in-line flow restriction/energy dissipation in sewer pipe

[amaya]

We are designing a sanitary sewer network on a new residential development. Within this development there is a steep slope (a drop of 25 meters in elevation over a length of 200 meters).the sewage would have to travel through this steep area. We were able to design and allocate a series of manholes to meet the maximum slope of 7% and maximum velocity of 10 feet per second (according to specification). We found out that because of the terrain in this particular area is going to be very expensive. So we were looking at installing sewer pipe/pipes to achieve a continuous flow. The problem with that is how we control the flow so it doesn’t exceed the 10 fps. In other word how do we dissipate the energy or control the kinetic energy. Can we have a venture tubing system or maybe a high velocity manhole at the bottom of the hill to withstand the kinetic energy? Any suggestion ?

 

[LeoSC]

Amaya,
You can let the sewage rip down the steep sewer, and then dissipate the energy at a properly designed energy dissipator structure. You must check a few things:
a. That the flow velocities in the sewer are within the maximum velocities permitted for the pipe material.
b. That you can construct the sewer properly so that the pipe segments won't separate; some slopes tend to move, and they move sewers with them. I recommend that you get a geotechnical engineer to assist you.
c. That the energy dissipator is appropriate for the flow rates and the velocities expected. I refer you to the U.S. Army Corps of Engineers or the FHWA for publications and manuals. It is a safer structure.
d. That if a hydraulic jump occurs within the pipe that it will have sufficient air, to prevent slug flow in the pipe.
e. The energy dissipator structure must be designed for the dynamic forces imposed by the flow.

Energy dissipators tend to make a lot of noise. If the structure is near residences or parks, it may not be welcome.

In general, a sewer arrangement where you dissipate energy in several steps, by providing mild slope sewer runs (less than 1%) and drops at manholes is my first choice, as I don't have to depend on a single structure to dissipate energy. A multiple manhole and drop structure arrangement provides redundancy.

 

[yogibear1]

Adding to what LeoSC has stated is another concern: odor. If the sewage is old the turbulence at the point of energy dissipation will release a lot of the gases from the flow stream. Most sewer gases have a large H2S component and the gas will escape the structure if not pulled into an odor control module. I would also urge you to think of corrosion protection for the structure- it would have to be lined or coated with a highly corrosion resitant material.

 

[cvg]

flatter slopes with frequent drops at manholes as recommended by Leo may actually improve the odor and H2S problems by aerating the sewage more.