How to Prevent Air Entrapment in Fire Pump Systems

Air in a fire pump system may sound harmless, but even a small pocket can compromise pump performance, reduce flow and pressure, and lead to cavitation or pump failure during an emergency. Preventing air entrapment is critical to maintaining the reliability of a fire protection system, and it is a challenge seen across facilities—from commercial buildings to industrial plants, warehouses, municipalities, and remote pumping stations.

As a manufacturer of fire pumps, we often see air-related issues cause unnecessary trouble during commissioning, annual testing, and even during real fire events. In this article, we explain why air enters fire pump systems, how it affects performance, and how design and maintenance steps can effectively prevent air entrapment.

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Why Air Entrapment Happens in Fire Pump Systems

Air can become trapped in fire pump systems for several reasons. Understanding the root causes is the first step to preventing failures.

1. Improper Suction Piping Configuration

Suction pipes with high points, dips, or unnecessary elbows allow air to accumulate. Horizontal piping runs that slope upward toward the pump also collect air pockets.

2. Leaks or Loose Connections

Any small leak on the suction side can draw air in, especially during pump operation when negative pressure develops.

3. Incomplete Priming

For pumps that require priming—such as end-suction or split-case pumps with above-grade water sources—failure to fully evacuate air results in low pressure or delayed start-up.

4. Low Water Level in the Water Supply

If the water tank, reservoir, or well level drops too low, vortexing occurs and draws air into the suction pipe.

5. System Draining After Maintenance

Fire pump systems that are drained for inspection or repairs often trap air in points that cannot automatically vent, especially in complex piping networks.

6. Vertical Turbine Pump Installation Errors

Vertical turbine fire pumps are designed to avoid air entrapment, but incorrect shaft alignment, column installation, or insufficient submergence can still introduce air.

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How Air Entrapment Affects Fire Pump Performance

Air pockets in a fire pump system can create serious operational hazards, including:

1. Reduced Pump Efficiency

Air reduces net positive suction head available (NPSHa), limiting the pump’s ability to deliver the required flow and pressure.

2. Cavitation

Air bubbles collapsing near the impeller generate shock waves, causing vibration, noise, and long-term damage to pump components.

3. Pump Overheating

When air interferes with proper cooling flow, pump bearings and seals can overheat.

4. Failure to Meet NFPA 20 Requirements

NFPA 20 mandates conditions for suction piping, priming, and pump operation. Air entrapment often leads to failure during acceptance or annual tests.

5. Delayed Fire Response

Air in the system may cause the pump to lose prime or delay delivering required water during a critical emergency.

The consequences are costly—not only for repair but also for system reliability. Preventing air entrapment is much easier than solving the problems it causes.

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Design Best Practices to Prevent Air Entrapment

Proper design and installation are the strongest defenses against air accumulation. Whether you are installing a diesel engine fire pump, an electric fire pump, a vertical turbine fire pump, or a complete fire pump set, consider the following engineering guidelines.

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1. Follow NFPA 20 Suction Piping Requirements

NFPA 20 recommends simple, stable, and properly sized suction piping with no air traps.

Key requirements include:

• Use straight, unobstructed suction piping.

• Avoid high points where air can accumulate.

• Maintain a constant slope toward the pump.

• Use eccentric reducers with the flat side on top.

Ensuring compliance reduces risk and improves certification results, especially for UL-listed fire pumps.

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2. Keep Suction Piping Short and Direct

Long suction runs increase the chance of air pockets forming. Use the shortest practical distance from the water supply to the fire pump and minimize fittings.

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3. Maintain Adequate Submergence

Too little submergence at the suction inlet causes air vortexing.

For tanks, reservoirs, and pits:

• Maintain the minimum recommended submergence based on flow rate.

• Use anti-vortex plates if space is limited.

For vertical turbine pumps:

• Ensure the pump bowl assembly is positioned deep enough to avoid air intake.

• Confirm correct pit depth and water level management.

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4. Install Air Release Valves

Air release valves automatically vent air during system filling and operation.

Install them at:

• The highest points in the suction line

• Pipeline peaks in long or uneven runs

• Pump discharge points where air can accumulate

Automatic air valves are especially valuable in large industrial installations.

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5. Use Proper Priming Systems

For pumps requiring priming, ensure:

• All priming lines are airtight and properly sized

• Priming chambers operate reliably

• Manual priming procedures are documented and followed

In some cases, converting to a self-priming design may help reduce risks.

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6. Choose the Right Pump Type

Vertical turbine fire pumps are often the best choice when water sources are below ground level because they eliminate suction lift and reduce air-related problems.

If using horizontal split-case pumps, ensure a positive suction head is maintained.

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7. Avoid Pipe Entrapments During Installation

Common installation mistakes include:

• Inverted U-shaped pipe sections

• Improper reducer orientation

• Using flexible connectors incorrectly

• Installing pipes without considering drainage and venting

A final slope check before commissioning helps prevent long-term issues.

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Maintenance Steps to Eliminate and Prevent Air Entrapment

Preventing air in fire pump systems is not only about design; maintenance teams play a major role in ensuring long-term system stability.

Here are the maintenance guidelines that keep pumps running reliably.

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1. Conduct Regular Suction Pipe Inspections

Check for:

• Corrosion

• Loose flanges

• Leaky joints

• Blockages

• Unusual vibrations during pump operation

Any suction-side air leak must be fixed immediately.

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2. Verify Water Levels in Tanks and Reservoirs

Air intrusion from vortexing happens when water levels are too low. Make it a routine to:

• Monitor tank water levels

• Inspect float valves and sensors

• Ensure minimum levels match design requirements

Water supply reliability directly affects fire pump performance.

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3. Test Air Release Valves Annually

Automatic air release valves can become clogged or non-functional over time. Regular testing ensures they open and close correctly.

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4. Conduct Weekly Fire Pump Testing

During the weekly churn test:

• Listen for unusual noises (aeration, cavitation)

• Check for pressure fluctuations

• Observe vibration levels

Air in the system often reveals itself through noise or inconsistent pressure readings.

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5. Perform Annual Flushing and Re-Priming

Systems that are drained for inspection or repairs must be properly re-primed. Use vent points to exhaust trapped air before returning the system to service.

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6. Inspect Vertical Turbine Pump Shafts and Bowls

Air-related issues in vertical turbine pumps often come from improper:

• Shaft tension

• Column length

• Bowl installation depth

Regular maintenance ensures these components remain in correct alignment and submergence.

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7. Monitor Cavitation Indicators

If you suspect air or cavitation, check for:

• Impeller wear

• Pitting on blades

• Metallic noises

• Excessive vibration

Early detection helps avoid catastrophic pump damage.

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Troubleshooting Air Entrapment Problems

Even with proper design and maintenance, air may still enter fire pump systems. Here’s how to identify and correct the issue.

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1. Pump Loses Prime or Fails to Start Properly

Likely causes:

• Air leaks

• Low tank levels

• Incomplete priming

• Incorrect pump type for application

Corrective actions include detecting leaks, restoring water level, or re-evaluating pump selection.

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2. Pump Vibrates or Produces Unusual Noise

This often indicates cavitation or air entrainment. Check suction piping for high points, leaks, or obstruction.

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3. Flow or Pressure Is Lower Than Expected

Air trapped in piping reduces delivery. Vent system high points and inspect automatic air valves.

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4. Vertical Turbine Pump Runs Rough

Possible causes include insufficient submergence or air intake around the well column. Verify water levels and pump depth.

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Why Preventing Air Entrapment Is Critical for Fire Safety

Air entrapment may appear minor, but its consequences can be serious. A fire pump is the heart of the fire protection system. If air compromises pump reliability, the entire system is at risk.

Preventing air entrapment ensures:

• Stable water supply

• Reliable operation under emergency conditions

• Longer pump lifespan

• Reduced cavitation damage

• Compliance with NFPA 20

• Improved safety for people and property

• Lower maintenance cost

For fire pump manufacturers, installers, and end users, focusing on air-related issues is one of the most cost-effective ways to enhance system performance.

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Conclusion

Preventing air entrapment in fire pump systems requires careful attention to design, installation, and maintenance. By following NFPA 20 guidelines, maintaining proper suction piping, using air release valves, ensuring adequate priming, and conducting regular system inspections, facilities can significantly reduce the risk of air entering the pump and compromising performance.

As a manufacturer of fire pumps— including UL-listed fire pumps, vertical turbine fire pumps, diesel fire pumps, and jockey pumps—our goal is to help customers build safe, efficient, and reliable fire protection systems. Managing air entrapment is an essential part of that mission.