Another question about Fire Hydrant Flow

[runoff]

I had posted a question about Fire Hydrant Flow a little while ago but I have another design question. Let me refresh my first post. I have a subdivision that is being designed by a local architect and land developer. The subdivision is small of only 11 homes with no expansion and it will be a culdesac road. From the city, they want the fire hydrant at the end of the road on the culdesac and have told me that my main line in the subdivision will be a 6", which I find annoying. Anyway, their flow test on their 8" maiin was conducted at the top of the road gave me a static pressure of 30 psi and residual pressure of 20 psi at 660 gpm. They did not give me a pitot pressure drop measurement but I can usually estimate that unknown. I have a 50' positive head pressure from the top to the bottom of the road due to elevation changes, which helps in my pressure issues.

However, I ran through my calculations and modeled the flow. The problem is that I am getting mix results. State requirements of residual pressure of 20 psi and 500 gpm at the fire hydrant cannot be met. I have hand calculated a pressure of 13 psi at the fire hydrant which will not meet the state requirements. Does not this pressure on the city's main that we will be tapping into seem a little low? I have usually seen 70-100 PSI on main loop systems. If it is low, what are some good approaches to getting the system pressure boosted without or minimal cost to my client?

Thanks in advance for the feedback.

 

[francesca]

There aren't any.

You might be able to get around the fire flow requirment if you have sufficient residual pressure without the 500 gpm fire flow. If the local fire department has water trucks that they fill to achieve sufficient water pressure you may be able to get away with a color-coded fire hydrant that indicates that the fire department cannot connect their hoses directly to it.

If that option is not available to you then it could well be that your 11 lot subdivision is not feasible.

 

[WILDWINGS27]

Seems to me that your losses are too great in a 6" pipe. I would tell the city that you need a larger water main. Otherwise, what is the point of the fire hydrant if you can not get the propper flow and pressure? I always try to have 1000 gpm w/ a min. residual pressure of 20 lbs. So, to me that system seems low.

 

[TravisMack]

The pressures do seem pretty low. Also, with that low, it would make sense to need a larger main than 6".

What if the developer provides fire sprinklers for the homes in accordance with NFPA 13D in the subdivision. Will the AHJ reduce some of the fire flow demands?

 

[cvg]

if the pressure is only 30 psi at the 8" main, it hardly makes any difference whether you connect with a 6" or larger main, your starting pressure is substandard. For example, I typically design city systems for 40 psi minimum pressure at all times (except for fireflow which is 20). I also agree that 500 gpm fireflow requirement seems low. I have never used less than 1,000 gpm fireflow for a residential area. In addition, 6 inch main size is also substandard in this part of the world and is generally only allowed for short runs to hydrants, not for mains.

 

[runoff]

Thanks CVG. I agree I have always designed fire hydrant flow greater than 1,000 gpm with residual pressure greater than 20 psi. The city I am dealing with is a small community of less than 7,000 people and the subdivision run is about 700 linear feet. Not a really long run. I think I am going to place an 8" on the drawing and meet with the architect, developer and city engineer and let them know that I do not feel comfortable designing the system the way the current pressures have been measured at the site. I have calculated out that with the positive head and the addition of the given pressure that I can get 34 psi at the hydrant but again I usually like to see that around 40 too. I think it is a matter of comfort for the engineer sealing the drawings.

Thanks for all the post they have been very helpful.