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Net Positive Suction Head Vapor Pressure

 

Net Positive Suction Head Vapor Pressure Formula

\( NPSH_v \;=\;   - \gamma \cdot \left( NPSH -  \dfrac{ v^2 }{ 2 \cdot g }  -  \dfrac{ p }{ \gamma }  \right)  \)     (Net Positive Suction Head Vapor Pressure)

\( NPSH \;=\;    - \dfrac{ p \cdot v }{ \gamma }  +  \dfrac{ v^2 }{ 2 \cdot g }  +  \dfrac{ p }{ \gamma^2 }  \)

\( g \;=\;    \dfrac{ 2 \cdot ( \gamma \cdot  NPSH - p  ) }{ v^2 }  -  \dfrac{ p }{ v^2 } \) 

\( p \;=\;    \dfrac{ 2 \cdot g \cdot \gamma \cdot  NPSH }{ v }  -  \dfrac{ \gamma \cdot v }{ 2 }  \) 

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 \(lbf \;/\; in^2\) \(Pa\)

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.

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