Centrifugal Pump MCQ Quiz in मराठी - Objective Question with Answer for Centrifugal Pump - मोफत PDF डाउनलोड करा

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 and clay, wherein the solid contents may be as high as 25%.

Thus, centrifugal pumps dealing with water containing slurry and sewage​ have an open impeller.

Explanation:

The figure shows the impeller and outlet velocity triangle of a pump. Let us assume that the impeller is rotating in a clockwise direction such that the positive sign shows the leading face and the negative sign shows the trailing face. 

When liquid passes through the passage of impeller vanes which is rotating in a clockwise direction, high pressure is created on the leading face and low pressure is created on the trailing face. This difference in pressure is known as vane loading.

On the high-pressure side: 

• The liquid follows the blade contour and it leaves the blade tangentially.

On the low-pressure side: 

• The liquid leaves the vane with a certain circumferential component.

Hence liquid which should leave the impeller at β₂ ideally leaves the impeller at β'₂ in the actual case.

Due to this deviation in the flow path, the tangential component gets reduced from V_w2 to V'_w2 which is known as a slip in the centrifugal pump which results in the reduction of energy transfer.

Slip factor (SF):

It is the ratio of the ratio of actual & ideal values of the whirl velocity components at the exit of the impeller.

\(SF=\frac{V_{w2}^{'}}{V_{w2}}\)

Explanation:

Priming: 

• Priming is the process of removing air from the pump and suction line. 
• In this process, the pump is been filled with the liquid being pumped, and this liquid forces all the air, gas, or vapour contained in the passageways of the pump to escape out.
• Priming may be done manually or automatically. Not all pumps require priming but mostly do.
• Priming reduces the risk of pump damage during start-up as it prevents the pump impeller to becomes gas-bound and thus incapable of pumping the desired liquid.

Surging: Sudden drop in delivery pressure and violent aerodynamic pulsations. Surging usually starts to occur in the diffuser passages.

Explanation:

Characteristic curves are necessary to predict the behaviour and performance of the pump when the pump is working under different flow rates, heads and speeds.

If the speed is kept constant, the variation of manometric head, power and efficiency with respect to discharge gives the operating characteristic curves of a pump.

Concept:

• For fluids in incompressible regime of operation such as backward curved vanes are used for maximum efficiency for radial and forward vanes, the power requirement increases monotonically as discharge increase.
• But backward curved the power requirement peaks at maximum efficiency.

Concept:

Tangential velocity at inlet \({u_1} = \frac{{\pi {D_1}N}}{{60}}\)

\(\left( {\tan \theta } \right) = \frac{{{V_{f1}}}}{{{u_1}}}\)

Calculation:

Given:

N = 1200 rpm, Inner Diameter = 200 mm = 0.2 m, Outer Diameter = 400 mm

Now,

Tangential velocity at inlet \({u_1} = \frac{{\pi {D_1}N}}{{60}}\)

\(= \frac{{\pi × 0.2 × 1200}}{{60}} = 12.56\;{m}/{s}\)

Now,

∴ Vane angle at inlet \(\left( {\tan \theta } \right) = \frac{{{V_{f1}}}}{{{u_1}}} = \frac{{V_{f1}}}{{10.47}}\)

∴ tan 20° × 12.56 = V_f1

∴ Vf1= 4.57 m/s

Explanation:

A Centrifugal pump works on the principle of force vortex where an external force is applied to the impeller with the help of a prime mover.

In a pump, there are two important parts, the first is the impeller which generates 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 experiences a rotational motion because of the torque provided by the prime mover, and the flow is characterized as a forced vortex flow.

When the fluid comes out from the rotating impeller at that time fluid also 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.

Explanation:

Net Positive Suction Head (NPSH):

• The Net positive suction head (NPSH) is defined as the absolute pressure head at the inlet to the pump, minus the vapor pressure head (in absolute units) plus the velocity head.
• It is the net head developed at the suction port of the pump, more than the head due to the vapor pressure of the liquid at the temperature in the pump.
• NPSH must be positive for preventing the liquid from boiling.
• Boiling or cavitation may damage the pump.
• If NPSH reaches zero then the liquid starts boiling and cavitation occurs.

​NPSH = \((\frac{P_A}{\rho g}~-\frac{P_v}{\rho g}~-~h_s ~-~h_f)\)

(a) It will not develop sufficient head to raise water

True. If the NPSH available is less than the NPSH required by the pump, then the pump may not provide the necessary head. This can lead to poor pump performance and the inability to raise or transport fluid effectively.

(b) Its efficiency will be low

Not necessarily true. While a pump operating under cavitation (which can occur if NPSH requirements aren't met) may be less efficient, it's not the NPSH itself that directly influences efficiency. Other factors, such as pump design, play a more direct role.

(c) It will deliver very low discharge

Not necessarily true. While NPSH can influence the pump's ability to raise water, it doesn't directly determine the pump's discharge rate. Cavitation can reduce the effective flow rate, but the primary relation is between NPSH and the ability to generate head, not discharge.

(d) It will be cavitated

True. If the NPSH available is less than the NPSH required, the pump can cavitate. Cavitation is the formation of vapor bubbles in the pump due to the local pressure falling below the vapor pressure of the fluid. When these bubbles collapse, they can cause damage to the pump impeller and reduce the pump's lifespan.
In summary, if the NPSH requirement of a pump is not satisfied, it may not develop the necessary head to function properly, and it can experience cavitation. Thus, the correct answer is option 3: (a) and (d).

Additional Information

Thoma's cavitation parameter (σ): 

• It is the ratio of the net positive suction head (NPSH) to the total head.
• \(\sigma~=~\frac{NPSH}{H}\) = \(\frac{(\frac{P_A}{\rho g}~-\frac{P_v}{\rho g}~-~h_s ~-~h_f)}{H}\)

• If σ > σc : No cavitation

• If σ < σc : Cavitation Occurs

Effect of Vapor Pressure on NPSH

• Vapor pressure is the amount of pressure required for a specific liquid to stay in liquid form.
• As the temperature of the liquid increases, the vapor pressure increases, decreasing the amount of NPSH.
• If consideration is not taken into the fluid temperature and vapor pressure met, the liquid could flash to a gas state near the pump suction.

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 directions.

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 free vortex. 

Explanation:

Characteristics curve of a centrifugal pump:

• These curves are plotted from the result of a test on the centrifugal pumps.
• It helps to predict the behavior and performance of the pump when the pump is working under different flow rates, head and speed.

There are 2 types of Characteristics curves.

• Main characteristic curve 
• Operating characteristic curve 

Main characteristic curve:

• This curve consists of a variation of head, power, and discharge with respect to speed.

Operating characteristic curve:

• This curve is plotted at a constant speed.