How to Avoid Fire Pump Overpressure in Low-Demand Systems?

Fire pump overpressure is a common but often underestimated risk in low-demand fire protection systems. While fire pumps are designed to deliver reliable pressure during emergency conditions, many systems operate for long periods under minimal flow. In these scenarios, excessive pressure can build up, leading to system damage, false alarms, leakage, or even catastrophic component failure.

For system designers, contractors, and facility owners, understanding how overpressure occurs and how to control it is critical for long-term system reliability and NFPA 20 compliance. This article explains why overpressure happens in low-demand systems and outlines practical, proven methods to avoid it.

Understanding Fire Pump Overpressure

Fire pump overpressure occurs when the system pressure exceeds the maximum allowable working pressure of piping, valves, sprinklers, or fittings. This situation is most likely to occur during low-flow or no-flow conditions, such as system testing, churn operation, or when only a small hose or test header is open.

Unlike domestic pumps, fire pumps are designed to prioritize reliability over efficiency. They operate at a fixed speed and are capable of generating pressures well above system demand when flow is restricted. If not properly controlled, this characteristic becomes a liability.

Why Low-Demand Systems Are More Vulnerable

Low-demand systems are especially prone to overpressure because they often combine high pump capacity with limited flow paths. Common examples include:

Facilities with small sprinkler zones but large fire pumps
Systems protected primarily by jockey pumps with infrequent fire pump operation
Installations with short piping runs and minimal friction loss
Test conditions where only one outlet is flowing

When the fire pump starts and system demand is low, most of the pump’s energy is converted into pressure rather than flow. This can quickly push pressure beyond safe limits.

Risks Associated With Fire Pump Overpressure

Uncontrolled overpressure can cause a wide range of operational and safety issues, including:

Damage to sprinklers, valves, and fittings
Frequent leakage at joints and flanges
Nuisance alarms and system instability
Increased wear on pump seals and bearings
Violation of NFPA 20 and insurance requirements

Over time, repeated exposure to excessive pressure shortens equipment lifespan and increases maintenance costs.

NFPA 20 Requirements Related to Overpressure

NFPA 20 recognizes overpressure as a serious concern and provides clear guidance to mitigate it. Key requirements include:

Pressure relief valves are required when a fire pump can generate pressure exceeding the rated system pressure
The relief valve must be sized to handle churn conditions
Relief valve discharge must be safely routed to prevent damage or flooding
System components must be rated for the maximum possible pressure

Ignoring these requirements can result in failed inspections, rejected projects, or denied insurance coverage.

Proper Fire Pump Selection Is the First Line of Defense

One of the most effective ways to avoid overpressure starts at the design stage. Oversized fire pumps are a leading cause of excessive pressure in low-demand systems.

Pump selection should be based on realistic hydraulic calculations, not conservative assumptions alone. Key considerations include:

Actual sprinkler and hose allowance demand
Static water supply pressure
Elevation differences
Future system expansion plans

Selecting a pump that is significantly larger than necessary may seem safer, but it often creates pressure control problems that require additional mitigation later.

Understanding Churn Pressure and Its Impact

Churn pressure refers to the pressure a fire pump generates at zero flow. This value can be substantially higher than rated pressure at 100 percent flow.

In low-demand systems, the pump may operate close to churn conditions during testing or partial flow events. If churn pressure exceeds system limits, overpressure becomes unavoidable without additional control measures.

Designers should always evaluate churn pressure during pump selection and verify that downstream components can safely withstand it.

Using Pressure Relief Valves Effectively

Pressure relief valves are the most common and widely accepted method for controlling fire pump overpressure.

A properly designed relief valve system should:

Open automatically when pressure exceeds a preset limit
Be sized to pass the full churn flow if required
Discharge water to a safe location such as a drain or reservoir
Be set and tested according to NFPA 20 guidelines

Relief valves should not be treated as optional accessories. In low-demand systems, they are often essential for protecting both equipment and piping.

Correct Placement and Setting of Relief Valves

The effectiveness of a pressure relief valve depends heavily on correct installation. Poor placement or incorrect settings can render the valve ineffective.

Best practices include:

Installing the valve on the pump discharge side
Setting the opening pressure below the weakest system component rating
Avoiding excessive discharge piping restrictions
Ensuring the valve is accessible for inspection and testing

Relief valves should be tested periodically to confirm proper operation under real pressure conditions.

Coordinating Jockey Pump and Fire Pump Pressures

In low-demand systems, improper coordination between the jockey pump and fire pump can contribute to pressure issues.

The jockey pump should maintain normal system pressure without triggering the fire pump unnecessarily. If the jockey pump pressure setting is too close to the fire pump start pressure, frequent cycling and pressure spikes may occur.

Recommended practices include:

Setting jockey pump cut-out pressure below fire pump churn pressure
Maintaining a sufficient pressure differential between pumps
Verifying control settings during commissioning

Proper coordination reduces wear, improves system stability, and minimizes pressure fluctuations.

Control Valves and Pressure Regulating Devices

In some systems, additional pressure control devices may be required, especially when protecting sensitive downstream equipment.

Pressure regulating valves can be used to limit pressure in specific zones or branches. These devices are particularly useful in high-rise or mixed-use systems where different areas have different pressure requirements.

However, pressure regulating devices must be carefully selected to ensure they do not restrict required fire flow during an emergency.

System Testing and Commissioning Are Critical

Many overpressure issues are only discovered during testing or after the system is placed into service. Thorough commissioning helps identify problems early.

During testing, attention should be paid to:

Pressure readings at churn, partial flow, and full flow
Relief valve operation and discharge behavior
Pressure stability during pump start and stop
Interaction between jockey pump and fire pump

Documenting these results provides valuable reference data for future maintenance and inspections.

Ongoing Maintenance and Monitoring

Even a well-designed system can develop overpressure issues over time due to component wear, valve adjustments, or system modifications.

Regular maintenance should include:

Verifying relief valve settings
Inspecting pressure gauges for accuracy
Reviewing pump performance curves
Checking for signs of leakage or stress

Monitoring system behavior during routine tests helps detect trends before they become serious problems.

Common Mistakes to Avoid

Several recurring mistakes increase the risk of overpressure in low-demand systems:

Selecting fire pumps based solely on maximum possible demand
Ignoring churn pressure during design
Omitting pressure relief valves to reduce initial cost
Improper jockey pump pressure settings
Assuming overpressure will not occur because demand is low

Avoiding these mistakes requires coordination between designers, manufacturers, and installers.

The Manufacturer’s Role in Overpressure Prevention

Fire pump manufacturers play a critical role in helping customers avoid overpressure problems. Providing accurate pump curves, clear churn pressure data, and application guidance enables better system design.

Manufacturers should also support customers with:

Technical consultation during pump selection
Clear documentation on pressure limits
Guidance on relief valve sizing and installation
Support during testing and commissioning

A well-matched pump is not only safer but also more reliable over its service life.

Conclusion

Fire pump overpressure in low-demand systems is not a rare edge case—it is a predictable outcome when high-capacity pumps operate under minimal flow. Fortunately, it is also a problem that can be effectively managed through proper design, correct pump selection, NFPA 20 compliant pressure relief, and careful system coordination.

By understanding the causes of overpressure and applying proven control strategies, system designers and owners can protect equipment, ensure compliance, and extend the life of their fire protection systems. Prevention begins long before installation and continues throughout the system’s operational life.