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Cross-section area equals flow divided by velocity; solving for diameter gives the handy form d = √(0.408 × GPM ÷ v) in inches and ft/s. Choose the velocity from the service — suction lines run slow to avoid cavitation, discharge lines faster to save pipe cost, within erosion limits.
- 100 GPM at 6 ft/s.
- d = √(0.408 × 100 ÷ 6) = 2.6 in → 3-inch pipe.
Reference tables
Approx. pipe size for water at 6 ft/s
| GPM | Pipe (in) |
|---|---|
| 20 | 1.25 |
| 50 | 2 |
| 100 | 3 |
| 200 | 4 |
| 400 | 6 |
| 800 | 8 |
Recommended water velocity
| Service | ft/s |
|---|---|
| Pump suction | 2–4 |
| Pump discharge | 5–8 |
| Long transmission | 3–5 |
A number the tables do not give: 100 GPM at 6 ft/s wants about a 3-inch line; at 2 ft/s (a suction line) it jumps to roughly 4-inch.
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FAQ
Where does 0.408 come from?
It converts GPM and ft/s to square inches: area(in²) = 0.408 × GPM ÷ v, from unit conversions of the flow equation.
Why keep suction velocity low?
High suction velocity drops pressure and can cavitate the pump; slower flow protects NPSH.
Does the fluid matter?
Yes — viscous or slurry services use lower velocities to limit friction and wear; the water ranges are a starting point.
Velocity or pressure-drop sizing?
Velocity is quick; for long runs, also check friction pressure drop (e.g., Hazen-Williams) to protect the pump head.
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