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

Explanation:

Theoretical Power Required to Drive a Single-Acting Reciprocating Pump:

• The theoretical power required to drive a single-acting reciprocating pump is derived based on the fundamental principles of fluid mechanics and pump operation. The power required is a function of the work done by the pump in lifting the liquid through the given head, considering the weight of the liquid, the stroke volume, and the number of strokes per minute.

The theoretical power for a single-acting reciprocating pump is calculated by multiplying the volume of fluid delivered per second by the specific weight and total head (suction + delivery).

\( P = \frac{w \times A \times L \times N}{60} (H_s + H_d) \)

Where:

• \( w \): Specific weight of the fluid
• \( A \): Cross-sectional area of the piston
• \( L \): Length of stroke
• \( N \): Number of delivery strokes per minute
• \( H_s \): Suction head
• \( H_d \): Delivery head

Concept:

• The specific speed of a centrifugal pump is defined as the speed of a geometrically similar pump which would deliver one cubic metre of liquid per second against a head of one metre.
• \({N_s} = \frac{{N\sqrt Q }}{{H_m^{3/4}}}\)

Important Points

• The specific speed (Ns) of a turbine is defined as, the speed of a geometrically similar Turbine that would develop unit Power (P) when working under a unit Head (H) (1 m head).
• Specific speed for the turbine is given by:
• \({N_s} = \frac{{N\sqrt P }}{{{H^{\frac{5}{4}}}}}\)
•  

Explanation:

Priming of a Pump

• Priming of a pump refers to the process of removing air from the pump casing and suction line to ensure that the pump operates efficiently. This process is crucial for the proper functioning of the pump, especially in cases where the pump is used to lift fluids from a lower level to a higher level.
• Pumps are designed to move fluids by creating a pressure differential. However, when air is trapped in the pump casing or suction line, it can disrupt the pressure differential and prevent the pump from functioning correctly. Priming involves filling the pump casing and suction line with the fluid to be pumped, ensuring that there is no air trapped in the system. This allows the pump to create the necessary pressure differential and move the fluid efficiently.

Methods of Priming:

• Manual Priming: This method involves manually filling the pump casing and suction line with the fluid to be pumped. This can be done using a priming pump, a priming chamber, or by pouring the fluid directly into the pump.
• Automatic Priming: Some pumps are equipped with automatic priming systems that use a small auxiliary pump or a priming chamber to remove air from the pump casing and suction line. These systems are designed to maintain the prime automatically, ensuring continuous operation.

Importance of Priming:

• Prevents Cavitation: Cavitation occurs when air bubbles form in the pump and collapse, causing damage to the pump components. Priming ensures that there is no air in the system, preventing cavitation and extending the life of the pump.
• Ensures Efficient Operation: Air in the pump casing or suction line can disrupt the pressure differential and reduce the pump's efficiency. Priming ensures that the pump operates at its optimal efficiency, providing consistent performance.
• Prevents Damage: Running a pump without priming can cause it to run dry, leading to overheating and damage to the pump components. Priming ensures that the pump is always filled with fluid, preventing damage and ensuring reliable operation.

Explanation:

Single Volute Pump Casing:

• A single volute pump casing is a type of pump casing design in which the volute is a single spiral-shaped chamber that surrounds the impeller. The primary function of the volute is to convert the kinetic energy imparted to the fluid by the impeller into pressure energy.
• In a centrifugal pump, the fluid enters the pump impeller along or near to the rotating axis and is accelerated by the impeller, flowing radially outward into a diffuser or volute chamber, from where it exits into the downstream piping system. The single volute design ensures that the fluid is smoothly transitioned from the high-velocity region near the impeller to the lower-velocity region in the volute, helping in maintaining the flow characteristics.

Advantages:

• Uniform Flow Distribution: The single volute design helps maintain a uniform flow distribution around the impeller, reducing the chances of flow separation and energy losses. This uniformity in flow minimizes the radial forces acting on the impeller, leading to smoother operation and reduced wear and tear on the pump components.
• Efficiency: By maintaining a uniform flow distribution, the single volute design helps in achieving higher pump efficiencies. The smooth transition of fluid reduces turbulence and hydraulic losses, ensuring that a significant portion of the energy imparted by the impeller is converted into pressure energy.
• Cost-Effectiveness: Single volute pump casings are generally simpler and less expensive to manufacture compared to more complex designs like double volute casings. This cost-effectiveness makes them a preferred choice for many industrial applications where cost considerations are critical.

Disadvantages:

• Limited Operating Range: Single volute pumps may have a restricted operating range compared to double volute designs. At off-design conditions, single volute pumps can experience higher radial forces, leading to increased vibration and wear.
• Potential for Cavitation: In certain conditions, single volute designs can be more prone to cavitation, especially at low flow rates. Cavitation can cause significant damage to the pump impeller and casing, leading to reduced pump life and efficiency.

Explanation:

Double Volute Casings

• A double volute casing is a type of pump casing where the volute (the spiral-shaped casing that collects fluid discharged from the impeller) is divided into two separate channels or volutes. These channels are designed to balance the hydraulic forces on the impeller, thereby reducing radial loads and prolonging the life of the pump components.
• In a double volute casing, the fluid is discharged from the impeller into two separate volute channels. These channels help to balance the pressure around the impeller, reducing radial thrust and minimizing the mechanical stress on the pump bearings and shaft. This design is especially beneficial in high-capacity and high-pressure applications.

Advantages:

• Reduced radial thrust on the impeller, leading to longer bearing and shaft life.
• Improved hydraulic balance, which enhances the overall reliability and performance of the pump.
• Better handling of high-pressure and high-flow applications.

Disadvantages:

• Increased complexity in design and manufacturing, which can lead to higher production costs.
• More challenging alignment and assembly processes due to the additional components and tighter tolerances required.

Applications: Double volute casings are commonly used in industrial and municipal applications where high flow rates and pressures are required, such as in water treatment plants, chemical processing, and power generation.

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.​

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: