Optimizing Water Supply System Design for High Demand Scenarios

[jasonyp]

can anyone tell me how to find the pressure head and residual head?

the Q is depend on which?

 

[Topher925]

Find the pressure head and residual head of what?

 

[jasonyp]

thx for the reply
i am design a water supply system for a small development about 82 unit of houses
in order to design the pipe , we need to find out the pressure/ hydraulic head or residual head , and also the flow rate right
i think i got the flow rate from the water demand about 330 gallon/days
emmm,
would u mind giving me some suggestion?
thx

 

[Mech_Engineer]

330 gallons per day seems very low for 82 houses (that's an average of 4 gallons per day per house). I would guess that with showers, dishes, and everything else each house could go through 25 gallons easy, which works out to over 2000 gallons per day for the neighborhood. Not to mention landscaping or other water-intensive tasks like washing a car.

As for pressure head, you need to know the difference in altitude between the water supply and the neighborhood.

 

[minger]

330 gallons per day seems very low for 82 houses (that's an average of 4 gallons per day per house)

Hell I flush more than 4 gal a day.

 

[Bob S]

Typical suburban indoor water use in the U.S. is 65 gpcd (gallons per capita per day) so a typical house indoor usage for 3 people would be 200 gpd (gallons per day). Including outdoor for garden would easily be 300 gpd. Static head pressure at homes is probably 50 psi (pounds per square inch) minimum to 75 psi maximum. You should check these numbers with local utility. Is this water gravity fed from a water tank, or is this a pressurized system with a pneumatic ballast tank?

 

[Mech_Engineer]

Typical suburban indoor water use in the U.S. is 65 gpcd (gallons per capita per day) so a typical house indoor usage for 3 people would be 200 gpd (gallons per day). Including outdoor for garden would easily be 300 gpd. Static head pressure at homes is probably 50 psi (pounds per square inch) minimum to 75 psi maximum. You should check these numbers with local utility. Is this water gravity fed from a water tank, or is this a pressurized system with a pneumatic ballast tank?

So based on those numbers he's got to be looking at somewhere between 11,000 and 25,000 gallons per day for the neighborhood. That's starting to sound more reasonable IMO.

 

[jasonyp]

eemmmm, actually this 330 gallon is approve by authorities, no need to change
1 more thing is, this 82 house is low cost single storey houses
thx for reply

 

[russ_watters]

No, I'm pretty sure you mistook gal/min for gal/day... 330 gpd is .22 gpm, 1/10 of one usage point (a single sink is 2.5 gpm). Water pipe sizing is done based on flow rates in units of gpm.

But let's back up here a little: The tone and nature of the question imply some disturbing things with serious legal implications that we need to get clear. The tone and nature of the question implies you have little or no engineering experience and are not working directly for a registered professional engineer. Perhaps you work for a developer who doesn't want to spend the money to hire an engineer? The task you are doing is one that in most places in the world is required by law to be done by a registered professional engineer or by someone in direct supervision of a registered professional engineer. If you aren't following the legally required engineering process, I highly recommend you re-examine the process you are following. The legal liability involved in doing the work illegally and making a mistake on this (like sizing a water pipe based on two orders of magnitude too small flow rate...) are vast. This particular problem involves exposure to several million dollars in legal liability in addition to loss of operating licenses, which together would force the closure of even a medium sized construction company.

Now, the nice thing about plumbing systems is they, like electrical systems, are designed mostly by the code (unlike air conditioning, which isn't required by code to keep a building cool). So there is no ambiguity to the design process either. Here's how it works:

1. Get yourself the relevant code book, probably the International Plubming Code.
2. Build a flow diagram of the complex's piping infrastructure layout.
3. Start from inside to out, building a spreadsheet for required flow rates from each house, cluster of houses, etc. The code book has tables of required flow rates to assume for each usage point and diversity to assume based on house size and number of houses on each main. Just plug and chug - it is a half hour exercise, tops.

The second task is then to size the pipe. You need the flow rate you just calculated (perhaps you already have it if you just got the units wrong - 330 gpm sounds about right), but you also need to know the pressure of the water available at the utility connection (adding pressure for the height of the buildings and difference in height from the utility entrance to the houses). Typically, you would want to design for an absolute max of 10 fps in the pipe and 10 ft/100ft of pressure loss (I'm not going to explain those units/terms: if you don't know what they are, it emphasizes my point that you need to hire an engineer). There are lots of books out there with tables/graphs of gpm vs pressure drop for pipes (probably on the net too, but never looked). But those are just rough rules of thumb - if you don't have much pressure available, you might want to size the pipe for less pressure drop. Or you may need to design yourself a booster pump.

How much static head you need at a faucet depends on how generously sized the piping is. Ie, 50 psi should be plenty if you have well sized pipes, but if you don't, it might not give you enough flow.

Just a little more emphasis on the point that you really need a qualified engineer to design this: if you size the pipe for 4" and it needs to be 5" (for example), that's only 20% too small in diameter, but 36% too small in area (and therefore too high in water velocity) and your pump will end up 74% too small (1/4 the size it should be).

 

[Bob S]

No, I'm pretty sure you mistook gal/min for gal/day... 330 gpd is .22 gpm,
The second task is then to size the pipe. You need the flow rate you just calculated (perhaps you already have it if you just got the units wrong - 330 gpm sounds about right),

330 gpd (gallons per day) is a reasonable value per household. It include both interior (about 65 gpcd (gallons per capita per day) and exterior use (summer irrigation). The 330 gpd is probably used in a 12 hour period, so the distribution pipes should be sized for

(82 houses) x (330 gallons)/(12 hours) = 2300 gallons per hour

 

[russ_watters]

Yes, 330 gpd is probably a reasonable estimate for a house, but water piping systems are not sized based on gpd (or gph), they are sized based on gpm. Using gpd does not properly account for usage patterns, even if you spread it over a 12 hour period instead of a 24 hour period. 330 gal/12 hr = 0.46 gpm and as I said, a single usage point is 2.5 gpm, so you're off by more than an order of magnitude for the service size to a house - a a more reasonable estimate for a house would be on the order of 10 gpm. The diversity factor for a development like that is probably on the order of 50%, so I'd estimate the required gpm of the development is closer to 400 gpm, not 38 gpm as your estimate would say. So you're probably off by an order of magnigude. But again: the calculation is laid-out for you by law, so there is no good reason to throw around such speculation.

It is easy to envision real-world scenarios where an 82 house development could exceed 100 or 200 gpm. There are specific times during the day when usage is much higher than other times. From 5:30-7:30 am, for example(and peaking from 6-7), a very high fraction of the houses will have multiple people taking showers at a flow rate of 2.5 gpm per shower for the length of the shower. In addition, there will be several toilet flushes per house at 2.5 gal/flush and faucets running for a few minutes at a time at 1gpm.

 

[Bob S]

It is easy to envision real-world scenarios where an 82 house development could exceed 100 or 200 gpm. There are specific times during the day when usage is much higher than other times. From 5:30-7:30 am, for example(and peaking from 6-7), a very high fraction of the houses will have multiple people taking showers at a flow rate of 2.5 gpm per shower for the length of the shower. In addition, there will be several toilet flushes per house at 2.5 gal/flush and faucets running for a few minutes at a time at 1gpm.

You are right. If a worst case scenario was a simultaneous 5 gpm per house, then the total demand would be over 400 gpm for the 82 houses.