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Sunday, 1 September 2019

MULTISTAGE CENTRIFUGAL PUMP

We were discussing the pumps and basic pumping systemtotal head developed by the centrifugal pumpparts of centrifugal pump and their function, heads and efficiencies of a centrifugal pump, work done by the centrifugal pump on water and expression for minimum starting speed of a centrifugal pump in our previous post. 

Now we will find out the multistage centrifugal pumps with the help of this post. First of all we will see here the basics of multistage centrifugal pumps and their functions. Further, we will find out here multistage centrifugal pumps for high heads and multistage centrifugal pumps for high discharge. 

Multistage centrifugal pumps 

If a centrifugal pump will have two or more than two impellers, that centrifugal pumps will be termed as multistage centrifugal pump. 

In simple we can say that in case of multistage centrifugal pumps, there will be two or more than two impellers in the pump. 

These impellers might be fixed on the same shaft or on the different shafts. 

Function of a multistage centrifugal pump

There are following important functions of a multistage centrifugal pump as mentioned here. 
  1. To produce a high head 
  2. To discharge a large quantity of liquid 

Depending on the demand i.e. whether we need high head or high discharge, impellers will be connected or assembled in series or parallel. 

If our demand is high head, impellers will be connected in series i.e. impellers will be fixed on same shaft. 

If our demand is high discharge, impellers will be connected in parallel i.e. impellers will be fixed on different shaft.  In simple, pumps will be connected in parallel to secure high discharge. 

Multistage centrifugal pumps for high head   

In order to secure high head, a number of impellers will be assembled in series or impellers will be fixed on same shaft as displayed here in following figure. 

The water from suction pipe will enter in to the first impeller at its inlet and will be discharged at the outlet of this impeller with increased pressure. 

Water with increased pressure, discharged from first impeller outlet, will be taken to the inlet of second impeller i.e. water with increased pressure coming out from outlet of first impeller will enter in to the second impeller inlet with the help of a connecting pipe as displayed here in following figure. 

Pressure of water coming out from outlet of second impeller will be more as compared to the pressure of water coming out from the outlet of first impeller. 
Therefore, we can conclude that pressure at the outlet could be increased further if more impellers mounted on same shaft. 

There is one equation, as mentioned below, which will provide us the total head developed by a multistage centrifugal pump. 

Total head developed by a multistage centrifugal pump = n x Hm

n = Number of identical impellers mounted on same shaft
Hm = Head developed by each impeller 

We must note it here that discharge passing through each impeller will be same. 

Multistage centrifugal pumps for high discharge    

In order to secure the high discharge, pumps will be connected with each other in parallel as displayed here in following figure. 

Each pump of such multistage centrifugal pump will take water from a common sump and will deliver the water to a common pipe. Delivery pipe of each pump will be connected with this common pipe as displayed here in figure. 
Each pump will work here against same head. 

So, we have seen here the basic concept of multistage centrifugal pump and their function in pumping system. We have also seen here the arrangement of impellers in multistage centrifugal pump depending on demand of high head or high discharge. 

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

Further we will find out, in our next post, Specific speed of a centrifugal pump

Reference: 

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

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