Hydraulic Line Fluid Velocity and Minimum ID
The hose-sizing check the hydraulics bench never had.
Example
You enter
- Flow (gpm) 20
- Line inside diameter (in, true ID) 0.625
- Line type pressure
You get
- Area (in²) 0.306796
- Velocity (fps) 20.9151
- Minimum ID at the ceiling 0.674 in
- Max flow through this line 17.2 gpm at the ceiling
- Band max (fps) 18
Details, formula, and sources
V = 0.3208 Q / A, with the constant derived from 231 in^3 per gallon rather than tabulated. 20 gpm through a 5/8-in ID runs 20.9 ft/s - over the pressure ceiling, needing 0.674 in. Suction is the band that punishes you: too fast and the pump cavitates. Published bands DISAGREE (return 4-13 vs 10-15, pressure 7-18 vs 15-20), so the ceiling is editable and both sets are named. Also gives the minimum ID and the max flow for the line you have.
V (ft/s) = [231 / (60 x 12)] x Q (gpm) / A (in^2) = 0.3208333 Q / A, A = pi/4 ID^2; minimum ID at a ceiling = sqrt(4 (K Q / Vmax) / pi); maximum flow = Vmax A / K.
Continuity with derived unit conversion (231 cubic inches per gallon, 60 s/min, 12 in/ft) plus industry recommended-velocity bands, named as conventions rather than a standard.
The conversion is arithmetic; the recommended bands are published across hydraulic design references and component catalogs.
Estimate. AHJ and licensed professional govern.
Field names used by the API: flow_gpm, inside_dia_in, line_type, area_in2, velocity_fps, min_dia_in, max_flow_gpm, band_max_fps
- Derived constant 0.3208333 is written in source as 231/(60 x 12) so it can be checked, not recalledunit arithmetic
- Bands are conventions editable ceiling seeded by line type; both published sets namedindustry hydraulic design references
- Velocity only pressure drop, heat rejection, and hose pressure rating are separate checksstated scope limit