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NCEES 2008 Water Resources No. 117


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#1 biofem2011

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Posted 12 September 2011 - 05:07 AM

QUESTION
Given a net total dynamic head of 1200ft and a flow of 10cfs, which type of single stage impeller would be the most efficient to use?

The solution used 1800 as the impeller speed. I need help figuring out where the 1800rpm in the solution came from. It is not mentioned in the problem. The solution also stated that "must select a pump speed (typically between 500-1800rpm)"

I have checked several textbooks and still cannot figure out WHERE 1800 is from, WHY I must select a pump speed and WHY I was not given a pump speed.

Thank you.


#2 Jacob_PE

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Posted 12 September 2011 - 05:58 PM

QUOTE (biofem2011 @ Sep 12 2011, 12:07 AM) <{POST_SNAPBACK}>
QUESTION
Given a net total dynamic head of 1200ft and a flow of 10cfs, which type of single stage impeller would be the most efficient to use?

The solution used 1800 as the impeller speed. I need help figuring out where the 1800rpm in the solution came from. It is not mentioned in the problem. The solution also stated that "must select a pump speed (typically between 500-1800rpm)"

I have checked several textbooks and still cannot figure out WHERE 1800 is from, WHY I must select a pump speed and WHY I was not given a pump speed.

Thank you.

This type of problem sounds really familiar, is 'specific speed' mentioned at all in the solution? I have a copy of the 10th CERM here at the office and on page 18-13, Example 18.5 solves for specific speed using equation 18.28 and uses table 18.9; for a specific speed of 595 - radial vane is the impeller type, for 1800 rpm, (U.S. units) the Fancis (mixed) vane might be appropriate since it ranges from 2000 - 3000.; radial vanes are appropriate at ranges from 500 to 1000. See, Table 18.9 on CERM pg 18-12. Any of that help?

#3 thebettersmith

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Posted 12 September 2011 - 08:31 PM

QUOTE (Jacob @ Sep 12 2011, 01:58 PM) <{POST_SNAPBACK}>
QUOTE (biofem2011 @ Sep 12 2011, 12:07 AM) <{POST_SNAPBACK}>
QUESTION
Given a net total dynamic head of 1200ft and a flow of 10cfs, which type of single stage impeller would be the most efficient to use?

The solution used 1800 as the impeller speed. I need help figuring out where the 1800rpm in the solution came from. It is not mentioned in the problem. The solution also stated that "must select a pump speed (typically between 500-1800rpm)"

I have checked several textbooks and still cannot figure out WHERE 1800 is from, WHY I must select a pump speed and WHY I was not given a pump speed.

Thank you.

This type of problem sounds really familiar, is 'specific speed' mentioned at all in the solution? I have a copy of the 10th CERM here at the office and on page 18-13, Example 18.5 solves for specific speed using equation 18.28 and uses table 18.9; for a specific speed of 595 - radial vane is the impeller type, for 1800 rpm, (U.S. units) the Fancis (mixed) vane might be appropriate since it ranges from 2000 - 3000.; radial vanes are appropriate at ranges from 500 to 1000. See, Table 18.9 on CERM pg 18-12. Any of that help?


TRY CHAPTER 18 OF CERM - PUMP SIMILARITY AND AFFINITY LAWS SHOULD HELP - OTHER TEXTS/REFS SHOULD HAVE STD DATA FOR IMPELLER TYPES

#4 biofem2011

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Posted 12 September 2011 - 11:01 PM

Thank you.
Unfortunately you haven`t answered my question. I have read Chapter 18 of CERM and I didn`t pick anywhere where it was said that that to calculate the specific speed, the speed of the pump motor has to be assumed.

Equation 18.28 of CERM was used in the NCEES solution and the value of "n" was taken to be 1800. The problem did not mention the speed of the motor (n).

Example 18.5 in CERM 11th edition is similar. However, the CERM question geve the value of "n" which was inserted in the equation to calculate n sub s- the specific speed.

Is there a table that gives the different speed of pump motor?

Thank you !

#5 biofem2011

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Posted 12 September 2011 - 11:05 PM

Also, Table 18.9 on page 18-12 gives different SPECIFIC SPEED versus impeller design. The specific speed is "n sub s", the speed of the motor is "n".


#6 Jacob_PE

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Posted 13 September 2011 - 12:04 AM

Table 18.8 on pg 18-10 of the 11th CERM shows common synchronous speeds (n values), in the U.S. The frequency is typically 60Hz. So from this table highest assumption would be 3600 instead of 1800.  For Q=10cfs (4488.3 gpm) and TDH = 1200', specific speed is 591 for n=1800 or 1183 for n=3600. But as explained on pg 18-12, for best efficiency, specific speed should be greater than 650. So if we assumed 3600, radial vane is best. 
And to add to the confusion, take a look at figure 18.8 on pg 18-9, we are approx on curve E, max efficiency at specific speed = 2500, at which case a Francis (mixed) vane impeller would be most efficient. What does the text say is the correct type?

#7 biofem2011

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Posted 13 September 2011 - 12:43 AM

Thanks Jacob. The NCEES solution calculated "n sub s" as 591 and selected a radial flow impeller.

Thanks for adding to the confusion sharkattack.gif

#8 Jacob_PE

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Posted 13 September 2011 - 01:11 AM

QUOTE (biofem2011 @ Sep 12 2011, 07:43 PM) <{POST_SNAPBACK}>
Thanks Jacob. The NCEES solution calculated "n sub s" as 591 and selected a radial flow impeller.

Thanks for adding to the confusion sharkattack.gif

No hay de qué, a tu servicio. At least on exam day if we're given Q and TDH we'll know exactly what to do.




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