Short Answer:

Cavitation in hydraulic machines occurs when the pressure of the flowing liquid drops below its vapor pressure, causing the formation of vapor bubbles. These bubbles travel with the flow and collapse suddenly when they enter a high-pressure region, creating strong shock waves.

In hydraulic machines like pumps and turbines, cavitation usually happens near the blades or suction side, where velocity is high and pressure drops. This leads to noise, vibration, and damage to machine surfaces, reducing efficiency and lifespan of the equipment.

Detailed Explanation:

Cavitation in Hydraulic Machines

Cavitation is a serious problem in hydraulic machines, such as centrifugal pumps, hydraulic turbines, and propellers. It involves the formation, growth, and sudden collapse of vapor-filled bubbles within the liquid flowing through the machine. This occurs when local pressure in the fluid drops below the vapor pressure of the liquid at that temperature.

Hydraulic machines are designed to handle fluids under pressure, but high-speed flow or poor design can create low-pressure zones, especially at the suction side or blade surfaces. These areas are where cavitation is most likely to start.

How Cavitation Occurs in Hydraulic Machines

1. Pressure Drop at Inlet or Blade Surfaces

In pumps, especially centrifugal pumps, fluid enters the impeller eye (center), where it is supposed to be pushed outward by the rotating blades. However, if the inlet pressure is too low, the pressure at the impeller eye may fall below the vapor pressure of the fluid.

Similarly, in turbines, water passing over the runner blades may accelerate sharply, causing local pressure drops.

2. Bubble Formation

At these low-pressure regions, the liquid vaporizes, forming vapor bubbles. These bubbles are carried along with the flowing liquid.

3. Bubble Collapse

As the fluid flows into higher-pressure zones further along the machine, the bubbles collapse or implode suddenly. This collapse releases strong micro-jets and shock waves that strike the nearby solid surfaces.

4. Surface Damage and Energy Loss

The repeated impact of collapsing bubbles causes pitting, erosion, and cracks on blades, casings, or runners. It also produces noise and vibrations, leading to loss of mechanical strength and lower hydraulic efficiency.

Common Zones of Cavitation in Machines

• Centrifugal Pumps: At the impeller eye or suction side where fluid enters.
• Francis Turbines: On the back side of the blades or near the runner outlet.
• Pelton Wheels: Near the needle nozzle or at bucket surfaces.
• Propellers: Along the low-pressure side of the blade.

Effects of Cavitation in Hydraulic Machines

• Metal surface erosion: Repeated bubble collapse causes material loss.
• Vibration and noise: Machines become unstable and noisy.
• Reduced efficiency: Energy is lost in bubble formation and collapse.
• Failure of parts: Long-term exposure leads to cracking and breakdown of components.

Prevention of Cavitation in Hydraulic Machines

1. Maintain adequate Net Positive Suction Head (NPSH) to keep pressure above vapor pressure.
2. Avoid sharp bends and restrictions in piping near inlets.
3. Use proper blade design and angles to ensure smooth flow.
4. Install air relief valves or venting systems to remove trapped gases.
5. Operate machines within rated flow and pressure ranges.

By following these design and operation precautions, engineers can minimize the chances of cavitation and protect hydraulic machines from premature failure.

Conclusion:

Cavitation in hydraulic machines occurs when fluid pressure drops below vapor pressure, causing bubble formation and collapse. This damaging process often happens in pumps and turbines at high-speed, low-pressure zones. It leads to surface damage, noise, and efficiency loss. Understanding how cavitation forms and applying proper design and operation measures helps in extending the life and reliability of hydraulic machines.