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 and cavitation in centrifugal pumps in our
previous post.

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Now we will
find out here the maximum suction lift of centrifugal pump with the help of
this post.

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**Maximum suction lift of centrifugal pump **

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.

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,

P

_{1}= Absolute pressure at the inlet of pump = 0
V

_{1}= Velocity of liquid through suction pipe = V_{S }
Now we
must note it here that in order to secure the maximum suction lift, pressure at
the inlet of the pump must not be less than the vapour pressure of liquid
otherwise there will be problem of cavitation. Therefore, for limiting case, we
will consider pressure at the inlet of the pump equal to the vapour pressure of
liquid.

i.e. P

_{1}= P_{V }= Vapour pressure of the liquid in absolute units
P

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

_{V}/ρg + [V^{2}_{S}/2g + h_{S}+ h_{fs}]
Where,

Pa /ρg = Atmospheric pressure head = Ha

P

_{V}/ρg = Vapour pressure head = H_{V }
Ha = H

_{V}+ V^{2}_{S}/2g + h_{S}+ h_{fs}
h

_{S}= Ha - H_{V}- V^{2}_{S}/2g - h_{fs }
Above
equation will provide the maximum suction lift or maximum suction height for a
given centrifugal pump.

Therefore
we must note it here that suction lift for any centrifugal pump should not be
more than the value of suction lift secured from above derived equation.

If the
suction lift of centrifugal pump will be more than the value of suction lift
secured from above derived equation, vapourisation of liquid flowing through
pump will take place at inlet of the centrifugal pump. Hence, there will be a
great probability of cavitation.

Therefore
in order to avoid the problem of cavitation, suction lift for any centrifugal
pump should not be more than the value of suction lift secured from above
derived equation.

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

Further we
will find out, in our next post, net positive suction head of pump.

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**Reference: **

Fluid
mechanics, By R. K. Bansal

Image
courtesy: Google

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