Introduction to Pump Cavitation

Pump Cavitation: What It Is? Cavitation in a pump occurs when there is insufficient fluid to fill the spaces around the impeller, leading to vaporization of the liquid. This can cause damage to the pump's components, inconsistent flow, and decreased performance. Common causes of pump cavitation include a small inlet pipe, thick fluids, high impeller speed, air on the suction side, and insufficient suction pressure.

Symptoms of Pump Cavitation

Cavitation can be identified through its characteristic noises and physical signs. A severely cavitating pump may exhibit the following symptoms:

Severe Pointer Shaking: In pressure gauges without glycerine, the pointer may shake severely. Vibrations and Noise: You can feel intense vibrations, and you might hear a sound similar to "pea gravel" going through the pump. Air Lock: Cavitation can sometimes be described as an "air-lock," indicating that the system is experiencing significant air entrapment. Reduced Suction Lift: A pump with a required NPSHr (Net Positive Suction Head Required) of 3.0m will allow for a negative suction lift of 7m or -0.7 barg vacuum.

Causes of Pump Cavitation

Cavitation often occurs due to insufficient suction pressure, which can be influenced by several factors:

Small Inlet Pipe Diameter: A smaller inlet pipe diameter can lead to a drop in pressure, making it difficult to fully saturate the impeller with liquid. Thick Fluid: Pumping dense or thick fluids can create localized low-pressure areas, which can cause cavitation. High Impeller Speed: Excessive impeller rotation can accelerate fluid velocity, leading to vapor formation. Air on the Suction Side: Air entrained in the suction line can reduce the fluid volume entering the pump, leading to cavitation. Inadequate Pressure: If the atmospheric pressure is low due to high altitude or reduced suction pressure, cavitation is more likely to occur.

Treatment of Pump Cavitation

Due to these causes, several methods can help mitigate and prevent cavitation:

Increasing Suction Head: Reducing the suction pipe length or increasing its diameter can help maintain higher suction pressure. Using an Impeller Inducer: An inducer can help maintain higher suction pressure by increasing the initial flow rate. Reducing Impeller Speed: Lowering the RPM of the impeller can reduce the risk of cavitation. Installing a Booster Pump: A booster pump can increase the overall pressure in the system, helping prevent cavitation.

Prevention of Pump Cavitation

To prevent cavitation, it is crucial to calculate the suction pressure and ensure it is greater than the required NPSH (Net Positive Suction Head):

Atmospheric Pressure: The atmospheric pressure at sea level is 14.6 psi (approximately 101.325 kPa). Altitude and any additional pressure or suction from the pump itself will affect this value. Vapor Pressure of the Liquid: For highly volatile substances like petrol, this must be taken into account. Higher vapor pressure increases the likelihood of cavitation. Viscosity and Specific Gravity: High viscosity or high SG (specific gravity) of the fluid can increase the risk of cavitation due to the increased energy required to move the fluid. Friction Loss Calculation: Carefully measure and account for the friction loss in the suction pipe to ensure that the NPSH is sufficient.

In summary, understanding the symptoms, causes, and treatment methods of pump cavitation is crucial for maintaining the efficiency and longevity of industrial pumps. By carefully managing suction pressure, ensuring proper fluid flow, and taking preventive measures, pump cavitation can be effectively controlled and mitigated.