Max. flow in a 4" fire feed main (below ground) for a combined system

[MYHVAC]

I need to supply 800 GPM (540 GPM to sprinklers and 250 GPM hose allowance)to a combination fire protection system. Can 4" line be used between street main and a riser? System's riser was sized to be 4". NFPA 13, section 8-2.1 requires a flow rate no less than 390 GPM thru 4" line to obtain at least 10 ft/s velocity, but it does not set an upper limit. Does anyone know what it is?
I know that this rate of flow thru 4" pipe will generate 20.2 ft./s velocity and pressure loss of 0.18 psi/100 ft. of pipe. I wonder if 4" line can be used with these flow parameters, assuming of course, that it will deliver required GPM at required min. pressure. Thx!

 

[DavidCR]

I don´t know about the maximum speed limit or recomended value, I´ve wondered about it also, and if you find it, please post it here.

I can only add some related points.

On NFPA 14, 5-6 indicates minimun sizes for standpipes. For example, for combined systems (5-6.2) the minimun size is 6" or 4" if the system is hidraulucally calculated. (For class I or II it is 4 in.).

Check also NFPA 24, 7-6.1.

Also on NFPA 20 they allow a maximum speed on the pump discharge of 20 ft/s (6.2 m/s).

Hope this helps.

DavidCR

 

[MYHVAC]

David,
Thanks for the references you listed in your response. I'll look them up. See the answer below sent to me by one of the FP experts, Mark Bromann, who publishes in PM Engineer Magazine.

I don't know where this is actually written in the code, but the NFPA benchmark for maximum water velocity is 32 fps, be it in the main, riser, branch lines, or underground piping supply. Several insurance companies, most notably Factory Mutual, do not permit a water velocity exceeding 20 fps in any one piece of piping throughout the system, reasoning that when water travels that fast inside the piping, it "bullets" with the water that hugs the pipe wall "clinging" at a slower rate. This throws off the accuracy of the calculations. It's a very fine point, but as I said, certain insurance companies adhere to the 20 fps maximum, and will reject calculations that show those higher velocities. It's easy to check - they look for the lines that contain a very high psi/ft. friction loss factor (over .50), and then they calculate the velocity pressure for that particular GPM flow vs. the inside pipe diameter.

 

[Mikhail]

I have read the question and the answers and i have the folowing comments:
1. If the system is fully sprinklered you donot have to add the flow of the sprinkler and the hose allowance, you just take the larger, (NFPA 14 Section 5-9.1.3.1) in your case it will be 540 gpm.
2.The minimum velocity listed in the pipes is not for the design of the system, but rather for flushing in the system (NFPA 13 Section 10.10.2)
3.The 20 fps limit on the discharge of the pump is applicable if the pump will be operating at 150 % of its rated capacity. (NFPA 20 Section A-2-10.13)

As for the maximum velocity in the pipes the answer you got sounds very reasonable but i tried to find the 20 fps limit in any of the NFPA publications but i failed. If you find it please post it here because i am working in projects in the middle east where we follow NFPA and i donot think the insurance companies regulations is of importance here.

 

[Caryatid]

The 1989 edition of NFPA (page 334 of the handbook edition with the brown interpretation text) noted that "While this standard does not stipulate a maximum limit on velocities, some insurance companies modify this standard and incorporate such limits. Twenty feet per second (6.1 m/sec) maximum velocities may result in superior designs. At velocities beyond this, the Hazen-Williams formula is not as conservative as other methods, such as Darcy-Weisbach.

It is not a "rule" it is more prudent/conservative engineering practice.

 

[Slowzuki]

Re: the hose and sprinkler allowances, you can only leave out the hose demand in fully sprinklered light and I think ordinary hazzard.

The 20 fps (we use 26 fps) limit is to try and enhance the accuracy of the Hazen-Williams formula. It isn't a hard rule because it based roughly on Moody diagram and laminar vs turbulent flows. Obviously a 1" pipe at 20 fps isn't gonna have the same friction factor as a 24" pipe at 20 fps.

So if you are designing a long yard pipeline, you may want to use Darcy Weibach which will always give more accurate results.

The other often quoted reason for max velocities is due to water hammer and limiting pipe reactions at turns. Again, the fps should really be different for each size of pipe to limit these but one number is easier for people.
Ken

 

[kieffer]

The 20 fps is no longer used even by most insurance companies. As mentioned in am=nother reply it was formerly in the NFPA 13 handbook for guidance. It is now felt that the friction loss in the pipe is a better design guideline. However, if you get too much over 20 fps you will find that you begin to lose pressure due to friction at too fast a rate to be efficient.

I don't know where you got your friction loss figures but for a pipe with a Hazen Williams C factor of 100 the friction loss in a 4 inch pipe, for a flow of 400 gpm is 68.8 psi per 1000 ft and for 1000 gpm it is 375 psi per 1000 ft. As you can see, for your flows you would lose to much in friction loss to provide adequately for your sprinkler system.

 

[hacksaw]

don't for get the include the fitting losses at these felocities.

a single 4" elbow is equivalent to a considerable length of straight run pipe