What the calculator is for
Line sizing, flowmeter selection and checks on erosion, noise and pressure drop all start with the velocity in the pipe. For a full circular pipe the mean velocity is the volumetric flow divided by the bore area. The Reynolds number then tells you whether the flow is laminar or turbulent, which matters for pressure-drop correlations and for most flowmeters.
- v = Q / A with A = π D² / 4, D the inside diameter
- Re = ρ v D / μ: laminar below about 2,300, turbulent above about 4,000
Worked example
50 m³/h of water flows in a 4" schedule 40 pipe, inside diameter 102.3 mm. The bore area is 8,213 mm², so the velocity is 1.69 m/s, comfortably inside the usual range for a liquid line. With water at 20 °C the Reynolds number is 173,000: fully turbulent.
The same flow in a 6" schedule 40 pipe (154.1 mm bore) runs at only 0.745 m/s. Velocity falls with the square of the diameter, so one pipe size makes a large difference.
Velocity and flowmeters
Flowmeters care about velocity and Reynolds number more than flow rate. A magnetic flowmeter loses accuracy at very low velocity, so a meter one or two sizes smaller than the line is common on slow flows, with reducers either side. Vortex meters need a minimum Reynolds number to shed vortices at all, and DP meters such as orifice plates lose signal with the square of velocity at low flow. Check the meter vendor’s velocity and Reynolds limits against the minimum and maximum of your operating range, not just the normal flow.
Typical velocities
| Service | Typical velocity |
|---|---|
| Pump suction | 0.5 to 1.5 m/s |
| Pump discharge, process liquid | 1.5 to 3 m/s |
| Gravity drains | below 1 m/s |
| Cooling water headers | 1 to 2.5 m/s |
Common mistakes
- Using the nominal size as the bore. A 4" schedule 40 pipe has a 102.3 mm bore; schedule 80 has 97.2 mm. The difference is 11 % in velocity.
- Mixing up gpm. US and imperial gallons differ by 20 %. The calculator uses US gallons.
- Partly full pipes. The formula assumes a full pipe. Gravity drains and open channels need other methods.
- Gases at actual conditions. For gas, enter the actual volumetric flow at line pressure and temperature, not a standard or normal flow.
Basis
Continuity for incompressible flow in a full circular pipe, and the Reynolds number with the stated laminar and turbulent limits commonly used for pipe flow. Velocity guidance is indicative; your project’s design basis governs.