NPSH Calculator Online: Check Cavitation Risk Before Buying Your Pump
Online NPSH calculator guide: calculate NPSHa step by step, compare vs NPSHr, and check cavitation risk. Vapor pressure table and worked example included.
Online NPSH Calculator for Cavitation Prevention
NPSH (Net Positive Suction Head) determines whether your pump will cavitate. Calculating it BEFORE buying the pump saves you from a $1,000-$5,000 mistake.
NPSHa Formula
NPSHa = Pa/(ρg) ± Hs - hf_suction - Pv/(ρg)Input Data You Need
| Parameter | How to Get It |
|---|---|
| Atmospheric pressure (Pa) | 10.34 m at sea level. Subtract 0.12 m per 100 m elevation. |
| Static suction head (Hs) | Measure: + if pump below water, - if pump above water |
| Suction pipe friction (hf) | Calculate using Darcy-Weisbach or Hazen-Williams |
| Suction fitting losses | Sum of K × V²/(2g) for suction fittings |
| Vapor pressure (Pv) | From temperature table below |
Vapor Pressure Table
| Temp °C | Pv (m water) | Temp °F |
|---|---|---|
| 10 | 0.125 | 50 |
| 15 | 0.175 | 59 |
| 20 | 0.239 | 68 |
| 25 | 0.324 | 77 |
| 30 | 0.434 | 86 |
| 40 | 0.756 | 104 |
| 50 | 1.264 | 122 |
| 60 | 2.043 | 140 |
| 80 | 4.858 | 176 |
Step-by-Step Example
System: Pump 2.5 m above water surface. Suction pipe: 3 m of 3" PVC, 1 foot valve (K=2.5), 1 elbow (K=0.3). Flow: 4 L/s. Water at 20°C. Sea level.
Pa/(ρg) = 10.34 m
Pv/(ρg) = 0.239 m
Hs = -2.5 m (pump above water)
V_suction = 0.004/0.00456 = 0.877 m/s
hf = 0.06 m (3 m PVC friction)
hm = (2.5 + 0.3) × 0.877²/(2×9.81) = 0.11 m
NPSHa = 10.34 - 0.239 - 2.5 - 0.06 - 0.11 = 7.43 mNow check the pump catalog. If NPSHr at 4 L/s = 3.0 m:
Margin = 7.43 - 3.0 = 4.43 m ✅ SAFE (margin > 1.0 m)Traffic Light System
| Margin | Status | Action |
|---|---|---|
| > 1.0 m | 🟢 Safe | Proceed with this pump |
| 0.3 - 1.0 m | 🟡 Caution | Consider lowering pump or increasing suction pipe |
| < 0.3 m | 🔴 Danger | DO NOT buy this pump for this system |
What If NPSHa Is Too Low?
5 solutions, ranked by cost:
- Lower the pump (cheapest — just reposition)
- Increase suction pipe diameter by one size
- Shorten suction pipe and remove unnecessary fittings
- Choose a pump with lower NPSHr
- Pressurize the source (most expensive, last resort)
Automate NPSH Analysis
HydroApp Pro calculates NPSHa automatically and compares it against the pump's NPSHr curve. A traffic-light indicator immediately shows whether cavitation is a risk.
No manual calculation needed. No forgotten terms. No cavitation surprises.
Try HydroApp Pro — NPSH calculator with cavitation warnings. $99 one-time.
Related
¿Cansado de calcular esto a mano?
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