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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...
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.
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.
Colleen
9/12/2018 02:34:09 pm
Thank you for the explanation! That explains why I was unable to solve the problem.
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.
Brian Jarvis
9/12/2018 03:54:06 pm
Hello JOHN,
GD
9/12/2018 03:21:12 pm
A per SFPE Handbook Ph. 4-151 (4th edition)
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?
Pari
9/13/2018 12:44:01 am
I have a hard time even with the mention figure to get the right answer 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. Leave a Reply. |
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