System Conditions
Fluid
Suction head z_s-2.0 m
Negative = suction lift above pump
Flow rate Q0.050 m³/s
Suction pipe dia. D_s0.100 m
Suction pipe length L_s10.0 m
Pipe roughness ε0.046 mm
Pump Characteristics
Max head H_max50.0 m
Design flow Q_design0.100 m³/s
Safety margin Δ_min1.0 m
Speed ratio n/n₀1.00
Affinity law — speed change effect
—
NPSH_A [m]
—
NPSH_R [m]
—
Margin Δ [m]
—
Cavitation
—
Head H [m]
—
Suction vel. v_s [m/s]
—
Vapor pressure [kPa]
—
Pipe loss h_f [m]
NPSH Equations
$NPSH_A = \dfrac{P_s - P_v}{\rho g} + \dfrac{v_s^2}{2g} + z_s - h_f$
$h_f = f \cdot \dfrac{L_s}{D_s} \cdot \dfrac{v_s^2}{2g}$ (Darcy-Weisbach)
Friction factor (Colebrook-White): $\dfrac{1}{\sqrt{f}} = -2\log\!\left(\dfrac{\varepsilon/D}{3.7} + \dfrac{2.51}{Re\sqrt{f}}\right)$
Affinity laws: $Q \propto n$, $H \propto n^2$, $NPSH_R \propto n^2$
Engineering tip: For hot water and hydrocarbons, elevated vapor pressure rapidly depletes NPSH_A. The most effective remedies are increasing suction pipe diameter or installing the pump below the tank (positive z_s).