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

Explanation:

Cavitation:

• It is the formation of gas bubbles of a flowing liquid in a region where the pressure of the liquid falls below its vapour pressure i.e. Vapor pressure of liquid is greater than local pressure.
• It is the phenomenon of 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 the cavitation may occur.

Centrifugal Pumps:

• Centrifugal pumps are a type of dynamic pump that use a rotating impeller to increase the velocity of a fluid.
• 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 (casing), from where it exits.

Cavitation:

• Cavitation occurs when the local fluid pressure drops below the vapor pressure, causing the formation of vapor bubbles within the liquid.
• These bubbles can collapse violently when subjected to higher pressures, causing damage to the impeller and other components of the pump.
• Cavitation can result in noise, vibration, decreased performance, and ultimately, physical damage to the pump.

Reducing Cavitation in Centrifugal Pumps:

• To reduce cavitation, it is important to maintain a sufficient suction head (Net Positive Suction Head Available - NPSHA).
• Reducing the suction head: Lowering the suction head can increase the NPSHA, reducing the risk of cavitation. This can be achieved by positioning the pump closer to the fluid source or by reducing the height difference between the pump and the fluid level.
• Other methods include increasing the diameter of the suction pipe to reduce flow velocity and thereby lowering the pressure drop, and ensuring that the pump is operating within its design parameters to avoid excessive flow rates that can contribute to cavitation.

Incorrect Methods to Reduce Cavitation:

• Increasing flow velocity: This would decrease the pressure at the pump inlet, increasing the risk of cavitation.
• Reducing the discharge: This might help in some scenarios but is not a primary method for reducing cavitation.
• Throttling the discharge: This could reduce the flow rate but might not effectively address the cavitation issue and can cause other operational problems.

Explanation:

The Net Positive Suction Head (NPSH) is defined as the absolute pressure head at the inlet to the pump, minus the vapour pressure head (in absolute units) plus the velocity head.

\({\rm{NPSH}} = \frac{{{P_1}}}{{\rho g}} - \frac{{{P_v}}}{{\rho g}} + \frac{{{v_s}^2}}{{2g}}\)

Where,

P1: Absolute pressure at the inlet of the pump

Pv: Vapour pressure at the inlet

vs= velocity of the fluid in the suction pipe

As the pump speed increases, vs will also increase. But NPSH may increase or decrease because it depends upon other factors also

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 NPSHA.
• 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:

Priming

• It is an operation in which the suction pipe, casing of the pump, and a portion of the delivery pipe are completely filled up by an water from an outside source before starting the pump.
• In other words, priming is the process in which the impeller of a centrifugal pump will get submerged in liquid without any air trap inside. It is always advisable to start pump only after priming.
• Priming is an operation that generally that happens in the centrifugal pump.
• Priming is required in order to drive out the air voids present, which otherwise would make the operation of the pump ineffective.

 Additional Information

Centrifugal pump:

• It works on the principle of the forced vortex, which means that when a certain mass of liquid is rotated with the external torque, the rise in pressure head of the rotating fluid takes place.
• Due to this high pressure the liquid can be lifted to a higher level.
• The action of the centrifugal pump is the reverse of radially inward flow reaction turbine ie radially outward flow, which takes water from the tailrace and delivers it to the headrace.

Explanation:

• 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 the sudden collapse of these bubbles in a region of higher pressure.
• In reaction turbines, the pressure of the working fluid changes gradually as it passes through the runner along with the change in its kinetic energy based on absolute velocity due to the impulse action between the fluid and the runner.
• At the exit of the rotor, the pressure of the working fluid is lowest, if the pressure falls below the vapour pressure of the working fluid then cavitation may occur at the outlet of the rotor.
• Due to cavitation, the metal of the runner vanes is gradually eaten away, which results in lowering the efficiency of the turbine.

Additional Information

Cavitation in Centrifugal Pump: 

• 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.
• Cavitation also occurs in reciprocating pumps if there is a high-velocity suction or discharge.

Explanation:

Cavitation

• Cavitation is the phenomenon of the formation of vapor bubbles of a flowing liquid in a region where the pressure of the liquid falls below the vapor 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 pump where the pressure is considerably low. So to avoid cavitation pressure on the suction side should be high. 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 is calculated.
• The hydraulic machines subjected to cavitation are reaction turbines and centrifugal pumps.

Thoma’s cavitation factor:

• Thoma's cavitation parameter (σ): It is the ratio of the Net Positive Suction Head (NPSH) to the total head.
• NPSH: It is defined as the net head developed at the suction port of the pump, in excess of the head due to the vapor pressure of the liquid at the temperature in the pump.

From the definition of σThoma,

\({\sigma _{Thoma}} = \frac{{{H_a} - {H_{vp}} - H_s}}{H}\)

• So, the correct answer is option 4.

Where Hvp is the vapor pressure head, Ha is the atmospheric pressure head, and Hs is the suction head

• If, Hs = Hs, max then σ = σC

​\({\sigma _{c}} = \frac{{{H_a} - {H_{vp}} - ​​H_{s, max}}}{H}\)

• Condition for no cavitation, Hs ≤ Hs, max

⇒ σ ≥ σC

• Where, σC is the critical cavitation factor
• The turbine manufacturer will specify the critical value of Toma's cavitation factor after performing testing. If the height of the Draft tube is increased then the pressure of the pressure at the exit of the turbine decreases and if it falls below the vapor pressure then the cavitation starts. The critical cavitation factor is the cavitation factor corresponding to the 10% drop in the efficiency of the turbine due to cavitation.

 Additional Information

• if Thoma's cavitation factor is greater than its critical value then there is no cavitation.
• The draft tube is used in the reaction turbine to convert the exit velocity head into the pressure head, the height of the draft tube should be such that pressure at the inlet of the draft tube is greater than vapor pressure.
• if pressure is less than vapor pressure then cavitation takes place.
• Draft tube is not used in impulse. impulse turbines have no cavitation because they are open to the atmosphere. everywhere there is atmospheric pressure. 

Concept:

Cavitation

Cavitation is the formation and collapse of vapour bubbles in a flowing liquid in a region of very low pressure.

It occurs when the static pressure of the liquid falls below its vapour pressure.

Cavitation is a common problem encountered in pumps and control valves; one that causes serious wear and tears and can reduce a component's time - in - service dramatically.

Cavitation parameter or number is,  \(\sigma = \;\frac{{{\rm{P}} - {{\rm{P}}_{\rm{V}}}}}{{\frac{{{\rm{\rho }}{{\rm{V}}^2}}}{2}}} = \frac{{{\rm{Pressure}} - {\rm{vapor\;pressure}}}}{{{\rm{Inertial\;Pressure}}}}\)

Explanation:

Cavitation

• Cavitation is the phenomenon of 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 pump where the pressure is considerably low. So to avoid cavitation pressure on the suction side should be high. 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 is calculated.
• The hydraulic machines subjected to cavitation are reaction turbines and centrifugal pumps.

Cavitation in Centrifugal Pump:

• 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.
• So to avoid cavitation pressure on the suction side should be high.
• 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 is calculated.

Explanation:

Cavitation:

• 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.
• Cavitation starts only when the pressure of the liquid falls below the vapour pressure of that liquid.
• The hydraulic machines subjected to cavitation are reaction turbines and centrifugal pumps.
• 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 is calculated.
• The cavitation damage in turbine runner occurs near the outlet on the convex side of blades.
• Cavitation in turbines affects the turbine's irregular collapse of vapour cavities, Vibration of turbine parts, loss of turbine material, and reduction in efficiency of the turbine.

Explanation:

• 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, 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 the 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.

\(\sigma = \frac{{{H_{atm}}~ - ~{H_V} ~- ~{H_S}~ - ~{h_{LS}}}}{H}\)

• 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 σ_c (Critical cavitation factor), the cavitation will not occur in that pump.

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 vapour pressure head (in absolute units) plus the velocity head.

\({\rm{NPSH}} = \frac{{{P_1}}}{{\rho g}} - \frac{{{P_v}}}{{\rho g}} + \frac{{{v_s}^2}}{{2g}}\)

where,

P1 = Absolute pressure at the inlet of the pump, Pv = Vapour pressure at the inlet, vs = Velocity of the fluid in the suction pipe.

• Vapour pressure is the amount of pressure required for a specific liquid to stay in liquid form.
• When the pressure of the fluid falls below the vapour pressure then cavitation occurs.
• NPSH must be positive to prevent cavitation.
• As the temperature of the liquid increases, the vapour pressure increases, decreasing the amount of NPSH.

​Cavitation:

It is defined as the phenomenon of formation of vapour bubbles of a flowing liquid in a region where the pressure of the liquid falls below its vapour pressure and the sudden collapsing of these vapour bubbles in a region of higher pressure.

Effects of cavitation:

• The metallic surfaces are damaged and cavities are formed on the surfaces.
• Due to the sudden collapse of the vapour bubble, considerable noise and vibrations are produced.
• Due to cavitation, the metal of the runner vanes and the draft tube is gradually eaten away, which results in lowering the efficiency of the turbine.

Explanation:

Cavitation:

• It is the phenomenon in which formation of vapour bubbles are takes place in a region where the pressure of flowing fluid is less than its vapour pressure, and then the vapour bubbles collapse in the high-pressure region which creates very high pressure on the surface on which vapour bubbles collapse.
• The high pressure exerted on the surface causes pitting or erosion of surface, thus cavitation occurs which causes noise and vibration also.

Methods to prevent cavitation:

• The pressure of fluid flowing through any hydraulic system should not be less than the vapour pressure.
• Special materials like stellite, bronze, nickel, stainless steel, are cavitation resistant material, it should be used.

​Effects of cavitation:

• Damage of metallic surface on which cavitation occurs.
• Noise and vibration due to sudden collapse of vapour bubble.
• It reduces the efficiency of turbine.

Thus, option (4) is correct answer.

Concepts:

Cavitation occurs when the liquid in a pipe turns to a vapor at low pressure.  When Cavitation takes place, air bubbles are formed when this low pressure reduces to the vapor pressure of liquid. As the liquid passes from low pressure to high pressure region, the air bubbles explode. This creates a shockwave that hits the pipe material and leading to bursting of pipe.

Important point:

Prevention of Cavitation:

1. Lower the temperature.

2. Raise the liquid level at low pressure region.

3. Increase the diameter of pipe.

4. Use pipes in parallel.

Explanation:

Thoma's cavitation parameter (σ): It is the ratio of Net Positive Suction Head (NPSH) to the total head.

\(\sigma = \frac{{NPSH}}{H} = \frac{{\left( {\frac{{{P_A}}}{{\rho g}} - \frac{{{P_V}}}{{\rho g}} - z - {h_f}} \right)}}{H}\)

NPSH: It is defined as the net head developed at the suction port of the pump, in excess of the head due to the vapour pressure of the liquid at the temperature in the pump. NPSH must be positive for preventing the liquid from boiling. Boiling or cavitations may damage the pump. If NPSH reaches zero then the liquid starts boiling and cavitation starts.

Cavitation: It is defined as the phenomenon of formation of vapour bubbles of a flowing liquid in a region where the pressure of the liquid falls below its vapour pressure and the sudden collapsing of these vapour bubbles in a region of higher pressure. When the vapour pressure collapse, very high pressure is created.

Effects of cavitation:

(i) The metallic surfaces are damaged and cavities are formed on the surfaces.

(ii) Due to the sudden collapse of the vapour bubble, considerable noise and vibrations are produced.

(iii) Due to cavitation, the metal of the runner vanes and the draft tube is gradually eaten away, which results in lowering the efficiency of the turbine.

Explanation:

• 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 and sudden collapsing of these vapour bubbles in a region of higher pressure. 
• When the vapour bubbles collapse, very high pressure is created. 
• The metallic surfaces, above which the liquid is flowing, is subjected to these high pressure which causes pitting action on the surface.
• Thus cavities are formed on the metallic surface and hence the name is cavitation.
• 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 the cavitation may occur.