Calculating the Flow Through a Pipe

Quick answer:

The maximum economic average velocity in any pipe is usually taken as 3ms^-1 for liquids. Multiply this by the cross-sectional-area of the pipe to get the maximum economic flowrate. Pocket calculator?

For a partially-filled pipe, the figure is going to be lower than this, and without knowing the level in the pipe at any point, that is the end of the analysis.

You can't. Use a flow meter to get a rough idea.

Ask one of your co-workers who has some experience.

First question that I am going to ask you: Is this a gravity sewer or storm sewer, or a pressure main?

2nd Question: What is IL?

3rd Question: IF this is a gravity sewer, is the pipe inlet allowed to surcharge (water above pipe crown)?

IF you have a gravity sewer with NO inlet surcharge, then use "MANNING'S FORMULA" to calculate the flow rate. You need to determine "n", the "roughness coefficient", for the pipe material utilized and the interior pipe wall condition first.

NOTE: Be fully aware that for a gravity sewer w/o inlet submergence, the Maximum Flow that a given pipe size can convey water through it is called the CRITICAL DISCHARGE. Manning's Formula is only valid up to up to that value, or approximately 0.87d. You could stand the pipe on end and not get any more flow through the pipe. For sanitary sewers and storm sewers limit your flow velocity between 3 fps minimum and 10 fps maximum.

IF it is a gravity sewer with inlet surcharge and/or outlet submergence, then you must treat the pipe as a culvert.

IF it is a pressure sewer, use "Bournelli's Formula" to calculate the pipe flow.

IF you don't know how to do any of the above, then take a hydraulics course, because you're way over your head computing the flow!

Hi apologies,
It is for a storm sewer design, and IL is the invert level of the pipe...i know the ks - roughness coefficient and i - hydraulic gradient value...i was currenly using table for the hydraulic design of pipes and sewers to calculate the discharge of the pipes... I have done the pipe/culvert to stream discharges, what was confusing me was the pipe to manhole connection (for which I just had a cover level and not invert level), is there a way to assume this invert level...

The Invert Elevation = MH rim elevation - Pipe Cover (ie, depth of bury) - Pipe Wall Thickness- Pipe ID.

You need to calculate the pipe Invert Elevations at both the upstream and downstream Manholes.

ks? What formula are you using? and why rely on flow tables or nomographs when there is a much more precise formula available?

Manning Formula (U.S. Units):

Q = (1.486/n) * A * R^0.667 * S^0.5; flow rate will be in Cubic Feet per Second (CFS)

but, Critical Discharge (Maximum Allowable Flow Rate):

Qcr = 2.58 * d^2.50; flow rate will be in Cubic Feet per Second (CFS)

....whatever the low rate (discharge) that you calculate using the Manning Formula cannot exceed this value for a gravity sewer (ie, water level in upstream MH is at or below the pipe inlet crown elevation....or in other words it is unsubmerged inlet).

ARE YOU A STUDENT?