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PE Problem #72

9/12/2018

10 Comments

 
What is the foam friction loss (in head per 100 ft) and flow velocity for a 4:1 expansion foam flowing 130 gpm in a 2-1/2" schedule 40 pipe (with a c-factor of 100) most nearly?
a. 4.5 ft head/100 ft pipe and 4.75 ft/s
b. 8 ft head/100 ft pipe and 4.75 ft/s
c. 10 ft head/100 ft pipe and 4.5 ft/s
d. 30 ft head/100 ft pipe and 5.5 ft/s"

Solution | Posted 09/13/18
So much for trying to stump you all. As several people point out in the comments, the tables used to write this problem is not in the required reference material, so this problem needs to be scratched. NFPA 11 does address expanded foam velocities (see NFPA 11-2016 Figure A.5.2.6.2(a) on page 11-54).

On a side note, had the foam been a 1:1 expansion and Hazen-Williams been applicable, it could be solved by the Hazen-Williams formula or via the Hazen-Williams nomograph of page 1394 in the 5th Edition of SFPE.

Great discussion and feedback, keep up the great work everyone.

​                                      - Joe
10 Comments
Colleen
9/12/2018 11:08:02 am

I'm really confused on how to solve this...

when I used Hazen-Williams (inside diameter or 2.469"), I got 0.0900 psi/1ft, then multiplying by 2.317 ft per 1 psi, and 100, I got 20.858 ft of head/100 ft...so I guess I would guess either C or D??

Did anyone else figure this out? I've never done a calculation specific to foam systems.

Reply
KC
9/12/2018 11:09:45 am

A. Used figure 4-5.A-1 in SFPE, 4th. Not sure if that's in the 5th. Meyer 18, pg 272 solution references 4th, but not 5th.

Reply
John Frank
9/12/2018 02:11:46 pm

Colleen, you cannot use HW for this. KC found the correct figure in HFPE 4 but it is not in HFPE 5. HFPE 5 says to consult NFPA 11 for the figure. Therefore it cannot be directly answered with HFPE 5. However, the fact that HW or Darcy cannot be used is still important to know. The figure in NFPA 11 (the same one KC found in HPFE 4) is the only want to do it that I know of. I have never seen an actual equation. This is only done with subsurface injection and forcing foam makers where you have expanded foam flowing through pipes. Unexpanded (1:1) foam solution (by far the most common situation) uses HW and foam concentrate (not mixed with water) uses Darcy or manufacturer's charts.

Reply
Colleen
9/12/2018 02:34:09 pm

Thank you for the explanation! That explains why I was unable to solve the problem.

Reply
John Frank
9/12/2018 02:45:54 pm

If anyone wants real photos of what these systems look like, e-mail me at john.frank@xlcatlin.com. I can't paste a photo into the comment block.

Reply
Brian Jarvis
9/12/2018 03:54:06 pm

Hello JOHN,
Good to see you are helping us on here, thank you.

Reply
GD
9/12/2018 03:21:12 pm

A per SFPE Handbook Ph. 4-151 (4th edition)

I doubt this question would be on the test since I don't think this figure is in the 5th edition

Reply
MHK
9/12/2018 07:26:58 pm

Clause 10.7.1 of NFPA 11 (2016 ed.) says that foam solution shall be considered to have the same hydraulic characteristics as water. Since the expansion ratio 4:1 is in the rage of low expansion, I guess the caluse 10.7.1 can be valid for the problem. By the way, I simply calculated the velocity as V=Q/A, and this brought me with 8.72 ft/s, which seems not close to any of the choices. Can anyone tell me how to calculate the velocity?

Reply
Pari
9/13/2018 12:44:01 am

I have a hard time even with the mention figure to get the right answer
Foam velocity is in p 4-152 sfpe book but is not easy to find the answer
Probably with higher flow and lager pipe sizing these figures should be useful.

Reply
John Sternitzke link
9/15/2018 12:23:24 pm

Hi Joe, would somebody be able to send me (i dont have SFPE 4) a copy of Appendix figure 4-5.A.1, p-451 and 4-5.A.3 p4-152, I have NFPA 11, I read John Frank comment, a I just want this if it is not too much trouble.

Thanks.

Reply



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  • Blog
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