NPSH Calculator

Calculate Net Positive Suction Head to prevent pump cavitation

Calculate NPSH Available

Reservoir Type

Reservoir Position

Atmospheric Pressure (p_atm)

Standard: 101.325 kPa at sea level

Height Difference (Z)

Distance between liquid surface and pump level

Friction Head Loss (H_l)

Head loss due to friction in suction pipe

Fluid Density (ρ)

kg/m³

Water at 20°C: 998.16 kg/m³

Temperature (for water vapor pressure)

°C

Auto-updates vapor pressure for water

Vapor Pressure (p_vapor)

Water at 20°C: 2.34 kPa

NPSH Required (from manufacturer)

Minimum NPSH required by pump manufacturer

NPSH Calculation Results

10.11

NPSH Available (m)

10.35

Liquid Surface Head (m)

0.24

Vapor Pressure Head (m)

Formula: NPSH = ((P_surface - P_vapor) / (ρ × g)) - Z - H_l

Surface Pressure: 101.33 kPa

Calculation: ((101.33 - 2.34) / (998.16 × 9.81)) - 0.0 - 0.0 = 10.11 m

ℹ️ Enter NPSH required from pump manufacturer to assess cavitation risk

Example Calculation

Water Pumping System

System Configuration:

• Open reservoir (atmospheric pressure)

• Water 1 meter below pump level

• Water temperature: 20°C

• Friction loss: 1.7 m

Parameters:

• Atmospheric pressure: 101.33 kPa

• Vapor pressure (20°C): 2.34 kPa

• Water density: 998.16 kg/m³

• Height difference (Z): 1 m

Calculation Steps

1. Surface pressure: P_surface = 101.33 kPa (open to atmosphere)

2. Pressure difference: 101.33 - 2.34 = 98.99 kPa

3. Pressure head: 98.99 × 1000 / (998.16 × 9.81) = 10.11 m

4. NPSH Available: 10.11 - 1 - 1.7 = 7.41 m

Water Vapor Pressure

0°C0.611 kPa

5°C0.872 kPa

10°C1.228 kPa

15°C1.706 kPa

20°C2.339 kPa

25°C3.169 kPa

30°C4.246 kPa

35°C5.628 kPa

40°C7.384 kPa

45°C9.593 kPa

50°C12.35 kPa

55°C15.76 kPa

60°C19.94 kPa

65°C25.03 kPa

70°C31.19 kPa

75°C38.58 kPa

80°C47.39 kPa

85°C57.83 kPa

90°C70.14 kPa

95°C84.55 kPa

100°C101.325 kPa

Click temperature input to auto-fill vapor pressure

Prevent Cavitation

💧

Reduce height difference between pump and fluid reservoir

🔧

Minimize friction losses in suction piping

🌡️

Lower fluid temperature to reduce vapor pressure

⚡

Reduce pump speed to decrease pressure drop

📏

Use larger diameter suction pipes

🏗️

Install pump below fluid level when possible

System Components

Pump

Creates suction and pressure differential

Reservoir

Fluid source with surface pressure

Suction Line

Piping with friction losses

Understanding NPSH and Cavitation

What is NPSH?

Net Positive Suction Head (NPSH) is the difference between the suction pressure at a pump's inlet and the fluid's vapor pressure at the system temperature. It represents the absolute pressure available to prevent the fluid from vaporizing.

Why is NPSH Critical?

•Prevents cavitation damage to pump components
•Ensures reliable pump operation and performance
•Reduces noise, vibration, and maintenance costs
•Maintains pump efficiency and flow rate

NPSH Formula

NPSH = ((P_surface - P_vapor) / (ρ × g)) - Z - H_l

P_surface: Pressure at fluid surface (kPa)
P_vapor: Vapor pressure at system temperature (kPa)
ρ: Fluid density (kg/m³)
g: Gravitational acceleration (9.81 m/s²)
Z: Height difference between fluid surface and pump (m)
H_l: Friction head loss in suction piping (m)

Safety Rule: NPSH Available should exceed NPSH Required by at least 0.5-1.0 m

What is Cavitation?

Cavitation occurs when the local pressure in a flowing liquid drops below the vapor pressure, causing the liquid to vaporize and form bubbles. When these bubbles collapse in higher pressure regions, they create shock waves that can severely damage pump internals.

Cavitation Effects

• Material erosion
• Noise and vibration
• Reduced efficiency
• Mechanical damage

Warning Signs

• Unusual noise
• Excessive vibration
• Reduced flow rate
• Higher power consumption

Prevention

• Ensure adequate NPSH
• Minimize suction losses
• Control fluid temperature
• Proper system design