In centrifugal pump, cavitation is reduced by:a)Increasing the flow velocityb)Reducing dischargec)Throttling the discharged)Reducing suction headCorrect answer is option 'D'. Can you explain this answer?

Question

Answer

Cavitation is the phenomenon of the formation of vapour bubbles of a flowing liquid in a region where the pressure of the liquid falls below the vapour pressure of the fluid and sudden collapsing of these bubbles in the region of higher pressure.

In centrifugal pumps the cavitation may occur at the inlet of the impeller of the pump or at the suction side of the pumps, where the pressure is considerably reduced.

Hence if the pressure at the suction side of the pump drops below the vapour pressure of the liquid then cavitation may occur.

In order to determine whether cavitation will occur in any portion of the suction side of the pump, the critical value of Thoma’s cavitation factor (σ) is calculated.

H = Head developed by the pump

H_S = Suction pressure head in m of water

H_V = Vapour pressure head in m of water

h_LS = Head lost due to friction in the suction pipe

If the value of σ is greater than σc (Critical cavitation factor), the cavitation will not occur in that pump.

So, the cavitation can be reduced by reducing the suction head.

Answer

Cavitation occurs in the hydraulic head at the pump and when it's unit is too low for operation .

While pumping water , if the pressure at any point inside a pump drops below the vapour pressure, corresponding to the temperature of liquid ,the liquid will vapourize and form cavities of vapour .

The bubbles of vapour are carried along with the stream untill a region of higher pressure is reached,when they collapsed or explode with tremendous shock on adjacent walls.
These phenomenon is called cavitation.

The energy required to accelerate the flow of water to fill the hollow spaces result in loss of water . Thus, cavitation is accompanied by reduction in efficiency of the pump.

Cavitation untill takes places primarily at the vane inlet portion of the impeller,on the vanes and the shrouds .

Cavitation is quantified through the cavitation coefficient,

Sigma = Hsv / H

where ,
Hsv = net positive suction head at critical point (m).
H = total head ,(m) .

Answer

Cavitation is a phenomenon that occurs in centrifugal pumps when the pressure at the suction side of the pump drops below the vapor pressure of the liquid being pumped. This causes the formation of vapor bubbles in the liquid, which then collapse when they reach areas of higher pressure. These collapsing bubbles create shock waves that can damage the pump impeller and other components, leading to reduced pump efficiency and potential failure of the pump.

To reduce cavitation in a centrifugal pump, it is important to maintain a sufficient suction head to ensure that the pressure at the suction side of the pump does not drop below the vapor pressure of the liquid. This can be achieved by reducing the suction head, which is the correct answer option 'D' in this case. Let's understand why reducing the suction head helps in reducing cavitation:

1. Understanding suction head:
- Suction head is the vertical distance between the centerline of the pump and the free surface of the liquid in the suction tank.
- It represents the energy available to the pump at the suction side.
- If the suction head is reduced, the pressure at the suction side decreases, increasing the likelihood of cavitation.

2. Effects of reducing suction head:
- By reducing the suction head, the pressure at the suction side of the pump is increased, preventing it from dropping below the vapor pressure of the liquid.
- This reduces the formation of vapor bubbles and eliminates the risk of cavitation.
- As a result, the pump operates more efficiently and with less risk of damage.

3. Other options and their effects:
- Increasing the flow velocity (option 'A') can actually increase the likelihood of cavitation as it lowers the pressure at the suction side.
- Reducing discharge (option 'B') does not directly affect the pressure at the suction side and, therefore, does not have a significant impact on cavitation.
- Throttling the discharge (option 'C') may help to some extent by reducing the flow rate, but it does not address the root cause of cavitation, which is the pressure drop at the suction side.

In conclusion, reducing the suction head is the most effective way to reduce cavitation in a centrifugal pump. This helps to maintain the pressure at the suction side above the vapor pressure of the liquid, preventing the formation of vapor bubbles and the associated damage.

Answer

Cavitation is the phenomenon of the formation of vapour bubbles of a flowing liquid in a region where the pressure of the liquid falls below the vapour pressure of the fluid and sudden collapsing of these bubbles in the region of higher pressure.

In centrifugal pumps the cavitation may occur at the inlet of the impeller of the pump or at the suction side of the pumps, where the pressure is considerably reduced.

Hence if the pressure at the suction side of the pump drops below the vapour pressure of the liquid then cavitation may occur.

In order to determine whether cavitation will occur in any portion of the suction side of the pump, the critical value of Thoma’s cavitation factor (σ) is calculated.

H = Head developed by the pump

H_S = Suction pressure head in m of water

H_V = Vapour pressure head in m of water

h_LS = Head lost due to friction in the suction pipe

If the value of σ is greater than σc (Critical cavitation factor), the cavitation will not occur in that pump.

So, the cavitation can be reduced by reducing the suction head.

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