Pump selection connects fluid mechanics, machine performance, and system design. This cheat sheet helps students choose an appropriate pump by comparing required flow, head, power, efficiency, and suction limits. It is useful for engineering design projects, water systems, HVAC examples, and process piping problems.
The goal is to match the pump curve to the system curve without causing cavitation or wasting energy.
The most important ideas are total dynamic head, hydraulic power, brake power, efficiency, net positive suction head, and specific speed. Pump affinity laws predict how flow, head, and power change when speed or impeller diameter changes. Specific speed helps classify pump geometry and performance behavior.
Good pump selection uses both calculations and curve reading, not one formula alone.
Key Facts
- Flow rate is the volume of fluid moved per time, given by Q = V/t or Q = A v for steady flow in a pipe.
- Total dynamic head is the total energy added per unit weight of fluid, often estimated as H = static head + pressure head + velocity head + friction losses.
- Hydraulic power delivered to the fluid is P_h = rho g Q H, where rho is fluid density, g is gravitational acceleration, Q is flow rate, and H is head.
- Brake power required by the pump shaft is P_b = P_h/eta = rho g Q H/eta, where eta is pump efficiency as a decimal.
- Pump efficiency is eta = hydraulic power output/brake power input, so eta = P_h/P_b.
- Net positive suction head available must exceed the required value, so NPSH_A > NPSH_R, with a safety margin to reduce cavitation risk.
- For a fixed impeller diameter, pump affinity laws give Q2/Q1 = N2/N1, H2/H1 = (N2/N1)^2, and P2/P1 = (N2/N1)^3.
- Pump specific speed in U.S. customary form is N_s = N sqrt(Q)/H^(3/4), where N is rotational speed, Q is flow at best efficiency, and H is head per stage.
Vocabulary
- Flow rate
- Flow rate is the volume of fluid passing a point per unit time, commonly measured in m^3/s, L/s, or gal/min.
- Head
- Head is energy per unit weight of fluid expressed as an equivalent height of fluid, commonly measured in meters or feet.
- Pump curve
- A pump curve shows how pump head, efficiency, power, or NPSH requirement changes with flow rate at a given speed and impeller size.
- System curve
- A system curve shows the head required by a piping system at different flow rates, including elevation change and friction losses.
- NPSH
- Net positive suction head is the pressure energy available at the pump inlet above the fluid vapor pressure, used to evaluate cavitation risk.
- Specific speed
- Specific speed is a pump classification number based on speed, flow, and head that indicates the general impeller type and operating behavior.
Common Mistakes to Avoid
- Using pressure instead of head without converting units is wrong because pump curves are often based on head, not pressure. Use p = rho g H to convert between pressure and head for a given fluid.
- Forgetting efficiency in power calculations is wrong because the motor must supply more power than the fluid receives. Use P_b = rho g Q H/eta, not just P_h = rho g Q H.
- Assuming the pump always operates at the rated point is wrong because the actual operating point is where the pump curve intersects the system curve. Always compare both curves for the same speed and impeller size.
- Ignoring NPSH is wrong because a pump can meet flow and head requirements but still cavitate at the inlet. Check that NPSH_A is greater than NPSH_R with an appropriate safety margin.
- Applying affinity laws outside similar operating conditions is wrong because they assume the same pump geometry and similar flow behavior. Large changes in speed, viscosity, or operating region can make the estimates inaccurate.
Practice Questions
- 1 A pump moves water at Q = 0.040 m^3/s through a total dynamic head of H = 25 m. If rho = 1000 kg/m^3 and g = 9.81 m/s^2, what hydraulic power does the pump deliver?
- 2 A pump requires hydraulic power of 8.0 kW and has an efficiency of 0.72. What brake power must be supplied to the pump shaft?
- 3 A pump running at 1750 rpm delivers 60 gal/min at a head of 40 ft. If the speed is increased to 2100 rpm with the same impeller, estimate the new flow rate and head using the affinity laws.
- 4 Two pumps can both provide the required flow and head, but one operates near its best efficiency point and has a higher NPSH margin. Explain why that pump is usually the better selection.