Wednesday, January 7, 2009
• Transition between the low pressure tunnel and reservoir and high pressure penstock and power house.
• No surge tank required if the distance between the power house and headwork is short.
• No surge tank required if the headrace is open channel flow.
Why surge tank?
• To protect low pressure conduit system from the high internal pressures.
• Helps to prevent/intercept/dampen the high internal pressure waves created due to the sudden closure of valve after penstock during load rejection by turbines (also called water hammer effect) for entering into the low pressure system.
• Thus it gives a facility to design entire headrace pipe as a low pressure system.• Storage functions of surge tank.
Point to be noted:
• Surge tank doesn’t directly influence the magnitude of water hammer pressure.
• Since pressure being transient and oscillatory in character, frequency of water hammer oscillation would depend upon the location of surge tank.
• Nearer the surge tank to power house greater frequency of oscillation
• Though water hammer pressure oscillation is dampened by the surge tank it gives rise to frequency of mass oscillation of water in surge tank and reservoir connected by low pressure tunnel.
• Design of surge tank is done by taking account of mass oscillation
• It’s a conveyance from fore bay or surge tank to turbine.
• Usually steep ground profile is selected for its location.
• Design is based on the total pressure occurred inside it (i.e. static pressure created by the water and dynamic pressure due to the water hammer effect)
• Pipe(penstock) bursting is dangerous because its usually constructed at steep slope and the water emerging out of the bursted penstock has the potential of causing landslide.
• Optimization of penstock for the selection of particular diameter is required since its installation requires significant portion of construction cost.
• Its design means the selection of pipe material, economical diameter, and safe thickness.
LOCATION OF PENSTOCK:
• Steeper slope 1:1 or 1:2 V:H because flatter the ground longer the penstock and hence uneconomical
• If steeper than 1:1 additional cost on slope stabilization for construction of support piers and anchor blocks will be required.
• The profile starting from gentle slope to steeper slope is avoided as it gives rise to negative pressure causing sub-atomic pressure.• Minimum bends selected as more bends means more head loss and more requirements of anchor blocks.
INTERESTING: When the velocity of the water inside the penstock is increased then the diameter designed will be decreased. If we see it through the general point of view, it would be see that for passing out the higher velocity thicker diameter of penstock should be used but the mathematical formulation shows somewhat different which is the actual case. The reason is defined by the conservation of total energy.
For any system, TOTAL HEAD (ENERGY HEAD) IS CONSTANT.i.e. VELOCITY HEAD + PRESSURE HEAD = CONSTANT
This shows if the velocity is increased then the pressure head is decreased. As the penstock thickness is designed considering the pressure head hence the thickness is decreased ultimately.
Thursday, November 20, 2008
With the objectives of reconnaissance of the selected site for the hydropower project and determination of available alternatives, the field visit of Naubise Khola Hydro power Project was carried out on 25th of October 2008, Friday. Our team includes four students, a supervisor, Er. Ashish Subedi, a helper, Ms. Usha Shrestha, and a technician for discharge measurement by salt dilution (conductivity) method, Mr. Maharjan. The proposed site of our project, Naubise Khola Small Hydro Power Project, is the upper part of Naubise Khola of Dhading District near Dobhan Village just opposite of Khawa of Naubise VDC. It is about 1.5 km upside from Naubise Bridge of Prithivi Highway of Dhading District. According to the desk study of previous report, it was proposed the water from Khawa Khola is to be diverted to the Naubise Khola through 200m long open cannel. After combination of the discharge, the intake structure should be construct in the Naubise Khola
with de-sanding basin including fore bay, steel convey pipe, penstock pipe, power house and tail race will be construct to the left bank of Naubise Khola in Naubise VDC at ward no-8 near Dobhan village just 1.375 km -1.2 km actual distance up stream from Naubise bazaar of Dhading district. Our aim of field visit was to get an overview idea about the suggested hydro power project site and the search for other possible alternatives around the project area.
The overviews of the subheads acquired by the field visit are compiled as follows:-
The topography is the key factor that determines the head of any hydropower project which ultimately defines the power for fixed discharge, if the head can be increased then the power produced is increased. From our field visit, the proposed site of headwork in prefeasibility study report was found appropriate to divert the water away from the
Appropriate site for intake or head works
river and towards the head race. The water course is stable and no meandering nature of river was observed. The river follows almost straight path to a distance of about 300m. A stable foundation or bed material composed of rocks was found which makes it suitable for the construction of dam.
The river width at proposed headwork side is 10.4cm having stable side slopes and this width doesn’t seem to vary through the gear due to the presence of stable beds and rocky and permanent side materials. It is located approx. 50m from the suspension bridge. As the bridge is position very high, the water can be increase to required level by the construction of weir across if to divert towards the intake site. The river doesn’t have tendency to carry boulders and therefore the risk of damage of constructed bridge is minimize or there is zero risk. Also the side piers of the bridge are situated at far end on stable grounds which saves it from the possibility of it being washed away by the water after weir .The water from the khawa Khola is being diverted and opened just above the head works and the topography is suitable for diversion. Discharge measurement of the khawa Khola was carried out by salt dilution method which was found to be 196lps.
The water is transported from the intake following the already existing Ghatta canal which can be expanded. It passes through Ghatta and there is also a small project generating about 20kw.The headrace follows the habitant area and according to the pre-feasibility report the agreement between the locals and funding consultants has already been done.
A small temporary diversion is to be constructed in Khawa Khola in order to divert the water towards Main River where the intake will be constructed. The settling basin can be constructed just near the presently constructed Ghatta which is very near to intake as well. The headrace can be provided on the slightly sloping ground as given by topography. Canal can be used to transport water in the most of the places and in some places the piping should be done as there is the possibility of landslides. The canal surface can be lined by the available rocks in the side to prevent the landslide through seepage. Just at the uphill of the position of the power house, there is the suitable condition for the provision of fore-bay. The fore-bay can be constructed after the water from the head race passes through an aqueduct construction of one hill to another. The valley formed in between can be used for the safe discharge of water in case of fore-bay malfunction. A straight steep slope is also available in between the position of power house and fore-bay hence the bends in the penstock will also be minimal. Fairly level ground is available for the construction of power house which minimizes the excavation cost and the space available is enough for the installation of the mechanical instruments. The tail race can be provided easily and the location is high enough for the prevention from the flood.
Above all the proposed site is suitable for the hydropower project. The construction of road is being carried out which makes it easy for the construction and transportation work. The stones are abundant in the site which can be used as the construction material. The other material required for the construction can be transported as the road is being constructed. The unskilled labors are available but for the skilled ones the manpower should be taken from the near by Naubise VDC.
1.2 HYDROLOGY AND WATER AVAILABILITY
For the collection of hydrological data or flow measurement, the field visit is to be done in dry season between the months of November to May which means our field visit is appropriate. Our project river is ungauged-type where site specific hydrological data is lacking. For the ungauged catchments two techniques can be used to predict flows namely WECS/DHM and MIP method. For the MIP method, at least one measurement of flow should be taken to generate the average monthly flow of the river. For this, discharge is measured on the purposed river- in Khawa Khola just above Dobhan village and on Nigalle Khola using ‘Salt Dilution Method’. This method is appropriate for river having discharge greater than 30 ltr/sec. Calculation in these two methods (MIP & WECS) are shown at the annexure. The MIP and WECS methods are not suitable for snow fed river and as our purposed river is not snow fed type hence this method can give nearly appropriate result. In consideration of seepage losses the soil available along the headrace are rocky type and hence the seepage loss is less than 0.5 ltrs/sec per thousand of wetted area.
1.3 SOCIO ECONOMIC ASPECT
Our interaction with the locals revealed that they are waiting for their houses to be enlightened. Till now they have been using kerosene lamps. According to one of the locals, Mrs. Maili Tamang, we came to know that a lot of research or feasibility works have been carried out in Naubise Khola but they are yet to be implemented. The locals depend on agriculture and it was found some of the locals have also gone abroad in search of employment. This shows that both skilled and unskilled labors can be easily available on the proposed area.
1.3 ALTERNATIVE SITE
After the field visit, another possible intake site was found adjoining to the Naubise bridge of the highway. The old dilapidated bridge abutment was found when one of the abutments is still good enough to be used for the bank protection for our possible intake site. A weir can be constructed some hundred meters far from the bridge and there is also a suitable space for the headrace along the currently-used canal. The construction of the weir will also help protect the existing foundation by decreasing the velocity of water and therefore it will be easy to get the permission from the government. The only constraint for this alternative site is that it is located near the agriculture area and the head race canal or pipe should be carried out along the farmland. The agreement between the land owner and the investor must be made to use the canal for transporting water. An underground piping system can also be used if there is a good agreement. The drawback in using canal may be the stealing of water by the land owner for their irrigation purpose. The alternative site is supposed to draw water from Naubise Khola and discharge is thrown after power station from tail race to the Mahesh Khola. The discharge measurement was also carried out in this site by salt dilution method which came out to be 531lps.
The discharge measurementS are as follows:DISCHARGE MEASUREMENT USING SALT DILUTION METHOD