Turbomachinery MCQ Quiz - Objective Question with Answer for Turbomachinery - Download Free PDF

Explanation:

Centrifugal Pump

• A centrifugal pump is a mechanical device designed to move fluid by converting rotational kinetic energy into hydrodynamic energy of the fluid flow. The power required to drive a centrifugal pump depends on several factors related to the fluid properties and operating conditions. These factors include flow rate, fluid density, fluid viscosity, and total head (which includes suction and delivery heads). However, atmospheric pressure does not directly influence the power requirement for driving the pump under standard operating conditions. Below, we will explore why this is the case and analyze the other options in detail.

Correct Option Analysis:

The correct option is:

Option 1: Atmospheric Pressure

Atmospheric pressure does not directly affect the power required to drive a centrifugal pump. Here's why:

The power required to drive a centrifugal pump (often referred to as "brake horsepower" or BHP) is calculated using the following formula:

BHP = (Flow Rate × Total Head × Fluid Density) / (3960 × Pump Efficiency)

• From this equation, it is evident that the power requirement is directly proportional to the flow rate, total head, and fluid density, while inversely proportional to the pump efficiency. Atmospheric pressure does not appear in this formula because it is typically a constant factor that affects both sides of the pump system equally (i.e., the suction side and the discharge side). Unless the system involves conditions where atmospheric pressure significantly varies (e.g., high-altitude operation or vacuum systems), it does not play a direct role in determining the power required to drive the pump.

In most practical applications, centrifugal pumps are designed to operate under standard atmospheric conditions, and any minor variations in atmospheric pressure are negligible in terms of their impact on pump power requirements. Therefore, atmospheric pressure is not a direct factor in the calculation of pump power.

Explanation:

Overall Efficiency of a Centrifugal Pump:

A centrifugal pump operates by converting mechanical energy into hydraulic energy. The overall efficiency of the pump is determined by comparing the hydraulic power output to the mechanical power input. The formula for efficiency is expressed as:

Efficiency (η) = (Hydraulic Power Output / Mechanical Power Input) × 100

In this case, the hydraulic power output is 10 kW, and the mechanical power input is 15 kW. Substituting these values into the formula:

η = (10 / 15) × 100

Performing the calculation:

η = 0.6667 × 100

η = 66.67%

Explanation:

Double Volute Casing with Vaned Diffuser

• A double volute casing with a vaned diffuser is a specific design in centrifugal pumps that enhances hydraulic performance and efficiency. The double volute casing is designed to reduce radial forces acting on the impeller, leading to a balanced operation and improved efficiency. The vaned diffuser, on the other hand, helps to convert the velocity energy of the fluid into pressure energy more effectively, minimizing energy losses and improving the overall hydraulic performance.
• In comparison to single volute casing designs, the double volute casing provides additional benefits. The double volute casing divides the flow into two symmetrical paths, which helps to balance the pressure around the impeller. This results in reduced radial forces acting on the impeller, leading to less vibration, lower wear, and enhanced longevity of pump components. The incorporation of a vaned diffuser further contributes to improved efficiency by streamlining the flow paths and reducing turbulence.
• The double volute casing, combined with a vaned diffuser, offers superior hydraulic performance and efficiency compared to a single volute casing. The double volute design balances the radial forces, which reduces the mechanical stresses on the pump components. This balance minimizes vibration and wear, leading to smoother operation and longer service life.
• The vaned diffuser contributes significantly to hydraulic performance by aiding the efficient conversion of velocity energy into pressure energy. This results in reduced turbulence and energy losses in the flow path, increasing the pump's overall efficiency. Pumps with double volute casings and vaned diffusers are particularly advantageous in applications requiring high efficiency and reliability, such as in industrial processes, water treatment, and power generation.

Additional Information

Important points to note:

• Reduced Radial Forces: The double volute casing design splits the flow into two symmetrical paths, reducing the radial forces on the impeller. This results in smoother operation and less wear on pump components.
• Improved Efficiency: The vaned diffuser minimizes energy losses by converting velocity energy into pressure energy efficiently. This enhances the overall hydraulic performance.
• Applications: Double volute casings with vaned diffusers are widely used in industries where high efficiency and reliability are critical, such as chemical processing, water treatment, and power generation.
• Durability: Reduced mechanical stresses and wear lead to longer service life and lower maintenance requirements for the pump.

Explanation:

Radial Flow Pumps:

• Radial flow pumps are a type of centrifugal pump where the fluid enters axially into the impeller but exits radially, perpendicular to the pump shaft. These pumps are designed to develop high pressures with relatively low flow rates, making them suitable for applications where a significant pressure head is required.

Working Principle: In a radial flow pump, fluid is drawn into the center of the impeller along its axis (axial direction). The rotating impeller imparts kinetic energy to the fluid, converting it into pressure energy as the fluid moves outward in a radial direction. The fluid exits the pump casing at a 90-degree angle to the shaft.

Advantages:

• Capable of generating high pressures, making them ideal for applications requiring a large pressure head.
• Compact design and relatively easy to maintain.
• Well-suited for handling clean liquids with low viscosity.

Disadvantages:

• Limited flow rate capabilities compared to axial flow pumps.
• Not suitable for handling large volumes of fluid or highly viscous liquids.

Applications: Radial flow pumps are commonly used in industries such as water supply, chemical processing, boiler feed applications, and irrigation systems where high pressure and low flow rates are required.

Explanation:

Manometric Efficiency:

• The manometric efficiency of a centrifugal pump is defined as the ratio of the manometric head to the head imparted by the impeller.
• Manometric head is the actual head against which a centrifugal pump works.

To understand this better, let's break down the terms:

Manometric Head:

• It is the total head developed by the pump minus the losses in the pump.
• It is essentially the head that is effectively used to lift the fluid.

Head Imparted by the Impeller:

• This is the theoretical head that the impeller would impart to the fluid if there were no losses.

The formula for manometric efficiency (ηm) is given by:

ηm = (Manometric Head) / (Head Imparted by the Impeller)

Important Points

• The manometric efficiency is always less than 100% due to losses in the pump.
• Higher manometric efficiency indicates a more effective pump with fewer losses.

Additional Information

Other efficiencies related to centrifugal pumps include:

(1) Volumetric Efficiency:

• It is the ratio of the actual discharge to the theoretical discharge.
• It accounts for the leakage losses within the pump.

(2) Mechanical Efficiency:

• It is the ratio of the power available at the impeller to the power at the shaft.
• It accounts for the mechanical losses in the pump.

(3) Overall Efficiency:

• It is the product of volumetric efficiency, mechanical efficiency, and manometric efficiency.

Explanation:

Positive displacement pump:

• Positive displacement pumps are those pumps in which the liquid is sucked and then it is pushed or displaced to the thrust exerted on it by a moving member, which results in lifting the liquid to the required height.
• Reciprocating pump, Vane pump, Lobe pump are the examples of positive displacement pump whereas the centrifugal pump is the non-positive displacement pump.​

Concept:

Pelton wheel:

It is a tangential flow impulse turbine in which the pressure energy of water is converted into kinetic energy to form a high-speed water jet and this jet strikes the wheel tangentially to make it rotate.

Taygun's formula:

It is used to determine the number of buckets on the runner in the Pelton wheel turbine. It is given by the below formula:

Where D = Pitch or mean diameter, d = Nozzle or Jet  diameter

Calculation:

Given,

D = 1 m, d = 0.1 m

The number of buckets on the runner by Taygun's formula

= 15 + 5 = 20

Concept:

The overall efficiency η_o of turbine = volumetric efficiency (η_v)× hydraulic efficiency (η_h)× mechanical efficiency (η_m)

Overall efficiency: 

Water Power = ρ × Q × g × h 

Calculation:

Given:

η_o = 0.8, Head h = 10 m, and Q = 1 m³/s.

Explanation:

Flow ratio

The flow ratio of Francis turbine is defined as the ratio of the velocity of flow at the inlet to the theoretical jet velocity.

In the case of Francis turbine,

Flow ratio varies from 0.15 to 0.3

Speed ratio varies from 0.6 to 0.9

Calculation:

Explanation:

Draft tube

It is a conduit which connects the runner exit to the tailrace where the water is being finally discharged from the turbine.

Hence, Draft tube at the exit of a reaction turbine used for the hydroelectric project is always immersed in water.

Function

The primary function of the draft tube is to reduce the velocity of the discharged water to minimize the loss of kinetic energy at the outlet. 

Non-dimensional specific speed is given by

The Francis turbine is a type of reaction turbine, and it can operate over a wide range of water flows and height differences, which makes it suitable for a specific speed value of 1. It is more flexible in terms of operation conditions compared to the Pelton wheel.

Explanation:

Specific speed:

• It is defined as the speed of a geometrically similar pump that would deliver one cubic meter of liquid per second against the head of one meter.
• It is used to compare the performances of 2 different pumps.
• Its dimension is M⁰L^3/4T^-3/2 and given by the formula and is given by
   

Where N_S = Specific speed, Q = Discharge, H = Head under which the pump is working, N = Speed at the pump is working.

Additional Information

(specific speed for turbines) = 

Explanation:

Overall Efficiency (η): It is defined as a ratio of the power output of the pump to the power input to the pump.

The overall efficiency of the pump will be given as,

Calculation:

Additional Information

Manometric Efficiency (ηman): It is the ratio of the manometric head to head imparted by the impeller to the water.

Mechanical Efficiency (ηm): It is the ratio of the power available at the impeller to the power at the shaft of the centrifugal pump.

Explanation:

Runoff river plants:

• A runoff river plant is defined as a plant that utilized minimum flow in a river and has no appreciable pondage on its upstream side. The excess water is temporarily stored in the pond on the upstream side when the discharge at the site is more than the demand.

Additional Information

Tidal plant:

• A tidal power plant is a plant that produces electricity using a source of renewable energy tides.

Pumped storage plant:

• A pump storage plant is a hydroelectric system in which electricity is generated during periods of high demand by the use of water that has been pumped into a reservoir.

Storage plant:

• A storage plant is defined as a plant used for the storage of energy in the form of water.