Reading Pump Curves Reference
Why this matters
Pump curves tell you exactly what a pump will do - how much flow at how much head, at what efficiency, with what NPSH requirement. Selecting a pump without reading the curve is selecting blind. Most "this pump isn't moving enough water" complaints trace to a pump sized only by horsepower instead of by the operating point on its curve. Reading the curve is straightforward once you understand the axes.
What's on a pump curve
Most centrifugal pump curves plot:
- X-axis (horizontal): flow rate (GPM = gallons per minute, or m³/hr in SI)
- Y-axis (left, primary): total dynamic head (TDH in feet of water, or meters)
- Y-axis (right, secondary): efficiency (%), brake horsepower (BHP), and/or NPSH required (NPSHr)
The pump's performance is a single line on the curve - at any given flow, the pump produces a specific head.
Some curves show multiple curves at different impeller diameters (when the impeller can be trimmed to alter performance) or at different speeds (when variable speed is an option).
How to read the operating point
- Find your required flow on the X-axis.
- Move up vertically to the pump curve.
- Read across to the Y-axis for the head the pump produces at that flow.
This intersection is the pump's "operating point" - what it will actually do in the application.
What head means
Head is the total resistance the pump must overcome, in feet of water column. Total head includes:
- Static head: vertical lift from suction surface to discharge surface
- Friction head: resistance from pipe friction (longer pipe, smaller diameter = more friction)
- Pressure head: any system pressure at the discharge (e.g., pressure required at a downstream fixture, or backpressure from a tank)
- Velocity head: kinetic energy of flow (small in most slow-flow systems)
1 psi = 2.31 ft of head. So a pump producing 30 ft of head at 50 GPM can lift water 30 ft vertically, OR push it through a long pipe whose friction loss is 30 ft equivalent, OR push it against 13 psi of system pressure, OR any combination that adds to 30 ft.
The system curve
The pump curve shows what the pump CAN do. The system curve shows what the system DEMANDS at each flow rate.
The system curve is plotted on the same axes as the pump curve. It starts at:
- Zero flow → just static head (vertical lift only; no friction)
As flow increases, friction head grows (typically as a square of flow - double the flow, 4× the friction head).
The system curve is roughly parabolic, opening upward.
Where the pump curve and system curve intersect = the actual operating point.
If the pump curve and system curve don't intersect well - pump is at a poor efficiency, or there's no flow because the pump can't develop enough head against the system, etc. - the pump is wrong for the system.
Best Efficiency Point (BEP)
Pumps have an efficiency curve. Efficiency peaks at one specific flow - the BEP. Operate too far below BEP: recirculation, bearing wear, vibration. Operate too far above BEP: high NPSH demand, possible cavitation, high power draw.
Target: operating point within ±10% of BEP. Within 20% is acceptable.
If your selection puts you at 50% of BEP, you've over-sized. If at 130% of BEP, you've undersized.
NPSH - Net Positive Suction Head
NPSH required (NPSHr): what the pump itself needs at its inlet to function without cavitation. Specified by manufacturer; published on the curve.
NPSH available (NPSHa): what the system actually provides at the pump inlet:
NPSHa = atmospheric pressure (ft water) + static suction head (positive if pump below source, negative if above) − friction in suction piping − vapor pressure of fluid
For NPSHa to be sufficient:
NPSHa > NPSHr (with safety margin of 2-3 ft typical)
If NPSHa < NPSHr → cavitation. Pump experiences vapor bubbles collapsing on the impeller. Noise (gravel-like sound), pump damage, eventual failure.
Common cavitation causes:
- Pump mounted too high above water source (lift too much)
- Suction line too small or too long
- Suction line plugged or partially obstructed
- Hot fluid (high vapor pressure)
- Atmospheric pressure low (high elevation)
Brake horsepower (BHP)
BHP curve shows the power required at the pump shaft at each flow. As flow increases, BHP often increases - so a pump running far above its BEP can draw more power than its motor is rated for, tripping overloads.
Motor HP should match or exceed the maximum BHP at any operating point in the system's range.
Service factor (SF): a motor's SF is its tolerance for overload. SF 1.0 = nameplate is the maximum. SF 1.15 = 15% over nameplate is acceptable for limited periods.
Single-speed vs variable-speed pumps
Single-speed (most older / residential): one pump curve. The pump runs at this curve any time it's on.
Variable-speed: family of curves at different RPMs. Pump can throttle down to match low-demand conditions, saving energy AND wear.
For variable-speed, the operating point is selectable - you choose a control mode (constant pressure, constant flow, proportional pressure, etc.) and the pump finds the appropriate speed.
How to use a pump curve to select a pump
Application: residential well pump pulling water 25 ft up to a pressure tank at 50 psi. Required flow: 10 GPM at peak demand.
- Calculate total head:
- Static lift: 25 ft
- Discharge pressure: 50 psi = 50 × 2.31 = 115 ft
- Friction (estimate from pipe sizing chart for 10 GPM in 1" PVC over 100 ft): ~5 ft
- Total head required: ~145 ft
Find a pump that produces 145 ft of head at 10 GPM on its curve.
Check that 10 GPM is near the pump's BEP (not at the edge of the curve where efficiency drops).
Verify NPSHa > NPSHr for the application.
Verify motor HP is sufficient for max BHP at any operating point.
Check the curve at minimum demand (e.g., 3 GPM if low flow is common) - make sure pump doesn't shut off or operate at very poor efficiency.
Common interpretation mistakes
- Sizing by GPM only, ignoring head. A "10 GPM pump" is meaningless without knowing what head it produces at 10 GPM.
- Ignoring NPSH. Pump on the curve will cavitate if NPSHa is insufficient.
- Operating far from BEP. Customer's pump is "the right size for the flow" but at 40% efficiency.
- Using a pump curve from a different impeller diameter. Curves apply only to the specific impeller; trimmed impellers produce less flow / head.
- Reading static head only and forgetting friction. A 25-ft lift through 200 ft of 1/2" pipe has friction head far higher than the static.
- Not factoring in the system pressure at the discharge (e.g., back-pressure from a tank or downstream restriction).
Pump troubleshooting using the curve
If a pump is installed and not performing:
References
- Hydraulic Institute Standards (HI 1.1 / 2.1 / 3.1, etc.)
- Pump manufacturer technical bulletins (Grundfos, Goulds, Bell & Gossett, Wilo, Taco)
- ASHRAE Handbook - Fundamentals (pump basics)
- "Pump Handbook" (Karassik et al.) for deeper theory