Distribution Question

[runoff]

I am working on a water distribution line for a small subdivision of 11 homes. The architect has been working with the municipality and they have asked for a 6" DI line to the fire hydrant. I was planning on designing the line to the fire hydrant with a 2" blow off assembly for dead end systems. The architect has asked me to run the line on past the fire hydrant but what I was taught is that I cannot do that on a dead end system of less than 1,000 foot of run. That can only be done on a loop. Any input????

Thanks for the help.

 

[francesca]

Really, the loop is only a guide for maintaining pressure. If you can show that you have adequate residual pressure at the end of the line then you'll be okay. The 6" diameter will help you because the friction head loss won't be large. Also, if your end is at a lower elevation than your hydrant, it'll help.

You need to calculate your head losses.

 

[cvg]

first of all, I would consider using a larger pipe than 6 inch. Unless this is a very short line, you will have trouble meeting your fireflow requirement. As far as installing a 2 inch blowoff, why can't you use the fire hydrant as your blowoff?

 

[rbcoulter]

I would model the entire system with a software package like EPANET. EPANET can be downloaded free of charge. That is what I did with a subdivision of over 125 homes. You also need to account for other loads on the system, not just the new expansion.

 

[ccor]

Surge pressures can be a problem on long dead-end mains.

 

[francesca]

Oh, and what if the subdivision is extended in a future phase? Is there a masterplan available for the land? Even if there aren't current plans to extend the subdivision, if the land is eminently developable you might want to put in an 8" line, particularly if it's running uphill into your subdivision.

We regularly used fire hydrants as blow offs in the subdivisions I designed. Most of them were on 6" lines and many were not on loops to begin with, but some were connected at a later date when another phase was added to the subdivision. Fortunately the city has its own WaterCAD model and evaluated the system for us, removing (some) liability if the hydrants don't work.

 

[runoff]

Thanks francesca. After completing the friction loss calculations, the 6" will work. I will be applying the pressure down hill and just using the hydrant as a blow off. That was my thought. I could go by the fire hydrant but I think I will be able to 90 off of the main 6" and leave the existing design the same. I spoke to the muncipality about the 8". I am coming off of an 8" and they feel that it would be better for just 11 homes to go with the 6". So, I will comply with their request.

Thanks again for all the responses.