Understanding Cavitation in Centrifugal Pumps: Causes and Prevention

II JAY SHRI KRISHNA II

Hello, friends, as we understand the working of Centrifugal Pump in previous blog, here we discuss about Cavitation which is a critical concern in Centrifugal Pumps, impacting their performance and longevity. It is an unwanted condition that occurs in hydraulic pumps due to excess air in the pump. Here's a comprehensive look at what Cavitation is, its causes and how to prevent this damaging phenomenon.

Cavitation occurs when the suction is greater than the pressure levels, which causes air bubbles collapse at the outlet.

Understanding Cavitation in Centrifugal Pumps: Causes and Prevention

The formation of bubbles in a liquid typically by the movement of propeller through it. Cavitation is a common issue in Centrifugal Pumps that can lead to severe consequences if not addressed. It occurs when low pressure at the pump's inlet causes the liquid to vaporize, forming bubbles. These bubbles then collapse in high-pressure regions and creating destructive shock waves.

Cavitation in Centrifugal Pump

In simple words, Cavitation means that cavities or bubbles are forming in the liquid that we are pumping. These cavities form at the low pressure or suction side of the pump casing. Several things to happen all at once:

• The cavities or bubbles will collapse when they pass into the higher regions of pressure, causing noise, vibration and reason to damage to many of the components.

• Loss in capacity

• The pump can no longer form the same head (Pressure).

• Pump’s efficiency drops

Consequences of Cavitation:

• Impeller Damage: The collapse of vapor bubbles on the impeller can erode and damage its surface over time.

• Reduced Pump Efficiency: Cavitation decreases the pump's efficiency as it disrupts the smooth flow of liquid.

• Noise and Vibration: The collapsing bubbles generate noise and vibrations, indicating cavitation issues.

To preventing this Cavitation, we should understand its causes to solve it.

Causes of Cavitation:

• Insufficient NPSHa: Net Positive Suction Head Available (NPSHa) must exceed the Net Positive Suction Head Required (NPSHr) to prevent Cavitation. Low NPSHa can lead to vaporization and bubble formation.

• High Liquid Velocity: Excessive liquid velocity in the pump impeller can lower pressure, creating conditions conducive to cavitation.

• Elevated Fluid Temperature: High temperatures can reduce the fluid's ability to handle low pressures, promoting cavitation.

Preventing Cavitation:

A fluid vaporized when its pressure becomes too low or its temperature too high. All Centrifugal Pumps have required head (Pressure) at the suction side of the pump to prevent this vaporization. This head (Pressure) is given by pump manufacturer and is calculated with the assumption that fresh water at 68℉ (20℃) is the fluid being pumped.

Since, there are losses in the piping which leading from the source to the suction of the pump. We must determine the head after these losses are calculated. i.e. Net Positive Suction Head is required to prevent the fluid from vaporization.

We take the NPSHa subtract the vapor pressure of the product we are pumping and this number must be equal to or greater than the NPSHr.

NPSHa – Vapor Pressure of the Product => NPSHr

Eccentric Reducer (FSU- Flat Side Up) at the pump suction of pumps to ensure air does not accumulate in the pipe.

• Proper NPSHa: Ensure that NPSHa is always greater than NPSHr by optimizing system design and minimizing suction lift.

• Control Liquid Velocity: Adjust pump speed or use throttling devices to control liquid velocity within recommended limits.

• Temperature Control: Maintain the fluid at suitable temperatures to prevent Cavitation due to reduced vapor pressure.

• Impeller Design: Selecting impellers with the right geometry and material can mitigate Cavitation risks.

Net Positive Suction Head (NPSH):

Net Positive Suction Head (NPSH) is a critical parameter in the operation of Centrifugal Pumps. It represents the margin of pressure at the Pump suction over the vapor pressure of the pumped fluid. NPSH is crucial in preventing cavitation, a phenomenon that can cause damage to pump components and reduce overall efficiency.

There are two main types of NPSH:

NPSH required (NPSHr) and NPSH available (NPSHa)

NPSH Required (NPSHr):

Net Positive Suction Head Required (NPSHr) is the minimum pressure required at the pump inlet to prevent cavitation.

• NPSHr is the amount of suction head required by the pump to operate without Cavitation.

• It is influenced by factors such as the pump design, impeller type, speed and fluid properties.

• Manufacturers provide NPSHr values for specific pump models at various operating conditions.

• If the actual NPSH available is less than the NPSHr, Cavitation can occur, leading to reduced pump performance and potential damage.

NPSH Available (NPSHa):

Net Positive Suction Head Available (NPSHa) is the actual pressure available at the pump inlet.

• NPSHa represents the total suction head available at the pump inlet, accounting for both static and dynamic components.

• Static head includes the pressure created by the liquid's elevation above the pump centerline.

• Dynamic head considers the velocity head of the fluid entering the pump, which can vary based on pipe configuration and fluid velocity.

• The goal is to ensure that the NPSHa is greater than the NPSHr to prevent Cavitation.

Maintaining NPSHa above NPSHr is essential to prevent Cavitation and ensure optimal Centrifugal Pump performance.

Factors affecting NPSHa and NPSHr include:

• Suction Piping Design: Proper suction piping design minimizes friction losses and promotes smooth flow to the pump.

• Elevation Difference: The vertical distance between the fluid level and the pump centerline affects NPSHa.

• Fluid Temperature: Higher temperatures reduce the fluid's vapor pressure, impacting NPSHa.

• Impeller Design: Certain impeller designs may have higher NPSHr requirements.

Maintaining an adequate NPSHa is essential for preventing Cavitation and ensuring optimal Centrifugal Pump performance. Engineers carefully consider system design, fluid properties & pump characteristics to ensure that NPSHa exceeds NPSHr under all operating conditions. Regular monitoring and maintenance practices are crucial to sustaining proper NPSH margins throughout the pump's life cycle.

Types of Impeller:

Impellers play a vital role in the performance of Centrifugal Pumps, and different types are designed to suit various applications and fluid characteristics. Here are some common types of Impellers:

Types of Impeller

1. Open Impeller:

• This type of impeller has vanes that are open, exposing the fluid to the impeller hub.

• Open impellers are suitable for handling clean fluids with minimal risk of clogging.

• They are easy to clean and maintain.

2. Closed Impeller:

• Closed impellers have shrouded or enclosed vanes, providing better efficiency compared to open impellers.

• They are more suitable for handling fluids with suspended solids or higher viscosity.

• Closed impellers are less prone to wear and are often used in industrial applications.

3. Semi-Open Impeller:

• Semi-open impellers have vanes on one side and are partially shrouded on the other.

• This design offers a compromise between the advantages of open and closed impellers.

• Semi-open impellers are often used in applications where a balance between solids handling and efficiency is required.

4. Vortex Impeller:

• Vortex impellers are designed to handle fluids with a high solids content.

• They create a vortex or whirlpool effect, allowing the pump to handle solids without clogging.

• Vortex impellers are commonly used in wastewater and slurry applications.

5. Multistage Impeller:

• Multistage impellers consist of two or more impellers mounted on a single shaft.

• Each impeller adds energy to the fluid, increasing pressure in multiple stages.

• These impellers are suitable for applications requiring high-pressure pumping, such as boiler feedwater systems.

6. Mixed Flow Impeller:

• Mixed flow impellers combine features of both radial and axial flow impellers.

• They generate a flow that is a combination of radial outflow and axial inflow.

• Mixed flow impellers are often used in applications where a balance between high flow rates and moderate head is required.

7. Axial Flow Impeller:

• Axial flow impellers create a flow parallel to the pump shaft.

• They are suitable for applications requiring high flow rates with low head, such as flood control and irrigation.

• Axial flow impellers are known for their energy efficiency in handling large volumes of fluid.

The selection of the appropriate impeller type depends on factors such as the nature of the fluid being pumped, the presence of solids, required flow rates and the system's head requirements. Engineers carefully consider these factors to optimize pump performance for specific applications.

Conclusion:

As pressure reduces it become easier for water to boil. During Cavitation, air particles within the water will expand as they reach their boiling point. These bubbles will then collapse in on themselves very rapidly on the surface of impeller or the components which removes small parts of metal from the surface, if not addressed all this happens then pumps can lead to causing damage of the impeller and reduced pump performance.

Understanding Cavitation in Centrifugal Pumps is crucial for maintaining reliable and efficient operations. By addressing the root causes and implementing preventive measures, industries can extend the lifespan of their pumps, reduce maintenance costs and ensure consistent performance.

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