NPSH simply explained: Formula, example & tips against cavitation

What is the NPSH value, and why is it so important?

Anyone dealing with pump technology will sooner or later come across the term NPSH. But what’s behind it, why is this value so important — and how is it calculated?

NPSH — explained briefly

NPSH stands for Net Positive Suction Head. It describes the energy head at a pump’s inlet, expressed as meters of liquid column. Put simply: the NPSH value indicates whether sufficient pressure (energy head) is present at the pump inlet so that the liquid does not vaporize.

This is crucial, because as soon as the pressure at the pump suction nozzle falls below the vapor pressure of the medium, the fluid starts to boil at the pump inlet and vapor bubbles form. These bubbles implode as soon as the pump raises the pressure again — an effect known as cavitation. Cavitation causes loud noise, loss of performance, and typically severe damage to the pump.

NPSHA vs. NPSHR — what’s the difference?

NPSHA (“Available”): The actually available suction head in the system under the given operating conditions. It depends on the pump installation, the fluid, the temperature, and the pressure losses in the suction line.
NPSHR (“Required”): The minimum suction head required by the pump (specified by the manufacturer). It indicates how much inlet pressure the pump needs to operate without cavitation.

The simple rule is: NPSHA > NPSHR + safety margin (typically 0.5–1 m for small systems, ≥ 1–3 m for critical media, high temperatures, or long-term operation).

How is NPSHA calculated?

The available NPSH (NPSHA) results from the balance of various pressures and elevations in the system. In simplified form, the calculation looks like this:

${NPSH}_{avail.} = \frac{p_{e} + p_{b} - p_{D}}{ρ \cdot g} + \frac{{v_{e}}^{2}}{2 g} - H_{VS} \pm H_{Sgeo}$

Meaning of the individual terms:

p_e – Overpressure in the supply tank (gauge) (Pa)
p_b – Atmospheric pressure at the installation site (absolute) (Pa)
p_D – Vapor pressure of the liquid at operating temperature (Pa)
ρ – Density of the liquid (kg/m³)
g – Acceleration due to gravity (m/s², standard value 9.81 m/s²)
v_e² – Velocity of the fluid at the pump inlet (m/s)
H_VS – Pressure loss in the suction line (m)
H_Sgeo – Geodetic suction head between liquid level and pump inlet (m)
- + for flooded suction (tank above pump)
- − for suction lift (tank below pump)

The result is given in meters of head.

Example calculation

Term	Value	Unit	Description
NPSH_req.	1.3	m	NPSH value of the pump
p_e	0	N/m²	Overpressure in the tank
p_b	~95200	N/m²	Atmospheric pressure (491 m a.s.l.)
p_D	48300	N/m²	Vapor pressure (~20°C)
ρ	905	kg/m³	Density
g	9.81	m/s²	Acceleration due to gravity
v_e	0.05	m/s	Velocity at pump inlet
H_VS	0.1	m	Pressure loss in suction line
H_Sgeo	0.73	m	Geodetic height difference
${NPSH}_{avail.} = \frac{0 \frac{N}{m^{2}} + 95200 \frac{N}{m^{2}} - 48300 \frac{N}{m^{2}}}{905 \frac{kg}{m^{3}} \cdot 9.81 \frac{m}{s^{2}}} + \frac{{(0.05 \frac{m}{s})}^{2}}{2 \cdot 9.81 \frac{m}{s^{2}}} - 0.1 m + 0.73 m$
NPSH_avail.=	5.91 m	NPSH value of the system
NPSH_avail. ≥ NPSH_req. + 0.5 m NPSH_avail. − NPSH_req. − 0.5 m = 4.11 m (= 0.365 bar)

If the manufacturer specifies an NPSHR value of 1.3 m for this pump, the system is clearly above the required value — the pump therefore operates safely without cavitation.

Conclusion

NPSHA = available suction head (depends on the system)
NPSHR = required suction head (specified by the manufacturer)
Important: NPSHA must always be greater than NPSHR (plus a suitable safety margin).

Anyone who understands these relationships can design pump systems safely and prevent cavitation from reducing a pump’s service life and efficiency.