Net positive suction head, abbreviated as NPSH, is a concept in fluid mechanics and pump engineering, used to analyze and prevent cavitation in pumping systems. It quantifies the margin by which the pressure at the pump inlet exceeds the vapor pressure of the liquid being pumped, ensuring the fluid remains in a liquid state as it enters the pump impeller. Expressed in units of head, NPSH represents the total suction head available at the pump inlet minus the vapor pressure head of the fluid.
Net Positive Suction Head formula |
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\( NPSH \;=\; \dfrac{ v^2 }{ 2 \cdot g } + \dfrac{ p }{ \gamma } - \dfrac{ p_v }{ \gamma } \) (Pump Suction Head and Cavitation) \( v \;=\; \dfrac{ 2\cdot g \cdot NPSH - \dfrac{ 2 \cdot g \cdot p }{ \gamma } }{ v - \dfrac{ 2 \cdot g \cdot p }{ \gamma } } \) \( g \;=\; \dfrac{ v^2 }{ 2 \cdot \left( NPSH - \dfrac{ p }{ \gamma } + \dfrac{ p \cdot v }{ \gamma } \right) } \) \( p \;=\; \gamma \cdot \dfrac{ NPSH - \left( \dfrac{ v^2 }{ 2 \cdot g } \right) }{ 1 - v } \) \( \gamma \;=\; \dfrac{ p }{ NPSH - \dfrac{ \dfrac{v^2 }{ 2 \cdot g} }{ 1 - v } } \) \( pv \;=\; v^2 + \dfrac{ p \cdot \gamma }{ 2 \cdot g } - \gamma \cdot NPSH \) |
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| Symbol | English | Metric |
| \( NPSH \) = Net Positive Suction Head | \(lbf \;/\; in^2\) | \(N \) |
| \( g \) = Gravitational Acceleration | \(ft \;/\; sec^2\) | \(m \;/\; s^2\) |
| \( p \) = Pressure | \(lbf \;/\; in^2\) | \(N \) |
| \( \gamma \) (Greek symbol gamma) = Specific Weight | \(lbf \;/\; ft^3\) | \(N \;/\; m^3\) |
| \( p_v \) = Vapor Pressure | \(lbf \;/\; in^2\) | \(N \) |
| \( v \) = Velocity | \(ft \;/\; sec\) | \(m \;/\; s\) |
\(NPSH_a\)
Net Positive Suction Head Available, abbreviated as \(NPSH_a\) - NPSH measured in feet or meters of liquid determined by the characteristics of the pumping installation with the liquid at rated flow and normal pumping temperature.
\(NPSH_a\) formula |
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\( NPSH_a \;=\; p_2 \;=\; p_1 - p_v - \Delta p_f - \Delta h \) (Pressure at Pressure Source) \( p_1 \;=\; p_2 + p_v + \Delta p_f + \Delta h \) \( p_v \;=\; p_1 - p_2 + \Delta p_f + \Delta h \) \( \Delta p_f \;=\; p_1 - p_2 - p_v - \Delta h \) \( \Delta h \;=\; p_1 - p_2 + p_v + \Delta p_f \) |
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| Symbol | English | Metric |
| \( NPSH_a \) = Available Net Positive Suction Head | \(lbf \;/\; in^2\) | \(N \) |
| \( p_1 \) = Initial Pressure | \(lbf \;/\; in^2\) | \(N \) |
| \( p_v \) = Vapor Pressure | \(lbf \;/\; in^2\) | \(N \) |
| \( \Delta p_f \) = Pressure Loss due to Friction | \(lbf \;/\; in^2\) | \(N \) |
| \( \Delta h \) = Pressure Moss due to Height Elevation Change | \(lbf \;/\; in^2\) | \(N \) |
In the photo above, the NPSH that is available at the pump suction, p2 is characterized by pressure at the pressure source, p1, minus pressure drop due to change of elevation, \(\Delta h\), and friction loss \(\Delta p_f \). It can be calculated using a modified form of the Bernoulli Equation:
\(NPSH_r\)
Net Positive Suction Head Required, abbreviated as \(NPSH_r\) - NPSH in feet or meters and is determined by characteristics within the pump. This is evaluated usually with water by the pump manufacturer. \(NPSH_r\) is measured at the suction flange and corrected to the datum elevation.
\(NPSH_r\) is not the pressure at which there is no cavitation in the pump. \(NPSH_r\) is the minimum NPSH at rated capacity required to prevent a head drop of more than 3% (first stage head in multistage pumps) due to cavitation within pump.
