Fire hydrant standard

[ceseng]

get a question:

Can fire hydrant be installed on 4-in water line? all the fire marshall regulated that it shall be installed on at least 6-in water line.

Some of my friends told me that it used to be OK to install FH on 4-in water line.

can anyone give me a more detailed introduction? Thanks.

 

[rconner]

While I don't doubt there are some "hydrants" on smaller water lines out there, I believe AWWA Manual M31, "Distribution System Requirements for Fire Protection" states under a table heading "Typical Minimum Values", for "Smallest pipes in networks" as "6 in.", and for "Smallest branching pipes(dead ends) "8 in.". There are many other references and organizations with requirements that dictate in many cases (incidentally I think with many engineering and other aspects to consider) fire flow far greater than the flow that can be comfortably achieved in any four inch piping!

 

[bimr]

Older communities may still have some smaller piping 4" in size that was installed many years ago.

Modern water systems are installed with a minimum size of 8" piping. Fire flow is typically a minimum of 1500 gpm and i susually a larger flow depending on the application (residential/commercial).

 

[jartgo]

I have to disagree with bimr on the minimum size. I've designed many water lines of 6 inches, admittedly small towns or rural areas. The rule will likely vary with your local regulators. Where I'm at, fire hydrants don't go on lines smaller than 6 inches. In addition, if your line (no matter the size) can't support 500gpm at 20psi residual, you can't put a fire hydrant on the line, blowoff hydrants only.

 

[bimr]

The minimum size of water main which provides for fire protection and serving fire hydrants shall be 6 inches in diameter. Larger main are generally required when it is necessary to allow the withdrawal of the required fire flow while maintaining the minimum residual pressure of 20 psi.

Most insurance companies and rating agencies typically require a fire flow of a minimum of 1500 gpm or higher depending on the building zoning mix (residential and or commercial).

That is why modern water systems are installed with a minimum size of 8" piping. It is difficult to get the required fire flow in a 6" water main.

To answer your question, 4 inch pipe is too small for a fire hydrant.

Since the difference in cost between 6” and 8” in minimal, it is recommended to use 8” minimum size of pipe.

 

[civilman72]

I agree that a 6" water main is the smallest you can reasonably use to maintain fire flows and for FH connections. Plus, most all fire hydrant assemblies come with a 6" main connection.

If you actually perform a fire flow analysis for your system, and are trying to use a 6" main, you will usually exceed the maximum allowable velocity, and/or will have so much head loss that you will not be able maintain the minimum residual pressures during a fire flow analysis. It only gets worse if you run the same analysis when using a 4" main.

 

[tomwalz]

The NFPA group here is really good on this sort of thing.

Thomas J. Walz
Carbide Processors, Inc.

http://www.carbideprocessors.com

Good engineering starts with a Grainger Catalog.

 

[stookeyfpe]

NFPA 24 permits it. However, the IFC requires a minimum fire flow of 1500 GPM @ 20 PSIG for commerical buildings. Your hydraulic analysis and site geometry will most likely drive the pipe diameter.

 

[rconner]

For at least many years I believe NFPA 24 contained the requirement,

"Public mains shall be of ample size, in no case smaller than 6 in. (152 mm)."

Has this been somehow changed to allow smaller pipes?

 

[786392]

I don't know exactly the point of difference;

However in the Petroleum Refining set-up I served

Witnessed the basic hydrant's design; almost invariably the

6" dia pipe was used having one 4" dia take-off and two 2.5" dia take-offs

Near the capped end.(Pillar Type were mostly employed)

Hope this helps

Best Regards
Qalander(Chem)

 

[cvg]

hydrant nozzle size has very little to do with the capacity of the hydrant to deliver fireflow. Remember, the fire department may hook up 3 hoses to one hydrant and run them all as needed. If a new line is being installed, it should meet NFPA code with a 6 inch line minimum. If an existing 4" watermain, then I could consult with the fire marshal to get a ruling. Water system network analysis is generally required to verify the capacity.

 

[786392]

Ring Main Pipeline sizes were all much more than 8"dia.

even from 20" dia varying to 10"dia designed to cater Flowrate needs;

however the hydrants were almost always fabricated with 6" dia pipe.

If I recall correctly.

Best Regards
Qalander(Chem)