Maximum Flow Through 6" Underground?

10/23/2019

What is the max gpm you can get with a 6” dr 18 underground?

We had a flow test of static 68 psi residual 48 psi flowing 1950 gpm. With 3111 gpm @ 20 psi. Some one is telling us the max gpm is 800 gpm. How is this possible?

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7 Comments

10/23/2019 08:39:02 am

The easiest way to explain available flow is using Freeman's formula. Q=29.84*c*(d^2)*sqrt(p) where p is the pitot pressure and c is a coefficient related to the outlet. This formula was likely used to calculate the 1950 gpm you were given from your flow test.

6" underground can flow significantly more water than 800 gpm, although it is a little more complicated than just giving a max gpm. Flow tests evaluate friction loss in the pipe system and velocity pressure. Your water available curve showing 3111 gpm at 20 psi sounds reasonable.

Annex C of NFPA 24 gives a very in depth explanation of fire flow testing. It also has a table that show theoretical discharges through circular orifices. Perhaps sharing this annex with the individual would help them learn about hydraulics.

CRB

10/23/2019 09:07:26 am

The 800 gpm might be coming from the manufacture specification.

HDPE underground piping has velocity limits and below is one manufacture specification for class 200:

For 150 psi operating pressure (working pressure), allowable sudden velocity change for "occasional surge pressure" condition is 15.49 ft/sec (calculated).

Others manufactures state 10 ft/sec (calculated) for normal operation.

10/23/2019 10:30:27 am

NFPA 291 is the Recommended Practice for Flow Flow Testing and Marking of Hydrants. Chapter 4 probably has the information you are looking for.

Cliff

10/23/2019 12:23:10 pm

Sounds like someone is confusing the flushing requirements with the flow capacity. NFPA requires you to flush 6" underground pipe by flowing a minimum of 880 gpm.

Kelly

10/23/2019 01:49:51 pm

Came here to say this. I think that is probably where it came from.

Dennis R

10/28/2019 10:16:10 am

AHJ's around here get this confused all the time!

Franck Orset

10/30/2019 10:42:04 am

Fully agree with what was said.

I addition:

The only limitation is related to the friction losses.
In 6" pipes, you have approx. a loss of 2.5 psi by 100 ft with a flow of 800 gpm (using the Hazen Williams formula for friction losses). With 3000 gpm, the pressure loss is approx. 28 psi by 100 ft.
So depending on the length of your pipe and your initial pressure (static - no flow), you will end up with a residual pressure that will greatly depend on your flow.

It then all depends on what are your minimum requirements in terms of flow/pressure for your needs.

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