Choosing the correct suction configuration for a fire pump is one of the most important decisions in fire protection system design. If a fire pump cannot receive adequate and reliable water supply, even the best UL-listed or NFPA-compliant equipment will fail to deliver the required flow and pressure during an emergency. Poor suction design can cause cavitation, unstable pressure, pump overheating, or complete system failure. That is why determining fire pump suction requirements must be approached with careful engineering, proper calculations, and strict compliance with NFPA 20.
This article provides an in-depth guide for engineers, contractors, facility owners, and fire safety professionals to understand the essential factors that determine suction requirements for electric fire pumps, diesel engine fire pumps, and vertical turbine fire pumps. As a manufacturer of fire pumps, we see many common design mistakes in the field—most of which can be avoided with proper planning. This guide aims to help you build safer, more reliable, and more efficient fire protection systems.
The first step in determining fire pump suction requirements is identifying the water source. The type of source you choose directly affects suction arrangement, pump selection, and installation layout.
Municipal water mains
Ground storage tanks
Above-ground tanks and reservoirs
Lakes, rivers, or other static water sources
Dedicated fire water ponds
Cisterns
Each water source has different characteristics that influence suction design:
A municipal supply typically provides pressurized water. The fire pump suction here is usually a positive pressure suction type. The key considerations include:
Minimum and maximum static pressure
Peak demand pressure drop during fire conditions
Required flow rate from the water authority
Distance and losses from the connection point to the pump
Municipal suction is often straightforward, but pressure fluctuations must be considered. Nighttime pressure may be high, but daytime office-hour pressure drops can be significant.
When drawing water from a tank or reservoir, the pump may need to lift water vertically or operate with low suction pressure. For this reason, vertical turbine fire pumps are commonly used when the pump needs to operate under a negative or flooded suction condition.
Key considerations for tanks:
Minimum water level (worst case)
Strainer design at the tank outlet
Suction pipe submergence to avoid vortexing
Reliability of refill rate
NFPA 20 provides clear guidelines for minimum suction performance. For horizontal split-case and end-suction fire pumps, NFPA 20 requires positive suction pressure at the pump inlet whenever possible.
The pump must have adequate suction pressure to prevent cavitation.
Pump suction supply must maintain the rated flow at required pressure.
A minimum of 15 psi is often recommended at the pump suction while flowing 150% of the rated flow.
When suction lift is unavoidable, a vertical turbine pump is required.
Cavitation is one of the leading causes of fire pump damage, and it occurs when suction pressure is insufficient, causing vapor bubbles to form and collapse inside the pump. Therefore, ensuring stable suction pressure is fundamental.
Suction piping design affects flow quality, friction loss, pump life, and overall system performance. Improper pipe sizing or excessive bends can cause turbulence at the pump inlet.
At least as large as the pump suction flange
Sized to maintain water velocity below 15 ft/s (4.6 m/s)
As straight as possible before entering the pump
The general recommendation is to provide:
10 pipe diameters of straight pipe upstream of the pump
This minimizes turbulence and ensures a stable flow.
Using reducing elbows at the pump suction
Installing too many fittings or dramatic direction changes
Running suction pipe overhead and then dropping down
Using undersized valves or strainers
Smooth, straight, and properly sized suction piping ensures the pump receives stable flow.
The most technical aspect of suction requirements is calculating the Net Positive Suction Head Available (NPSHa) and comparing it with the NPSH Required (NPSHr) specified by the pump manufacturer.
If not, cavitation will occur, which can severely damage the impeller and reduce performance.
Atmospheric pressure
Static suction water level
Friction losses in suction piping
Water temperature
Elevation above sea level
For municipal supplies, NPSHa is usually sufficient due to positive pressure. For tank supplies, NPSHa must be calculated carefully, especially at lowest water level.
Proper NPSH evaluation ensures pump reliability during peak demand.
NFPA 20 permits several suction configurations, each suited to different water sources and site conditions.
Used with municipal water or tanks with adequate water elevation.
Advantages:
Stable pressure
Lower risk of cavitation
Suitable for electric or diesel pumps
When the pump is above the water level and must lift water, only vertical turbine fire pumps are acceptable.
Advantages:
Ideal for deep wells or open water
Eliminates cavitation risk due to design
Often requires careful design to maintain pressure and flow at minimum water level.
Important considerations:
Proper strainer design
Anti-vortex plates
Low-level cutouts
Suction lines must use valves and accessories that do not create excessive friction or cause blockages.
Recommended components include:
Outside screw and yoke (OS&Y) gate valves
Check valves installed on discharge only
Suction strainers only when absolutely necessary
Butterfly valves only when permitted and properly tested
The suction line should generally avoid unnecessary fittings because every fitting adds friction loss and increases turbulence.
For critical facilities such as airports, data centers, industrial plants, and high-rise buildings, water supply reliability is essential.
You may need:
Dual water supply
Redundant tanks
Separate connections to different mains
Suction control valves
Monitoring equipment to detect low levels
NFPA 20 supports dual suction designs when risk profile is high.
Diesel engine fire pumps require special consideration because the engine’s performance depends on stable flow and pressure.
Key points:
Suction pressure variations can affect diesel RPM stability.
Water supply must support the pump at all loads (100%, 150%).
Diesel pumps are more sensitive to cavitation due to high engine torque.
Ensuring adequate suction pressure is essential for long-term reliability.
Vertical turbine pumps are used when water supply comes from wells, sumps, open bodies of water, or when suction lift is required.
Their key advantages:
Operate reliably even with negative suction
Impellers are submerged, avoiding cavitation
Suited for deep water sources
When designing suction for vertical turbine pumps:
Determine pump setting depth
Ensure enough submergence to avoid vortexing
Evaluate seasonal water level fluctuations
Even the best design must be verified during installation and commissioning.
Flow testing at 0%, 100%, and 150% load
Suction pressure monitoring
Vibration and cavitation evaluation
Pump alignment and pipe support inspection
Suction problems often show up during testing as:
Noise
Pressure fluctuation
Reduced flow
Overheating
Early detection ensures long-term safety.
Determining fire pump suction requirements is essential for creating a reliable fire protection system. Proper suction design ensures that the fire pump performs at its rated capacity during an emergency. Whether the water source is a municipal supply, a storage tank, or an open reservoir, the same principles apply: maintain stable pressure, provide adequate flow, minimize friction loss, and comply with NFPA 20.
By carefully evaluating suction pressure, pipe sizing, fittings, NPSH, and the type of fire pump used, you can avoid many common installation problems and guarantee long-term system reliability. As a manufacturer of fire pumps—including horizontal split-case, end-suction, diesel engine pumps, vertical turbine fire pumps, and complete fire pump packages—we strongly recommend investing time in proper suction design before installation. A well-designed suction supply protects lives, property, and the integrity of the entire fire safety system.
