Calculating Peak flows

[swazimatt]

I would just like to get some clarification on calculating peak flows in a residential township.

the average annual daily demand is 1200l/d

now if the instantaneous peak factor of 5 is used, does this mean that the flow rate at peak demand will be:

1200x5/(24x60x60)=0.09 l/s

seems a bit low, should i assume that the water usage will only be for 12hours instead of the 24 that i used for the calc?
thanks

 

[swazimatt]

this is at each plot/erf. I will also need to design for fire flows of 20 l/s

 

[bimr]

The peak flow is an hourly flow. The peak hour flow divided by the peaking factor is the average daily flow.

The peaking factor is based on population. The larger the population, the smaller the peaking factor.

 

[CrazyHorse81]

What is instantaneous flow being used for in your case? Instantaneous flow is really a pie in the sky number that may happen but only for a split second, instantly. The reason I ask is because our DEQ requires that the hydropneumatic tank be sized for 20 minutes of instantaneous flow - they have a chart that identifies the instantaneous flow and looks kind of like this...

# Services IF (gpm)
1 20
2 22.1
... ...
50 105.9
et cetera

So for a new subdivision with 50 homes the hydropneumatic tank would be 105.9 gpm * 20 min = 2118 gallons minimally. Average Day Demand is 24 hrs/day 365 days/year
and Average Day Demand of the Peak Month is 24 hrs/day for the month of most use like June (in the northern hemisphere) and Peak Hourly Demand may happen once or twice per day not lasting for more than an hour like at 6 a.m. and 6 p.m.

Each flow scenario has its purpose in designing particular water system components.

 

[bimr]

You need to assume a typical water demand of 378 liters per day for a project in the industrialized countries.

Where did you get 1200 l/d?

 

[cvg]

378 liters/day may be valid demand per person, 1200 per household (which I assume is a plot/erf). What is an erf?

 

[swazimatt]

thanks, seems like i was on the right track.

yes 1200 is the household demand as specified for that class of township by the South African standards. I have no idea where the word erf comes from but it basically means a plot or stand in a township development

 

[gerhardl]

I could not resist perhaps an un-necessary comment:

If you start your thinking at the extreme firefighting condition, you will have to first sketch your distribution net based on this, and determine and locate your points for firefighting support in the net, and how many points simultanously, and what speed/(pipeline size)/pressure combination you need to support this situation.

On top of this comes the evaluation you have presented here:
average use and top use of a group of housholds, combined with considerations of how many housholds per firefighting post/station (10?-20?-100?), the statistical possibillity of firefighting demand from several/all firefighting posts at the same time, and the statistical average consumption on top of firefighting, or possibillity to close down parts of the system while firefighting.

I am sure you already are aware of this, my point is that
even if you maximize pipeline speed for firewater, you will cover the peak-performance for a considerable number of homes within the pipeline size needed for firewater.

I doubt the system would be dimensioned for longtime peak performance consumption at top of max firefighting.

The philosophy for firewater solutions will probably be dominating large parts of your system, and if the lifetime of the system is 25 - 50 years, how is the possible increase in water usage handeled in the consideration?

 

[jgailla]

swazimatt,
You can check the U.S. military Unified Facilities Criteria for a methodology in determining peak and average flows and storage volume required.

It won't apply specifically to your situation and it's not in SI units, but the overview of the methodology may be helpful, as well as checking whether your numbers are reasonable.

Try 3-230-03A, 04-A, 07A, 09A, and 10A at:

http://www.wbdg.org/ccb/browse_cat.php?o=29&c=4

In general, your fire flow will control the size of your mains. The laterals are typically a standard size (I think 1.5") which are a little oversized by calculation.