Revit computes pressure losses in piping based on the geometry and roughness of piping, fluid density, and fluid viscosity. Values for density and viscosity are specified in the Mechanical Settings dialog. Roughness is specified in the type properties for pipe/pipe fitting component families.
The following example shows how Revit calculates the pressure drop for a 100 foot segment of 4" carbon steel pipe, containing water at a temperature of 60 degrees F, with a flow rate of 100 GPM.
- Fluid Viscosity - u = 0.0007533333 lb/ft-s.
- Fluid Density - p = 62.36 lb/ft3.
- Roughness - e = 0.00015 ft (inside diameter, D = 0.3355 ft).
- Relative roughness (e) is calculated as D/e = 0.3355 / 0.00015 = 2236.67.
Revit uses the following formula to calculate pressure drop:
Revit uses the following formula to calculate the average fluid velocity (V):
- V = 2.520241077 FPS.
Revit uses the following formula to calculate the Reynolds Number (Re):
- Re = V * D * p / u = 2.520241077 * 0.3355 * 62.36 / 0.0007533333 = 69992.82.
Revit uses the following formula to calculate the Friction factor (f):
- Re > 4000, so friction factor (f) is: f = (1 / (2 * log10 (3.7*e)) ^ 2 = 0.0162875.
Revit calculates the Darcy-Weisbach equation as follows:
dpf=f*L*p*V*V/(D*gc*2)=0.0162875*100*62.36*2.520241077*2.520241077/(0.3355*2*32.2*144)=0.207 psi.