| name | pump-selection |
| description | Pump selection and sizing — centrifugal, positive displacement, pump curve, system curve, operating point, NPSH, cavitation, affinity laws, specific speed, parallel/series operation. |
| metadata | {"priority":7,"promptSignals":{"phrases":["pump","pump selection","pump curve","NPSH","cavitation","centrifugal pump","pump sizing","head"],"minScore":4}} |
Pump Selection & Sizing — Complete Skill
Pump Types Selection Guide
| Type | Flow | Head | Application |
|---|
| Centrifugal (radial) | High | Low-Medium | Water supply, HVAC, irrigation |
| Centrifugal (axial/propeller) | Very high | Very low | Drainage, large flow |
| Centrifugal (mixed flow) | High | Medium | Storm water, sewage |
| Gear pump | Low-Medium | High | Viscous fluids, hydraulics, lubrication |
| Vane pump | Medium | Medium-High | Hydraulics, fuel |
| Piston pump | Low | Very high | Hydraulic systems, chemical injection |
| Diaphragm pump | Low | Medium | Chemical, slurry, metering |
| Peristaltic | Very low | Medium | Sanitary, lab, slurry |
| Screw pump | Medium-High | Medium | Viscous, multiphase |
System Head Calculation
H_system = H_static + H_friction + H_velocity
H_static = (z_discharge - z_suction) [m or ft] — elevation difference
H_friction = h_f,total (Darcy-Weisbach + minor losses)
H_velocity = (V_d² - V_s²)/2g (usually negligible if pipe sizes similar)
Total Dynamic Head (TDH):
TDH = (p_d - p_s)/(ρg) + (z_d - z_s) + (V_d² - V_s²)/2g + h_L
System curve: H_sys = H_static + R·Q² (R = system resistance coefficient)
Pump Curve & Operating Point
Pump head curve: H_pump = H_shutoff - b·Q^c (manufacturer's curve)
Operating point: H_pump(Q) = H_system(Q) → solve simultaneously
If no curve available, estimate: H_shutoff ≈ 1.2·H_design, Q_max ≈ 1.4·Q_design
Specific Speed (determines pump type)
N_s = N·Q^0.5/H^0.75 [N in rpm, Q in gpm, H in ft] — US customary
N_s = N·Q^0.5/H^0.75 [N in rpm, Q in m³/s, H in m] × 51.6 → SI
| N_s (US) | Pump Type |
|---|
| 500-2000 | Radial centrifugal |
| 2000-4000 | Francis mixed flow |
| 4000-10000 | Axial propeller |
Specific speed for PAT (pump as turbine) applications: reverse centrifugal
Affinity Laws (Scaling Laws)
For the same pump at different speeds (N₁ → N₂):
Q₂/Q₁ = N₂/N₁
H₂/H₁ = (N₂/N₁)²
P₂/P₁ = (N₂/N₁)³
For geometrically similar pumps (scale factor D₂/D₁):
Q₂/Q₁ = (D₂/D₁)³
H₂/H₁ = (D₂/D₁)²
P₂/P₁ = (D₂/D₁)⁵
VFD control: Speed varies with demand; P ∝ N³ → significant energy savings at reduced flow
Pump Efficiency
η_hydraulic = ρgQH/P_shaft (shaft power input to impeller)
η_volumetric = Q_actual/Q_theoretical (leakage losses)
η_mechanical = P_hydraulic/P_motor (bearing, seal friction)
η_overall = η_h × η_v × η_m (total efficiency, typically 60-90%)
Wire-to-water efficiency = η_overall × η_motor
NPSH (Net Positive Suction Head)
NPSH_available (NPSHA):
NPSHA = (p_atm - p_vapor)/(ρg) + h_s - h_fs
h_s = suction head (positive if pump below reservoir, negative if above)
h_fs = friction losses in suction pipe
NPSHA = (p_inlet - p_vapor)/(ρg) + V_s²/2g [at pump inlet]
NPSH_required (NPSHR): from manufacturer's curve (minimum head to avoid cavitation)
Cavitation condition: NPSHA < NPSHR → bubbles form → implosion → pitting, noise, vibration
Safety margin: NPSHA ≥ NPSHR + 0.5-1.0 m (0.6-1.2 m for water)
Improving NPSHA:
- Lower pump elevation (increase h_s)
- Increase suction pipe diameter (reduce h_fs)
- Cool fluid (reduce p_vapor)
- Use pressurized suction tank
Series & Parallel Operation
Series (same Q, heads add):
H_total = H_pump1 + H_pump2 at same Q
Use when: high head required, single pump insufficient
Parallel (same H, flows add):
Q_total = Q_pump1 + Q_pump2 at same H
Use when: high flow required, or redundancy needed
Note: parallel pumps are effective only if system curve is steep (H-dominated)
Power and Motor Sizing
Hydraulic power: P_hydraulic = ρgQH [W] = Q[m³/s]·H[m]·ρg
Shaft power: P_shaft = P_hydraulic/η_pump
Motor power: P_motor = P_shaft/η_motor
Safety factor on motor sizing: 1.15-1.25× (for startup, overload)
Viscosity Correction (Centrifugal)
For ν > 20 cSt: use HI correction charts
C_Q = Q_viscous/Q_water, C_H = H_viscous/H_water
Efficiency drops significantly with high viscosity → consider positive displacement
Output
Provide: TDH [m or ft], Q [m³/s or gpm], P_hydraulic [kW or hp], NPSHA vs. NPSHR, pump type recommended, operating efficiency estimate.
