What is the difference between NPSHa and NPSHr?

NPSHa (Net Positive Suction Head Available) is a system parameter determined by tank pressure, suction line friction losses, liquid vapor pressure, and liquid elevation relative to the pump. NPSHr (Net Positive Suction Head Required) is a physical property of the pump supplied by the manufacturer to prevent 3% head loss due to cavitation.

What safety margin should be kept between NPSHa and NPSHr?

Standard industrial practice (Hydraulic Institute Standards) recommends that NPSHa should exceed NPSHr by at least 0.6 meters (2 feet) or provide a ratio of NPSHa / NPSHr >= 1.15 to 1.30 to ensure long pump impeller life and avoid cavitation noise or damage.

Pump NPSH Calculator (Online + Excel) - Available Net Positive Suction Head Sizing Tool

Section summary
1. Definition of NPSH
2. Interactive NPSH Calculator
3. NPSH Calculation Formula
4. Effect & Cavitation

1. Definition : What is pump NPSH

Meaning of NPSH : the term pump NPSH stands for Net Positive Suction Head.

It represents the pressure drop in between the suction point of the pump and the minimum pressure that is reached at the pump inlet.

If the pump NPSH available ($NPSH_a$) is less than the NPSH required ($NPSH_r$), the pump will not work properly. Either the liquid will not be pumped (not possible to prime the pump) or the operation will generate cavitation.

⚡ Interactive Pump NPSH Calculator

⚠️ ENGINEERING NOTICE & EDUCATIONAL DISCLAIMER: This interactive calculator is provided exclusively for preliminary estimation and educational purposes. It is not intended for detailed design or equipment procurement without certified vendor rating. No warranty, expressed or implied, is provided, and no liability is assumed.

SI Metric (m, m³/h, Pa, kg/m³) US Customary (ft, gpm, psi, lb/ft³)

Volumetric Flow Rate (Q): m³/h

Suction Pipe Internal Diameter (D): mm

Static Head (h₁ - liquid level to pump centerline): m (positive if tank above pump)

Tank Surface Pressure (p₁): Pa (absolute)

Liquid Vapor Pressure (pᵥ): Pa (absolute)

Liquid Density (ρ): kg/m³

Friction Head Loss (ΔH_f): m

Required NPSH (NPSHr - Pump Vendor): m

Suction Line Velocity (u₁): 0.98 m/s

Velocity Head (u₁² / 2g): 0.05 m

Pressure Head [(p₁ - pᵥ) / (ρ·g)]: 10.11 m

                NPSH Available (NPSHa):
                12.16 m
              

NPSH Margin (NPSHa - NPSHr): 9.16 m

2. NPSH Calculation : How to calculate the pump NPSH

NPSH required - NPSHr

The NPSH required for a given pump is given by the pump manufacturer. It can be read on the pump characteristic curve.

Pump NPSH

Figure 1 : Pump NPSH, required and available

NPSH available - NPSHa

The NPSH available needs to be calculated from the actual suction pipe layout of the pump. The following case is considered below :

Pump circuit

Figure 2 : Pump circuit

The NPSH formula is given below :

\[NPSH = h_1 + \frac{p_1 - p_v}{\rho g} + \frac{u_1^2}{2g} - \left|\Delta H_f\right|_1^a\]

NPSH Formula Image

Equation 1 : NPSH calculation [Midoux]

Note : in many cases the velocity head $u_1^2/2g$ is negligible which simplifies the relation [Sihi]

With :
- $h_1$ = height from the level of liquid to centrifugal pump axis (m)
- $p_1$ = pressure of the tank from which liquid is pumped (Pa)
- $p_v$ = vapor pressure of liquid pumped (Pa)
- $u_1$ = velocity of liquid in the suction pipe (m/s)
- $\Delta H_f$ = pressure drop by friction in the suction pipe (m)
- $\rho$ = specific gravity/density of liquid pumped (kg/m³)
- $g$ = gravitational acceleration ($9.81\text{ m/s}^2$)

If the NPSH available is very low, it is important to select a low NPSH pump so that it can still suck the liquid without cavitation.

💡 Plant Engineering Best Practices & NPSH Rules of Thumb

Suction Line Velocity: Sizing suction lines for liquid velocities between 0.6 and 1.2 m/s (2 to 4 ft/s) prevents cavitation, minimizes dynamic turbulence, and restricts friction pressure drops. Avoid velocities above 1.5 m/s (5 ft/s).
NPSH Safety Margin: Always maintain a absolute safety margin: $NPSH_a \ge NPSH_r + 0.6\text{ m}$ (2 ft) or a ratio of $NPSH_a / NPSH_r \ge 1.15$ to $1.30$ (per Hydraulic Institute standards).
Suction Piping Geometry: Install eccentric reducers with the flat side on top to eliminate high-point air pockets. Provide a minimum straight pipe length of 5 to 10 pipe diameters directly before the pump inlet flange.
Hot/Volatile Liquids: For fluids near their boiling point (e.g. boiler feedwater, hot condensates, hydrocarbons), increase static suction head $h_1$ by elevating tanks or sub-cooling the feed liquid.

NPSH calculation example

Refering to figure 2, a pump is installed at the outlet of a tank, to transfer water to another tank. It pumps 10 m³/h.

The tank is at atmospheric pressure, the pipe internal diameter is 60 mm, the height difference in between the level of liquid in tank 1 and the pump is 2 m. Water is at 20°C.

STEP 1 : calculate the liquid vapor pressure

The vapor pressure can be either read from tables or calculated thanks to an Antoine law. For water at 20°C : 2339 Pa.

STEP 2 : calculate the velocity in pipe

The velocity is 10/(π*0.06²/4)= 3536 m/h = 0.98 m/s.

Note : in this example, the pipe is short and the velocity low, friction losses are neglected, if it is not the case and you wish to calculate the pressure loss, please refer here.

STEP 3 : calculate the NPSH

NPSH = 2+(101325-2339)/(9.81*998)+0.98²/(2*9.81)+0=2+10.11+0.05=12.16 m

3. Effect

An NPSH available too low will lead to operation issues for the pump, especially a decrease in flowrate and the risk of cavitation.

Source

[Midoux] Mecanique et Rheologie des Fluides, Midoux, 1993
[Sihi] Basic Principles for the Design of Centrifugal Pump Installations, 2003

Pump NPSH Calculator & Technical Guide

Net positive suction head (NPSHa vs NPSHr)