We were
discussing the pumps and basic pumping
system, total head developed by the centrifugal
pump, parts of centrifugal pump and
their function, heads and efficiencies of a
centrifugal pump, work done by the centrifugal
pump on water, expression for minimum
starting speed of a centrifugal pump, multistage centrifugal pumps, cavitation in hydraulic
machine,
specific speed of a
centrifugal pump,
cavitation in hydraulic turbines, cavitation in centrifugal pumps and maximum suction lift of centrifugal pump in our previous post.

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NPSH = Ha - h

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Now we
will find out here the net positive suction head of centrifugal pump with the
help of this post. After reading this post, we will be able to understand the
meaning and importance of net positive suction head in installation of a
centrifugal pump for transporting the liquid up to a desired height from a
given sump.

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**Net positive suction head of
centrifugal pump **

Net
positive suction head i.e. NPSH is a very important term which is used in pump
industries. Net positive suction head of centrifugal pump will be of two types.
One is net positive suction head required and another is net positive suction
head available.

Net
positive suction head required is basically provided by manufacturer of pump. Pump
manufacturer will provide the minimum suction condition of a pump in terms of
net positive suction head or NPSH. Net positive suction head required will be dependent
over the design of pump, speed of the pump and capacity of the pump.

Net
positive suction head available will be determined during pump installation. We
must note it here that in order to avoid the problem of cavitation, net
positive suction head available should not be less than the net positive
suction head required.

Net
positive suction head available should always be greater than or equal to the
net positive suction head required for smooth operation of a centrifugal pump.
Otherwise there will be problem of cavitation and metallic surface of pump will
be eroded.

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**Net positive suction head (NPSH):
Definition **

Net
positive suction head is basically defined as the total head required to make
the liquid flow through the suction pipe to the pump impeller.

Net
positive suction head could be determined as the absolute pressure head at the
inlet of the pump minus vapour pressure head of liquid plus the velocity head. Vapour
pressure head of liquid will be used in absolute unit.

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**Net positive suction head = Absolute
pressure head - vapour pressure head + velocity head **

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**Determination of net positive
suction head**

Let us
consider a centrifugal pump as displayed here in following figure. Centrifugal
pump will lift the water from a reservoir i.e. sump.

There will be one centrifugal pump, as
displayed in figure, which will lift the liquid (say water for example) from sump
and will deliver it to the higher reservoir.

There will be one inlet pipe and one
outlet pipe. Inlet pipe will connect the sump with the inlet of the centrifugal
pump and outlet pipe or discharge pipe will connect the discharge or outlet of
centrifugal pump with the higher reservoir.

Liquid or water will enter in to the
inlet pipe and will go to the centrifugal pump. Centrifugal pump will provide
the energy to the liquid. At the outlet of the pump, liquid will be discharged
with a high pressure head and therefore liquid could be lifted up to high level
and will be discharged to higher reservoir.

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**Net positive suction head = Absolute
pressure head - vapour pressure head + velocity head**

Net positive suction head = P

_{1}/ρg - P_{V}/ρg + V^{2}_{S}/2g
Now we will find out the value of term P

_{1}/ρg i.e. Absolute pressure head and it could be determined as followed.
Let us consider the following terms from
above figure

h

_{S}= Suction lift of suction height i.e. the vertical height or depth between free surface of liquid and center of centrifugal pump impeller eye
V

_{S}= Velocity of liquid flowing to centrifugal pump through inlet or suction pipe of centrifugal pump
Now we will apply the Bernoulli’s
equation at the free surface of liquid in the sump and section 1 in the suction
pipe just at the inlet of the pump. We have also considered the free surface of
liquid as datum line.

Where,

Pa = Atmospheric pressure on the free
surface of liquid

Va= Velocity of liquid at the free
surface of liquid

Za = Height of free surface from datum
line

P

_{1}= Absolute pressure at the inlet of pump
V

_{1}= Velocity of liquid through suction pipe = V_{S }
Z

_{1}= Height of inlet of pump from datum line = h_{S}
Pa /ρg = P

_{1}/ρg + V^{2}_{S}/2g + h_{S}+ h_{fs}
P

_{1}/ρg = Pa /ρg – [V^{2}_{S}/2g + h_{S}+ h_{fs}]
Because,

Za = Height of free surface from datum line = 0

Va= Velocity of liquid at the free surface of liquid = 0

Va= Velocity of liquid at the free surface of liquid = 0

V

_{1}= Velocity of liquid through suction pipe = V_{S }
Now we
will use the value of P

_{1}/ρg i.e. Absolute pressure head in NPSH equation and we will have following equation for net positive suction head as mentioned here.
Net positive suction head = Pa /ρg – [V

^{2}_{s}/2g + h_{s}+ h_{fs}] - P_{v}/ρg + V^{2}_{s}/2g
NPSH = Pa /ρg - h

_{s}- h_{fs}- P_{v}/ρg
NPSH = Ha - h

_{s}- h_{fs}- H_{v}_{ }###
NPSH = Ha - h_{s} - h_{fs} - H_{v}

Where,

Ha = Atmospheric pressure head

H

_{V}= Vapour pressure head
h

_{S}= Suction lift of suction height i.e. the vertical height or depth between free surface of liquid and center of centrifugal pump impeller eye
h

_{fs}= Loss of head due to friction
Above
equation will provide the net positive suction head available for a given
centrifugal pump.

Therefore,
we have seen here the definition, importance and calculation of net positive suction
head of centrifugal pump. We have also discussed above that in order to secure
the cavitation free operation of centrifugal pump, net positive suction head
available must be larger than the net positive suction head required.

Do you
have any suggestions? Please write in comment box.

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**Reference: **

Fluid
mechanics, By R. K. Bansal

Image
courtesy: Google

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