Net Positive Suction Head

Definition: Net positive Suction head is excess of sum of velocity head and pressure head in suction line over vapour pressure head of liquid. 

                                    

The main objective of NPSH theory is to design a fluid delivery pump with suitable head by considering all assumption.

When pump installation is designed available net positive suction head (NPSH)_A must be equal to or greater than (NPSH)_R

(NPSH)_A should be calculated by process engineer where (NPSH)_R should be specified by pump supplier.

When (NPSH)_A < (NPSH)_R then Cavitation take place. 

                     

In theoretical terms Cavitation occurs when suction pressure fall below the vapour pressure then some of liquid flashes into vapor form these phenomena is called Cavitation. 

 

As a process engineer step to calculate (NPSH)_A;

(NPSH)_A= hss - hfs - Pv

 where, hss = static suction head           

            hfs=friction loss in suction line

            Pv=vapour pressure of liquid at suction temp.

hss = p + z   when pump installed below liquid level shown fig.1

hfs = p - z    when pump installed above liquid level shown fig.2

    

             

                                 fig.1                                                                          fig.2