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newSchoolWorks.com › VoTechWorks › Manufacturing & Marine › Industrial Maintenance › Pumps: Selection, Installation, Operation, and Maintenance › Lesson 4
MFG-INDU16-04 · Lesson 4 · 10.5 clock hours · theory

Reading Pump Performance Curves and System Curves

Objective. Students will read a manufacturer's pump performance curve to determine head, flow, efficiency, brake horsepower, and NPSH required at a given operating point, and overlay a system curve to identify the operating point.

⚠ Pending professional review

This lesson, its worked examples and its practice questions has not yet been verified by a licensed engineer or certified journey-level tradesperson or by a certification or approval body. It is a study draft and may contain errors. Do not use it as instructions for work on a job site or on live equipment. Always follow your instructor, your supervising professional, and current official references and regulations.

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What a Pump Performance Curve Tells You

A pump performance curve (also called a pump curve or H-Q curve) is a graph published by the manufacturer that shows how a specific pump behaves across its full range of flows. The horizontal axis is flow rate, usually in gallons per minute (GPM). The vertical axis is total dynamic head (TDH), usually in feet. Additional curves printed on the same chart show efficiency (%), brake horsepower (BHP), and net positive suction head required (NPSHr)—all plotted against flow rate.

Understanding how to read this chart is one of the most practical skills in industrial pump work. It lets you verify that a pump is right for a job before it is installed, and it helps you diagnose problems once the pump is running.

Key Points on the Pump Curve

Shutoff head is the TDH value where the flow rate is zero—the left end of the H-Q curve. This is the maximum pressure the pump can produce when its discharge valve is completely closed. Operating at shutoff for more than a few seconds is harmful: the liquid recirculates inside the casing, generating heat rapidly.

Best Efficiency Point (BEP) is the flow rate at which the pump converts the most input power into useful fluid energy. Manufacturers mark it with a dot or a dashed line. Running a pump at or near BEP produces the least vibration, the lowest bearing loads, and the longest seal and impeller life. As a rule of thumb, staying within roughly 70–110% of BEP flow keeps the pump in an acceptable operating range; exact limits depend on the manufacturer.

Maximum flow (or run-out) is the right end of the usable H-Q curve. At this point head is low, efficiency drops sharply, and NPSHr rises. Operating here can cause cavitation and motor overload.

NPSHr curve is plotted below the H-Q curve on many charts, using its own scale. NPSHr rises as flow increases. If the system cannot supply enough net positive suction head (NPSHa ≥ NPSHr + a safety margin), the pump will cavitate—vapor bubbles form at the impeller, collapse violently, and erode metal. Always compare NPSHa to NPSHr at the actual operating flow, not at the design point alone.

BHP curve shows the shaft power the pump demands at each flow rate. Use it to select the correct motor and to check that the motor will not be overloaded if the system curve shifts.

The System Curve

A system curve describes the resistance that the piping system imposes on the pump. It is not a pump property—it belongs to the installation. Total dynamic head in a system has two parts:

Because of this relationship, a system curve always has a parabolic shape on an H-Q graph. At zero flow it starts at the static head value. As flow increases, friction losses climb rapidly.

To plot a system curve, calculate TDH at three flow rates—zero, 50% of design, and 100% of design—then draw a smooth curve through the points. The formula is:

TDH = Static Head + (Friction Head at design flow) × (Q/Q_design)²

Finding the Operating Point

When you overlay the system curve on the pump curve, the point where the two curves cross is the operating point. This is where the pump will actually run. At that intersection, read off the flow, TDH, efficiency, BHP, and NPSHr. Then compare NPSHr to your calculated NPSHa. If NPSHa < NPSHr, the pump will cavitate and must not be used as configured.

Off-BEP Operation and Corrective Actions

If the operating point lands far from BEP, you have several options:

Safety Notes

Before working on any pump or its associated piping, apply lockout/tagout (LOTO) per OSHA standards and your facility's energy control procedure—electrical, pneumatic, and hydraulic energy sources must all be isolated and verified zero-energy before hands-on work begins. Wear appropriate PPE for the fluid handled: at minimum safety glasses and chemical-resistant gloves when dealing with process fluids. Never open a pump casing under pressure. When interpreting curves in the field, confirm the actual impeller diameter and speed match the curve you are reading—installed conditions often differ from the original design sheet.

Key terms10

Total Dynamic Head (TDH)The total equivalent height of fluid the pump must overcome, combining static head and all friction losses in the system, expressed in feet.
Best Efficiency Point (BEP)The flow rate at which a pump operates at its highest hydraulic efficiency; running near BEP minimizes wear, vibration, and energy waste.
Shutoff HeadThe maximum TDH a pump produces at zero flow; operating at shutoff for extended periods causes dangerous heat buildup inside the casing.
NPSHr (Net Positive Suction Head Required)The minimum suction-side pressure, above the vapor pressure of the liquid, that the pump manufacturer requires to prevent cavitation at a given flow rate.
NPSHa (Net Positive Suction Head Available)The actual suction-side pressure above vapor pressure delivered by the system to the pump inlet; must exceed NPSHr to avoid cavitation.
CavitationThe formation and violent collapse of vapor bubbles inside a pump caused by insufficient suction pressure; causes noise, vibration, and rapid impeller erosion.
Brake Horsepower (BHP)The actual shaft power input required to drive the pump at a given flow rate; used to size the driver motor.
System CurveA parabolic graph of the TDH required by the piping system at each flow rate; its intersection with the pump curve identifies the operating point.
Operating PointThe flow rate and head at which the pump actually runs, determined by the intersection of the pump performance curve and the system curve.
Impeller TrimMachining the impeller to a smaller diameter to reduce the pump's head and flow output, shifting the pump curve downward without replacing the pump.

Worked examples4

1. A pump curve shows a shutoff head of 120 ft. At 200 GPM the pump delivers 100 ft TDH at 72% efficiency. Calculate the BHP at 200 GPM. (Use: BHP = (GPM × TDH) / (3,960 × efficiency))
  1. Write the formula: BHP = (GPM × TDH) / (3,960 × efficiency).
  2. Substitute values: BHP = (200 × 100) / (3,960 × 0.72).
  3. Numerator: 200 × 100 = 20,000.
  4. Denominator: 3,960 × 0.72 = 2,851.2.
  5. Divide: 20,000 / 2,851.2 = 7.02 BHP.
Answer: BHP ≈ 7.0 hp at 200 GPM.
2. A system has a static head of 40 ft and friction losses of 35 ft at the design flow of 300 GPM. Calculate the TDH the system requires at 150 GPM (50% of design flow) using the system-curve formula.
  1. Identify known values: Static head = 40 ft; friction head at design = 35 ft; Q_design = 300 GPM; Q = 150 GPM.
  2. Write the formula: TDH = Static Head + Friction Head_design × (Q / Q_design)².
  3. Calculate the flow ratio: 150 / 300 = 0.50.
  4. Square the ratio: 0.50² = 0.25.
  5. Multiply friction head by the squared ratio: 35 × 0.25 = 8.75 ft.
  6. Add static head: TDH = 40 + 8.75 = 48.75 ft.
Answer: TDH required by the system at 150 GPM ≈ 48.75 ft.
3. Using the same system as above (static head 40 ft, design friction 35 ft at 300 GPM), calculate TDH at 0 GPM and 300 GPM so you have all three points needed to plot a system curve.
  1. At 0 GPM: Friction term = 35 × (0/300)² = 35 × 0 = 0 ft. TDH = 40 + 0 = 40 ft.
  2. At 150 GPM (from previous example): TDH = 48.75 ft.
  3. At 300 GPM: Flow ratio = 300/300 = 1.0; squared = 1.0. Friction term = 35 × 1.0 = 35 ft. TDH = 40 + 35 = 75 ft.
  4. Three points for the system curve: (0 GPM, 40 ft), (150 GPM, 48.75 ft), (300 GPM, 75 ft). Plot these and draw a smooth parabolic curve through them.
Answer: System curve points: 40 ft at 0 GPM, 48.75 ft at 150 GPM, 75 ft at 300 GPM.
4. The system curve above intersects the pump curve at 280 GPM / 72 ft TDH. The pump curve shows NPSHr = 14 ft at 280 GPM. The calculated NPSHa for the installation is 11 ft. Will the pump cavitate? What is the minimum corrective action needed?
  1. Compare NPSHa to NPSHr: NPSHa = 11 ft; NPSHr = 14 ft.
  2. Since NPSHa (11 ft) < NPSHr (14 ft), the available suction head is insufficient.
  3. The pump will cavitate at this operating point.
  4. Minimum corrective action: increase NPSHa by at least 3 ft—options include lowering the pump elevation relative to the suction source, increasing suction pipe diameter to reduce friction losses on the suction side, or reducing the operating flow rate to a point where NPSHr drops below 11 ft.
Answer: Yes, the pump will cavitate (NPSHa 11 ft < NPSHr 14 ft). The system must be modified to raise NPSHa or reduce flow until NPSHr ≤ NPSHa.

The math in this lesson3

Learners evaluate BHP = (GPM × TDH) / (3,960 × efficiency) at a pump operating point.
A-SSE.1F-IF.2F-IF.4
Brush up in the math halls at newSchoolWorks.com:
In class: Instructor-led worked examples using two actual manufacturer pump curves; students identify best efficiency point (BEP), maximum flow, shutoff head, and required NPSH at three different flow rates, recording answers on a structured worksheet. · Students plot a system curve on graph paper using three calculated TDH values at 0, 50%, and 100% of design flow, then overlay it on the pump curve to locate the operating point and determine whether the pump will cavitate at that point. · Partner activity: each pair receives a mismatched pump-system scenario (pump operating far off BEP) and must recommend a corrective action—impeller trim, speed change, parallel pumping, or system modification—and justify the choice using the curve.
Materials: Printed manufacturer pump curves (minimum two different pump models), graph paper, rulers, scientific calculators, colored pencils, NCCER Industrial Maintenance pump performance curve handout
Check: Instructor reviews plotted system curves and operating-point identification; students must correctly identify the operating point within 5% of instructor-determined value and score 80% or higher on the worksheet.

Practice check8

Answer each question, then check it. Nothing is recorded; this is your own practice.

NCCER Industrial Maintenance – Pumps
1. On a pump performance curve, what does the Best Efficiency Point (BEP) represent?
NCCER Industrial Maintenance – Pumps
2. A pump is operating far to the right of BEP at a very high flow rate. Which condition is most likely to occur?
MSSC CPT – Manufacturing Processes and Production
3. A system has a static head of 50 ft and friction losses of 20 ft at the design flow of 400 GPM. What is the TDH required at zero flow?
NCCER Industrial Maintenance – Pumps
4. Using the formula TDH = Static Head + Friction Head_design × (Q/Q_design)², what is the system TDH at 200 GPM if static head = 30 ft, design friction = 40 ft, and design flow = 400 GPM?
NCCER Industrial Maintenance – Pumps
5. The operating point of a pump is found by:
NCCER Industrial Maintenance – Pumps
6. A pump's BHP at 300 GPM and 90 ft TDH is closest to which value, given an efficiency of 75%? (BHP = GPM × TDH / (3,960 × efficiency))
MSSC CPT – Manufacturing Processes and Production
7. A pump is operating significantly to the left of BEP because system resistance is higher than expected. Which corrective action directly lowers system resistance to move the operating point toward BEP?
NCCER Industrial Maintenance – Safety
8. Before reading measurements from gauges on an installed pump or opening any part of the pump casing, a technician must first:
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