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Wednesday, 11 September 2019

NET POSITIVE SUCTION HEAD 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, cavitation in centrifugal pumps and maximum suction lift of centrifugal pump in our previous post. 

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. 

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. 

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. 

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

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. 

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

Net positive suction head = P1 /ρg - PV /ρg + V2S/2g

Now we will find out the value of term P1 /ρg i.e. Absolute pressure head and it could be determined as followed. 

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, 

Za = Height of free surface from datum line = 0 
Va= Velocity of liquid at the free surface of liquid = 0
V1= Velocity of liquid through suction pipe = V

Now we will use the value of P1 /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 – [V2s/2g + hs + hfs ] - Pv /ρg + V2s/2g 

NPSH = Pa /ρg - hs - hfs - Pv /ρg 

NPSH = Ha - hs - hfs Hv 

NPSH = Ha - hs - hfs Hv


Where, 

Ha = Atmospheric pressure head 
HV = Vapour pressure head 
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 
hfs = 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.  

Further we will find out, in our next post, axial flow pump working principle

Reference: 

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

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