intermittent high velocities during fire flows 2

[foley6]

I have a project that was supposed to have a 16" watermain in front of the development. It turns out that the watermain is under a major roadway and is really deep. There is a hydrant in front of the development that we are considering taking
off and extending a 10" main into the development. The hydrant line is 6" in diameter, 20 ft long and connects to the 16" main under the road. The pressures and flows from the flow test will produce a 20 fps velocity through the 6" main ONLY during fire flows. Is the high velocity in the 6" main a major concern since it's just for fire flows, it's a short run and there are no bends?

 

[RWF7437]

It is very unlikely the water system owner will allow you to do what you propose. Have you asked them? Water mains are usually designed to operate at around 4-5 ft/sec. Some agencies may allow up to 10 ft/sec during fire flows but 20 ft/sec is not only uneconomical it can lead to serious problems with water hammer.

 

[foley6]

Yes, we have spoken with the water owner and they approved it.

 

[RWF7437]

AMAZING!

 

[dicksewerrat]

What happens to the hydrant you took out? Are you going to install more hydrants in your development?

Richard A. Cornelius, P.E.

http://WWW.amlinereast.com

 

[bimr]

APPENDIX I
LIMITING WATER VELOCITIES IN THERMOPLASTICS PIPING SYSTEMS
The maximum water velocity in a thermoplastic piping system depends on the specific details of the system, the character of the flow stream, and the system operating conditions. In general, design velocities of 5 to 10 feet per second are being used and are considered normal.
Higher flow velocities are common in certain applications including gravity and slurry flow. However, in all instances, careful consideration should be given to the effect that flow velocity will have on overall piping system performance in light of valve, pump, and system operation. Particular attention should be given to possible effects of excessive velocity on pipe abrasion rate
and on pressure surges that may be generated by sudden or rapid changes in flow velocity. Recommendations for pressure surge design, which are given in design standards or offered by piping manufacturers, should be followed.
In the case of a polyethylene piping system, the working pressure of the system plus recurrent surge pressure associated with a specific piping arrangement or operation should not exceed 150% of the pipe pressure rating. Occasional surge pressures in excess of this limit are allowable so long as the total of the expected surge plus the working pressure of the system does not exceed 200% of the pipe pressure rating.

AWWA Standard for Polyethylene (PE) Pressure Pipe and Fittings, 4 in. through 63 in., for Water
Distribution, C-906, American Water Works Association, Denver, CO, 1990.

http://www.plasticpipe.org/pdf/tr-14_waterflow_characteristics.pdf

 

[foley6]

Yes, we're going to relocate the existing hydrant and add some more in the development.

 

[RWF7437]

And just how many hydrants might you be adding ?
What are the required fire flows for what duration for you development ?
Will the ten inch line be looped back to the 16-inch line or to some other line or lines in the existing distribution system ?
Are there significant elevation differences across your development ?
If the six inch line failed, say because of water hammer, what would be the consequences ?

 

[foley6]

At least 6 hydrants will be added.
The required fire flow is 1500 gpm.
No, the line will not be looped back to the existing main.
There are no significant elevation changes.
The development would be without water until the main is fixed.

Thanks

 

[sjohns4]

Since it's a short run (less than a full joint) and if it has good rodding or restrained joint DIP I think I'd be OK with it. I worked on a pump station that had a short piece of 12" DIP operating around 20 FPS, it was only a few feet in length but was above grade and flanged.

Restraint would be the key. I'd have a good thrust collar poured at the transition from the existing 6" to the 10".

Did you actually do a flow test from the hydrant to verify you will get your 1500 GPM? If so you've already tested the line at that kind of velocity. Most if not all hydrants are connected to the mains with 6" pipe, so it's not really that uncommon to scream water through a short 6" run.

 

[foley6]

yes..the hydrant was tested and we got 1,250 gpm out of one eye. All of the joints will be restrained with megalugs and a concrete collar will be poured at the 6" to 10" transition. All of the valves on this main are gate valves that close pretty slowly.

Thanks

 

[bimr]

The velociy of 20 ft/sec would indicate that your fire flow is 1750 gpm. How did you arrive at that flow?

The needed fire flow is usually developed by the ISO or the fire chief and depends on the type of building and the building size.

http://www.isomitigation.com/downloads/ppc3001.pdf

You have not stated what type of development that you have. If your development consists of a Walmart, you will need 3000 gpm.

You need to be sure that you design for the correct fire flow.

 

[foley6]

You're right the flow is approx. 17 ft/sec. The development is an apartment complex and we the used the 2006 IFC requirements for the fire flow. The construction type is V-A, the buildings are sprinkled and around 75,000 sf for each building.

 

[foley6]

I need to clarify that the buildings are four stories and each floor is 18,750 sf.

 

[bimr]

According to the ISO requirement in Chapter 6, you need only 1000 gpm for a sprinklered apartment building.

That would only be around 11 feet/sec fire flow.

 

[stookeyfpe]

Actually, read the IFC a little closer. A minimum fire flow of 1,500 GPM is required. The ISO requirement is not applicable unless it has been specifically adopted by the jurisdiction.

 

[bimr]

Good point. Any Code is not applicable unless formally adopted.

Online Link to the California Fire Code (based on IFC):

http://publicecodes.citation.com/st...t_b300v07_appb_sec005.htm?bu=CA-P-2007-999999

 

[rconner]

While I suspect there have been cases where much higher flow velocities have apparently been accommodated without incident, at least with ductile iron piping and sufficiently strong restraints, it is probably best to limit velocities where possible to no more than say 14 fps (that one can see is suggested by DIPRA in the FAQS-Hydraulics one can read at

http://www.dipra.org/

). That being said, and while I have no idea of any practical applicability to the current inquiry, I am aware that at least some hydrant manufacturers e.g.

http://www.acipco.com/afc/hydrants/

have the ability to furnish 8” (~200mm) diameter base connections for this size lead piping in high flow areas, and that much reduces velocity in the lead piping (effects can incidentally be observed in Table No. 17-1 at

http://www.acipco.com/adip/products/Sect17.pdf

or independent calculations with actual pipe involved etc.) Of course, 8" lead piping might make little sense if it had to be connected in other applications to smaller mainline piping.
I hope this information is of help in this or at least similar future issues.