Step 1: Define the Duty Point
The duty point is the specific flow rate and total dynamic head (TDH) the pump must achieve. Calculate TDH using the method in our TDH guide. Define both the normal duty point and the maximum design flow (system at minimum resistance, e.g. all valves open, lowest system head). The pump must perform acceptably across the full operating range.
Step 2: Define Fluid Properties
Collect data on the fluid being pumped:
- Density (kg/m³) — affects motor power calculation
- Viscosity (cSt or cP) — centrifugal pumps derate above 200 cSt
- Temperature (°C) — affects vapour pressure (NPSH), material selection, elastomer choice
- Solids content — type, size, and concentration of any suspended solids
- Chemical compatibility — pH, corrosiveness, oxidising or reducing nature
- Vapour pressure at pumping temperature — needed for NPSH calculation
Step 3: Select Pump Type
Based on fluid properties and duty point:
| Fluid type | Pump type |
|---|---|
| Clean water, low-viscosity liquid, moderate flow | End-suction centrifugal (ISO 2858 / ANSI B73.1) |
| High flow, low head | Mixed-flow or axial centrifugal |
| High head, moderate flow | Multistage centrifugal |
| Viscous liquid (>200 cSt) | Gear, lobe, or screw pump |
| Abrasive slurry | Rubber-lined centrifugal or peristaltic |
| Dosing / metering | Diaphragm, peristaltic, or plunger pump |
| Explosive, toxic, or zero-leakage duty | Magnetic-drive or canned-motor centrifugal |
Step 4: Check NPSH
Calculate NPSHa for your system (see our NPSH guide) and ensure it exceeds the pump's NPSHr (from the manufacturer's pump curve) by at least 0.5 m — ideally 1–2 m. If NPSHa is marginal, consider a pump with lower NPSHr, a double-suction impeller, or lower speed operation.
Step 5: Select Seal Type
| Service | Seal type |
|---|---|
| General water, clean non-hazardous fluid | Single mechanical seal |
| Hazardous, toxic, or flammable fluids | Double mechanical seal with barrier fluid (API Plan 53/54) |
| Abrasive fluid | Single seal with external flush (API Plan 32) or double seal |
| Zero-emission requirement | Magnetic-drive or canned motor (sealless pump) |
Step 6: Verify Operating Point vs BEP
Plot the system curve on the pump curve. The intersection (operating point) should fall within 70–110% of BEP flow. Operating far from BEP causes: increased radial loads on the shaft and bearings; seal face instability; elevated vibration; premature wear. If the operating point is consistently outside this range, select a different pump size or adjust impeller diameter.
Step 7: Select Motor Power
Motor power (kW) = (ρ × g × Q × H) / (pump efficiency × motor efficiency × 1000)
Apply a 1.1–1.25× service factor to allow for density variations, future flow increases, and bearing/seal mechanical losses. Always check that the selected motor can start the pump under full liquid load (high-inertia or viscous starts may need a larger motor).
Step 8: Specify Installation Requirements
- Baseplate type (grouted, anti-vibration mounts)
- Coupling type (flexible jaw coupling, gear coupling, or rigid spacer coupling)
- Alignment tolerances (typically <0.05 mm offset, <0.05 mm/m angular)
- Inlet and outlet pipe support — avoid loading pump flanges with pipe weight
- Instrumentation: pressure gauges suction and discharge, flow meter, vibration monitoring