All questions of Power Plant Engineering for Mechanical Engineering Exam

Fire tube boilers are generally used in locomotives like trains so they are also called as locomotive boiler

Explanation:

Importance of delivery valve in starting an axial flow pump:
The delivery valve in an axial flow pump is responsible for controlling the flow of fluid from the pump to the piping system. It plays a crucial role in the startup process of the pump.

Reason for keeping the delivery valve open:
- When starting an axial flow pump, the delivery valve should be kept open to allow the fluid to flow freely through the pump and into the piping system.
- Keeping the delivery valve open ensures that there is no obstruction to the flow of fluid, allowing the pump to operate efficiently.

Consequences of closing the delivery valve:
- If the delivery valve is closed during the startup process, it can lead to increased pressure within the pump, causing damage to the pump components.
- Closing the delivery valve can also result in cavitation, which can further damage the pump and reduce its efficiency.

Conclusion:
In conclusion, it is important to keep the delivery valve of an axial flow pump open when starting the pump. This ensures smooth flow of fluid and prevents any damage to the pump components.

Given Data:
- Pump speed at 1000 rpm: Power = 1 kW, Head = 10 m
- Pump speed at 2000 rpm: ?

Calculation:
- Power Consumption (P) is directly proportional to the cube of the speed (N) of the pump. The formula is P2/P1 = (N2/N1)^3
- Power at 2000 rpm = (2000/1000)^3 * 1 kW = 8 kW
- Head generated by the pump is directly proportional to the square of the speed of the pump. The formula is H2/H1 = (N2/N1)^2
- Head at 2000 rpm = (2000/1000)^2 * 10 m = 40 m
Therefore, when the pump is operated at 2000 rpm, the power consumption would be 8 kW and the head generated would be 40 m of water, which corresponds to option 'D'.

Pressure Rise per Stage in Centrifugal Compressor
Pressure rise per stage in a centrifugal compressor is restricted to a ratio of 4:1. This means that for every stage in the compressor, the pressure is increased by a factor of 4.

Explanation:

Centrifugal Compressor Stages
- A centrifugal compressor typically consists of multiple stages, each designed to increase the pressure of the gas being compressed.
- The pressure rise per stage is limited in order to prevent excessive temperature rise and to maintain efficiency.

Pressure Ratio of 4:1
- The 4:1 pressure rise per stage ratio is a common design parameter for centrifugal compressors.
- This ratio allows for efficient compression of the gas while preventing excessive temperature rise, which could lead to issues such as compressor surge.

Importance of Pressure Ratio Limitation
- By restricting the pressure rise per stage to 4:1, the compressor can effectively handle the compression process without compromising performance or reliability.
- This limitation also helps in achieving the desired compression ratio across multiple stages of the compressor.

Conclusion:
In a centrifugal compressor, the pressure rise per stage is restricted to a ratio of 4:1 to ensure efficient compression of the gas while maintaining performance and reliability.

Given: Temperature at which gas turbine operates, T1 = 50°C and T3 = 8390°C

To find: Pressure ratio at which cycle efficiency equals Carnot cycle efficiency

Concepts used: Brayton cycle, Carnot cycle

Explanation:

- The Brayton cycle is the ideal cycle for gas turbine engines. It consists of four processes: isentropic compression, constant pressure heat addition, isentropic expansion, and constant pressure heat rejection. The cycle is shown below:


- The efficiency of the Brayton cycle is given by:

ηBrayton = 1 - (1/r)^(γ-1)

where r is the pressure ratio and γ is the specific heat ratio.

- The Carnot cycle is the ideal cycle for heat engines. It consists of four processes: isothermal heat addition, adiabatic expansion, isothermal heat rejection, and adiabatic compression. The cycle is shown below:


- The efficiency of the Carnot cycle is given by:

ηCarnot = 1 - T1/T3

where T1 is the temperature at which heat is added and T3 is the temperature at which heat is rejected.

- To find the pressure ratio at which cycle efficiency equals Carnot cycle efficiency, we need to equate the two efficiencies and solve for r:

1 - (1/r)^(γ-1) = 1 - T1/T3

(1/r)^(γ-1) = T1/T3

r = (T3/T1)^(1/(γ-1))

- Substituting the given values, we get:

r = (8390+273)/(50+273)^(1.4-1) = 128

Therefore, the pressure ratio at which cycle efficiency equals Carnot cycle efficiency is 128. Answer: (c)

In a reaction turbine, a stage is represented by:

A stage in a reaction turbine refers to each row of blades that the steam passes through as it flows through the turbine. It is an important concept in understanding the operation and design of reaction turbines.

Explanation:

A reaction turbine is a type of turbine where the steam expands and changes direction as it passes through the blades. It is a continuous-flow machine, meaning that steam enters the turbine at one end and exits at the other, while the blades extract energy from the steam. The steam pressure decreases gradually as it passes through each stage, and the velocity of the steam increases.

Each row of blades:

- In a reaction turbine, the blades are arranged in multiple rows, which are referred to as stages.
- Each stage consists of a set of stationary blades (nozzles or guides) and a set of rotating blades (buckets or rotor blades).
- The steam passes through each stage, and as it does so, it undergoes a change in pressure and velocity, transferring energy to the blades and causing the rotor to rotate.

Number of exits of steam:

- The number of exits of steam does not represent a stage in a reaction turbine.
- In a reaction turbine, there is typically only one exit for the steam, which is at the exhaust end of the turbine.
- The steam flows through multiple stages, but it is a continuous flow, and there is no separate exit for each stage.

Number of bleeding:

- Bleeding refers to the extraction of steam from an intermediate stage of the turbine to be used for other purposes, such as heating or feedwater heating.
- Bleeding does not represent a stage in a reaction turbine.
- It is a separate feature that can be added to the turbine design, but it does not define a stage.

Conclusion:

In a reaction turbine, a stage is represented by each row of blades. The steam passes through multiple stages, with each stage consisting of a row of stationary blades and a row of rotating blades. The concept of stages is important for understanding the flow of steam and the extraction of energy in a reaction turbine.

Superheating in Rankine cycle provides several advantages, such as longer turbine blade life, less erosion, and an increase in efficiency. However, it does not offer the advantage of better sealing and less leakage.

Here is a detailed explanation of why superheating does not provide better sealing and less leakage:

1. Introduction to the Rankine Cycle:
The Rankine cycle is a thermodynamic cycle that is commonly used in steam power plants to generate electricity. It consists of four main components: a boiler, a turbine, a condenser, and a pump. The cycle operates by converting heat energy into mechanical work, which is then used to drive a generator.

2. Superheating in the Rankine Cycle:
Superheating is the process of heating the steam beyond its saturation point. This is typically done by passing the steam through additional heating surfaces after it leaves the boiler. Superheating increases the temperature of the steam, which in turn increases the available energy for conversion into work.

3. Advantages of Superheating:
a) Longer turbine blade life: Superheating the steam reduces the moisture content, which helps to prevent blade erosion and corrosion. This can significantly extend the life of the turbine blades, reducing maintenance and replacement costs.
b) Less erosion: Superheating reduces the moisture content in the steam, which reduces the erosion caused by the high-velocity steam particles. This helps to protect the turbine blades and other components from wear and tear.
c) Increase in efficiency: By superheating the steam, the average temperature at which heat is added in the cycle increases. This results in a higher thermal efficiency of the power plant, as more heat energy is converted into useful work.
d) Improved cycle performance: Superheating allows for a higher temperature difference between the heat source (boiler) and the heat sink (condenser). This increases the overall efficiency of the Rankine cycle.

4. Lack of better sealing and less leakage:
Superheating does not directly contribute to better sealing and less leakage in the Rankine cycle. The sealing and leakage issues in the cycle are primarily related to the design and maintenance of the equipment, such as valves, seals, and joints. While superheating can indirectly help reduce leakage by reducing the moisture content in the steam, it does not directly address the sealing and leakage issues.

In conclusion, while superheating in the Rankine cycle provides several advantages such as longer turbine blade life, less erosion, and an increase in efficiency, it does not directly contribute to better sealing and less leakage.

The given problem deals with the flow velocities at the eye tip of a centrifugal impeller. We are given the blade velocity at the eye tip as 200 m/s, the uniform velocity at the inlet as 150 m/s, and the sonic velocity as 300 m/s. We need to find the inlet Mach number of the flow.

To solve this problem, we can use the equation for Mach number:

Mach number = Velocity / Sonic velocity

Let's calculate the inlet Mach number step by step:

1. Calculate the inlet velocity relative to the blade velocity:
Inlet relative velocity = Blade velocity - Inlet velocity
Inlet relative velocity = 200 m/s - 150 m/s
Inlet relative velocity = 50 m/s

2. Calculate the inlet Mach number:
Inlet Mach number = Inlet relative velocity / Sonic velocity
Inlet Mach number = 50 m/s / 300 m/s
Inlet Mach number ≈ 0.1667

Therefore, the inlet Mach number of the flow is approximately 0.1667.

Now let's compare the calculated Mach number with the given options:

a) 0.75
b) 0.66 (Correct Answer)
c) 0.90
d) 0.83

The calculated Mach number is less than all the given options. Therefore, the correct answer is option 'b' (0.66), as it is the closest value to the calculated Mach number.

In conclusion, the inlet Mach number of the flow is approximately 0.1667, and the correct answer is option 'b' (0.66).

Impulse Turbine:
An impulse turbine is a type of steam turbine where the steam expands solely in the nozzle. It is commonly used in power generation plants to convert the kinetic energy of a high-velocity steam jet into mechanical energy.

Working Principle:
The working principle of an impulse turbine is based on the Newton's second law of motion, which states that the change in momentum of an object is equal to the force acting on it multiplied by the time during which the force is applied. In an impulse turbine, the steam is accelerated to a high velocity in the nozzle, and then directed onto the blades of the rotor.

Expansion in the Nozzle:
In an impulse turbine, the steam expands only in the nozzle. The nozzle is designed to provide a high-velocity steam jet by gradually increasing the cross-sectional area in the direction of flow. This causes the steam to undergo a controlled expansion, resulting in an increase in its velocity.

Velocity and Kinetic Energy Conversion:
As the steam expands in the nozzle, its velocity increases. This is due to the conservation of mass principle, which states that the mass flow rate of a fluid remains constant throughout a flow system. Therefore, as the cross-sectional area of the nozzle increases, the velocity of the steam increases to maintain the constant mass flow rate.

The high-velocity steam jet then impacts the blades of the rotor, causing a change in momentum. This change in momentum results in a force being exerted on the blades, which causes them to rotate. As the blades rotate, they convert the kinetic energy of the steam into mechanical energy.

Blades:
The blades in an impulse turbine are designed to efficiently capture the kinetic energy of the steam and convert it into rotational motion. They are typically curved or shaped in a way that allows the steam to flow smoothly over them, minimizing any losses due to friction or turbulence.

Nozzle Design:
The design of the nozzle is crucial in an impulse turbine as it determines the expansion of the steam and the resulting velocity. The nozzle should be carefully designed to achieve the desired steam velocity and ensure that the steam expands completely within the nozzle before it impacts the blades.

Conclusion:
In an impulse turbine, the steam expands only in the nozzle. The nozzle is designed to provide a high-velocity steam jet, which impacts the blades and causes them to rotate. The expansion of the steam in the nozzle and its subsequent impact on the blades result in the conversion of kinetic energy into mechanical energy, making impulse turbines an efficient choice for power generation.

Statement 1: The speed of rotation of the moving elements of gas turbines is much higher than of steam turbine.
This statement is correct. Gas turbines typically operate at much higher rotational speeds than steam turbines. Gas turbines can reach speeds of up to 30,000 revolutions per minute (RPM), while steam turbines usually operate at speeds around 3,600 RPM. The high rotational speed of gas turbines is necessary to achieve high power output and efficiency.

Statement 2: Gas turbine plants are heavier and larger in size than steam power plants.
This statement is incorrect. Gas turbine plants are generally smaller and lighter than steam power plants. This is because gas turbines have a higher power-to-weight ratio compared to steam turbines. Gas turbine plants also have a simpler design and fewer components, which contributes to their compact size and lower weight. Steam power plants, on the other hand, require large boilers, condensers, and other equipment, making them bulkier and heavier.

Statement 3: Gas turbines require cooling water for their operations.
This statement is incorrect. Gas turbines do not require cooling water for their operations. They are air-cooled machines, meaning that they use ambient air to cool down their components. Air is drawn into the turbine and used to cool the hot gases and the turbine blades. This eliminates the need for a separate cooling water system, simplifying the turbine plant design and reducing water consumption.

Statement 4: Any kind of fuel can be used with gas turbines.
This statement is correct. Gas turbines are versatile machines that can operate on a wide range of fuels. They can burn natural gas, diesel, kerosene, biofuels, and even synthetic fuels. The flexibility in fuel choice makes gas turbines suitable for various applications, including power generation, aviation, and industrial processes. The combustion process in gas turbines is highly efficient, resulting in lower emissions and better fuel utilization.

In conclusion, the correct statements are 1 and 4. The speed of rotation of gas turbines is indeed higher than that of steam turbines, and gas turbines can utilize a wide range of fuels. However, gas turbine plants are generally smaller and lighter than steam power plants, and they do not require cooling water for their operations.

Impeller diameter and speed are the key parameters

Head developed by a centrifugal pump depends primarily on the impeller diameter and the speed of rotation.

Impeller diameter
- The impeller diameter plays a crucial role in determining the head developed because it directly affects the velocity and pressure of the fluid being pumped.
- A larger impeller diameter typically results in higher head development due to increased fluid flow and pressure.

Speed
- The speed of rotation of the impeller also significantly impacts the head developed by a centrifugal pump.
- Higher speeds generally lead to greater head development as the impeller can impart more energy to the fluid.

Therefore, the correct answer is option 'A' - Head developed by a centrifugal pump depends on impeller diameter and speed. Other parameters such as fluid density and type of casing may have some influence on the performance of the pump, but they are not as directly related to head development as impeller diameter and speed.

Explanation:

Fission of Uranium-235:
- Uranium-235 is a fissile isotope of uranium, meaning it can undergo nuclear fission.
- Nuclear fission is a process in which the nucleus of an atom splits into two smaller nuclei, releasing a large amount of energy in the process.

Energy Released during Fission:
- The energy released during the fission of one atom of U-235 can be calculated using the famous equation E=mc², where E represents energy, m represents mass, and c represents the speed of light.
- The difference in mass between the original uranium atom and the two resulting nuclei, known as the mass defect, is converted into energy according to Einstein's equation.
- In the case of U-235, the average mass of the two resulting nuclei is slightly less than the mass of the original uranium atom.
- This mass difference is converted into energy according to the equation E=mc².
- The energy released during the fission of one atom of U-235 is approximately 200 MeV (million electron volts).

Answer:
- The correct answer is option 'B' which states that the energy released during the fission of one atom of U-235 is about 200 MeV.

The correct answer is option C: 3 only.

Impulse turbines are commonly used in hydroelectric power plants to convert the kinetic energy of water into mechanical energy. These turbines operate at high speeds due to the high velocity of water flowing through them. However, in practical applications, it is necessary to bring down the speed of the turbine to ensure safe and efficient operation. Several methods are adopted to achieve this:

1. Compounding:
Compounding is a technique used to reduce the speed of an impulse turbine. It involves dividing the turbine into multiple stages, each with its own set of nozzles and blades. The high-speed jet of water is directed onto the first set of nozzles and blades, where it imparts a partial amount of its kinetic energy. The remaining energy is then directed onto the next set of nozzles and blades, and so on. This process is repeated in multiple stages, gradually reducing the speed of the turbine. Compounding is an effective method to bring down the speed of an impulse turbine while maintaining high efficiency.

2. Use of Governor:
A governor is a device used to regulate the speed of a turbine. It consists of a set of rotating weights connected to the turbine shaft. As the speed of the turbine increases, the centrifugal force acting on the weights increases. This force causes the weights to move outward, which in turn actuates a mechanism that reduces the flow of water to the turbine, thereby reducing its speed. Conversely, when the speed decreases, the weights move inward, allowing more water to flow into the turbine and increase its speed. Governors are commonly used in conjunction with impulse turbines to maintain a constant speed and prevent overspeeding.

3. Increasing the Load:
Increasing the load on the turbine is another method to bring down its speed. By connecting more devices or generators to the turbine shaft, the torque required to drive these devices increases. As a result, the turbine slows down to match the increased load. This method is commonly used in power plants where the electricity demand varies, allowing the turbine to adjust its speed according to the load.

In conclusion, the methods adopted to bring down the speed of an impulse turbine to practical limits include compounding, the use of a governor, and increasing the load. These methods are essential to ensure safe and efficient operation of the turbine in various applications.

The nozzle angle in a single-stage impulse turbine is typically set at 20-30 degrees. This angle is important as it determines the direction and velocity of the steam as it enters the turbine blades. A nozzle angle of 15 degrees would result in a less efficient turbine operation as the steam flow would not be optimally directed towards the blades.