How to Prevent Cavitation in Vertical Turbine Fire Pumps

Vertical turbine fire pumps play a critical role in delivering reliable water supply during fire emergencies. These pumps are widely used in high-rise buildings, industrial plants, refineries, data centers, and other facilities where large volumes of water are needed quickly.

However, one of the most common problems that can damage these pumps and compromise fire safety systems is cavitation. Cavitation occurs when the pressure inside the pump drops below the vapor pressure of the liquid, causing vapor bubbles to form and collapse violently. This phenomenon can lead to severe impeller erosion, vibration, noise, reduced performance, and even catastrophic pump failure.

In this article, we’ll explain:

• What cavitation is and why it happens in vertical turbine fire pumps

• Early warning signs of cavitation

• Key design considerations to prevent it

• Maintenance tips to keep your pump running smoothly

• Best practices according to NFPA 20 standards

By understanding cavitation and how to prevent it, you can significantly increase the lifespan, efficiency, and reliability of your vertical turbine fire pump systems.

1. What Is Cavitation in Vertical Turbine Fire Pumps?

Cavitation is a hydraulic issue that occurs when the liquid inside the pump experiences a rapid pressure drop, causing small vapor bubbles to form near the impeller or bowl assembly. When these bubbles collapse, they create shockwaves that damage the impeller surfaces, bowl assemblies, and columns.

In vertical turbine fire pumps, cavitation is particularly dangerous because these pumps operate in high-demand fire protection systems where uninterrupted water delivery is critical. A pump suffering from cavitation may fail to provide sufficient water flow and pressure during emergencies, posing significant fire safety risks.

2. Causes of Cavitation in Vertical Turbine Fire Pumps

Cavitation usually results from improper design, installation, or operational conditions. The most common causes include:

2.1. Insufficient Net Positive Suction Head (NPSH)

If the available Net Positive Suction Head (NPSHa) is less than the required NPSH (NPSHr), the pump may experience cavitation. This typically occurs when:

• The water level in the wet well or reservoir is too low.

• The pump column is too long for the given submergence.

• Friction losses in the suction pipe are excessive.

2.2. Incorrect Pump Bowl Submergence

Vertical turbine pumps rely on sufficient bowl submergence to avoid vortexing and air entrainment. If the water level falls below recommended limits, air pockets can form, leading to severe cavitation.

2.3. High Pump Speed or Oversizing

Operating the pump at speeds higher than designed can lower suction pressure and cause vapor bubble formation. Similarly, oversized pumps often operate at reduced efficiency points, increasing cavitation risks.

2.4. Air Entrapment

Air leaking into the suction column or through faulty seals can cause bubbles to form within the impeller, triggering cavitation.

2.5. Inadequate Suction Conditions

Poorly designed intake structures, undersized suction pipes, or blocked strainers can restrict water flow and lead to low-pressure zones inside the pump.

3. Signs and Symptoms of Cavitation

Detecting cavitation early is crucial for preventing severe damage to your vertical turbine fire pumps. Key warning signs include:

• Unusual Noise: A rattling or “marbles in a can” sound.

• Excessive Vibration: Continuous or intermittent pump vibration.

• Reduced Pump Performance: Lower flow rate and discharge pressure.

• Damaged Impeller Blades: Pitting, erosion, or cracks visible during inspections.

• Frequent Seal or Bearing Failures: Caused by unbalanced hydraulic forces.

Monitoring these signs during regular maintenance can save your pump from costly downtime.

4. How to Prevent Cavitation in Vertical Turbine Fire Pumps

Preventing cavitation requires a combination of correct pump selection, proper installation, and regular maintenance. Below are key strategies:

4.1. Ensure Adequate NPSH Margin

• Always verify that the available NPSH (NPSHa) is at least 3 feet greater than the pump’s required NPSH (NPSHr) as recommended by NFPA 20.

• Reduce friction losses by using properly sized suction piping.

• Keep the pump bowl adequately submerged below the minimum level specified by the manufacturer.

4.2. Proper Pump Selection

• Select pumps based on the actual system demand rather than oversizing.

• Choose an impeller diameter and pump speed optimized for your required flow range.

• Use computational fluid dynamics (CFD) or factory testing data when possible.

4.3. Maintain Proper Bowl Submergence

• Follow manufacturer guidelines on minimum water levels.

• Install water level monitoring sensors to prevent the pump from running when submergence is insufficient.

4.4. Prevent Air Entrapment

• Inspect column pipe joints and packing for leaks.

• Ensure the intake structure is designed to minimize vortex formation.

• Keep suction strainers clean and properly sized.

4.5. Optimize Operating Conditions

• Avoid throttling discharge valves excessively, as this can increase suction lift.

• Operate pumps within their Best Efficiency Point (BEP) range whenever possible.

• Regularly monitor pressure gauges, flow meters, and vibration sensors.

4.6. Routine Maintenance and Inspections

• Perform quarterly inspections on bowl assemblies and impellers for pitting or erosion.

• Check bearings, seals, and shaft sleeves to ensure smooth operation.

• Clean suction strainers and wet wells regularly to maintain unobstructed flow.

5. NFPA 20 Guidelines for Vertical Turbine Fire Pumps

The NFPA 20 Standard for the Installation of Stationary Pumps for Fire Protection provides detailed recommendations to avoid cavitation:

• Ensure sufficient NPSHa above the NPSHr.

• Maintain recommended minimum water levels and bowl submergence.

• Use high-quality strainers and screens to prevent air entrainment.

• Include pressure monitoring points at the suction and discharge to detect low-pressure issues early.

Following these standards helps ensure your pump system complies with international fire safety codes while maintaining operational reliability.

6. Consequences of Ignoring Cavitation

If cavitation is not addressed, vertical turbine fire pumps may suffer:

• Severe Impeller Damage – Pitting and erosion reduce hydraulic efficiency.

• Reduced Fire Pump Performance – Lower flow rates can compromise fire suppression systems.

• Increased Downtime and Costs – Repairs and replacements are expensive and time-consuming.

• System Failure During Emergencies – Inadequate water supply can lead to catastrophic fire losses.
   

7. Conclusion

Cavitation is one of the most common and destructive issues affecting vertical turbine fire pumps. By understanding its causes and implementing preventive measures—such as ensuring adequate NPSH, proper bowl submergence, optimal pump selection, and regular inspections—you can significantly extend your pump’s lifespan and improve overall fire safety performance.

As a manufacturer of vertical turbine fire pumps, we strongly recommend integrating NFPA 20 guidelines and proactive maintenance practices into your fire protection strategy. Reliable pumps are the backbone of any fire suppression system, and preventing cavitation is key to keeping them running at peak performance.