# MERM CENTRIFUGAL PUMP PROBLEM HELP!!!!!



## annie (Jul 15, 2007)

I cannot understand how they used Power ( Brake Horse Power ) to calculate the Total Dynamic Head in Example Problem 18.4 MERM. I

thought that the equation is USED to calculate Hydraulic Power.

Any help is appreciated.


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## JohnNevets (Jul 16, 2007)

Annie,

If this is the same sample that is in MERM 11, and by context it appears to be, they just used algebra to reverse the equation. The original equation was P=(h*mdot/550)*(g/gc) they solved it for head to give h=(550*P/mdot)*(gc/g). Now if that wasn't what you were asking, let us know, I'm sure someone else may be able to give a better explanation of the details, I'm a MD guy not a fluids guy.

Good Luck, and keep asking,

John


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## annie (Jul 16, 2007)

JohnNevets said:


> Annie,
> If this is the same sample that is in MERM 11, and by context it appears to be, they just used algebra to reverse the equation. The original equation was P=(h*mdot/550)*(g/gc) they solved it for head to give h=(550*P/mdot)*(gc/g). Now if that wasn't what you were asking, let us know, I'm sure someone else may be able to give a better explanation of the details, I'm a MD guy not a fluids guy.
> 
> Good Luck, and keep asking,
> ...



John,

Thanks for responding. The original equation used was P=(h*mdot/550)*(g/gc). This is correct but the P used here is the Hydraulic Power (TABLE 18.4) but in the example 18.4 they used Brake Horse Power (BHP) . This does not seem correct to me since BHP * Pump Efficiency = HP So I think they forgot to multiply by Pump Efficiency.

Annie


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## NEplantengineer (Jul 16, 2007)

annie said:


> John,
> Thanks for responding. The original equation used was P=(h*mdot/550)*(g/gc). This is correct but the P used here is the Hydraulic Power (TABLE 18.4) but in the example 18.4 they used Brake Horse Power (BHP) . This does not seem correct to me since BHP * Pump Efficiency = HP So I think they forgot to multiply by Pump Efficiency.
> 
> Annie


I don't believe that they did forget the pump efficiency. If you look at part b, MERM calculates the pump efficiency by using the theoretical torque multiplied by the given pump rpm (I think it was around 21 HP?), with the whole thing divided by the 30 HP that the problem statement says that the motor is developing/supplying. I think the pump efficiency came out to around 71%.

So in part c if you want to use the power actually delivered by the impeller to the water, use the 21 HP. If you want to start from the motor end of the machine, take the 30 HP and multiply it by the efficiency you calculated in part c.

Hope this helps.


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## annie (Jul 16, 2007)

NEplantengineer said:


> I don't believe that they did forget the pump efficiency. If you look at part b, MERM calculates the pump efficiency by using the theoretical torque multiplied by the given pump rpm (I think it was around 21 HP?), with the whole thing divided by the 30 HP that the problem statement says that the motor is developing/supplying. I think the pump efficiency came out to around 71%.
> So in part c if you want to use the power actually delivered by the impeller to the water, use the 21 HP. If you want to start from the motor end of the machine, take the 30 HP and multiply it by the efficiency you calculated in part c.
> 
> Hope this helps.



Thanks a lot. This helps.


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