What is a Fire Pump Sensing Line and How it Works

A fire pump sensing line is a critical component in fire protection systems, specifically designed to monitor the water pressure in the system. This small-diameter pipe, often made of copper or stainless steel, connects the discharge side of the fire pump to a pressure-sensing device, such as a pressure switch or a pressure transducer. The purpose of the sensing line is to provide accurate pressure readings, ensuring that the fire pump operates correctly during a fire emergency.

According to NFPA20,

SENSING LINE (Fire Pump Controller); One end of the fire pump Sensing Line is connected to a pressure sensor (Mercoid Switch or pressure transducer) within the controller. The other end of the sensing line is connected to the Fire piping between the discharge check valve and discharge control valve of the fire pump assembly. This Provides the controller the ability to sense pressure change in the fire protection system when the fire pump is fully in service, but also allowes testing of the pressure settings even if the pump discharge control valve is closed.

SENSING LINE (Pressure Maintenance Pump); The Sensing Line performs the same function as the fire pump sensing line , but for the pressure maintenance pump.

To understand the necessity of pressure monitoring in fire and jockey pump controllers, it's essential to grasp the primary functions of each pump:

• Fire Pumps: These are designed to deliver substantial amounts of water for fire suppression.
• Jockey Pumps: These maintain system pressure by replenishing normal water loss, not for fire suppression.

When an automatic fire sprinkler head activates during a fire, water is immediately discharged because the system is typically pressurized. The fire pump controller detects this sudden drop in water pressure and signals the pump to start, supplying a large volume of water for fire suppression. The same response occurs if a firefighter or trained operator opens a fire hose valve.

For minor water losses, such as minimal leakage within the system, the pressure drop will not reach the fire pump controller's cut-in pressure. Instead, the jockey pump controller, programmed to activate first, will engage the jockey pump to restore the system pressure to its normal level.

Fire Pump Pressure Settings (Cut-In and Cut-Out)

The National Fire Protection Association (NFPA) recommends setting the jockey pump to shut off at the "pump churn" pressure plus the minimum static pressure on the suction side of the pump. The jockey pump start pressure should be 10 psi lower than its shut-off pressure. The main pump should start about 5 psi below the jockey pump start pressure and does not have an automatic shut-off; it must be turned off manually.

Recommended Pressure Settings

For fire pumps connected to the city water main or receiving water from elevated tanks, the minimum static supply pressure should be added to the system’s maintaining pressure to prevent water hammer or sudden pressure increases when the fire pump activates.

Example of Fire Pump Settings: Consider a 1000 gpm pump with a 100 psi rating and a churn pressure of 115 psi, connected to a cistern tank:

• The jockey pump should shut off at the churn pressure plus the minimum static pressure.
• The jockey pump should start at 10 psi below its shut-off pressure.
• The main pump should start 5 psi below the jockey pump start pressure and be manually shut off.

These settings are recommended by NFPA and not obligatory so following them can help avoid sudden pressure surges and water hammer when the fire pump activates. Slight deviations may be permissible based on engineering judgment.

How Does it Work in Different Pumps?

1. Centrifugal Fire Pumps:

• Pressure Monitoring: In centrifugal fire pumps, the sensing line monitors the pressure at the pump discharge. When the system pressure drops below a set threshold (typically due to an open sprinkler head or hose valve), the pressure-sensing device signals the fire pump controller to start the pump.
• Control: The pressure switch or transducer sends a signal to the pump controller, which activates the pump motor to maintain the necessary water pressure in the fire suppression system.
• Stop Operation: Once the fire event is over and the system pressure stabilizes above a certain level, the sensing line helps to signal the pump to stop.

2. Vertical Turbine Fire Pumps:

• Similar Operation: In vertical turbine pumps, the sensing line works similarly centrifugal fire pumps, monitoring the discharge pressure to control the pump operation.
• Installation Differences: Due to the vertical nature of these pumps, the installation of the sensing line may differ slightly, often requiring additional support to ensure accurate pressure readings.

3. Positive Displacement Fire Pump:

• Consistent Pressure: In positive displacement pumps, the sensing line helps ensure consistent pressure delivery. These pumps deliver a constant volume of water regardless of pressure variations, but the sensing line ensures the system pressure stays within the required range.

How to Install a Fire Pump Sensing Line

1. Material Selection: Choose appropriate materials (e.g., copper or stainless steel) for the sensing line to ensure durability and corrosion resistance.
2. Location: Install the sensing line on the discharge side of the fire pump, typically after the pump discharge valve. Ensure the sensing line is not exposed to high vibration or physical damage.
3. Connection: Connect one end of the sensing line to the pump discharge pipe using appropriate fittings. The other end should be connected to the pressure-sensing device (pressure switch or transducer).
4. Support and Protection: Secure the sensing line using clamps or brackets to prevent movement and vibration. Protect the line from potential physical damage by routing it away from high-traffic areas or using protective covers.
5. Testing: After installation, test the system to ensure that the sensing line accurately detects pressure changes and that the fire pump responds correctly. Regularly inspect and maintain the sensing line and associated components to ensure reliable operation.

Key Considerations

• Accurate Placement: Ensure the sensing line is placed correctly to provide accurate pressure readings.
• Regular Maintenance: Periodically inspect and maintain the sensing line to prevent blockages, leaks, or other issues.
• Compliance: Follow all relevant standards and regulations (e.g., NFPA 20) to ensure the installation meets fire safety requirements.

Fire Pump Sensing Line Diagram

Below is an example arrangement from the NFPA20 (Section 4.32.1, Ed 2024):

For all pump installations, including Jockey pumps, each controller shall have its own individual pressure sensing line:

Proper fire pump sensing line installation is crucial for ensuring the reliable and efficient operation of fire protection systems. Here are some of the key advantages:

1. Accurate Pressure Sensing

• Consistent Pressure Readings: Properly installed sensing lines ensure accurate pressure readings, allowing the fire pump controller to operate the pump effectively.
• Avoids Pressure Loss: Correct installation prevents pressure loss in the sensing lines, which can otherwise lead to false readings and improper pump operation.

2. Reliable Pump Operation

• Timely Activation: Accurate sensing lines ensure that the pump activates promptly in response to a pressure drop in the system, ensuring rapid firefighting response.
• Prevents False Starts: Proper installation reduces the likelihood of false starts or stops, which can be caused by incorrect pressure readings.

3. Enhanced Safety

• Optimal Fire Protection: Ensuring the pump activates and deactivates at the correct pressures improves overall fire protection, reducing the risk to life and property.
• Compliance with Standards: Proper installation helps meet fire safety codes and standards (e.g., NFPA 20), ensuring the system is legally compliant and safe.

4. Longevity and Maintenance

• Reduced Wear and Tear: Accurate pressure sensing leads to fewer unnecessary pump cycles, reducing wear and tear on the pump and associated components.
• Simplified Maintenance: Well-installed sensing lines are easier to inspect and maintain, leading to quicker diagnostics and repairs.

5. Cost Efficiency

• Energy Savings: Proper pump operation avoids unnecessary running, leading to energy savings and lower operational costs.
• Reduced Repair Costs: By minimizing wear and tear and preventing malfunctions, the overall maintenance and repair costs are reduced.

6. System Reliability

• Consistent Performance: Properly installed sensing lines contribute to the consistent performance of the fire pump system, ensuring it works correctly when needed.
• Minimizes Downtime: Reliable operation reduces the risk of system downtime, ensuring continuous fire protection.

7. Improved Response Time

• Quick Reaction: Accurate pressure detection allows the fire pump to respond quickly to drops in system pressure, improving the overall response time in an emergency.

Best Practices for Installation

To achieve these advantages, consider the following best practices for fire pump sensing line installation:

• Correct Sizing: Use appropriately sized pipes for the sensing lines to ensure accurate pressure transmission.
• Avoiding Air Traps: Install lines with a continuous slope to prevent air traps that can cause inaccurate readings.
• Secure Connections: Ensure all connections are secure and leak-free to maintain pressure integrity.
• Regular Testing and Maintenance: Periodically test and maintain the sensing lines to ensure they remain in good working condition.

By following these guidelines and ensuring proper installation, fire pump sensing lines can significantly enhance the effectiveness and reliability of fire protection systems.

1. Proper Installation

Material Selection:

• Use corrosion-resistant materials like copper or stainless steel for the sensing line to ensure durability and longevity.
• For environments with high corrosion potential, consider additional coatings or specialized materials.

Sizing:

• Ensure the sensing line has the correct diameter (typically 1/4" to 1/2") to avoid delays or inaccuracies in pressure transmission.

Routing:

• Install the sensing line with minimal bends or elbows to reduce the risk of pressure loss and blockages.
• Avoid long runs or unnecessary loops that can cause inaccurate readings.

Connections:

• Securely connect the sensing line to the discharge side of the pump and the pump controller.
• Use threaded or compression fittings to ensure a leak-proof connection.

Support and Protection:

• Support the sensing line to prevent sagging or vibration, which could lead to fatigue or damage.
• Protect the line from mechanical damage, such as impacts or accidental disconnection.

Isolation Valves:

• Install isolation valves on both ends of the sensing line to facilitate maintenance without affecting the system's operation.

Strainers or Filters:

• Add strainers at the pump discharge to prevent debris from entering the sensing line.

2. Monitoring

Pressure Readings:

• Regularly monitor the pressure gauge installed along the sensing line to ensure the system is maintaining proper pressure.
• Compare the readings from the gauge to the controller's pressure sensors to verify accuracy.

System Alerts:

• Ensure the fire pump controller is equipped with an alarm or notification system to alert operators of abnormal pressures.

Flow Checks:

• Periodically simulate pressure drops (e.g., by opening a test valve) to ensure the pump activates as expected.

Visual Inspections:

• Look for signs of wear, corrosion, leaks, or physical damage during routine inspections.

Environmental Considerations:

• Monitor for environmental factors like freezing temperatures or vibrations that could affect sensing line performance.

3. Maintenance

Cleaning and Debris Removal:

• Clean strainers or filters in the sensing line regularly to prevent blockages.

Leak Repairs:

• Promptly repair any leaks in the sensing line to prevent pressure loss or false readings.

Testing:

• Conduct weekly or monthly pressure drop tests (as per NFPA 20 or local codes) to ensure the sensing line and pump respond correctly.

Calibration:

• Periodically calibrate pressure gauges, switches, and transducers to maintain accuracy.

Insulation:

• In cold climates, insulate or heat-trace the sensing line to prevent freezing, which could block pressure sensing.

Replacement:

• Replace aging or damaged components, including tubing, gauges, and switches, to ensure reliable operation.

Best Practices for Reliable Fire Protection

1. Compliance with Standards: 1)Follow guidelines from authorities like NFPA 20 (Standard for the Installation of Stationary Pumps for Fire Protection). 2)Adhere to manufacturer recommendations for installation and maintenance.
2. Documentation: 1)Maintain a log of all inspections, tests, and maintenance activities. 2)Document any repairs or component replacements.
3. Qualified Personnel: Ensure that installation and maintenance are performed by trained and certified professionals.
4. Testing Schedules: 1) Perform weekly churn tests (no-flow conditions) to verify the fire pump and sensing line functionality. 2) Conduct annual performance testing to evaluate overall system efficiency.
5. Emergency Readiness: Always verify sensing line integrity during fire pump readiness checks to prevent failures during emergencies.

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