Headloss, Darcy

[jrs]

A pump delivers 3.0 cfs of 70° F water through a 1000 foot, 12-inch steel discharge pipe. Assume a pipe roughness (e) of 0.0002 feet, and a Reynolds number of 3.0 * 10^5 . Use the Darcy equation to find the head loss (hf, feet) for the pipe.

The book answer is 3.851. Can anyone confirm?

 

[IlPadrino]

A pump delivers 3.0 cfs of 70° F water through a 1000 foot, 12-inch steel discharge pipe. Assume a pipe roughness (e) of 0.0002 feet, and a Reynolds number of 3.0 * 10^5 . Use the Darcy equation to find the head loss (hf, feet) for the pipe.
The book answer is 3.851. Can anyone confirm?

Using a Moody diagram (for water at 60°F, not the 70°F of the problem) I come up with a friction factor of 0.016. Then just a quick conversion of Q to V (3.8 fps assuming inside diameter is 12") and the Darcy-Weisbach equation gives me an answer close to 3.6 ft.

Close enough?

 

[petobe]

I get 3.74 ft..

 

[IlPadrino]

I get 3.74 ft..

What did you get for f?

 

[petobe]

What did you get for f?

.0163

 

[jrs]

I also used f=0.0163, from the CERM appendix and came up with 3.74'.

Calculating Re, I get an even lower 3.62'

Can't get to the book answer!

 

[snickerd3]

This is a good example of why most of the time, not always, the problem statement gives you everything and more you need to answer the problem. Since people interpret charts differently, the answers could range over too wide of a span for accurate multiple choice test.

 

[pinkpig]

A pump delivers 3.0 cfs of 70° F water through a 1000 foot, 12-inch steel discharge pipe. Assume a pipe roughness (e) of 0.0002 feet, and a Reynolds number of 3.0 * 10^5 . Use the Darcy equation to find the head loss (hf, feet) for the pipe.
The book answer is 3.851. Can anyone confirm?

I remember during my time taking PE, either f or C has always been given in the description.

f coming from Re will differ too much among various sources.

 

[IlPadrino]

This is a good example of why most of the time, not always, the problem statement gives you everything and more you need to answer the problem. Since people interpret charts differently, the answers could range over too wide of a span for accurate multiple choice test.

Snickerd3,

I'm not sure I understand... this problem is essentially about finding the friction factor - and there are lots of ways to do that. Charts, nomographs, tables, or equations... they all should work well enough to get you close enough to the right answer, even on a multiple choice test. I'd forgotten the CERM had a table - in general, I think tables are the easiest to use in an exam situation.

 

[snickerd3]

Snickerd3,
I'm not sure I understand... this problem is essentially about finding the friction factor - and there are lots of ways to do that. Charts, nomographs, tables, or equations... they all should work well enough to get you close enough to the right answer, even on a multiple choice test. I'd forgotten the CERM had a table - in general, I think tables are the easiest to use in an exam situation.

maybe it varies by test type, but on the chemical exam for these same type of headloss problems, the problem statements gave me everything I needed to solve the problem and more. Unless it was specifically a chart reading question, I didn't have to look up information on tables and charts...which surprised me.

 

[IlPadrino]

maybe it varies by test type, but on the chemical exam for these same type of headloss problems, the problem statements gave me everything I needed to solve the problem and more. Unless it was specifically a chart reading question, I didn't have to look up information on tables and charts...which surprised me.

Got it...

 

[petobe]

I also used f=0.0163, from the CERM appendix and came up with 3.74'.Calculating Re, I get an even lower 3.62'

Can't get to the book answer!

i also worked the problem using different inside diameters and areas (based on the table for 12" steel pipe in the CERM Appendix) and could not replicate the answer that the book gave you, but I only tried a couple. I am pretty confident that they way we did it is correct though... I've found several mistakes in books before, wouldn't surprise me if this is one as well...

 

[jrs]

Thanks petobe. Glad to know it's not just me!

 

[livingwaterman]

If the answers are bunched close together for mutiltiple choice in a future problem I suggest avioding the Moody diagram. Use the table or the Swamme Ja eq (page 17.5). Frankly in a test situation with the pressure and everything I would use the table. Remember also they may even be thinking about a different roughness value than you are as well because sometimes they are given as a range.

 

[livingwaterman]

Forgot to say this but very possible the person calculating it for the book used a Moody diagram and was a little off creating this error.