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Centrifugal pump sizing

Describe the liquid, suction and discharge to get total dynamic head, NPSH available, power and motor size, with cavitation and velocity warnings.

Pre-sizing and estimating tool: results do not replace design, calculations or reports signed by a licensed professional, nor verification against the applicable codes and standards.

Pre-sizing
Liquid and flow
Suction

Level above the centerline is positive (flooded suction); below it is negative (suction lift). Negative pressure means vacuum.

Discharge
Fittings list (paste from Excel)

One row per fitting: line (suction or discharge); fitting; quantity; K (optional, otherwise the typical value by name is used). When filled in, it replaces the counts above.

See what was read
Pump and motor
—Selection head
—

Head breakdown

Components the pump must overcome at design flow.

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System curve

Head the system requires versus flow. The selected pump should cross this curve at the design point, near its best efficiency.

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NPSH available versus flow

NPSH available falls with flow as suction losses grow; the pump requirement rises.

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Suction and discharge

Next step

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Operating point and VFD on the pump curve →Size the motor for this pump →

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Method, assumptions and references

Equations used

H = (z_d − z_s) + (p_d − p_s) ÷ (ρg) + Σ (f · L ÷ D + ΣK) · v² ÷ 2g + h_equip; H_selection = H · (1 + margin)
f from Colebrook–White (64 ÷ Re if laminar); typical K: entrance 0.5; 90° elbow 0.45; gate 0.15; check 1.5; globe 6; butterfly 0.7; foot valve 2.5; Y strainer 2; exit 1
NPSHa = (p_atm + p_s − p_v) ÷ (ρg) + z_s − h_losses,suction; p_atm = 101.325 · (1 − 2.25577·10⁻⁵ · z)^5.25588 kPa; recommended NPSHa ≥ max(1.2 · NPSHr; NPSHr + 2 ft)
Ns = n · √gpm ÷ ft^0.75 (nq metric); η estimated by Gülich: η = 1 − 0.095 · (1/Q)^m − 0.3 · (0.35 − log(nq ÷ 23))² · (1/Q)^0.05; P = ρ g Q H ÷ η; motor ≥ P · 1.25 (to 30 hp), 1.15 (to 75 hp) or 1.10
Viscous (ANSI/HI 9.6.7): B = 16.5 · ν^0.5 · H^0.0625 ÷ (Q^0.375 · n^0.25) (cSt, m, m³/h, rpm); C_Q = C_H = 2.71^(−0.165 · (log B)^3.15); C_η = B^(−0.0547 · B^0.69)

Assumptions and limits

  • ASME B36.10M steel pipe, steady flow, incompressible liquid; equipment loss varies with flow squared.
  • NPSH required at other flows is estimated proportional to Q²; use the manufacturer curve for the final decision.
  • Estimated efficiency for a single-stage radial centrifugal pump in good condition; final selection is made from the manufacturer catalog.

Standards and technical documents

  • ANSI/HI 9.6.1: Rotodynamic Pumps — Guideline for NPSH Margin
  • ANSI/HI 9.6.7: Effects of Liquid Viscosity on Rotodynamic Pump Performance
  • ANSI/HI 14.3: Rotodynamic Pumps for Design and Application
  • Gülich, J. F. Centrifugal Pumps, 3rd ed. Springer (efficiency correlation, Table 3.9)
  • Crane Co. Technical Paper No. 410: Flow of Fluids through Valves, Fittings and Pipe
  • ISO 9906: Rotodynamic pumps — Hydraulic performance acceptance tests
  • API 610: Centrifugal pumps for petroleum, petrochemical and natural gas industries (motor sizing margins)

U.S. regulations

  • DOE 10 CFR Part 431, Subpart Y: energy conservation standards for clean water pumps (Pump Energy Index)
  • OSHA 29 CFR 1910.219: mechanical power-transmission apparatus guarding

Frequently asked questions

How do I calculate pump total dynamic head?

Add the elevation difference between discharge and suction, the pressure difference between the tanks converted to feet of liquid, and all friction losses in pipe, fittings and equipment. Add a 5 to 10% margin for selection.

What is NPSH available?

It is the absolute pressure at the pump inlet above the liquid vapor pressure, in feet of liquid. It depends on elevation, liquid level, temperature and suction losses. It must exceed the catalog NPSH required with a margin, or the pump cavitates.

How do I size the pump motor?

Calculate shaft power (flow × head × specific gravity ÷ 3960 ÷ efficiency in hp) and add a 10 to 25% margin depending on size, then pick the next standard NEMA rating. Also check power at the end of the pump curve, where it is often higher.