Net Positive Suction Head

Net Positive Suction Head, abbreviated as NPSH - Total suction head in meters (feet) of liquid absolute determined at suction nozzle and referred to datum elevation, minus the vapor pressure of liquid in meters (feet) absolute.  The datum elevation is the shaft centerline for horizontal pumps, the suction nozzle centerline for vertical in-line pumps, and the top of the foundation for other vertical pumps.

 

Net Positive Suction Head formula
\( NPSH \;=\; ( v^2 \;/\; 2 \; g ) + ( p \;/\; \gamma ) - ( p_v \;/\; \gamma ) \)     (Pump Suction Head and Cavitation) \( v \;=\; [\; 2\; g \; NPSH - ( 2 \; g \; p \;/\; \gamma ) \;] \;/\; [\; v - ( 2 \; g \; p \;/\; \gamma ) \;]  \) \( g \;=\; v^2 \;/\; [\; 2 \; (\; NPSH - ( p \;/\; \gamma ) + ( p\;v \;/\; \gamma ) \;) \;] \) \( p \;=\; \gamma \; [\; NPSH - ( v^2 \;/\; 2 \;g ) \;/\; 1 - v \;]  \) \( \gamma \;=\; p \;/\; [\; NPSH -  (v^2 \;/\; 2 \; g ) \;/\; 1 - v \;] \) \( pv \;=\; v^2  + ( p \; \gamma \;/\; 2 \; g ) - \gamma \; NPSH  \)
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.

 

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: 

 

\(NPSH_a\) formula
\( NPSH_a  \;=\; p_2 = p_1 - p_v - \Delta p_f - \Delta h \)     (Pressure at Pressure Source) \( p_1 \;=\; p_2 + p_v + \Delta p_f + \Delta h \)  \( p_v \;=\; p_1 - p_2 + \Delta p_f + \Delta h \)  \( \Delta p_f  \;=\; p_1 - p_2 - p_v - \Delta h \) \( \Delta h  \;=\; p_1 - p_2 + p_v + \Delta p_f  \)
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	\(lbf / in^2\)	\(N \)

 

\(NPSH_r\)

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.