Sizing Force Main 2

[robotfood540]

Hey guys, need some help. I was asked to size some gravity sewer feeding into a lift station, and also size the force main. A pump station handles 62.84 gpm downstream, but I need to figure what the peak flow is once it reaches the gravity sewer which flows to the main lift station in order to get the total peak flow going to the main lift station. How do I do that? I do not know pipe length, slope, or diameter. I am assuming the force main is Class 52 DI pipe.

 

[bimr]

Force mains are typically sized for 3-5 ft/sec. The peak hourly flow in a gravity sewer is typically 4 times the average daily flow.

 

[robotfood540]

ok thanks. i am using a peaking factor of 5.8 for the peak flow. also, how do i go about sizing gravity sewer. i have calculated all the flows, have the pipe length, and slope. how do i size the pipe, i believe there is a chart i can use to check and see if certain pipes can handle certain capacities at various slopes. thanks.

 

[bimr]

Gravity pipes are sized using the mannings formula.

 

[cvg]

unless surcharged, gravity sewers use mannings formula for partially full pipe. Force mains use the same formula except the pipe is usually assumed to flow full.

 

[jgailla]

cvg,
Could you clarify the use of Manning's for force main applications?
I've always used Hazen-Williams. I thought Manning's was only applicable for gravity flow.
Thanks.

 

[cvg]

Darcy Weisbach, Chezy, Mannings and Hazen Williams equations all are used to calculate energy loss due to friction in a pipe.

The primary benefit of Mannings is that hydraulic radius can be calculated for conduits of any shape as well as for open channels. The weakness is that it is only appropriate for water at normal temperatures.

Darcy Weisbach can be used for any fluid, however is particularly sensative to the assumption of roughness factor f which varies over a larger range than Manning's n value

Hazen Williams is often used in waterworks design as well as sewage and is perfectly acceptable for force main design as well unless the force main does not flow full.