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Wednesday, 11 September 2019

MAXIMUM SUCTION LIFT OF CENTRIFUGAL PUMP

We were discussing the pumps and basic pumping systemtotal head developed by the centrifugal pumpparts of centrifugal pump and their functionheads and efficiencies of a centrifugal pumpwork done by the centrifugal pump on waterexpression for minimum starting speed of a centrifugal pumpmultistage centrifugal pumpscavitation in hydraulic machine, specific speed of a centrifugal pump, cavitation in hydraulic turbines and cavitation in centrifugal pumps in our previous post. 

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

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 

hS = Suction lift of suction height i.e. the vertical height or depth between free surface of liquid and center of centrifugal pump impeller eye 

VS = 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 
P1 = Absolute pressure at the inlet of pump  
V1= Velocity of liquid through suction pipe = V
Z1 = Height of inlet of pump from datum line = hS

Pa /ρg = P1 /ρg + V2S/2g + hS + hfs

P1 /ρg = Pa /ρg – [V2S/2g + hS + hfs ]
Because, 

P1 = Absolute pressure at the inlet of pump = 0 
V1= Velocity of liquid through suction pipe = V

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. P1= PV = Vapour pressure of the liquid in absolute units 

PV /ρg = Pa /ρg – [V2S/2g + hS + hfs

Pa /ρg = PV /ρg + [V2S/2g + hS + hfs

Where, 
Pa /ρg = Atmospheric pressure head = Ha 
PV /ρg = Vapour pressure head = H
Ha = HV + V2S/2g + hS + hfs
hS = Ha - HV - V2S/2g - hfs 


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

Reference: 

Fluid mechanics, By R. K. Bansal 
Image courtesy: Google  

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