Not meeting minimum fire flows

[SkinnyDip]

I am doing a small waterline extension to serve a development in a rural town. The town code requires fire flows of 1000 gpm. The two nearest hydrants were tested and gave 650 gpm and 750 gpm. Do towns usually let you bend the rules since their system does not provide required flow? If not, what options do we have? Upgrade their system? Pumps? Also, fire chief gave me hydrant data of 52psi static, 34 psi residual, and 650gpm at 15psi. What is the 34psi residual? I thought you need to have a residual pressure at a flow. I usually model system as a pump curve (52psi,0gpm 34psi,Xgpm 15psi,650gpm). How do I use this data? Is this typically all the information you get? Thanks.

 

[coloeng]

Yes, there should be a flow associated with the 34 psi residual. If the town's existing system doesn't even provide 1000 gpm, typically it should be with a 20 psi residual, how do they expect new development to be able to provide it?

Without knowing anything about the town's existing system it would be difficult to make specific recommendations on what options you have. In order for a pump to work you would need to have the pump station at a location where you would be able to get at least a 1000 gpm flow while maintaining a residual of 20psi. You might have to provide some additional storage, either elevated or ground storage with a pump station. You may need to upgrade some water mains in the area of town where the new subdivision is. Depending on if the town is desiring the growth, and if they have the funds available, they may be willing to help pay the costs of any upgrades Or, as you suggested, ask the town if they will relax the requirement.

 

[bimr]

Fixing the town's water problems will depend on the relative size of your devleopment. If you are putting in 1000 lots, the town would probably want a contribution. If you are doing 3 lots, then the town should fix the situation or not approve your project.

Talk to the City Engineer and see what they have done in the past.

 

[jgailla]

I was involved in a similar situation project a few years ago. We could not get the city-required fire flow at the hydrant because the existing line was too weak.

We wrote a letter to the city stating our problem and they replied with a letter allowing the lower flow. The project was a sprinklered 50,000 square foot one story office building, so we didn't have an issue with NFPA, just the city requirement. Also, we had something like 1400 gpm at 20 psi when the requirement was 1500 gpm at 20 psi, so we were close.

You definitely would need a flow rate to go with the 34 psi residual, but I'm used to only getting one set of residual/flow numbers, not two.

Concur with coloeng on the 20 psi residual. I've never seen any residual lower than 20 psi be used to calc the flow.

If this is a residential development, you should be alright with the flow for NFPA, so this becomes a political issue with the town rather than a life safety code issue.

 

[RWF7437]

Confusing. Ask the Fire Chief to give you a copy of the actual hydrant flow test report. That may clear up the confusion.

 

[655321]

I did a subdivision several years ago in a rural area where the county water district couldn't supply fire flows. The fire department let us install underground tanks with dry hydrants that trucks could pump from. The tanks had a float valve in them to keep full.

 

[jartgo]

The 34psi residual, might be the residual pressure at another hydrant, although I wouldn't expect the flowing hydrant to be that much lower (15psi) than the second hydrant.

 

[stookeyfpe]

Since the building is sprinklered the discussion should focus on the efficacy of the automatic sprinkler system. I don't know what your building's occupancy is but we find in about 85% of all fires in sprinklered buildings that the fire is generally controlled with 5 sprinklers or less in over 90% of the fires. If the system is properly designed and maintained the fire service will essentially be performing salvage and overhaul as a result of the fire and the sprinkler's operation.

Talk to the City Engineer and the Fire Chief - their response needs to be coordinated.

 

[cvg]

stookeyfpe - who said anything about sprinklers or buildings?

 

[kwdwaterengineer]

Skinnydip,

You have been given some poorly presented data. If you have a copy of NFPA 291, equasion a) is for flow from a hydrant: Q=29.83*c*d^2*p^0.5
where: Q=flow in gpm
c=hydrant coefficient (0.90 for smooth & rounded outlet)
d= hydrant nozzle i.d.in inches(usually 2.5)
p= pitot pressure (or opposite cap) in psi

If you use c=0.90 and plug in the 15 psi and 2.5" nozzle, you get a flow of 650 gpm.

When they did the test, the static pressure was 52 psi, and the residual dropped to 34 psi during the test. Now you can use 52 psi for 0 gpm and 34 psi for 650 gpm for your modeling if that is the method needed. With this data, the flow at 20 psi 885 gpm. Other data that you are missing is if any pumps were running (and how many depending on the system) and what the tank level was when the test was done. I would request that the test be redone, preferably with some calibrated gauges and knowing the tank & pump status. If all else fails, I would apeal to the reviewer rather than have the client face an expence for upgrades.

Hopefully this in not always typical of what others are giving out for data.

 

[SkinnyDip]

Thanks a lot ya'll. I just got a call from somebody else in the town and they said they recorded a flow of about 850 gpm. Still not all the info I need but I am thinking KWDWaterEngineer is on target. I am going to wait to get more clarification before I go any farther with this.