Pump performance curves show how a pump behaves as flow rate changes, making them essential for selecting and operating pumps in real systems. The most important curve usually plots head, H, against flow rate, Q, with head decreasing as flow increases. Engineers use this graph to predict whether a pump can overcome pipe losses, elevation changes, and pressure requirements.
A clear pump curve helps prevent undersized pumps, wasted energy, and poor system performance.
The operating point occurs where the pump head-flow curve intersects the system resistance curve. At that point, the head supplied by the pump exactly matches the head required by the piping system at that flow rate. Efficiency curves, power curves, and net positive suction head requirements are often added to help choose a safe and economical pump.
Good pump selection balances required flow, required head, efficiency, cavitation margin, and possible future operating changes.
Key Facts
- Pump head curve: H decreases as Q increases for most centrifugal pumps.
- System resistance often follows Hsystem = Hstatic + kQ^2.
- Operating point: Hpump(Q) = Hsystem(Q).
- Hydraulic power: Ph = rho g Q H.
- Pump efficiency: eta = Ph / Pinput.
- Brake power required: Pinput = rho g Q H / eta.
Vocabulary
- Head
- Head is the energy per unit weight of fluid, usually expressed as an equivalent height of fluid.
- Flow Rate
- Flow rate is the volume of fluid moving through the pump or pipe per unit time.
- Pump Curve
- A pump curve is a graph showing how a pump's head, efficiency, power, or suction requirement changes with flow rate.
- System Curve
- A system curve shows the head required by a piping system at different flow rates.
- Operating Point
- The operating point is the flow rate and head where the pump curve and system curve intersect.
Common Mistakes to Avoid
- Reading the pump curve without the system curve. The pump does not operate at every point on its curve, only where the pump and system requirements balance.
- Ignoring static head. Static elevation or pressure difference can shift the entire system curve upward and greatly change the operating point.
- Assuming efficiency is constant. Pump efficiency changes with flow rate, so operation far from the best efficiency point can waste energy and increase wear.
- Using inconsistent units for flow and head. Mixing gallons per minute with cubic meters per second or feet of head with meters of head gives incorrect power and selection results.
Practice Questions
- 1 A pump curve is approximated by Hpump = 50 - 0.002Q^2, and a system curve is Hsystem = 10 + 0.001Q^2, where H is in meters and Q is in liters per second. Find the operating flow rate and head.
- 2 Water flows at Q = 0.040 m^3/s through a pump that adds H = 35 m of head. If the pump efficiency is 70 percent, calculate the required input power using rho = 1000 kg/m^3 and g = 9.81 m/s^2.
- 3 A technician partially closes a discharge valve, making the system resistance curve steeper. Explain how the operating point moves on the pump head-flow curve and what happens to flow rate and head.