What is NPSH for pumps ? Derive general expression for its calculation. Differentiate between NPSH and NPSHR.

What is NPSH for pumps ? Derive general expression for its calculation. Differentiate between NPSH and NPSHR. [10 Marks]

NPSH (Net Positive Suction Head)
> > The term NPSH is very commonly used in pump industry. Actually minimum suction conditions are more specified in terms of NPSH. NPSH may be defined as the total head required to make the liquid flow through the suction pipe to the pump impeller, or it is the amount by which the absolute pressure available at the suction point is in excess or greater than vapour pressure of liquid at pumping temperature.
> > Poor suction due to inadequate NPSH leads to physical erosion, damage to the impeller and this damage can become so severe as to completely destroy the impeller and create excessive clearances in the casing. >

Expression for NPSH

For deriving the expression for NPSH, let us consider the pumping system shown in figure below.

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Where $\Z\a$ and $\Z\b$ = Elevation of point A and B respectively, m

$\h{\SL}$ and $\h{\DL}$ = Friction losses for suction and discharge side of pump respectively, kgf-m/kg

W = Weight density of liquid = ρg

NPSh = (Absolute pressure head available at suction point A’) - (Vapour pressure head)

∴ NPSH = $({\V’\a^2}/{2\g} + {\P’\a}/{\W}) - {\P_\v}/{\W}$ —(1)

where $\P_\v$ = Vapour pressure of liquid to be pumped.

Applying Bernoulli equation between point A and A’.

$\Z\a + {\V’\a^2}/{2\g} + {\P\a}/{\W} = \Z\a’ + {\V’\a^2}/{2\g} + {\P’\a}/{\W} + \h_{\LS}$

Assuming $\Z\a = 0$ and $\V\α = 0$

$({\P’\a}/{\W} + {\V’\a^2}/{2\g}) = {\P\a}/{\W} - \h{\LS} - \Z’_\a$

Substituting this value for the bracketed quantity in equation (1) we get

NPSH = ${\P’\a}/{\W} - {\P\v}/{\W} - \h{\LS} - \Z’\a = {\P’\a - \P\v}/{\W} - \h{\LS} - \Z’\a$

If pump center-line is below the point A, then

NPSH = ${\P’\a - \P\v}/{\W} - \h{\LS} - \Z’\a$ —(2)

Or equation (2) can also be written as,

NPSH = ${\P’\a - \P\v}/{\ρ\g} - \h{\LS} - \Z’\a$ —(3)

Or in terms of energy per unit mass

NPSH = ${\P’\a - \P\v}/{\ρ} - \h{\LS} - \gZ’\a$ —(4)

where $\h{\LS}$ is in J/klg and $\gZ’\a$ is in J/kg.

Note : If NPSH = 0, suction pressure equals the vapour pressure and cavitation occurs. Thus NPSH must be greater than zero and is usually 2 meters or more.

NPSHA and NPSHR

• For any pump installation a distinction is made between required NPSH (NPSHR) and the available NPSH (NPSHA).

• The value of NPSHR is given by the pump manufacturer and this value can also be determine experimentally.

• For determining its value, the pump is tested and minimum value of suction head is obtained at which the pump gives maximum efficiency without any noise. (i.e. cavitation free.)

• The NPSHR varies with pump design, speed of pump and capacity of the pump.

• In order to have cavitation free operation of centrifugal pump, the NPSHA should be greater than the NPSHR.

• Generally NPSHR must be greater than 2 meters or more.