## Designing water reticulation systems

Hi,

Im currently doing an assignment where i have to design a water reticulating system for a small suburb.

I have planned out all my distribution mains and my reticulation mains and services

I also have the water demand(Q) for the houses using the Australian Standards for plumbing and drainage.

So i know my Q value from Australian standards given by the formula:

Q=3.637*10^-5 H^0.555 D^2.667

where:
Q=Flow rate (L/s)
D=pipe diameter

So for example (this is the calcs i did on one of the reticulation mains with a dead end serving 3 houses)
If i have
Q=1.03 L/s
L=50 M
h=1.1

1.03=3.637*10^-5 * 1.46^0.555 * D^2.667
D = 43.16 mm

now using this with V=Q/A i get V to be V=0.709 m/s

Am i doing this right??? because the velocity seems to be very slow(even though the maximum allowable is 3m/s anyway)

and am i going about it the right way? i thought start from the demands from the houses and work backwards to the distribution mains that way i can calculate all the pressures,velocities and flows needed at the entrances of the reticulation mains.

and if this is right, what formula do i use to work out the pressure needed to at the entrance of this pipe to supply this flow and velocity? ive been reading fluid mechanics books all week and i cant seem to crack it.

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 Recognitions: Science Advisor You seem to be on the right track. I didn't check all of your units though. Just by gut feel that velocity doesn't seem too far off. As far as the pressure needed, I would do as you are. You started at the demand and calculated back all of the losses due to fittings, line lengths, etc... You know what pressure you want at the end for the users, now add on the pressure losses due to your piping. That will give you the minimum required at the supply to produce that flow. You can look to a modified version of the Bernoulli equation for help: Look here under pipe flow calcs: http://www.roymech.co.uk/Related/Flu...uids_Pipe.html Also check here: http://odlpc.oum.edu.my/v2/tutorkits...ngineering.ppt
 Hi, Sounds like you are on the right track. I'm actually a graduate mechanical engineer designing water reticulation for a mine site at the moment and can tell you from experience that those sort of velocities are correct. Usually you would want to limit velocities to somewhere between 0.7m/s and 1m/s, this is done to limit head loss due to pipe friction and fittings, because as you no doubt know, head loss is proportional to velocity squared. Also keeping fluid velocity low is beneficial in terms of mitigatin any detrimental effects that may result from water hammer in the lines. On another note, you should probably do your final calculations using standard pipe sizes. The next suitable size to the diameter you indicated is 50mm nominal diameter. Hope this helps: P.S. I am interested as to which Australian Standard you found the equation you noted Q=3.637*10^-5 H^0.555 D^2.667

## Designing water reticulation systems

Sorry,

Missed the last part of your query. For pressure requirements, you should be investigating the most hyrdaulically disadvantage building in your network, in this case, the building farthest away. Usually city councils reccomend that around 200kPa - 220kPa is more than suitable to supply water amenties at a residential household, so getting this pressure to your most hydraulically disadvantaged facility is your goal after taking into account the pressure losses involved in getting the water to its location.

 Quote by James3849 P.S. I am interested as to which Australian Standard you found the equation you noted Q=3.637*10^-5 H^0.555 D^2.667
Its from the AS 3500 Plumbing & Drainage Standards, the reason im sort of worried about that formula is it says its for rapid sizing of pipes for residential areas.

i also just realized i cant use that formula because
H = h*100/L*1.5

and h is head loss which i cant work out without V or D
 There are no equations that will take you step by step through the design of a reticulation system because obviously water demand varies. The equations given in the standard are just guides for you to get your head around what sort of numbers you should be getting. My suggestion is to make a spreadsheet. If you are only supply 3 households, you should design your piping to accomodate the maximum expected demand for the houses plus a little bit extra (imagine all showers, taps are running and toilets flushing). AS 3500 will give you typical flow rates for showers and the like, but also gives you probable simultaneous demand for multiple dwellings in table 3.2. (3 dwellings = 0.88L/s). Therefore you should design your piping for 1.5L/s for example. With this flowrate you can select an appropriate pipe diameter that will limit your fluid velocity. With this velocity you can then calculate head loss due to friction and fittings, and if these losses are too great, tweek your pipe size a little until it is okay. Don't forget to use standard pipe sizes and take into account any change in elevation too (especially if the fluid has to travel uphill). Hope this helps
 yes it does help. I sort of figured out how to do it, made a spread sheet but then realized i had estimated my demand wrong. We have estimated each household uses 42.5 L/hr during the peak hour(8-9am). now the new problem i have is working out the flow rate in m^3/s because 42.5 L/hr is 0.0000118 m^3/s and iam convinced it isnt as easy as converting 42.5 to m^3/s(and because the pipes turn out to be absolutley tiny) but if i convert that to l/s i get 0.0118 l/s and my pipe sizes turn out pretty decent for 20 houses in 1 street with pressure losses turn out to be DN 350 as opposed to DN 600 we were getting earlier. because in Q=VA Q is m^3/s correct? or can i use l/s for Q?