Short Answer:
Cavitation is mainly caused by the formation and collapse of vapor bubbles in a liquid when pressure falls below the vapor pressure. The main causes of cavitation include high fluid velocity, low pressure at pump suction, high temperature, improper pump design, and sudden changes in direction of flow. When the liquid pressure drops, vapor bubbles form and later collapse violently, damaging nearby surfaces like impeller blades and valves.
In mechanical systems such as pumps, turbines, and propellers, cavitation happens when the local pressure is too low for the liquid to remain stable. This low-pressure condition may occur due to poor system design, improper installation, or excessive suction lift. Cavitation not only reduces efficiency but also causes noise, vibration, and serious mechanical damage over time.
Detailed Explanation:
Causes of Cavitation
Cavitation occurs when the pressure in a liquid drops below its vapor pressure, forming small vapor-filled bubbles. When these bubbles move into a region of higher pressure, they suddenly collapse, releasing a large amount of energy. This repeated bubble formation and collapse can cause severe damage to the surfaces in contact with the fluid, such as pump impellers, turbine blades, or valve seats. The main causes of cavitation are discussed below:
- Low Pressure at Pump Suction
One of the most common causes of cavitation is low pressure at the pump suction. When the pressure at the suction side of the pump falls below the vapor pressure of the liquid, vapor bubbles form. This may happen due to excessive suction lift, high fluid temperature, or improper system layout. The lower the suction pressure, the higher the chance of bubble formation. - High Flow Velocity
High velocity in the suction line or impeller passages causes a drop in pressure according to Bernoulli’s principle. When the fluid velocity increases too much, it reduces static pressure and can fall below vapor pressure. This phenomenon is particularly observed in pumps and turbines operating beyond their designed capacity. - High Liquid Temperature
As temperature increases, the vapor pressure of the liquid also increases. This means even a small drop in pressure can cause vaporization. High-temperature fluids are more prone to cavitation because they can easily form vapor bubbles. In boilers, condensate pumps, and cooling systems, high temperatures often contribute to cavitation. - Improper Pump Design or Selection
Pumps that are not correctly designed or selected for the operating conditions can experience cavitation. For example, impellers with sharp curvature or narrow passages can create local low-pressure zones. Similarly, using a high-speed pump for low-pressure applications can cause vaporization inside the casing. - Improper Pump Installation
Incorrect installation such as long suction pipes, too many bends, or inadequate pipe diameter can increase friction losses, lowering suction pressure. Air leakage into the suction line can also reduce the effective suction head and promote cavitation. - Sudden Changes in Flow Direction
Rapid changes in the direction of liquid flow, such as sharp turns or abrupt restrictions, can lead to localized pressure drops. These regions of low pressure encourage bubble formation and subsequent collapse, leading to erosion and noise. - Excessive Suction Lift
Suction lift refers to the vertical distance between the pump and the surface of the liquid being pumped. If the suction lift is too high, the pump may not be able to maintain sufficient pressure at its suction side, leading to vapor formation. - Air or Gas Entrainment
Air leaks or gas entrainment in the suction line can contribute to cavitation. These gases lower the local pressure and cause uneven flow, which promotes bubble formation and instability in the liquid stream. - Improper Operation or Overloading
Operating the pump far from its designed operating point (best efficiency point) can cause uneven flow distribution within the impeller. This leads to pressure fluctuations and increases the likelihood of cavitation. - Inadequate Net Positive Suction Head (NPSH)
Cavitation occurs when the available NPSH is less than the required NPSH of the pump. NPSH ensures that the liquid enters the pump without vaporizing. If the system cannot maintain sufficient NPSH, cavitation is almost certain to occur.
Conclusion:
Cavitation is primarily caused by a drop in local pressure below the vapor pressure of the liquid. Factors like high temperature, high flow velocity, low suction pressure, and poor pump design increase the chances of cavitation. To prevent it, proper system design, correct pump selection, maintaining adequate NPSH, and avoiding excessive suction lift are essential. Controlling these causes helps to enhance the efficiency, safety, and life span of pumps and turbines.