Centrifugal Pump MCQ Quiz - Objective Question with Answer for Centrifugal Pump - 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:

Design of a Semi-Open Impeller

• A semi-open impeller is an essential component commonly used in pumps. Its design is characterized by a combination of features from both open and closed impellers. A semi-open impeller typically consists of a series of vanes attached to a central hub, with one side exposed and the other side covered by a backplate. This hybrid design offers a balance between the advantages of open and closed impellers, making it ideal for specific applications.
• The design of a semi-open impeller makes it particularly suitable for fluids with moderate amounts of suspended solids while providing better efficiency than open impellers. This is because the semi-open design allows for easier passage of solids compared to closed impellers, while maintaining a level of efficiency that is higher than open impellers. The presence of the backplate provides structural support to the impeller vanes, reducing deflection and improving durability under moderate solid load conditions.

Key Features and Advantages:

• Handling of Suspended Solids: The semi-open impeller design ensures that fluids containing moderate amounts of suspended solids can pass through the pump without clogging. This is particularly advantageous in applications where the fluid contains impurities such as sand, sludge, or other particulate matter.
• Improved Efficiency: Compared to open impellers, semi-open impellers offer higher efficiency. This is because the backplate minimizes energy losses and helps in maintaining a more streamlined flow of fluid through the pump.
• Ease of Maintenance: Semi-open impellers are relatively easier to clean and maintain compared to closed impellers. This is because the exposed vanes are more accessible, allowing for quicker inspection and removal of any debris or blockages.
• Reduced Wear and Tear: The design reduces the likelihood of wear and tear caused by solid particles, as the backplate provides additional support to the vanes. This improves the overall lifespan of the impeller.

Applications:

Semi-open impellers are used in a wide range of applications, including:

• Industrial processes where the fluid contains moderate levels of suspended solids.
• Municipal wastewater treatment plants for handling sludge and slurry.
• Mining operations for pumping fluids with abrasive particles.
• Food processing industries where semi-solid materials need to be transported.

Explanation:

Single Volute Casing in a Centrifugal Pump

• A single volute casing is a critical component in a centrifugal pump, designed to perform the primary function of directing fluid flow and converting the kinetic energy of the fluid into pressure energy. The volute casing plays a central role in ensuring the efficient operation of the pump, as it collects the fluid discharged by the impeller and channels it toward the pump outlet. This process involves the gradual expansion of the volute's cross-sectional area, which helps in reducing the velocity of the fluid and, consequently, converting the kinetic energy into pressure energy.
• The centrifugal pump operates on the principle of imparting energy to the fluid through the action of the rotating impeller. As the fluid enters the impeller, it gains kinetic energy due to the high-speed rotation. However, this kinetic energy alone is not sufficient for most practical applications. The volute casing ensures that this kinetic energy is effectively transformed into pressure energy, which is required for the fluid to overcome system resistance and reach its intended destination.

Working of the Single Volute Casing:

1. As the impeller rotates, it accelerates the fluid outward due to centrifugal force. This increases the fluid's velocity and kinetic energy.

2. The volute casing, which surrounds the impeller, collects the high-velocity fluid discharged from the impeller.

3. The design of the volute casing includes a progressively expanding cross-sectional area. This design ensures a gradual reduction in the velocity of the fluid as it moves through the casing.

4. The reduction in velocity leads to an increase in pressure energy, as per Bernoulli’s principle.

5. The fluid exits the pump through the discharge nozzle at a higher pressure, ready to be delivered to the system.

Advantages of Single Volute Casing:

1. Simple Design: The single volute casing has a straightforward design, making it easier to manufacture and maintain.

2. Cost-Effective: Due to its simplicity, the single volute casing is typically more economical than other designs, such as double volute casings.

3. Suitable for Small and Medium-Sized Pumps: The single volute casing is ideal for applications where the pump size and flow requirements are moderate.

Applications of Centrifugal Pumps with Single Volute Casings:

1. Domestic water supply systems

2. Industrial cooling and heating systems

3. Irrigation and agricultural water distribution

4. Chemical processing plants

5. Municipal water treatment facilities

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
   

\(N_{s}=\frac{N\sqrt{Q}}{H_{m}^{3/4}}\)

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) = \({{\rm{N}}_{\rm{s}}} = \frac{{{\rm{N}}{\sqrt{P}}}}{{{{\rm{H_m}}^{5/4}}}}\)

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,

\({{\rm{\eta }}_{\rm{}}} = \frac{{{\rm{water\;power}}}}{{{\rm{shaft\;power\;}}}} = \frac{{{\rm{\omega QH}}}}{{\rm{P}}}\)

\(P = \frac{{{\bf{\omega QH}}\;}}{{{\eta }}}\)

Calculation:

\(\eta = \frac{{{\bf{\rho g QH}}\;}}{{{P }}} = \frac{{{\bf{1000\times10\times0.04\times25}}\;}}{{{16000}}}=0.625\)

Additional Information

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

\({\eta _{man}} = \frac{{{H_m}}}{{\frac{{{V_{w2}}{u_2}}}{g}}} = \frac{{g{H_m}}}{{{V_{w2}}{u_2}}}\)

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

\({\eta _m} = \frac{{{\rm{Power\;at\;the\;impeller}}}}{{{\rm{Power\;at\;the\;shaft}}}} = \frac{{\frac{W}{g}\left( {\frac{{{V_{w2}}{u_2}}}{{1000}}} \right)}}{{{\rm{SP}}}}\)

Concept:

Before manufacturing the large-sized pumps, their models which are in complete similarity with the actual pumps (also called prototypes) are made. Tests are conducted on the models and the performance of the prototype is predicted. The complete similarity between the model and actual will exist if the following condition is satisfied.

I)\(\left ( \frac{\sqrt{H}}{DN} \right )_m=\left ( \frac{\sqrt{H}}{DN} \right )_p\;\;\;\;(1)\)

II) \(\left ( \frac{Q}{D^3N} \right )_m=\left ( \frac{Q}{D^3N} \right )_p\;\;\;\;\;(2)\)

III) \(\left ( \frac{P}{D^5{N^3}} \right )_m=\left ( \frac{P}{D^5{N^3}} \right )_p\;\;\;\;\;(3)\)

From (1)

\(\sqrt{H}∝\;N\)

From (2)

Q ∝ N

Combining (1) and (2)

\(\sqrt{H}∝\;Q\)

∴ H ∝ Q²
∴ the head 'H' varies with the square of discharge 'Q'

The relation between head 'H' and discharge 'Q' can be better understood with the Operating characteristic curve as shown in the figure which gives the relation between the manometric head (H), power (P) and efficiency (η) with respect to the discharge when the speed (N) is kept constant.

Explanation:

• The centrifugal pump acts as a reverse of an inward radial flow reaction turbine.
• This means that the flow in centrifugal pumps is in the radial outward direction.

• The centrifugal pump works on the principle of forced vortex flow which means that when a certain mass of liquid is rotated by an external torque, the rise in pressure head of the rotating liquid takes place.
• In a centrifugal pump casing, the flow of water leaving the impeller is a free vortex. 

Explanation:

Centrifugal pump works on the principle of force vortex. Where external torque is provided to the impeller by the means of the prime mover.

In a pump, there are two important parts, first is the impeller which creates velocity through rotation. And the second is the casing which converts this velocity into pressure by the change in the area.

When fluid is inside the impeller, then the speed of the fluid experience a rotational motion because of the torque provided by the prime mover, and flow is characterized as a forced vortex flow. When the fluid comes out from the rotating impeller at that time also fluid has a vortex motion because of the inertia of the fluid. But since the external torque is absent outside the casing therefore it becomes free vortex flow.

Additional Information

There are two types of casing generally available for the centrifugal pump.

1.Volute casing:

In volute casing area gradually increase as you can see in the above figure. Because of the gradual increase in the area the velocity decreases and pressure increases.

2.Diffuser casing:

In diffuser casing impeller periphery is designed in such a way that its area gradually increases which promotes the rise in the pressure at the expense of the velocity.

Concept:

As per the affinity law, the relationship between the power and diameter of the impeller is given by:

\(p\propto~D^3\)

Where P is shaft  power, D is the diameter of the impeller,

Calculation:

Given:

P₁ = 12 hp, D₁ = 127 mm, D₂ = 254 mm

In the given question it is said that the only diameter is changed and other parameters are constant then

\(\frac{P_2}{P_1}=(\frac{D_2}{D_1})^3\)

\(\frac{P_2}{12}=(\frac{254}{127})^3\)

\(\Rightarrow {P_2} = {\left( {\frac{{254}}{{127}}} \right)^3} × 12 = {2^3} × 12\;hp = 96\;hp\)

Explanation:

Closed impellers (Two-sides shrouded):

• In the closed or shrouded impellers, the vanes are covered with shrouds (side plates) on both sides
• The back shroud is mounted into the shaft and the front shroud is coupled by the vanes
• This ensures full-capacity operation with high efficiency for a prolonged running period
• This type of impeller is meant to pump only clear water, hot water and acids

Semi-open impeller (One-side shrouded):

• It has a plate (shroud) only on the backside
• The design is adapted to industrial pump problems which require a rugged pump to handle liquids containing fibrous material such as paper pulp, sugar molasses and sewage water etc.

Open impeller:

• In open impeller, no shroud or plate is provided on either side i.e. the vanes are open on both sides
• Such pumps are used where the pump has a very rough duty to perform i.e. to handle abrasive liquids such as a mixture of water, sand, pebbles, muds and clay, wherein the solid contents may be as high as 25%.

Thus, centrifugal pumps dealing with muds have an open impeller.

Explanation:

In the case of a centrifugal pump, the power is decreasing from the shaft of the pump to the impeller and then from the impeller to the water. The following are the important efficiencies of a centrifugal pump:

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

\({\eta _{man}} = \frac{{{H_m}}}{{\frac{{{V_{w2}}{u_2}}}{g}}} = \frac{{g{H_m}}}{{{V_{w2}}{u_2}}}\)

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

\({\eta _m} = \frac{{{\rm{Power\;at\;the\;impeller}}}}{{{\rm{Power\;at\;the\;shaft}}}} = \frac{{\frac{W}{g}\left( {\frac{{{V_{w2}}{u_2}}}{{1000}}} \right)}}{{{\rm{SP}}}}\)

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

Concept:

Centrifugal Pump:

Centrifugal pumps are used to transport fluids by the conversion of rotational kinetic energy to the hydrodynamic energy of the fluid flow. The rotational energy typically comes from an engine or electric motor.

• The pressure in the rising main increases, it becomes almost double and the discharge remains constant.
• Head is also increased at a constant flow rate.

Important Points

Pumps in parallel:

Concept:
Velocity in pump:

\(\begin{aligned} & V=\frac{\pi \times D \times N}{60}=\sqrt{2 g h} \\ & h=\frac{\pi^2 \times D^2 \times N^2}{3600 \times 2 g} \end{aligned}\)

Discharge of a pump: 
Q = A × V 

\(Q=\frac{\pi^2 \times D^3 \times N}{240}\)

Power of a pump: 
P = ρ × g × Q × h

 P = K × N³ 

\(\frac{P_1}{P_2}=\frac{N_1^3}{N_2^3}\)

Calculation: 
Given: P1 = 2 kW, N1 = 1400 rpm, and N2 = 2800 rpm.
 From\(\frac{P_1}{P_2}=\frac{N_1^3}{N_2^3}\)

P₂ = 16KW (ANS)