Net Positive Suction Head Vapor Pressure

Net positive suction head vapor pressure, abbreviated as \(NPSH_v\), is used in fluid mechanics, particularly in the design and operation of pumps.  It is related to the pressure conditions at the suction side of a pump and helps ensure that the pump doesn't experience cavitation.  Cavitation is a phenomenon where the local pressure in a liquid drops below its vapor pressure, causing the formation of vapor bubbles. When these bubbles collapse or implode, it can lead to damage to the pump components and a reduction in pump efficiency.

This equation essentially compares the pressure available at the suction side of the pump with the vapor pressure of the liquid.  If the NPSH_v is too low, there's a risk of cavitation.  Pump manufacturers often provide NPSH_v requirements for their pumps, and it's crucial for engineers to ensure that the system meets these requirements to prevent cavitation issues.

 

Net Positive Suction Head Vapor Pressure formula
\( NPSH_v = - \gamma \; [\; NPSH - ( v^2 \;/\; 2 \; g ) - ( p \;/\; \gamma) \; ]  \)     (Net Positive Suction Head Vapor Pressure) \( NPSH = - ( p \; v \;/\; \gamma ) + ( v^2 \;/\; 2 \; g ) + ( p \;/\; \gamma^2 )  \) \( g =  [\; 2 \; ( \gamma \; NPSH - p ) \;/\; v^2 \;]  - ( p \;/\; v^2 ) \)  \( p =  ( 2 \; g \; \gamma \; NPSH \;/\; v )  - ( \gamma \; v \;/\; 2 )  \)
Symbol	English	Metric
\( NPSH_v \) = vapor pressure	\(lbf \;/\; in^2\)	\(Pa\)
\( \gamma \)  (Greek symbol gamma) = specific weight	\(lbf \;/\; in^3\)	\(N \;/\; m^3\)
\( NPSH \) = net positive suction head	\(lbf \;/\; in^2\)	\(Pa\)
\( v \) = velocity	\(ft \;/\; sec\)	\(m \;/\; s\)
\( g \) = gravitational acceleration	\(ft \;/\; sec^2\)	\(m \;/\; s^2\)
\( p \) = pressure	\(lbf \;/\; in^2\)	\(Pa\)