Hydaulic Design of WWTF- how account for losses

[tgavin]

Several Engineers and I have been having a discussion/dispute about the correct way to account for entrance/friction loss in pipes/channels for WWTF design.

The question boils down to the following...

When transitioning from a concrete channel to a pipe (gravity- meaning by this definition that the pipe entrance is not flooded) what method is best used to determine headloss?

Here are our thoughts...

Culvert Calculation for headwater- The program I use assigns a inlet loss coefficient of 0.5 for a concrete headwall (similar to channel to pipe). This seems to be overly conservative from a common sense standpoint.

Assume full pipe flow- use full pipe flow and hazen williams formula to calculate head loss through the pipe at a certain flow. This will return a lower headwater or less loss than the above option across the board.

I think that neither are right and the answer lies somewhere between. The culvert calculator has an inlet loss based on flow coming into a pipe from 180 degrees. When water is entering the pipe from a channel, the water is already "organized" and should enter the pipe much easier.

If you agree, do you have an equation that takes into account the difference in size and shape to use with the culvert calculator? If you don't, what do you think?

Thanks in advance for any responses...

TG

 

[bpattengale]

I'd treat the pipe as a culvert. Assuming that the as you say the inlet is unsubmerged, the pipe would be under inlet control. Now if the inlet is submerged it could also be operating under outlet control and that should be checked. Now assuming Headwater/diameter of the pipe is less than 1.5 the entrace would be operating under wier flow. You could then use this formula

Q=Cd*Ac*srqt(2g*(HW-Yc))
Cd=Coeefficient of Discharge
Ac=Cross sectional area at critical depth
g=gravity
HW=depth of headwater
Yc= Critical Depth

Now if you know the flow and channel geometery you could calculate the normal depth of the channel and use that as HW for the culvert.

Now for pipe culverts with a square edge in a vertical headwall Cd=0.93 for HW/d<0.4 to 0.80 for HW/d=1.5
You can interpolate Cd based on your HW/w Ratio

This gives you flow throught the pipe which is what sounds like you're ultimately looking for. If you still want Ke try the following formula with the Cd you got from above.

Coefficeind of discharge is
Cd=1/(1+Ke)^1/2
With Ke being the entrance loss coefficient.

 

[redbridge]

If the pipe entrance is submerged and the pipe is full flow then I would use Hazen Williams equation. Minor losses are going to be greater than the pipe losses so you will need to get that close (assuming low velocity through pipe). Add the losses to top of pipe and this will give the water elevation.