Short Answer:

Cavitation can be reduced by maintaining proper pressure levels in hydraulic systems and avoiding sudden changes in flow velocity. By ensuring that the pressure in any part of the system does not fall below the liquid’s vapor pressure, cavitation can be prevented. Proper design of pumps, turbines, and pipes helps minimize this problem.

Other effective methods include using smooth surfaces, controlling fluid temperature, and installing air removal systems. Cavitation can also be reduced by proper selection of operating speed and by submerging turbine or pump inlets sufficiently to maintain a steady fluid flow without pressure drops.

Detailed Explanation :

Reduction of Cavitation

Cavitation reduction is a major concern in the operation of pumps, turbines, and other hydraulic systems. Since cavitation causes serious damage such as erosion, noise, and efficiency loss, several preventive measures must be applied. The main aim is to ensure that the pressure in the liquid does not fall below its vapor pressure at any point of flow. This can be achieved by proper design, operation, and maintenance of the equipment. The following points explain the different methods to reduce cavitation in detail:

1. Maintain Sufficient Suction Head (NPSH):
   One of the most effective ways to prevent cavitation is to ensure that the Net Positive Suction Head (NPSH) available is greater than the NPSH required by the pump. The NPSH represents the energy level above the vapor pressure of the liquid at the pump suction. By maintaining enough suction head, the formation of vapor bubbles is avoided. This can be done by keeping the suction pipe short, avoiding sharp bends, and ensuring that the pump is placed below the fluid level in the reservoir.
2. Avoid Sudden Pressure Drops:
   Cavitation is directly caused by a rapid fall in pressure. Therefore, designing flow passages that provide a smooth transition in flow direction and cross-section helps reduce cavitation. Sharp corners, abrupt contractions, and expansions should be avoided in pipe design. Smooth, gradual curves ensure stable flow and prevent local pressure drops that cause vaporization.
3. Control the Operating Speed:
   High-speed operation of pumps and turbines increases velocity and decreases pressure at certain regions, especially around blades and impellers. By operating at the recommended speed and avoiding over-speeding, cavitation can be minimized. Manufacturers usually specify an optimum speed range to balance performance and safety.
4. Submerge the Inlet Properly:
   The suction inlet of a pump or turbine should be sufficiently submerged below the fluid surface to avoid air being drawn into the system. Proper submergence helps maintain a steady flow and prevents the formation of low-pressure zones that encourage cavitation.
5. Use of Cavitation-Resistant Materials:
   Even with preventive measures, some amount of cavitation may still occur. Using materials that can resist erosion, such as stainless steel, bronze, or special alloys, can reduce damage. These materials are strong and can withstand the repeated bubble collapse without quick wear.
6. Improve Surface Finish and Shape:
   The smoother the surface of the blades, impellers, and pipes, the less likely it is for cavitation to start. Polished surfaces and aerodynamic shapes reduce flow separation and minimize low-pressure pockets. Proper blade angle and curvature design also ensure a smooth flow path.
7. Reduce Fluid Temperature:
   Higher fluid temperatures reduce the vapor pressure margin and make cavitation more likely. By cooling the fluid or maintaining it at a lower temperature, vapor bubble formation can be avoided. In systems where temperature cannot be reduced, materials and designs must be chosen to tolerate the expected conditions.
8. Remove Air and Gas from the Fluid:
   The presence of air or dissolved gases can trigger cavitation, as these gases provide nucleation sites for bubble formation. Air traps, deaerators, or air separators should be used to remove entrained air from the system. Regular bleeding of air from pipelines also helps maintain proper flow.
9. Use of Draft Tube in Turbines:
   In reaction turbines, the use of a well-designed draft tube helps to recover pressure and reduce cavitation near the runner exit. A properly shaped draft tube allows smooth expansion of flow, which prevents local pressure drops and helps maintain efficiency.
10. Operate within Designed Conditions:
    Cavitation mostly occurs when a pump or turbine operates away from its design point. Each machine is designed for a specific head, discharge, and speed. Operating too far above or below these conditions can cause unfavorable flow patterns, resulting in cavitation. Following the manufacturer’s design limits ensures safe operation.
11. Regular Inspection and Maintenance:
    Cavitation can also be minimized by regular checking and cleaning of system components. Blockages, deposits, or wear can disturb the flow and create low-pressure zones. Periodic maintenance helps detect and correct issues before they lead to serious damage.
12. Proper Installation of Valves and Fittings:
    Incorrect placement of valves or fittings near the suction line may lead to turbulence and pressure loss. Valves should be installed away from the pump suction and must be fully opened during operation to avoid partial restriction that could trigger cavitation.

Conclusion:

Cavitation can be reduced effectively through proper system design, correct operation, and timely maintenance. The key is to avoid low-pressure zones, control fluid speed, and maintain enough suction head. Using smooth surfaces, suitable materials, and correct installation methods further ensures cavitation prevention. By implementing these measures, the efficiency, safety, and life span of pumps and turbines can be greatly improved, leading to reliable and trouble-free operation of hydraulic systems.