What is the shut off pressure of a fire pump?

In fire protection systems, maintaining the correct pressure is crucial to ensure reliable and efficient water delivery during emergencies. One critical concept in fire pump performance is the shut off pressure. Understanding what it is, how it is determined, and why it matters is essential for engineers, facility managers, and anyone responsible for fire protection system design and maintenance.

In this article, we will explain what shut off pressure means, discuss the relevant NFPA 20 requirements, analyze how it affects fire pump selection and performance, and provide practical guidance on monitoring and controlling shut off pressure in real-world applications.

1. Understanding Fire Pump Shut Off Pressure

The shut off pressure of a fire pump refers to the maximum pressure the pump can produce when there is no water flow. In other words, it is the point on the pump performance curve where the flow is zero, but the pump is still running at full speed.

Shut off pressure is a critical value because it determines the upper limit of pressure the fire protection system might experience. If the system is not designed to handle this pressure, components such as pipes, valves, sprinklers, and fittings may fail under stress.

Key Points to Remember:

• Shut off pressure occurs at 0 GPM flow.

• It represents the highest pressure the pump can generate.

• It must comply with NFPA 20 and local code requirements.

• It affects system safety, pump performance, and component longevity.

2. NFPA 20 Guidelines on Fire Pump Shut Off Pressure

The National Fire Protection Association (NFPA 20) provides detailed standards for the installation of stationary fire pumps. According to NFPA 20, the shut off (or churn) pressure must not exceed 140% of the rated pressure of the fire pump at its rated flow.

Example:

If a fire pump is rated for 100 psi at 1000 GPM, the maximum allowable shut off pressure should be:

100 psi × 140% = 140 psi

Exceeding this limit could compromise the safety of the entire system and potentially damage equipment.

3. Fire Pump Performance Curve and Shut Off Point

A fire pump performance curve visually illustrates the relationship between flow rate (GPM) and discharge pressure (psi or bar).

• At 0 GPM, the pump delivers its shut off pressure.

• As flow increases, pressure drops along the curve.

• At the rated flow, the pump delivers its rated pressure.

• Beyond 150% of rated flow, pressure continues to drop until it reaches the pump’s maximum capacity.

Understanding this curve is essential when selecting the right fire pump model for your system. Oversized or undersized pumps can cause significant operational issues, including excessive shut off pressure or insufficient system pressure during demand.

4. Why Shut Off Pressure Matters

Shut off pressure is not just a theoretical value—it has real-world implications for the safety, reliability, and cost-effectiveness of fire protection systems.

4.1. System Safety

If the shut off pressure exceeds system component ratings, it can cause pipe bursts, valve failures, or sprinkler damage. Ensuring that shut off pressure stays within safe limits prevents catastrophic failures during pump startup or system testing.

4.2. Equipment Protection

Fire pumps, jockey pumps, control panels, and pressure relief valves are designed to operate within specific pressure limits. Excessive shut off pressure can shorten the lifespan of these components, increase maintenance costs, and result in system downtime.

4.3. Compliance with NFPA and Local Codes

Failure to comply with NFPA 20 and local fire codes can lead to failed inspections, insurance issues, and project delays. Engineers must design systems to ensure shut off pressure remains within permissible limits.

5. How to Calculate Fire Pump Shut Off Pressure

Calculating shut off pressure involves analyzing the fire pump’s rated performance and applying NFPA 20 requirements.

Formula:

Shut Off Pressure ≤ Rated Pressure × 1.40

Steps:

1. Obtain the pump’s rated pressure from the manufacturer’s datasheet.

2. Multiply the rated pressure by 1.40 to find the maximum allowable shut off pressure.

3. Compare the calculated value with the system’s actual shut off pressure.

4. If the measured pressure exceeds the limit, adjustments are required.

6. Controlling and Managing Shut Off Pressure

Properly managing shut off pressure is critical to preventing overpressure conditions. Here are several effective strategies:

6.1. Use of Pressure Relief Valves

NFPA 20 recommends installing pressure relief valves on systems where shut off pressure may exceed safe limits. These valves automatically release excess pressure, protecting pipes and components.

6.2. Correct Pump Sizing

Oversized pumps often produce unnecessarily high shut off pressures. Selecting the right pump size based on system demand ensures safer operation and improved efficiency.

6.3. Incorporating Jockey Pumps

Jockey pumps maintain system pressure under normal conditions and reduce frequent fire pump starts. Proper jockey pump selection can minimize stress on the main fire pump and control excessive shut off pressures.

6.4. Variable Frequency Drives (VFDs)

VFDs allow real-time control of pump speed, helping maintain pressure within safe limits and reducing mechanical wear.

7. Common Problems Caused by High Shut Off Pressure

Excessive shut off pressure can create multiple operational and safety issues:

• Pipe damage due to overpressurization

• Valve and sprinkler head failures

• Frequent system leaks

• Increased maintenance costs

• Shortened pump lifespan

• Non-compliance with NFPA standards

8. Testing Fire Pump Shut Off Pressure

Regular testing is vital to ensure the system operates within safe pressure limits. NFPA 25, which governs fire pump testing, recommends:

• Annual flow tests to verify performance.

• Churn tests to measure shut off pressure at zero flow.

• Monitoring readings at the pump controller and discharge gauges.

• Comparing actual measurements against NFPA requirements and manufacturer specifications.

9. Selecting the Right Fire Pump to Control Shut Off Pressure

As a fire pump manufacturer, we design and produce pumps that comply with NFPA 20 and UL/FM standards. When selecting a pump, consider:

• Rated flow and pressure requirements

• Maximum allowable shut off pressure

• Compatibility with system components

• Integration with jockey pumps, VFDs, and pressure relief valves

Choosing the right fire pump not only ensures compliance but also improves system reliability, safety, and cost-efficiency.

10. Best Practices for Fire Pump System Designers

• Always design for maximum shut off pressure to avoid overpressurization risks.

• Review pump curves carefully before selection.

• Verify component pressure ratings exceed potential shut off pressures.

• Include safety devices like relief valves and VFDs.

• Test and document system performance regularly.
  
  

Conclusion

The shut off pressure of a fire pump is one of the most critical parameters in fire protection system design and maintenance. Understanding how it is defined, calculated, and controlled helps ensure compliance with NFPA 20, protects equipment, and guarantees system safety during emergencies.

By selecting properly sized pumps, incorporating pressure relief strategies, and following testing best practices, you can achieve a safe, reliable, and code-compliant fire protection system.