Causes of airlock in centrifugal pumps and preventive measures; the difference between airlock and cavitation.

Centrifugal pumps, as crucial fluid transport equipment, are essential for the stability of industrial production due to their efficient and reliable operation. However, airlock often leads to decreased pump performance or even prevents normal startup. Airlock is a fluid dynamics problem caused by gas accumulation in a localized area within the pump, typically manifesting as interrupted fluid flow and loss of pumping capacity. Although airlock is relatively common in practical applications, it is often mistaken for cavitation. To better understand and address this issue, this paper systematically analyzes the causes of airlock and its essential differences from cavitation, and proposes effective preventative measures.

 

 

• Causes of air lock in centrifugal pumps

Causes of Air Lock in Centrifugal Pumps Air lock occurs when gas accumulates within the pump chamber, obstructing normal liquid flow. The main causes include:

 

1. Incomplete Removal of Gas from the Pump

Before starting the centrifugal pump, a certain amount of gas may remain inside the pump. If this gas is not completely removed during startup, it will accumulate at high points in the pump or near the impeller, forming an air lock. This is particularly common during initial startup or restart after shutdown.

2. Inadequate Suction Pipe Design

The design of the suction pipe has a significant impact on air lock formation. If there are high points in the suction pipe, gas tends to accumulate there and enter the pump chamber. Furthermore, excessively long pipes or pipes with insufficient diameter can reduce liquid flow velocity, making it easier for gas to precipitate and accumulate.

3. Precipitation of Dissolved Gases in the Liquid

When the local pressure inside the pump drops below the pressure at which dissolved gases precipitate, dissolved gases will precipitate as bubbles. These bubbles may accumulate inside the pump, eventually forming an airlock.

4. Improper Operation

Improper operation can also lead to an airlock. For example, failing to perform adequate priming before starting the pump, or suddenly closing the suction valve while the pump is running, can cause gas to enter the pump and form an airlock.

5. Pump Installation Height Too High

If the pump is installed at a height exceeding the allowable suction head of the suction line, the negative suction pressure may cause gas to precipitate and accumulate, thus creating an airlock.

 

• Measures to prevent airlock

To effectively prevent airlock, the following measures can be taken:

 

1. Optimize the Suction Pipeline Design

The suction pipeline design should avoid high points as much as possible, and ensure the pipeline is short, straight, and has a sufficiently large diameter to reduce the possibility of gas release due to decreased flow velocity.

2. Thoroughly Exhaust Gas

Before starting the pump, thoroughly remove all gas from the pump body through the exhaust valve to ensure the pump is filled with liquid. Furthermore, during pump operation, regularly check and remove any accumulated gas.

3. Control the Pump Installation Height

The pump installation height should be within the allowable suction head range to avoid excessive suction negative pressure leading to gas release.

4. Improve Operation Procedures

Operators should strictly follow the operating procedures for pump startup and operation. For example, ensure the suction valve is fully open before startup and avoid suddenly closing the suction valve.

5. Use Anti-Airlock Devices

In certain critical situations, automatic exhaust valves or other anti-airlock devices can be installed at high points on the pump to automatically remove gas from the pump and prevent airlock.

 

• The difference between airlock and cavitation

Although airlock and cavitation share some similarities in their manifestations, their causes and mechanisms of action differ significantly.

 

1. Different Causes

An airlock is caused by gas accumulating in a localized area within the pump, obstructing liquid flow. Cavitation, on the other hand, occurs when the local pressure within the pump drops below the liquid vapor pressure, causing the liquid to vaporize and form bubbles. When these bubbles burst in high-pressure areas, they release energy, leading to equipment damage.

2. Different Affected Areas

Airlock typically occurs at high points in the pump chamber or near the impeller, primarily affecting liquid flow. Cavitation mainly occurs at the impeller inlet or other low-pressure areas, its destructive effect primarily manifesting as the impact and erosion of equipment surfaces by bursting bubbles.

3. Different Manifestations

The main manifestation of an airlock is a decrease in pump flow or a complete loss of pumping capacity. Cavitation, in addition to potentially causing a decrease in pump performance, also manifests as increased pump noise and vibration, accompanied by erosion and wear on the impeller blade surface.

4. Different Treatment Methods

The main treatment method for an airlock is to remove the gas from the pump, for example, by venting it through an exhaust valve. To address cavitation, it is necessary to increase the pump's suction pressure, for example, by lowering the pump's installation height, reducing the pump's speed, adding an inducer, or installing a booster pump.

 

The occurrence of airlock in centrifugal pumps is closely related to the accumulation of gas within the pump. Its causes include incomplete removal of gas from the pump chamber, improper design of the suction pipeline, and the precipitation of dissolved gases from the liquid. Airlock can be effectively avoided by designing a proper suction pipeline, ensuring adequate venting, and controlling the pump's installation height. Although airlock and cavitation share some similarities in their manifestations, their causes and mechanisms of action are significantly different. Correctly distinguishing between airlock and cavitation and taking targeted preventative and remedial measures is crucial for ensuring the reliable operation of centrifugal pumps.

Based on the above analysis and literature support, this paper provides a comprehensive theoretical basis and practical guidance for understanding and solving the airlock problem in centrifugal pumps, hoping to provide a reference for research and engineering applications in related fields.