The use of Manning's Equation in gravity flow system

[aumechengr]

Hello,

I am trying to determine if the Mannings's Equation in a gravity flow system takes into account various losses due to fittings (i.e., elbows, tees, diffusser header, etc.).

If not, would the applicable formula be the Hazen-Williams formula that takes into account these losses fittings? If so, how do you account for flow through the fittings when the pipe is not flowing full?

The system is gravity flow of water that will will be distributed via a header over the top of filter cells. I am concerned that the Manning's equation does not take into account the losses due to the fittings in the header and associated piping. Being gravity flow the pipes will not be at full flow. Any help with this would be appreciated.

Thanks in advance!

aumechengr

 

[RWF7437]

Neither the Manning Equation or the Hazen Williams Equations take into account losses through fittings, To account for those losses you'll need some empirical test data. Usually, if the "fittings" are bends, valves fully open, etc. such losses are minor and may be ignored. You may be able to find tables to convert fitting to equivalent lengts of straight pipe. The Hydraulics Institute Handbook has many such design aids.

Good luck

 

[semo]

Manning's works on open channel flow and does not take into account fittings. It only takes into account a cross sectional area of the open channel but, also uses the channel slope and friction coefficient of the channel.

Because the pipe (channel) is sloped, the additional head (hydraulic jump) created in bends, valves, etc. will not affect the hydraulic grade line for a long distance. You will have to calculate the hydraulic jump if you want the suface profile along the pipe.

Hazen Williams only works on full pipes and uses the pipe length (which is where the fittings come in), friction coefficient and the waterlevels on either side of the pipe. It does not care if the pipe is sloped or not. To include the fittings you must get a table to obtain an equivalent length of pipe for the fitting. Valves typically have a chart giving the headloss at various flows. You must combine this with the pipe headlosses for your calculation or backtrack into the equation for an equivalent length. Do not ignore these losses in your full pipe calculations. Not always will it; but, it can bite you big time if you do.

Now just because the water is flowing by gravity does not mean the pipes will not flow full. You will have to calculate their capacity and see what the actual flow depth is.

 

[aumechengr]

RWF7437/semo,

Thanks for the information that you supplied. I did a little more research and came to the same conclusions that you both mentioned. Again, thanks for the help.

aumechengr

 

[gibfrog]

There are velocity factors to account for such minor loses that may be used with Manning. You still need a table to look them up, but then you multiply them by (v^2)/(2g). These factors are usually <=1 and therefore neglible for low velocity applications.




Clifford H Laubstein
FL PE 58662