municipal water line extension for subdivision doesn't add up 3

[hopeengr]

I am working on my first water line extension project. It is a small extension serving 11 homes in a subdivision of approximately 12 acres total. I have been told my local officials that I need to consider 3 cases: 1)static or low flow must have 30 psi at each meter 2) average daily demand flow; and 3) peak demand plus fire flow demand must have 20 psi throughout entire distribution system. Is this the usual process? I read in a text that statistically you will not have a peak demand at the same time as a fire demand (how many people are going to be taking showers and washing cars while their neighbor's house is on fire?).

Also, let me verify that my methodology is correct. I am using Bernoulli's energy equation with Hazen-Williams headloss equation and the continuity equation (flow in equals flow out). Am I on the right track? Next, I have been using the pipe dimension to solve my velocity at each node (transitions and meter locations). With the static pressure, velocity and elevation known at the beginning and the calculated velocity, and known elevation at the node, I am calculating the pressure in the pipe just before the location of the meter. This gives me a pressure that I compare to the minimum required residual pressure, correct? As I progress through the system, I reduce the flow by the demand at each node (meter). I might add that this is not a closed loop system. It is essentially a y configuration.

Thanks for any insight you may have. I have the academic background, but converting that to a real world application has been frustrating.

 

[hopeengr]

Correction to original post: Actually, I have been using the Darcy Equation (not Hazen Williams) to calculate the headloss due to friction. Also, I have been neglecting minor losses.

 

[cvg]

you could save yourself a lot of headache by investing in software to perform a water network analysis. There are free models out there so the cost really is minimal.

I would guess that you are using peak daily demand plus fire flow which is commonly required. However, you site the case of peak hour plus fire flow which is not so common.

Statistically, fires happen any time of the day, and possibly more likely to happen when people are at home, using lots of water. For example, I watched a fire on the news this morning which occured while my wife and daughter were both in the shower. So, fire flow plus peak demand in my opinion is quite possible.

Hazen Williams is probably more commonly used than Darcy for this type of analysis

 

[hopeengr]

thanks for your reply. I recently downloaded EPAnet, free software for network analysis, but haven't quite mastered the details of it yet. I had already set up a spreadsheet to run my calculations. I should clarify that I am not actually designing this system but attempting to justify another engineer's design. I am not sure the system is adequate but I don't what to point fingers at the other engineer. I am trying to make sure that I haven't overlooked anything before I make a final decision.

Your statement of peak daily vs peak hourly makes sense, but the table of peak demand I am required to use is in gpm per home and doesn't really clarify if it is based on an hourly or daily peak. I'll just have to go with what they have given me to calculate, even though it seems to be too much for the system as is.

Thanks again.

p.s. Darcy is what I am using and sited HazenWilliams in error (due to sleep deprivation)!

 

[francesca]

If the developer is being reimbursed by the utility company, then you should consider future demand that may connect to the network. If it's a growing area and the neighboring property looks like a likely candidate for a subdivision, for instance, then the developer should lay pipe that will meet future demand, not just imminent demand.

 

[rconner]

I think francesca has made a great post including a point that may arguably have been overlooked in many areas in the past (I plunked a "star" down next to this one!). A lot of small pipe appears nowadays being replaced, in some cases I suspect at even much greater cost due to having to replace/patch much other development/infrastructure built over/around the lines since, mostly due to inability to deliver/satisfy current flow needs. I suspect this particularly includes a great deal of pipe <=4"(100mm) size that may not have been satisfactory/considered in the beginning for adequate fire protection.
This is of course a delicate balancing act, as the initial installed cost of larger pipe costs at least a little more and may require a little more cleaning/flushing etc. at least in its early life due to low flows etc. There are perhaps similar parallels in wastewater and hydrology etc. ("100-year" floods seem to be happening a little more frequently nowadays, due in at least some cases to clearing/development etc. that was also not anticipated/provided for??)

 

[hydrae]

"but the table of peak demand I am required to use is in gpm per home and doesn't really clarify if it is based on an hourly or daily peak. I'll just have to go with what they have given me to calculate, even though it seems to be too much for the system as is."

Is that the chart from APWA? if so I have used it as a peak hour, and it follows well in the peak hour of the year.
As for is it a reasonable number? think hottest hour on hottest day, when is that 4th of july with fireworks...


Hydrae

 

[cvg]

peak demand isn't necessarily affected by heat. In the arid regions of the country, this might be more likely due to irrigation, however in the more humid regions, it is probably more closely related to domestic use in the house such as showers, baths, clothes and dishwashing and cooking / food preparation.

 

[hopeengr]

hyrae: the chart was not taken from APWA, at least not directly. The chart I refer to is in the section of our state regulations concerning water line permiting requirements.

 

[biglil]

Have you researched whether the owner has any existing engineering design report for the system you're modifying or water master plan? These sources, as well as the Owner's current Engineering Design Standards should document whether "hourly or daily peak" is used.