet.
The rugged topographical features and the difficulties in finding a suitable short and
steep slope for pipe lines make it more economical to install the water conduit, the
machine, transformer hall and tailrace system underground.
Foundation costs for over ground power house become excessive in case of poor quality
surface layers. The construction of draft tube, spiral case and separating floors in loose
weathered rock is again more expensive than the excavation of corresponding parts
underground.
The costs of underground machine hall are lower than those of the superstructure of a
surface powerhouse of similar dimensions.

Chapter Six
Power house
Advantages
It overcomes the limitations of head available as per topography and provide compact
and economical layout.
This requires less land and consequently reduce rehabilitation and resettlement prob-
lems(R&R).
There is greater freedom in positioning stations
These stations are safer from defence point of view and are immune to bombardment.
They are also protected against such natural hazards as rain,wind,loose rocks etc.
There is saving in cost of land
Land acquisition problem is minimum etc.

Chapter Six
Power house
Disadvantages
Such power houses take greater amount of time and capital cost to construct.
Geological uncertainties resulting into indeterminate design and construction problems
leading to time and cost over runs.
The cost of construction of power station and accessories is more
Increased cost of lighting, special ventilation and air conditioning
The tailrace tunnels and additional surge chambers are costly
Additional cost of underground location of transformers etc.

Chapter Six
Electrical and mechanical equipments in a hydro plant
Electrical Equipments
In electrical equipment the following elements are included
Generators
Exciters
Voltage regulators
Transformers
Switch gear
Control room equipment

Chapter Six
Cont....
Generators: the generators employed in a hydro plant are usually 3 phase synchronous
machines and have either vertical shaft arrangement or horizontal shaft arrangement;
but vertical shaft arrangement is preferred.
Transformers: The generated voltage is stepped up by means of step up transformers.
They are oil ﬁlled for insulation and cooling purposes.
Switch gear: A switch gear consists of a switch, isolators, surge arrestors and circuit
breakers. Its main function is to make and break the circuits.

Chapter Six
Cont....
Control room equipment: It performs the following functions.
Machine starting and stopping
Generator and system voltage control
Machine loading control
Frequency control
Hydraulic control
Machine running control

Chapter Six
Mechanical Equipments
Mechanical equipments include the following
Compressors and air ducts
Shaft,coupling,bearings
Braking equipment for the generator
The oil circuits and pumps
Cranes and other lifting equipment
Ventilation and cooling systems
Equipment for water supply and drainage
Equipment for power housing lightning

Chapter Six
Safety measures in hydroelectric power plants
Safety measures which are usually provided for the safe and controlled operation of a
hydroelectric power plant are:
Surge tanks
Spillways
Relief valve
Trash screens
Fish pass
Jet dispersers
Sand traps

Chapter Six
Cont....
Surge tanks: Surge tanks are provided to reduce the water hammer effect in penstock
pipe, during fall of load.
Spillway: A spillway is also considered a safety valve for dam. It must have the
capacity to discharge major floods without damage to the dam.
Relief Valve: It is also a safety device, which is used to protect penstock against
excessive water hammer pressure. It may be used independently or in conjunction
with surge tank. When used independently it is a bypass device for excess water. It is
fitted in the penstock at the turbine inlet. If the power demand drops suddenly and
guide blades are quickly closed, the relief valve opens and directs the water to tailrace,
bypassing the runner.

Chapter Six
Cont....
⇒ This avoids any serious water hammer formation which could damage the governing
mechanism.
Trash Screens(Trash racks): These are provided to prevent logs, ice blocks, fishes
and other foreign matter from entering the conduits and then to turbines. Trash rack
is made up of steel bars.
Fish Pass: This allows the fishes to pass upstream without difficulty. Arrangements
for fish pass are to be made on the dams as per law also.
Jet Dispersers: These are provided to dissipate the energy possessed by the dis-
charged water at the bottom of high dams, which may cause damage to the foundation.
The disperser permits the jet to be broken up into a conical shower of drops and their
energy is absorbed by air.

Chapter Six
Cont....
Sand Traps:
After large stones and woody debris are held back by a screen at the weir, the sand
trap separates smaller solids such as gravel, sand, and silt from the river water, which
may otherwise cause rapid wear of nozzles, blades, spears etc., of the turbine. Solids
in suspension are allowed to settle down.
It thereby prevents sedimentary deposits from building up in the penstock.
This in turn, extends the hydropower plant’s operating time and maintenance intervals,
protects the turbine from excessive wear, and increases the facility’s service life.

Chapter Six
Questions
Thank You!

Conduits,intakes,power house and Accessories

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