The procedure at a glance
This guide follows the technical sequence an engineer or buyer typically works through once the basic question of pump type is settled: define the duty, build the system curve, read the pump curves, check suction conditions, and compare the survivors. It focuses on the numbers and the checks. The broader question of how to choose by fluid and application is handled in a separate article.
Step 1: Define the duty
Write down the required flow, including the normal value and the range, and the fluid properties: temperature, density, viscosity, solids and vapor pressure. Add the operating hours and any special conditions such as a hot start or a dry-running risk. Everything later depends on this record, so keep it in one place and revise it if the process changes.
Use consistent units. A common error is to mix flow in cubic meters per hour with head in feet, or pressure in bar with head in meters. Convert early and label every number.
Step 2: Calculate total head and draw the system curve
Total head is the sum of static head, any pressure difference between the source and the destination, and friction losses in pipes, fittings and valves. Friction losses grow roughly with the square of flow, so the system curve rises as flow increases, starting from the static and pressure components.
Plotting head against flow gives a system curve. Where the pump curve crosses it is the operating point. If the system has throttled valves, strainers that may foul, or variable levels, draw more than one curve, for example clean and fouled, high level and low level. A pump should behave acceptably across that whole band, not only at the nominal point.
Step 3: Read the pump curves
Manufacturers publish performance curves showing head against flow, and usually efficiency, power and the required suction head on the same chart. When reading one:
- Find the duty point on the head-flow curve and see which impeller diameter or speed covers it.
- Look at the efficiency curve and note how close the duty is to the best efficiency point. A common aim is to have the normal operating point near it rather than far to either side.
- Check the power curve to confirm the motor can supply enough power across the expected flow range, including the end of the curve where power can rise on some designs.
- Check the recommended operating region. Pumps run far outside it tend to suffer more vibration, recirculation and wear.
Curves are for a stated fluid, normally clean cold water. Viscous liquids shift the curves, and the manufacturer can supply corrected values.
Step 4: Check suction conditions with NPSH
NPSH stands for net positive suction head. It compares what the system provides with what the pump needs at the inlet to avoid vapor forming in the liquid.
- NPSH available (NPSHa) comes from the system: the pressure on the liquid surface, plus the height of liquid above the pump (or minus the lift), minus friction losses in the suction line, minus the liquid’s vapor pressure, all expressed as head.
- NPSH required (NPSHr) comes from the pump curve and rises with flow.
The rule is that NPSHa should comfortably exceed NPSHr, and many designers add a margin on top. The exact margin depends on the service and standards, so confirm it with the manufacturer.
Here is a hypothetical example with cold water. An open tank is at atmospheric pressure, which corresponds to roughly 10.3 meters of water head. The liquid surface is 2.0 meters above the pump inlet, suction friction loss is 0.5 meters, and the vapor pressure head is 0.24 meters. Then:
NPSHa = 10.3 + 2.0 − 0.5 − 0.24 = 11.56 meters.
Now suppose the liquid is hot, and the vapor pressure head rises to a hypothetical 4.8 meters, with other values unchanged:
NPSHa = 10.3 + 2.0 − 0.5 − 4.8 = 7.0 meters.
The same layout has lost more than four meters of margin simply because the liquid is hot. If NPSHr for the pump at that flow were, hypothetically, 5 meters, the margin would be thin. Raising the tank, shortening or enlarging the suction line, or choosing a pump with lower NPSHr are common remedies. Failing this check leads to pump cavitation, which damages impellers and shortens life.
Step 5: Compare candidate options
With two or three pumps that meet the duty and NPSH requirements, compare them on the same basis:
- Efficiency at the duty point and across the flow range.
- Motor size and power draw.
- Materials suited to the fluid, and the sealing arrangement.
- Whether speed control with variable speed drives fits the demand pattern.
- Size, weight, footprint and access for service.
- Availability of spares and support.
- Purchase cost together with energy and maintenance, in the sense of total cost of ownership.
Place the cheapest option in this table, then ask what it gives up. Often the answer is efficiency, materials or service life.
Step 6: Review the system around the pump
A pump that suits the duty on paper can still perform badly if installed poorly. Check suction piping for sharp bends close to the inlet and for air traps. Make sure line strainers can be cleaned without major disruption. Review pipe supports and nozzle loads so the pump casing is not strained. Good pump installation practices cover alignment, foundations and piping connections.
Step 7: Record and verify
After installation and start-up, compare actual flow, pressure, power and vibration against the selection data. The numbers on equipment data plates should match what was specified. These baseline readings are useful later for predictive maintenance, because they show when performance begins to drift.
A word of caution
The formulas above are simplified, and real selection involves details such as pipe roughness, fluid behavior and safety factors. Treat them as an aid to understanding, and have the final choice confirmed by the manufacturer or a qualified engineer. Follow manuals, trained and qualified personnel, lockout and tagout style procedures during service, and local regulations.