In the last
issue we read about the vanishing forests of tropical
Asia and the consequences thereof. One of the indirect consequences
of depleting forest cover on the Himalayan slopes is enhanced rate of
erosion and mass wasting. Each year mass wasting processes lead to
landslides during the monsoons when the hill slopes are wet and lose
rock mass is 'well lubricated' to slide. Simultaneously the eroded
surface material reaches the rivulets and rivers in bulk. A large
proportion of which, is the silt which travels with the river hurtling
down the slope.
Forests are like our life support system. They are the sinks of carbon
dioxide and a great provider to life forms. The Indirect advantage of
the forests is that the roots of trees hold the slope forming material.
They prevent it from erosion and check its movement to the river.
The fast pace of development of our nation relies mainly on electricity.
Total surface water potential of our country is estimated to be 1675
million cubic meters. It means that there is no dearth of water for
hydroelectric power generation. Water resource available is sufficient
for irrigation too. It is estimated that if utilized in a planned way
the available surface water alone can irrigate nearly 75 million
hectares of land.
Taking the above fact into picture several hydro-electric power projects
came up in the mountainous terrain of our country. Before a such a power
project is executed a detailed survey of the amount of silt carried by
the rivers is meticulously carried out by the geologists. Silt is mainly
composed of silica, which is quite a hard substance. We know that silica
grains are used in making sand paper because of great abrasive power of
the mineral. Therefore, lot of care is taken to evaluate the quantity of
silt that would reach the reservoir of the dam. It is natural that if
the water reaching the reservoir travels through areas with loose slope
forming material bereft of trees more silt would be produced. The eroded
material upon reaching the reservoir not only upsets the calculations
about the volume of water that would be available in the reservoir at
any given time, it manages to reach the turbines that generate the
power.
In a study carried out by the Geological Survey Of India (GSI) a few
years ago, it was estimated that in Ramganga and Bhakra reservoirs in
the Himalayas the annual siltation rates are 0.31% and 0.40%
respectively. Maithon and Panchet reservoirs in the Shield areas of the
peninsula show siltation at the rate of 0.80% and 0.845% respectively.
Koyna reservoir in the trap country has a siltation rate of 0.48% per
annum. Yes the rates of siltation vary, because of the varying
geological and geomorphological setup of the terrains on which the
dams/reservoirs are located.
How much damage the silt can do to the turbines is an interesting study.
Baira-Siul Power Station, a project envisaged in 1970-71 was constructed
on fast flowing tributaries of Ravi River, namely Baira, Siul and
Bhaledh. It is a run of the river scheme. The project started commercial
production of electricity in 1982.
National Hydro-electric Power Corporation (NHPC) carried out a study in
the Baira-Siul Power Station, Himachal Pradesh. It was found that river
waters carried 75-98% grains of quartz (sand). Quartz is third highest
in the grade of hardness of various minerals. Only Corundum and Diamond
have hardness more than quartz. These grains work as an abrasive when
they strike the moving turbines. Damage to the Plant was so severe that
the stainless steel guide vanes loose about 15% of metal from surface
after 1000 hours of operation in one year and after a few years the
vanes disappear.
A hydropower station is erected with an anticipated life of 100 years.
Therefore, the problem of corrosion of vanes of the turbine needs
attention. Various measures from designing de-silting chambers in such a
way that no or minimum silt reaches the power house to afforestation and
painting the vanes with non-corrosive paints have been suggested and are
being worked out.
Harsh Gupta, an expert engineering geologist with GSI studied the
problem in some of the power projects in northwest Indian Himalayas and
has come out with some solutions too. In one of his recently published
papers in the Journal of the Indian Society of Engineering Geologists he
has elaborated his views.
A major hydropower project with six turbines producing 250 MW
electricity each has four de-silting chambers each about half a km long,
16m wide and 27m deep, for exclusion of silt particles of 0.2mm and
above. All the six generating units faced problem of erosion of the
underwater parts during their maiden encounter with the monsoon. During
the second monsoon year, the stream flow rate and silt was catastrophic
on the generating units when the discharge in the river rose to 1500
cumecs from general flow of 400 cumec. During the floods the river
carried a maximum silt content of 1,51, 000 parts per million (ppm).
Another power plant designed for maximum silt content of 1200 ppm gets
around 12, 000 ppm silt with a quartz content varying between 40 to 78%.
It is imperative that the guide vanes, labyrinths and discharge ring all
are heavily scoured he says.
Another power project in H.P. had no de-silting chamber because during
the pre-construction surveys silt level was low and such a structure was
not warranted. Alas the rate of silting turned out to be rather high,
once the reservoir came up. Within six years the reservoir got silted up
with a maximum silt concentration of 3792 ppm. Condition became so bad
that silt had to be flushed out by opening the sluice gates during the
monsoon. Turbine vanes, turbine runner etc started showing the signs of
corrosion and began to wear out. Situation is such that repairs of the
turbine have to be carried out every year after the monsoon and a major
maintenance is done every three years. Naturally this enhances the cost
of power generation.
Yet another run of the river scheme showed 12000 ppm of silt with 75 to
95% quartz grains. Soon the turbines started to show abrasion.
Storage schemes on the other hand, showed practically no silt in the
powerhouse. This is mainly because silt in the impounded water in the
reservoir settles down and only clear water reaches the turbines.
Harsh Gupta's studies revealed that a particular type of turbine
(Francis turbine) was more prone to damage from the silt compared to
others like Peltan and Kaplan turbines. He also found that turbines with
larger diameter faced less silt abrasion than those with faster runner
speed and a higher head. The striking velocity of silk laden water being
higher in such cases, damage was more.
He recommends better routine management measurements in silt affected
turbines.
The NHPC and Bharat Heavy Electricals Ltd (BHEL) have tried several ways
to check the damage on the Hydro-turbine components due to silt
abrasion. Losses are tremendous. The NHPC alone spends about Rs One
Crore annually in replacing/repairing worn out parts, besides the
downtime of Power plants.
Government of India's TIFAC along with Carborundum Universal LTD, (CUMI),
Chennai and NHPC has tried coating of various materials on the
components endangered by abrasion. A plasma spraying technique is
usually done. This generates very high temperatures to melt ceramics so
that they can be impacted at high velocity to form a coating on metal
surface, without affecting the base material. Alumina-Titania Ceramic
powder in varying compositions is tried. Such studies take time, because
the impact of abrasion is a slow process.
The engineers are optimistic about the results and say that if proven
successful the technology will have a good export market too. The
Hydro-Power Plants in the Hindu Kush region and greater Himalayas have
similar problem of siltation.
Engineering solutions may temporarily offset the damage but the real
solution lies in control of erosion. Himalayan states have to be more
stringent about deforestation and mass wasting of the hill slopes.
Constant erosion of mountain slopes is bound to reduce the lives of the
hydroelectric projects. One must remember that such projects involve a
colossal amount of money and unless saved from erosion the maintenance
cost of turbines will become a recurring feature, adding to the cost of
power generated.
Save Forests to save turbines for a better tomorrow.
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Forests are like our life support system. They are the sinks of carbon
dioxide and a great provider to life forms. The Indirect advantage of
the forests is that the roots of trees hold the slope forming material.
They prevent it from erosion and check its movement to the river.