Low-pressure problems in a fire pump are among the most common and most dangerous failures that can occur in a fire protection system. When a fire pump cannot build or maintain the proper discharge pressure, the entire building or facility loses its ability to deliver an adequate water supply in an emergency. For system designers, maintenance teams, and facility managers, knowing how to diagnose low-pressure issues quickly can make the difference between a functional fire safety system and a catastrophic failure during a fire.
As a manufacturer of professional fire pumps, including UL listed fire pumps, diesel engine fire pumps, electric fire pumps, jockey pumps, and vertical turbine fire pumps, we understand exactly how low-pressure problems develop and what steps are required to resolve them efficiently. This article provides a comprehensive, practical guide to identifying, diagnosing, and fixing low-pressure issues in both new and existing fire pump installations.
Fire pumps are designed to maintain a specific rated pressure and flow based on NFPA 20 requirements. If the pressure drops significantly below the pump’s rating, several risks occur:
Hose streams may be insufficient
Sprinkler systems may not activate properly
Water may not reach upper floors or distant zones
The pump may experience cavitation or internal damage
Overall firefighting effectiveness is reduced
Low pressure is not only a performance issue—it can directly compromise life safety. Understanding the root causes ensures quick and effective resolution.
Low-pressure problems often fall into a few major categories: suction issues, pump mechanical issues, driver or control failures, system design problems, and environmental conditions. Below is a deep dive into the most common causes.
The majority of low-pressure complaints are ultimately traced back to suction problems. A fire pump cannot generate pressure if it is starved of water.
Water tank level too low
Municipal water supply pressure drop
Pipe diameter smaller than design specifications
Excessive friction loss in long suction piping
Partially closed suction valves
Debris blocking strainers or suction pipes
Air leakage into suction piping
Check suction gauge reading during pump operation
Inspect all valves for full open position
Confirm static water tank level
Listen for air leaks or irregular pump sound
Inspect strainers for blockages
Restore water supply to correct level
Clean or replace suction strainers
Repair pipe leaks or replace improperly sized pipes
Coordinate with municipal water supplier if pressure fluctuations occur
Air entry can significantly reduce pump performance and cause cavitation.
Poor priming or failed priming system
Vertical turbine pump pit air ingestion
Improper installation of suction piping
Loose fittings
Pump vibrates or makes rattling noise
Discharge pressure fluctuates
Sudden spikes in suction gauge
Re-prime the pump
Tighten all fittings
Improve suction piping slope and venting
Inspect pit or wet well for air entry points
Cavitation occurs when NPSH (Net Positive Suction Head) is insufficient. This can damage the impeller and drastically reduce pressure.
Loud cracking or grinding noise
Reduced pump efficiency
Excessive vibration
Metal pitting on impeller (long-term)
Inadequate suction pressure
Water temperature too high
Excessive elevation difference
Obstructions in suction line
Correct suction-side design
Lower pump installation elevation (if possible)
Increase water supply pressure
Replace damaged impeller if cavitation has caused wear
A newly installed pump may exhibit low pressure simply due to wrong motor rotation.
Pressure is significantly lower than expected
Flow does not match pump performance curve
Rotation arrow on pump mismatched with driver rotation
Reverse motor wiring phases (for electric pumps)
Recheck rotation before operating pump under load
Over time, impellers can wear out, especially if the water contains debris or sediment.
Normal wear from long-term operation
Corrosion from poor water quality
Debris entering pump casing
Cavitation damage
Compare actual pressure to original test results
Inspect impeller and wear rings
Check for uneven wear or cracks
Replace impeller
Replace wear rings
Clean casing and remove debris
Upgrade materials for corrosive environments
Even if the pump itself is fine, the driver can cause low-pressure output.
Low voltage
Motor running at reduced speed
Overheated motor
Incorrect motor rating
Insufficient engine RPM
Fuel supply obstructions
Weak governor or fuel injector problems
Air filter blockage
Compare driver RPM to nameplate
Check motor current
Inspect diesel engine speed under load
Observe automatic controller readouts
Restore proper electrical supply
Service diesel engine components
Replace damaged controllers or governors
If the system has severe leakage or valves are unintentionally open, pressure cannot stabilize.
Open test header valve
Standpipe system leaks
Underground pipe leak
Open drain or maintenance valve
Compare discharge pressure to system pressure
Monitor flow rate
Inspect system for unusual water discharge
Close valves properly
Repair leakage points
Conduct pressure-holding tests
Over-relieving can quickly reduce system pressure.
Improper factory preset adjustment
Relief valve stuck open
NFPA 20–noncompliant adjustments made onsite
Water discharging from relief valve during operation
Pressure cannot exceed a certain limit
Reset valve to required pressure
Replace faulty relief valve
Inspect system for overpressure events causing activation
Here is a structured and practical approach that fire pump technicians can follow:
Start by checking the suction gauge to confirm whether the pump is receiving adequate inlet pressure.
Check tank levels, verify municipal supply condition, and ensure all suction valves are open.
If the pump is newly installed or recently serviced, confirm that rotation matches pump arrow direction.
Bleed any trapped air from suction lines and pump casing.
If the above steps do not resolve the issue:
Inspect impeller
Check wear rings and casing
Look for debris
Ensure the motor or diesel engine is running at the correct speed.
Check discharge valves, test headers, and drains to ensure no unintended water path is open.
Ensure it is not opening prematurely and releasing discharge pressure.
Review the original certified pump test curve to determine how far the pump has deviated from expected conditions.
A full flow test helps verify:
Pump performance
System losses
Flow stability
Ventilation and cooling
If performance is still below standard, the pump may require overhaul or replacement.
Regular maintenance and correct installation practices are the best way to prevent low-pressure issues.
Conduct weekly no-flow tests
Perform annual flow and pressure tests
Maintain clean water supply sources
Ensure all valves are clearly tagged
Keep strainers clean
Inspect diesel engines regularly
Update control panel logs
Ensure NFPA 20 installation compliance
Some low-pressure issues indicate irreversible wear or design limitations.
Impeller and casing are severely damaged
Pump cannot meet rated pressure even after overhaul
Suction supply is permanently insufficient
New system changes require higher flow and pressure
In these cases, upgrading to a UL listed fire pump or a more efficient pump model is often the most reliable solution.
Low-pressure issues in fire pumps are not only inconvenient—they pose a significant risk to the fire protection readiness of a building. By understanding the root causes, inspecting the suction supply, verifying pump mechanics, and appropriately testing the entire system, maintenance teams can restore pump performance rapidly and safely.
