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Performance Curve For Centrifugal Pump


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#1 Afshin4451

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Posted 05 September 2016 - 01:24 PM

Dear All,

 

It is well know in centrifugal pump with increasing flowrate total pump head is reduce.I have some doubt about which equation can be used to describe such a behavior in centrifugal pumps .

 

To make question simple we can consider a fixed speed pump with known impeller size and shape.

 

Please share your knowledge and experience about my concern.

 

Cheers


Edited by Afshin4451, 05 September 2016 - 01:25 PM.


#2 samayaraj

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Posted 05 September 2016 - 01:47 PM

Hi,

 

Its simple energy conversion. You can relate Bernoulli equation to centrifugal pump. Centrifugal pump are kinetic pumps whose flow rare increases as the system resistance decreases. Its conversion of pressure energy into velocity (due to reduction in system resistance). So as the velocity increases, flow rate increases. Flow rate is proportional to power input thereby power required for the pump also increases. As the flow increases and crosses BEP (best efficiency point), efficiency will further drop and adds further load to the motor. This causes overload and ultimately pump will be tripped on overload. 



#3 Afshin4451

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Posted 05 September 2016 - 02:37 PM

Hi,

 

Its simple energy conversion. You can relate Bernoulli equation to centrifugal pump. Centrifugal pump are kinetic pumps whose flow rare increases as the system resistance decreases. Its conversion of pressure energy into velocity (due to reduction in system resistance). So as the velocity increases, flow rate increases. Flow rate is proportional to power input thereby power required for the pump also increases. As the flow increases and crosses BEP (best efficiency point), efficiency will further drop and adds further load to the motor. This causes overload and ultimately pump will be tripped on overload. 

 

Samyaraj,

 

Thank you for prompt reply. Let say pump suction and discharge nozzles size are same and in same elevation could you use Bernoulli equation to explain such a behavior? 


Edited by Afshin4451, 05 September 2016 - 11:33 PM.


#4 Francisco Angel

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Posted 05 September 2016 - 06:05 PM

Dear Afshin4451, if elevation and pressure at suction and discharge are the same, there are two cases:

*If you assume no friction, there will be no impediments for fluid flow.

*If you assume friction, the friction "head" will be fLv^2/(2D) where f: friction factor, L: equivalent length, v: fluid velocity and D: equivalent diameter.

 

The pump power (P) will be P=m*"head" , where m: mass flow. In this case, if the pump has constant power, as you increase mass flow, the available "head" decreases.

 

If there are differences in height, the power will be:

 

P=m(fLv^2/(2D)+dH)

 

Where dH is the difference in height, the same is true here, as you increase flow at constant power, available head decreases.

 

Best regards.



#5 Bobby Strain

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Posted 05 September 2016 - 09:32 PM

Why not do some research and tell us of your findings? Should probably be easy with Google search.

 

Bobby



#6 Afshin4451

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Posted 05 September 2016 - 11:43 PM


 

Francisco,

 

We can't say suction and discharge pressure are same because there is no meaning of pump head. In this regards, power change when pump working point change.


Edited by Afshin4451, 06 September 2016 - 02:33 PM.


#7 Afshin4451

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Posted 05 September 2016 - 11:46 PM

Why not do some research and tell us of your findings? Should probably be easy with Google search.

 

Bobby

Boby,

 

I searched a lot but I didn't find any equation to explain such a behavior. 



#8 Bobby Strain

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Posted 06 September 2016 - 09:05 AM

It took less than 5 minutes to find this. When you search Google, search for exactly what you want. In this case that would be "centrifugal pump design".

 

http://www.academia...._Method_and_CFD



#9 Francisco Angel

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Posted 06 September 2016 - 02:10 PM

Francisco,

 

We can't say suction and discharge pressure are same because there is no meaning of pump head. Inn this regards, power change when pump working point change.

 

Dear Afshin4451, when I say pressure at suction and discharge to be the same, I'm not referring to PUMP suction and discharge, but to a pumping line suction and discharge, please see image at https://drive.google...UEZZTDR1V1NqTkk .

Like I said, if no friction is considered, there would be free flow. But if you consider friction losses, the pump must compensate for them, in the pumping line the pressure will decrease from suction to pump, increase due to the pump presence, and finally decrease from pump until discharge as I understand.

Best regards



#10 Afshin4451

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Posted 06 September 2016 - 09:02 PM

It took less than 5 minutes to find this. When you search Google, search for exactly what you want. In this case that would be "centrifugal pump design".

 

http://www.academia...._Method_and_CFD

 

Bobby,

 

Thank you for sharing paper, I went through article and I found Equation 8 useful which explain relation of pump head and capacity and geometry of impeller but it including another parameter (U2) which I am sure can't be considered constant when capacity change, please correct me if I am wrong.


Edited by Afshin4451, 06 September 2016 - 09:04 PM.


#11 Afshin4451

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Posted 06 September 2016 - 09:10 PM

 

Francisco,

 

We can't say suction and discharge pressure are same because there is no meaning of pump head. Inn this regards, power change when pump working point change.

 

Dear Afshin4451, when I say pressure at suction and discharge to be the same, I'm not referring to PUMP suction and discharge, but to a pumping line suction and discharge, please see image at https://drive.google...UEZZTDR1V1NqTkk .

Like I said, if no friction is considered, there would be free flow. But if you consider friction losses, the pump must compensate for them, in the pumping line the pressure will decrease from suction to pump, increase due to the pump presence, and finally decrease from pump until discharge as I understand.

Best regards

 

Francisco,

 

Appreciated if you could express your explanation with using of equation similar article shared by Bobby.



#12 Francisco Angel

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Posted 07 September 2016 - 01:19 PM

 

 

Francisco,

 

We can't say suction and discharge pressure are same because there is no meaning of pump head. Inn this regards, power change when pump working point change.

 

Dear Afshin4451, when I say pressure at suction and discharge to be the same, I'm not referring to PUMP suction and discharge, but to a pumping line suction and discharge, please see image at https://drive.google...UEZZTDR1V1NqTkk .

Like I said, if no friction is considered, there would be free flow. But if you consider friction losses, the pump must compensate for them, in the pumping line the pressure will decrease from suction to pump, increase due to the pump presence, and finally decrease from pump until discharge as I understand.

Best regards

 

Francisco,

 

Appreciated if you could express your explanation with using of equation similar article shared by Bobby.

 

Dear afshin4451, in a case like the one shown in the image, you can say that the "head" (or energy) difference between suction and discharge are due equal to the sum of contributions and losses between those points.

d: discharge

s:suction

z:elevation

v:velocity

g:gravity accelaration

P:pressure

r:density

W:power supplied by the pump dividided by mass flow.

L: losses.

 

(z+0.5v^2/g+P/(r*g))_s-(z+0.5v^2/g+P/(r*g))_d=W-L

 

so, if elevation (z), pressure (P) and velocity (v) are the same at suction and discharge, then:

 

W=L

 

So, again revisiting my previous assertions, if losses are neglected, L=0 and W=0 , there is no power required from the pump. If losses are:

 

L=0.5fv^2T/(D*g)

 

T: equivalent piping lenght

f: friction factor

D: equivalent diameter.

 

We have:

 

W=0.5fv^2T/(D*g)

 

This way you can use an experimental piping circuit to determine a curve of head versus flow for a given pump. The power input from the pump is:

m: mass flow.

P=W*m

 

Assuming pump power is constant, is flow through the piping is to be increased, the "head" the pump is able to provide decreases. As to how you can determine this curve of head versus flow, using only pump data, like geometry of his internal parts, or characteristics of the associated motor, I don't have that knowledge.

 

Best regards.






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