Peaking factor 1

[Surcharged]

I am designing a 50 acre industrial park. I anticipate sewage demand at 1500 gallons per day per acre. I can probably assume that this 1500 gallons per day per acre is occurring in a 8 to 10 hour (say 9) window. So that will be 1500/9/60 * 50 acres = 139 gpm. Now I need to apply a peaking factor. Normally I would use a peaking factor of 3 but since I have already decided to divide my entire flow by 9 hours instead of 24 hours have I already applied the peaking factor to this? Or should I at least I reduce the peaking factor down to 2 or 2.5? Any thoughts would be appreciated.

 

[RWF7437]

The answer depends greatly on how much you know, or can confidently predict about what the industrial park will contain for the next 20 to 50 years. Industrial flows vary widely from industry to industry. They can also vary widely from hour to hour and can change markedly over a period of many years. Unless the park is zoned in a way that limits industries to those which use small amounts of water ( i.e. low demand ) you may want to be very conservative.

Published peaking factors are most reliable for large service areas where errors tend to average out.

Fortunately, the difference in capacity of a 10-inch pipe and an 8-inch pipe is approximately 56% while the total in place cost is less than that.

The only place "oversizing" might get you in trouble would be at the upper end of a branch line where, because of low flows, you might not be able to achieve self cleaning velocities at least once a day. There you would want to try to ensure that the pipe flow at least half full for some part of every day.

good luck

 

[RWF7437]

To answer your original question more directly; yes you have already applied a peaking factor of 24/9 = 2.67. A "day" in the parlance on wastewater flows is 24 hours.

But, can you assume all the industries in your park will only work 5 days a week, 8 hours a day ?

Assume is a word to use with great caution.

good luck

 

[bimr]

Agree with RWF7437l that you have more or less applied a peaking factor. The peaking factor is the relationship Peak Hour/ Average Day.

Because your project is relatively small, you need a slightly larger peaking factor (than 3) as shown in the attached table:

http://ilga.gov/commission/jcar/admincode/035/03500370ZZ9996dR.html

 

[cvg]

often times, industrial fire flow requirements dictate the size of the pipe more so than the actual peak daily demand. It's not unusual to have 2,000 gpm or greater fireflow requirement. This generally requires at least 10" if not 12" pipes to deliver it with required residual pressure. Most industrial areas I have seen have a network of 12" minimum lines.

 

[Surcharged]

cvg,

Good point on sizing the water mains but I still have to design my sewer lines.

Depending on my peak rate factor I need 10" or 12" sewer lines.

 

[bimr]

An 8" sewer at the minimum practicable slope of .08% is capable of handling approximately 155 gpm. One would not expect that you would have to use many sewer lines greater than 8".

 

[Surcharged]

Bimr
Look at my calculation in the first post. If I apply a peaking factor to 139 gpm then I am over 155 gpm. In my part of the world our minimum slope is .4% for an 8" sewer.

 

[bimr]

Sorry, you are correct, minimum slope for an 8" sewer is .4% for a flow of around 0.76 cfs or 341 gal/min.

50 acres * 1500 gal/day = 75,000 gal/ day

75,000 gal/day over 8 hours = 156 gal/min

As RWF7437 and others have noted, you do not need to use a peaking factor on the 8 hour day since you have equated the 24 hour flow to the peak hour flow.

If you want to figure the peak using the normal 3-4:1 factor, then:

75,000 gal/ day with a peaking factor of 3.8 = 197 ga/ min

Anyway, you are using only 8" sewers.

 

[jartgo]

I agree in theory with everything that has been stated above, however, I would caution you to check your local or state regulations.

In NC, the rules state that you must take hours of operation into account (as you have done) when designing the conveyance system. The resulting flow is your average daily flow. Your peaking factor (minimum of 2.5, higher for smaller populations), which accounts for diurnal fluctuations in the flow, must be applied on top of the ADF calculated above.

For example, if a restaurant has 100 seats and you assign (or are required to assign) 40 gallons/seat, the permitted flow to the treatment plant will be 4000gpd, however, in the conveyance system (gravity line or pump station), you must take hours of operation into account (assume 12 hours) to get an ADF of 5.56gpm, then you multiply that by 2.5 (minimum) to get a design peak flow rate of 13.9gpm for the conveyance system.

 

[RWF7437]

See also thread 181-27567 on this forum

 

[CivEnvEng98]

OK if converting the flowrates to account for hours of operation is really the peak hourly flow, then how do you determine the average daily flow?

 

[bimr]

Avergage daily flows are listed in a number of text books or states regulations like this:

http://ilga.gov/commission/jcar/admincode/035/03500370ZZ9996aR.html

http://www.ilga.gov/commission/jcar/admincode/035/03500370ZZ9996bR.html