Single-Stage Impeller

I am curious as to Problem #117 of the NCEES Civil Sample Questions & Solutions book. The problem provides a "net total dynamic head" and a flow. It asks you to determine which type of single-stage impeller would be the most efficient to use. Using the CERM, I know that I can solve this problem with the following equation:

ns = (n*SQRT(Q)) / (hA^0.75) [EQ: 18.28b]

Where:

ns = what I am solving for to determine the type of impeller

Q = flow (GIVEN as 10cfs)

hA = net total dynamic head (GIVEN as 1200ft)

n = speed in rpm

The answers reference that you should be able to find the solution problem with the HYDRAULIC DESIGN HANDBOOK, MAYS. And viewing the solution, apparently you are able to determine the missing "n" value as 1800 rpm. I do not have the aforementioned reference manual, and cannot seem to find with the CERM how to determine "n."

Can someone help me with this problem? Is there another way to "know" how to determine the rpm of an impeller?

wstahlm80 said:

I am curious as to Problem #117 of the NCEES Civil Sample Questions & Solutions book. The problem provides a "net total dynamic head" and a flow. It asks you to determine which type of single-stage impeller would be the most efficient to use. Using the CERM, I know that I can solve this problem with the following equation:
ns = (n*SQRT(Q)) / (hA^0.75) [EQ: 18.28b]

Where:

ns = what I am solving for to determine the type of impeller

Q = flow (GIVEN as 10cfs)

hA = net total dynamic head (GIVEN as 1200ft)

n = speed in rpm

The answers reference that you should be able to find the solution problem with the HYDRAULIC DESIGN HANDBOOK, MAYS. And viewing the solution, apparently you are able to determine the missing "n" value as 1800 rpm. I do not have the aforementioned reference manual, and cannot seem to find with the CERM how to determine "n."

Can someone help me with this problem? Is there another way to "know" how to determine the rpm of an impeller?

Click to expand...

I was wondering the same thing myself. even at 3500rpm the answer is still radial vane, i used 3500rpm because that's what the other example problems had in it, probably not a good method,

wstahlm80 said:

I am curious as to Problem #117 of the NCEES Civil Sample Questions & Solutions book. The problem provides a "net total dynamic head" and a flow. It asks you to determine which type of single-stage impeller would be the most efficient to use. Using the CERM, I know that I can solve this problem with the following equation:
ns = (n*SQRT(Q)) / (hA^0.75) [EQ: 18.28b]

Where:

ns = what I am solving for to determine the type of impeller

Q = flow (GIVEN as 10cfs)

hA = net total dynamic head (GIVEN as 1200ft)

n = speed in rpm

The answers reference that you should be able to find the solution problem with the HYDRAULIC DESIGN HANDBOOK, MAYS. And viewing the solution, apparently you are able to determine the missing "n" value as 1800 rpm. I do not have the aforementioned reference manual, and cannot seem to find with the CERM how to determine "n."

Can someone help me with this problem? Is there another way to "know" how to determine the rpm of an impeller?

Click to expand...

I don't know if this will help you, but I'm planning to use table 18.9 CERM ( with Specific speed (in rpm) this table direct you what type of impeller to get).

I'm just curios...do you have the new material from NCEES 2011?

chess5329 said:

wstahlm80 said:

I am curious as to Problem #117 of the NCEES Civil Sample Questions & Solutions book. The problem provides a "net total dynamic head" and a flow. It asks you to determine which type of single-stage impeller would be the most efficient to use. Using the CERM, I know that I can solve this problem with the following equation:
ns = (n*SQRT(Q)) / (hA^0.75) [EQ: 18.28b]

Where:

ns = what I am solving for to determine the type of impeller

Q = flow (GIVEN as 10cfs)

hA = net total dynamic head (GIVEN as 1200ft)

n = speed in rpm

The answers reference that you should be able to find the solution problem with the HYDRAULIC DESIGN HANDBOOK, MAYS. And viewing the solution, apparently you are able to determine the missing "n" value as 1800 rpm. I do not have the aforementioned reference manual, and cannot seem to find with the CERM how to determine "n."

Can someone help me with this problem? Is there another way to "know" how to determine the rpm of an impeller?

Click to expand...

I don't know if this will help you, but I'm planning to use table 18.9 CERM ( with Specific speed (in rpm) this table direct you what type of impeller to get).

I'm just curios...do you have the new material from NCEES 2011?

Click to expand...

Table 18.9 is for specific speed, to cal specific speed you need the shaft speed in RPM, which you either have to assume or look up, THis is the old 2008 book, maybe it's changed in the 2011 book? I didn't order one until monday cause I didn't know it was out there, i should get it tomorrow for a quick overview.

chess5329 said:

I don't know if this will help you, but I'm planning to use table 18.9 CERM ( with Specific speed (in rpm) this table direct you what type of impeller to get).
I'm just curios...do you have the new material from NCEES 2011?

Click to expand...

What "new material from NCEES 2011" are you referring to?

Pumps are a weak spot for me. I haven't even read ch 18, but when I took this NCEES practice test, I got that question right by examining the given values:

h = 1200 ft - very large head

Q = 10 cfs

Having no clue what radial/axial meant, I looked it up in the CERM index and was taken to p. 18-23, where a short written desription is given for types of turbines. Radial turbine have high heads and low flows. Answer solved.

Not that this helps explain the NCEES answer, for which I'm equally stumped on how they pulled the RMP out of a hat. But there are other ways to tackle a problem like this.

Happy said:

Pumps are a weak spot for me. I haven't even read ch 18, but when I took this NCEES practice test, I got that question right by examining the given values:h = 1200 ft - very large head

Q = 10 cfs

Having no clue what radial/axial meant, I looked it up in the CERM index and was taken to p. 18-23, where a short written desription is given for types of turbines. Radial turbine have high heads and low flows. Answer solved.

Not that this helps explain the NCEES answer, for which I'm equally stumped on how they pulled the RMP out of a hat. But there are other ways to tackle a problem like this.

Click to expand...

Pumps and turbines are different though

jillhill said:

Pumps and turbines are different though

Click to expand...

Great point, (I had mentioned that I didn't even read Ch 18).

If mastering pumps is bringing you down, like they did for me, consider this,

there are 6 topics under Closed Conduit, 7 under Open Channel, 8 for Hydrology and 3 under Wastewater/Water. Pumps are 1 topic out of 24. There will be 8 water questions in the morning.

Reading and understanding Chapters 17, 19 & 20 will get you 20/24 sections. Chapter 18 gets me 1 questions <maybe>. That why I skipped it. But if I do see a pump question I'll be the guy speed reading Ch 18 at the end of the exam.

I'll go away now.

Happy said:

jillhill said:

Pumps and turbines are different though

Click to expand...

Great point, (I had mentioned that I didn't even read Ch 18).

If mastering pumps is bringing you down, like they did for me, consider this,

there are 6 topics under Closed Conduit, 7 under Open Channel, 8 for Hydrology and 3 under Wastewater/Water. Pumps are 1 topic out of 24. There will be 8 water questions in the morning.

Reading and understanding Chapters 17, 19 & 20 will get you 20/24 sections. Chapter 18 gets me 1 questions <maybe>. That why I skipped it. But if I do see a pump question I'll be the guy speed reading Ch 18 at the end of the exam.

I'll go away now.

Click to expand...

Good point, I'm scared of the water. hope it's straight forward.

Good luck,