NET POSITIVE SUCTION HEAD

NPSH:

NPSH is the net positive suction head. It is a measure of pressure of fluid at suction of centrifugal pump. It is denoted in meters or feet rather than as Pa or atm.

NPSH-R:

NPSH-R is a pump property. NPSH-R is the minimum suction pressure required by pump in order to avoid cavitation. It is mentioned by pump manufacturer as a result of testing.

NPSH-A:

NPSH-A is a system property. NPSH-A is the pressure available at the suction of fluid in centrifugal pump. It is calculated as:

NPSH-A = P₁ + h₁(ρ/γ) + h_v – h_f -P_v

P₁ = Atmospheric Pressure (kPa or psi)

h1 = Static Head (m or ft)

ρ = Density of the fluid (kg/m³ or lb/ft³)

γ = Specific Weight of the fluid (ρ,g, N/m³ or lb/ft²)

h_v = Velocity Head (m or ft)

h_f = Frictional Losses (m or ft)

P_v = Vapor Pressure (kPa or psi)

NPSH Safety Margin:

According to Michael Smith Engineers Limited: The NPSH margin describes the safety factor by which NPSH-A must exceed NPSH-R to avoid cavitation. It can be quoted in two ways:

• As a ratio of NPSH-A to NPSH-R. For example, an NPSH margin ratio of 1.1 indicates that NPSH-A is 10% greater than NPSH-R.

• As the difference between NPSH-A and NPSH-R. As a rule of thumb, it is necessary to ensure that the NPSH margin is 0.5m or higher (that is: NPSH-A = NPSH-R + 0.5m) as shown in fig 1.

Graphically it can be represented as:

If NPSH-A is less than NPSH-R then it causes cavitation in pump.

wait for the next blog about cavitation.

Reference: https://www.michael-smith-engineers.co.uk/