What is Cavitation & its Effects in Turbine ?

What is Cavitation ?

Cavitation is a phenomenon that arises when the pressure of a liquid drops below its vapor pressure, causing the formation of vapor bubbles or cavities. Pressure drop may occur in the region of high flow velocities, for eg. at the exit of turbine runner. As the water flows through the turbine, its velocity increases. And according to Bernoulli's principle, an increase in flow velocity causes increase in velocity head and hence decrease in  pressure head since the total head always remains constant.

What Causes Cavitation in Turbine ?

When prevailing pressure falls towards vapour pressure of liquid, water starts vaporising and at the same time, normally dissolved gas gets liberated due to low ambient pressure. The water vapour and the liberated gas thus forms minute microscopic bubbles in the flowing water. When these bubbles get transported to the zone of higher pressure which is high enough to overcome the surface tension of bubbles, they get collapsed. When millions of such bubbles collapse simultaneously, a shock wave similar to water hammer but of short duration is produced which slowly causes erosion of concrete and metal surfaces.

What are Harmful Effects of Cavitation in Turbine ?

• Erosion of concrete and metal surfaces.
• Vibration and noise of machine parts
• Loss of material due to pitting
• Reducing the actual volume of liquid due to formation of bubbles

How to avoid Cavitation in Turbine ?

• A careful streamlined design of flow passage of the runner and draft tube.
• The sub atmospheric pressure at runner exit should be kept resonably above the vapour pressure limit.
• By using metals more resistance to cavitation damage.
• By periodic inspection and maintenance of turbine.

Working Principle & Functions of Draft Tube in Turbine

Draft tube is a pipe of gradually increasing cross-section that connects the outlet of turbine runner to the tailrace. It is used for discharging the water from exit of a reaction turbine to the tail pool and is provided only for reaction turbines eg. Francis turbine. Its cross section gradually expands and also changes its shape along its length from circular at inlet to the rectangular at the end. A draft tube plays an important role in optimizing the performance and efficiency of turbine.

Working Principle of Draft Tube

Applying Bernoulli's equation between runner exit (1-1) and draft tube outlet (2-2):

$$z_1+\frac{p_1}{\gamma}+\frac{v_1^2}{2g}=z_2+\frac{p_2}{\gamma}+\frac{v_2^2}{2g}+h_f$$

$$or, (H_s+h)+\frac{p_1}{\gamma}+\frac{v_1^2}{2g}=(\frac{p_{atm}}{\gamma}+h)+\frac{v_2^2}{2g}+h_f$$

$$or, \frac{p_1}{\gamma}=\frac{p_{atm}}{\gamma}-H_s-\frac{v_1^2-v_2^2}{2g}+h_f$$

$$or, \frac{p_1}{\gamma}=\frac{p_{atm}}{\gamma}-(H_s+\frac{v_1^2-v_2^2}{2g})+h_f \tag{1}$$

where,

$H_s$ = Static suction head

$\frac{v_1^2-v_2^2}{2g}$ = Dynamic suction head

$h_f = k\frac{v_1^2-v_2^2}{2g}$

$$or, \frac{p_1}{\gamma}=\frac{p_{atm}}{\gamma}-(H_s+\frac{v_1^2-v_2^2}{2g}-h_f )$$

$$or, \frac{p_1}{\gamma}=\frac{p_{atm}}{\gamma}-\left[H_s+(1-k)\frac{v_1^2-v_2^2}{2g}\right]\tag{2}$$

Now, draft tube efficiency can be written as:

$${\eta}_d=\frac{\text{Actual regain of pressure head}}{\text{Velocity head at entrance of draft tube}}$$

$$ =\frac{v_1^2-v_2^2}{2g}-h_f$$

$$=(1-k)\frac{v_1^2-v_2^2}{2g}$$

$$\therefore {\eta}_d=\frac{\frac{v_1^2-v_2^2}{2g}-h_f}{\frac{v_1^2}{2g}}$$

$$\therefore {\eta}_d=\frac{(1-k)\frac{v_1^2-v_2^2}{2g}}{\frac{v_1^2}{2g}}$$

From equation (2), it is clearly known that there exists a negative pressure at runner exit which is equal to $H_s+(1-k)\frac{v_1^2-v_2^2}{2g}$. From this, following two conclusions can be drawn:

1. Due to the use of draft tube, the turbine will not lose head $H_s$ becasue of equal reduction in pressure head at runner exit.
2. Due to use of draft tube of increasing cross-section, the pressure value at runner exit further reduced by $(1-k)\frac{v_1^2-v_2^2}{2g}$.

Purpose / Function of Draft Tube

1. It helps to achieve the recovery of velocity head at runner outlet which otherwise would have gone to waste as an exit loss.
2. It allows the turbine to be set at higher elevation without losing advantage of elevation difference.
3. It serves as a passage for water from runner exit to tail pool.

Factors Affecting Selection of Foundation

Selection of particular type of building foundation is affected by various factors which are explained below:

1. Type of soil

Shallow foundation are preferred if the soil close to the surface has good bearing capacity. If the soil is not capable of supporting structural loads then deep foundation are required.

2. Load from Superstructure

If the structural loading is relatively small, shallow foundation may withstand load from superstructure. In case of high rise building with intense loading, deep foundations may become the only choice.

4. Settlement

If the foundation settlement is not within the allowable limit, then choice of foundation type may vary accordingly.

5. Property Line

Due to restriction of property line, a column may have to be placed at the edge of footing creating an eccentricity. In such case, a cantilever footing (or strap footing) should be provided.

6. Stress Overlap

If the spacing between column is very small, then the stress from independent footings might overlap and become larger than allowable limit. Thus, combined footing have to be preferred.

7. Local Building Codes & Regulations

Building codes and regulations set by local authorities dictate the minimum standards and requirements for foundation design and construction. Compliance with these regulations is essential to ensure the safety and stability of the building.

8. Environmental Factors

Environmental conditions, such as seismic activity, flooding etc need to be considered when selecting a foundation system. Regions prone to earthquakes, for example, may require specialized foundation designs to withstand the seismic forces.

9. Type of Structure in Neighborhood

High rise buildings may cause uplift of nearby building due to soil heaving. So, a pile foundation may be the solution to safely transfer load to the deep strata and not to harm the nearby structures if any.

10. Other Factors

• Construction cost and time
• Service life of structure
• Safety Margin
• Ground water table
• Site topography
• Depth of hard strata

Requirements of Earthquake Resistant Building Construction

An earthquake is a sudden and rapid shaking of Earth's surface caused due to the movement of tectonic plates floating on the molten rock below the surface of earth. It causes vibrations of structures and induce inertial forces on them. As a result structure may collapse resulting into loss of property and lives. Earthquakes do not kill people, vulnerable buildings do so. Hence, there is need of designing earthquake resistant buildings, especially in the earthquake prone areas. The earthquake resistance of buildings may be increased by taking some precautions and measures in site selections, building planning and building constructions which are explained below:

Improving Earthquake Resistance of Small Buildings

• Avoid buildings in sloping grounds with different column heights.
• Provide simple and symmetric geometry in plan.

• Avoid too many doors and windows close to each other.
• Windows should be kept at same level.
• In sloping roof with span greater than 6m, use trusses instead of rafters.
• Building with four sided sloping roof is stronger than the one with two sided sloping, since gable walls collapse early.
• Restrict the projections of chajja and balcony to maximum of 1m. For larger projections, use beams and columns.
• Provide following RC bands:
1. Plinth Band
2. Lintel Band
3. Sill Band
4. Roof Band
5. Gable Band
• Offering retrofitting solutions to vulnerable structures ensures their resilience and safety is enhanced.

Improving Earthquake Resistance of High Rise Buildings

• Provide shear walls evenly throughout the building.
• Provide base isolation
• Provide seismic dampers
• Provide seismic gap in between neighbouring structures.
• The reinforcement within structural elements should ensure adequate strength and ductility.

Difference Between Pelton Turbine & Francis Turbine

Concept of IEE/EIA & Its Importance in Project Development

 Concept of IEE / EIA

The concept of IEE/EIA was first introduced in USA in 1970 AD under United States Environmental Law. This concept spread worldwide particularly after UN Earth Summit held in 1992 at Rio De Janeiro, Brazil. In the context of our country, government of Nepal introduced the National Environment Impact Assessment Guideline in 2050 BS. The Environment Protection Act 2053 and Environment Protection Regulation 2054 were then formulated. A new environment protection act was introduced by government of Nepal in 2076 BS. Such acts and guidelines provide a legal framework that requires developers to assess and mitigate the environmental impacts of any development project. IEE/EIA is a tool to identify and manage the effects of a development project to the environment. 

Initial Environmental Examination (IEE)

It is a preliminary environmental assessment for small projects with relatively low environmental risk. If IEE provides solutions to the identified environmental problems, then EIA is not necessary. If EIA becomes necessary, IEE serves as a valuable precursor to full EIA. The methodology involved in initial environmental examination (IEE) are as below:

• Project description
• Environmental screening
• Preparation of TOR
• Approval of TOR by concerned body
• Conducting Environmental Assessment
• Preparation of IEE Report
• Submission of IEE Report

If the IEE report submitted is approved by concerned authority, the project is implimented. If it donot get approved, EIA is needed.

Environmental Impact Assessment (EIA)

It is the more extensive environmental assessment process for relatively larger project with potentially significant environmetal impacts. Its methodology includes:

• Project description
• Environmental screening
• Preparation of TOR and Scoping
• Approval of TOR and Scoping
• Environmental Assessment
• Preparation of draft EIA report
• Disclosure of draft EIA report for comments and review
• Submission of EIA report

If the EIA report submitted gets approved then the project is implimented otherwise, redesigning of project is necessary.

Principles of EIA

1. Cost Effectiveness
2. Transparency
3. Certainity
4. Participation
5. Practicality

Difference between IEE and EIA

Importance of IEE / EIA

• To identify the environmental impacts
• To assess whether the impacts can be mitigated
• To recommend the corrective and preventive mitigation measures
• To examine the enviromental implications
• To inform the decision makers and concerned parties about the environmental implication
• To advise whether the development project should go ahead or not.

Specific Considerations for Hill Irrigation System in Nepal

Irrigation System in Nepal

The history of irrigation development in Nepal before 1992 shows that the irrigation system was developed, operated, and maintained by farmers called Farmer Managed Irrigation System (FMIS). After that, the government of Nepal started to make efforts for the development of irrigation infrastructures in Nepal. The irrigation system in Nepal can be broadly divided into the following two parts:

1. Irrigation System in Plains/Terai
2. Irrigation System in Hills/ Hill Irrigation System

The irrigation system in plains (Terai regions) is usually of canal irrigation type which consists of diversion headworks, canal networks, and canal structures (cross drainage structure, canal falls/drops, cross regulators, head regulators, outlets, escapes, etc). On other hand, the hill irrigation system is a small system that taps water from small streams & river tributaries & consists of narrow, deep, and long canals with steep slopes. Sprinklers & drip irrigation systems are usually suitable for hilly regions of Nepal.

Specific Consideration for Hill Irrigation System

1. Social Arrangements

The irrigation system adopted in hills needs to be appropriate to the culture of different ethnic groups living in hilly regions. Junior and senior water rights should also be kept in mind while designing irritation systems in hills. Village boundaries may also affect the layout of the distribution network of the irrigation system.

2. Managerial & Institutional Constraints

If farmers in the hilly regions lack managerial & institutional capabilities & cannot be trained effectively within the available time period, then the distribution system should be simplified and control structures should be minimized. The regulating structures should be made easy to operate by local farmers.

3. Agricultural Considerations

The irrigation system proposed should also be compatible with the type of soil present in the hilly region. Also, the irrigation system should be suitable for the type of crops to be cultivated in hilly areas.

4. Financial Considerations

The choice of technology, methods of construction, type of materials to be used, etc may be restricted due to financial limitations.

5. Design Considerations

• Farmers' Participation in Engineering Design

Because of intimate familiarity with the local conditions, farmers may help to better estimate the design flood discharge, flood level in the river, boulders & sediments carried by the river, etc. By providing these facts, farmers can help designers to avoid costly mistakes.

• Field-Based Design

Decisions regarding where to place the structure, what type of structure to be built, etc should be taken in the field since the structures in hills are affected by various topographical factors of the hilly region.

• Design Standards

Design standards while designing structures in hill irrigation systems should be considered wisely. Adoption of higher design standards (structural & operational) may be unrealistic as the structures built in hill irrigation systems are subject to frequent flooding, landslides, rock falls, soil erosion, etc.

• Canal Design

Large canal sections should not be built on unstable hill slopes. Canal beds may be steep to increase the flow velocity & thus to reduce the seepage loss via canal beds. Canal linings may be done with locally available materials in hilly regions.

Difference Between Flexible Pavement & Rigid Pavement

Importance & Requirements of Highway Drainage

Water has detrimental effects on the good performance of road & should be drained off as soon as possible. The process of quick removal of water out of surface & sub-surface region of the road is called Highway Drainage. The surface & sub-surface water of the road should quickly pass into the longitudinal drains if a proper highway drainage system is present. The surface water passes into longitudinal drains via gravity flow due to the cross-slope provided on the road surface. The subsurface water first goes into the perforated cross drains & then into the longitudinal drains, both being under the road surface in the areas of heavy rainfall. The surface drainage system prevents the surface water from percolating down to the sub-surface layers. The sub-surface drainage system takes the sub-surface water out of the subsurface layers.

What is the Importance of Highway Drainage?

The provision of a proper drainage system provides the following important functions:

1. It arrests the moisture variation in the subsurface layers thus preventing the reduction in bearing capacity of subgrade soil.
2. It prevents the erosion of side slopes.
3. It prevents the failure of formation slope caused by the poor drainage system.
4. It prevents the stripping of bitumen from aggregates in flexible pavements.
5. It prevents the mud pumping in rigid pavements.
6. It prevents the skidding of vehicles caused by a reduction in friction coefficient.
7. It prevents the frost action caused by the accumulation of water.

What are the Requirements of a Good Highway Drainage System?

A good highway drainage system should fulfill the following requirements:

1. The surface water on the carriageway and shoulder should be drained off effectively as soon as possible.
2. The surface water from the adjoining land should be prevented from entering into the roadway.
3. The groundwater table should be maintained well below the bottom surface of the subgrade soil.
4. The capillary rise & seepage water should be controlled effectively.
5. The longitudinal drains & cross drains should have sufficient capacity to carry the collected water.
6. The longitudinal drains & cross drains should have sufficient bed slope for gravity flow.
7. The flow of water across the road surface & in the drains should not cause erosion.
8. Complex & costly cross drainage structures should be avoided as much as possible.