1. Understanding Pump Curves
Understanding pump performance and selecting the right pump are critical steps in designing efficient and reliable fluid transfer systems. This section provides a detailed overview of pump curves, system head curves, selection factors. Understanding pump performance and selecting the right pump are crucial for ensuring both efficiency and reliability in fluid transfer systems. A pump that is well-matched to the specific application not only optimizes energy usage but also helps prevent common issues such as excessive vibration, overheating, or cavitation. Consequently, engineers and system designers must carefully examine each pump’s performance characteristics—represented by its pump curve—alongside the overall system head curve, which reflects the total energy requirements imposed by friction and other factors in the piping network.
This section delves into the fundamental principles behind pump curves and system head curves, illustrating how these two key tools work together to guide pump selection. Readers will learn about important selection factors—such as flow rate, head, fluid properties, and operating conditions—that should be taken into account to ensure a pump’s suitability for a given application.
Pump curves are essential tools for evaluating pump performance. They graphically represent the relationship between key parameters and help engineers ensure that the pump matches the system requirements.
2. Total Head vs. Flowrate
TDH (Total Head) indicates how much head (or pressure) the pump must generate to overcome all forms of resistance within the system. These resistances include:
- Static elevation differences – the height the fluid needs to be lifted.
- Friction losses – energy lost to friction in pipes, fittings, and other components.
- Discharge pressure requirements – any pressure that must be maintained at the discharge.
By comparing the TDH requirement of the system with the pump’s performance capabilities, engineers can select a pump that efficiently meets the operational demands.
Calculation (Simplified) TDH=(Discharge Pressure Head)−(Suction Pressure Head)+(Velocity Head Differences)+(Friction Losses). Although more comprehensive formulas may incorporate additional factors, this simplified version covers the primary terms that contribute to the overall head requirement.
3. Why Flow Rate Matters
Flow Rate refers to the volume of fluid (liquid or gas) that moves through a system per unit time. It is often measured in units such as gallons per minute (GPM), liters per minute (L/min), or cubic meters per hour (m³/h). Mathematically, flow rate can be expressed as the product of fluid velocity and the cross-sectional area of the pipe or channel through which it moves: Q = v × Q
Q = Flow Rate
v = Fluid Velocity
A = Cross-Sectional Area of the Flow Path
4. Pump Curve
A pump curve shows how a pump’s head (vertical axis) varies with its flow rate (horizontal axis). In other words, it tells you how much pressure—or “lift”—the pump can deliver at different flow rates:
Head vs. Flow Rate: Head (also called total head) typically decreases as the flow rate increases. This is because, at higher flow rates, the pump must push more fluid, causing a drop in the pressure or energy it can impart to each unit of fluid.
At the left (low flow) side of the curve, the pump delivers a high head; as you move to the right (higher flow rates), the head gradually falls.
Proper Pump Selection: By comparing the pump curve to the system’s head requirements (system head curve), you can find the operating point where the pump will run in actual conditions.
Operating Efficiency: Most pumps have a “best efficiency point” (BEP) on this curve, indicating the flow rate at which they run optimally—using less energy and minimizing wear. Operating too far left or right of this point may lead to issues like cavitation, excessive vibration, or high power consumption.
Match with System: You determine your system’s required head at a certain flow rate. Where that required head intersects the pump curve is the pump’s operating point.
Adjusting Flow: If you need more or less flow, the pump head will change accordingly—moving you along the curve.
Overall, the pump curve is a key tool for engineers and operators to ensure the chosen pump can meet both the pressure and flow demands of a specific fluid system.

5. Pump Efficiency
This diagram overlays two key pump characteristics—head versus flow rate (blue curve) and efficiency versus flow rate (orange curve). Here’s what each curve signifies and how they work together.

Head Curve (Blue): Shows how much head (or pressure) the pump can produce at different flow rates.
As the flow rate increases, the pump’s head generally decreases, since the pump is pushing a larger volume of fluid and therefore has less “energy” available to convert into pressure.
Efficiency Curve (Orange): Shows how much head (or pressure) the pump can produce at different flow rates.
As the flow rate increases, the pump’s head generally decreases, since the pump is pushing a larger volume of fluid and therefore has less “energy” available to convert into pressure.
By examining these two curves together, engineers can determine:
a. What flow rate and head the pump can deliver under various operating conditions.
b. Where the pump operates most efficiently (near its BEP).
Selecting and operating a pump such that the actual system demand (head and flow rate) aligns with the high-efficiency region helps ensure optimal performance, energy savings, and extended pump life.
6. Brake Power Curve
Brake Power (BP) or Brake Horsepower (BHP) (green curve) is the actual power delivered to the pump shaft by the driving motor or engine. It differs from theoretical or “water power” (the power required purely to move the liquid) because it also accounts for losses in the pump and drivetrain—such as friction, wear, and other mechanical inefficiencies.

In other words, BP or BHP is:
a. The input power the pump needs to achieve a certain flow rate and head.
b. Measured at the pump’s shaft, typically via a dynamometer or similar device.
c. Generally higher than the theoretical power required by the fluid (due to real-world inefficiencies).
Understanding BP or BHP helps ensure that your motor or engine can provide sufficient power for the pump’s operating conditions while also leaving room for efficiency losses and other operational factors.
7. NPSH explained
Now we will explain the relationship between a pump’s Net Positive Suction Head Available (NPSHa), the Net Positive Suction Head Required (NPSHr) by the pump, and the flow rate. The key idea is to compare how much suction head is available in the system (NPSHa) to how much the pump needs (NPSHr).

Vertical Axis (NPSH):
The chart plots values of NPSH on the y-axis (in units of length, typically feet or meters).
Two curves are shown: the green curve is NPSHa, and the red curve is NPSHr.
Horizontal Axis (Flow Rate):
NPSHa (Green Curve): “Net Positive Suction Head Available” depends on system conditions (tank level, fluid vapor pressure, pressure losses in suction piping, etc.).
In many systems, NPSHa tends to decrease as flow rate increases (more flow can increase friction losses, lowering suction pressure).
NPSHr (Red Curve): “Net Positive Suction Head Required” is a characteristic of the pump itself.
As flow increases, the pump generally requires more suction head to avoid cavitation (the NPSHr curve often rises with flow).
Cavitation Region: When NPSHa < NPSHr, the pump does not have enough suction head to avoid vapor bubbles forming and collapsing in the impeller (cavitation).
In the shaded region on the right, NPSHa is below NPSHr, indicating likely cavitation.
No Cavitation Region: On the left, the green line (NPSHa) is above the red line (NPSHr), meaning the system is providing enough suction head—there is an adequate “margin” between NPSHa and NPSHr, so the pump will operate without cavitation.
NPSH Margin: The difference between NPSHa and NPSHr (when NPSHa > NPSHr) is sometimes called the “NPSH margin” or “NPSH safety margin.”
Maintaining a good margin helps ensure quiet, reliable operation and extends pump life.
In short, this figure shows that as you increase flow, you both lose some of your available suction head (due to higher friction losses) and increase the pump’s required suction head. Past the point where these two curves meet (where NPSHa crosses below NPSHr), the pump is likely to cavitate. To avoid cavitation, system designers and pump operators often try to operate to the left of that intersection—keeping NPSHa comfortably above NPSHr.