DESIGN GUIDELINES
3.1.1.2 Desanding Basin General
This guidelines deals desanding basin only because in a logical sequence it appears as a part of the system of a conveyance system. More details of the desanding basin is dealt in the design guidelines of headworks for hydropower projects which are presently under preparation by DoED under a separate consultancy contract. Where run-off-the-river plants are located on rivers which transport substantial suspended sediment loads, it is a general requirement that a desanding basin be provided to trap and exclude sediment particles in excess of a selected size so as to minimize damage to the turbine and its accessories.
The performance of desanding basin is guided by its ability to trap suspended sediments and its ability to remove the trapped particles from the basin, i.e. qualities of the adopted sediment flushing system. The main function of the desanding basin is to:
- maintain the hydraulic transport capacity of the waterways - reduce the sediment load to the turbines,
- obtain the required power generation regularly.
Typical section, plan and elevation of a desanding basin are shown in Fig. 3.2 below.
Figure 3.2: Typical Section, Plan and Elevation of the Desanding Basin Design Consideration
Data Requirement: It is desirable that a regular program of suspended sediment sampling be initiated near the intake site from an early stage during site investigation to ensure that sufficient data is available for design. The sampling program should extend through the entire rainy season and should comprise at least two readings daily. On glacier fed rivers where diurnal flow variations may exist , the schedule of sampling should be adjusted to take this phenomenon into account and the scheduled sampling times be adjusted to coincide with the hour of peak daily flow with the other sampling taken about twelve hours later. A five year long sediment collecting program would be ideal and less than one monsoon season of data is considered unsatisfactory. Data normally collected in sediment sampling program would include:
• Mean daily concentration of suspended sediment
• Water temperature
• Flow in the river
The following additional information would be derived from collected samples:
• Particle size gradation curve
• Specific gravity of particles
• A sediment rating curve
Choice of Design Criteria: The rate of wear of turbine and accessories due to sediment abrasion is related to the following factors:
• Concentration of suspended particles
• Hardness of particles
• Size of particles
• Shape of particles
• Resistance of turbine parts
• Turbine head
These inter-relationships are complex and as yet there is no fully reliable method available for selecting the sediment size to be used for desanding basin design. Opinions of experts vary widely.
Bouvard, citing European experience, recommends that no desanding basin are required for plants having less than 100m head, while Varshney recommends that all particles 0.2 mm and greater be excluded without regards for plant head (based on Indian Practices). In general for a medium head hydropower plants the removal of particles larger than 0.2 to 0.5 mm is usually specified. According to Sokolov, sharp edged quartzite sediment with a particle size as small as 0.25 mm may seriously damage turbines. For the high head plants particle size of 0.1 to 0.2 mm and even smaller might be objectionable. The wear of mechanical equipments installed at power plants with very high head may be prevented only by removing particles of size as small as 0.01 mm to 0.05mm. The smaller the particle size to be removed, the larger the desanding basin should be and vice versa, therefore, a design choice will have to be made based on largely on engineering judgment. In this regard a conservative choice would be recommended for plants on rivers having large annual sediment loads, while a less conservative choice could be adopted for plants on rivers with lower sediment loads. It is also advisable to make an economical analysis of the size of the desanding basin with respect to the turbine replacement / maintenance for the optimum size of the desanding basin for the given head, discharge and sediment concentration.
Operating Consideration: In general there are two types of desanding basin:
• Continuous flushing desanding basin: Continuous flushing desanding basin, uses surplus water for flushing, about 10%-15% of the plant discharge. This type of desanding basin does not interfere power production during the flushing process. During the low flow season when sediment inflows are minimal, flushing can be done intermittently, as required, so that most of the available water can be used for power production. However, the design and operation of continuous flushing desanding basin is more complex than for the discontinuous type and much more care is required in their operation. The main problem is clogging of the sediment extracting system, manifold openings (Dufour type), vortex extractors or hopper openings. In order to improve reliability and enhance operating flexibility, continuous flushing desanding basins are usually constructed with a minimum of two basins. Continuous flushing desanding basins are used in Sunkoshi, Adhi Khola, Jhimurk, Modi Khola, Puwa Khola, Chilime hydropower plants.
• Discontinuous flushing desanding basin: Discontinuous flushing desanding basins are of much simpler design and are much less susceptible to blockage or clogging. The main operating inconvenience is that plant output must be cut back, for multi basin design, or shut down entirely for single basin design (e.g., Marsangdi and Trishuli). In muddy water this is not easy to determine. A variety of instruments have been suggested, however, sounding from a bridge across the basin would seem to be more practical on small basins. Typically, discontinuous desanding basin release much larger flushing flows than continuous flushing type and do so suddenly. The desanding basin if functioning according to design will trap most of particles down to the target size and some proportion of smaller sizes, thus minimizing turbine abrasion.
However, very fine particles of less dense minerals, such as mica will not be trapped. Flushing operation is conducted only when the desanding basin is full of silt.
A new type of desanding basin flushing system called “Serpentine” system –developed by Hakoon Stole at the Norwegian Institute of Science and Technology in Trondheim is being tried out at Andhi Khola, Jhimruk Khola and Khimti Khola hydropower plants in Nepal.
The main function of the desanding basin is to reduce the turbulence level in the water flow to allow suspended sediment particles to settle out from the water body and deposit on the bottom of the basin. The basic observation is that sediment particles, excepting very fine colloidal particles, will settle out in still water at a rate depending on the fall velocity of the particle. It will never be possible to trap all suspended sediment particles in the basin as the fall velocity of suspended silt and clay are too small compared with the turbulence level in the desanding basin. In essence then, a desanding basin should provide a sufficient retention time for a particle, to precipitate from the surface to the bottom of the basin.
The hydraulic design of a desanding basin arrangement shall secure:
- an even flow distribution between parallel basins, - an even flow distribution internally inside each basin, - efficient removal of deposits during flushing of the basin.
Basically there are two main approaches available to dimension the desanding basin with respect to the resulting trap efficiency of the basin:
Particle Approach: The particle approach to trap efficiency computation is assessing the probability of one particle being trapped or passed through the desanding basin. The particle approach is based on the simple relation. If there is no turbulence inside the basin, the ratio between the particles fall velocity, ‘w’, and the horizontal transit velocity in the basin, ‘v’, must be same as the ratio between the fall distance-‘D’ (depth) and the horizontal travel distance ‘L’ (length) in an ideal basin, i.e. a basin without any turbulence, all particles with a fall velocity larger or equal to ‘w’ will be trapped.
L v w= D*
--- (3.1) Where,
w = Fall velocity
D = Depth of the desanding basin
L
= Length of the desanding basin v = Horizontal transit velocityTrap efficiency for this method is determined using Camp’s diagram given in the Annex-1.
Concentration Approach: The concentration approach is the difference in average sediment concentration in the flow entering the basin and the flow leaving the basin. Trap efficiency of the basin is determined by the Vetter’s method using following formula:
⎟⎟⎠
⎜⎜ ⎞
⎝
−⎛
−
= Q
A w s
e
.
η
1 --- (3.2) Where,η
= Trap efficiency As = Net surface areaQ
= Discharge w = Fall velocityFor all simplified trap efficiency computation it is important to make reasonable assessment of effective surface area for settling. The net surface area is the area of the basin where the flow distribution is close to uniform. During the hydraulic design and dimensioning of the desanding basin, general practice is to ignore the effects of concentration on the rate of fall velocity of individual particles but the main parameter with respect to wear of turbine at high head run-of-river plant is the total amount of sediment load passing through the turbine and not the size of the individual particles in the sediment load. Standard design criteria for desanding basins are however linked to the basins’
ability to trap particles of a given size. It is relatively easy to prove that a design is satisfying the trap efficiency criterion, which is a more or less direct function of the size of the desanding basins. This design approach is popular due to lack of sufficient data required for the concentration approach design of the desanding basin. This approach is not strictly true but for concentrations below 2000 ppm the effect is minimal and may be ignored. However, this factor should be taken into account for desanding basin operating on river carrying suspended loads of more than 2000 ppm for a significant portion of the time, in accordance with concentration approach.
Removal of Deposited Sediment
It is necessary to provide some dead storage in desanding basin where sediment may accumulate between the flushing processes. The size of the dead storage is dependent on the sediment load as well as the adopted flushing method. Two basic methods (continuous flushing and discontinuous flushing) mentioned above are used for the removal of the deposited sediment from the basin.
The sediment concentration in Nepalese rivers particularly during rainy season is very high, due to which even surplusly designed desanding basins have not been able to settle down fully the designed size particles. For the projects with larger design flow and lower head the costs of desanding basins
are quite large and, therefore, it is cost-effective to change the turbine blades more frequently than constructing a desanding basin. Such comparative analysis has been conducted during feasibility study of Sapta-Gandaki Hydroelectric Project and has been recommended the following instead of desanding basin:
i. To provide an intake wall in front of the power intake and take only surface water in which most of the large size particles are already settled.
ii. To use the 13 Chromium Hi-Nickel Steel as the material of the turbine blades. This material is durable against abrasion and the life of the turbine blades is expected to extend up to around 10 years.
iii. To repair the damaged turbine blades due to abrasion by the build-up-welding.
Inlet and Outlet Zone Inlet Zone
The need for good inlet design cannot be overemphasized. Poor inlet design is probably the factor most responsible for “poor” basin performance. To achieve good hydraulic efficiency and effective use of the settling zone, the inlet strictly needs to distribute inflow and suspended sediment uniformly over the vertical cross-sectional area of the settling zone.
Horizontal velocity variations across the width of a desanding basin affect the hydraulic efficiency considerably more than velocity variations in depth, provided always that bed scour is avoided.
Principal attention therefore needs to be given to uniform inflow distribution in the horizontal plane.
Methods which are commonly adopted to achieve good flow distribution are:
• Submerged weir;
• Gradual open channel expansion, possibly using guide vanes;
• Troughs with slots or orifices in walls or bottom;
• Baffle walls; and
• Tranquiller racks
Orifices or baffled inlets are generally used only when extremely low flow through velocities is needed for water treatment. As a general rule, the inlet layout should either follow an existing proven design, or be model tested.
Outlet Zone
The operating water level of the desanding basin is generally controlled at the outlet or further downstream. If the outlet is narrower than the basin, the outlet control requires an appropriate approach transition to avoid short circuiting and to maintain an even flow distribution. The outlet contraction may be more abrupt than the inlet expansion.
Where it is practical decanting the outflow from the desanding basin over a weir is recommended.
This method will generally provide a higher water quality than by the simple transition structure. The design engineer will have to decide the merits of the above approach taking into account the implications of additional construction costs and head losses for a weir take off.
Desanding Basin Hopper
Desanding basins are constructed to settle the suspended particles greater than a particular size containing in the water and to successfully flush out the settled particles through a flushing channel.
Settled particles are initially stored at the bottom level of the desanding basin which is called a hopper of the basin and the hopper are constructed in such a way that its bottom width is relatively smaller than the width of the desanding basin. The slope angle of the basin and the bottom of the hopper is arranged slightly greater than the repulse angle of the sand which is usually 450. The narrow width at the bottom of the hopper is to create high velocity during the flushing operation to flush out all the deposited sediments in it.
3.1.1.3 Headrace Canal