Gravity flow velocity into MH? 1

[sjohns4]

I'm bringing a 20" diameter C-905 PVC forcemain to discharge into a 10' wet well. Due to the ground surface elevation, the forcemain will gravity flow several hundred feet from the high point in the line. Velocity will be around 13 fps just prior to discharging into the MH. What can I do to reduce the velocity prior to discharge into the MH?

Thanks,

Mike

 

[ccor]

Layouts that have the last section of forcemain going downhill can be really problematic. If the steady state gravity flow velocity is higher than the pumping velocity the water column can separate which can rapidly change the pumping head causing the pump to "search" for steady state. Of course you have to select the pump to get over the high point as well as be in operating range with flow to the outlet. Then there is the issue of air relief at the high point which is tricky for wastewater.

What I do in this situation is end the forcemain at the highpoint and run gravity down to the terminal manhole. This alleviates these problems and is usually cheaper. If you do this you can control velocities by running drops at each manhole.

 

[sjohns4]

The gravity flow of the forcemain is being accounted for, there is a different system curve below a paticular GPM.

I've seen exactly what you are talking about proposed, but I dont see how gravity sewer with man holes will be cheaper?

Mike

 

[cvg]

you didn't ask for cheap, you asked for a reduced velocity. The drop manholes will do the trick. Yes it will be expensive to install a larger pipe and manholes. How else would you propose to dissipate the energy? (cheaply)

 

[ccor]

The projects I have done this on, the price has been a wash or slightly cheaper. This is because: the larger pipe diameter is offset by the thinner wall, the manholes are offset by the elimination of restraint and air release station (also frost line is about 7' here so depth of excavation was about the same). None had to use the drop manhole detail, which will cost.

 

[sjohns4]

The current setup has a forcemain that discharges to gravity sewer with free-fall drops before entering the wet well. The gravity man holes are in pretty bad condition from corrosion, also odor is a concern. The velocities in this portion of gravity appear to be quite high. The other part of this is there is a desire to allow the pump station to reverse flow and pump back up the influent forcemain (with changes in system valuing) to re-direct flows in emergency situations.

The solution doesn't have to be cheap, just work.

Any ideas other than several joints of gravity pipe?

Thanks,

Mike

 

[dicksewerrat]

Part of the odor problem is the velocity of the gravity portion. H2S is coming out of suspension due to the turbulence. 13 fps will shoot the sewage across the MH and corrode the back side very quickly. We installed a vortex configured out fall for a force main that worked fairly well for a deep drop. The flow was directed into a vortex that kept the flow on the from breaking up as it spiraled down the drop structure.

Richard A. Cornelius, P.E.

http://WWW.amlinereast.com

 

[sjohns4]

I was looking at the same vortex inserts. The manufacturer said they are confident they can come up with a design to handle the 13 fps.

In your application for the vortex inserts, was it a new system, or were you fixing an existing problem area? It sounds as if you were pleased with the way they worked?

Thanks,

Mike

 

[dicksewerrat]

We were rehabing aforce main drop MH. It was a good application. the sewage sprialed down the drop pipe. Very little odor compared to the old system

Richard A. Cornelius, P.E.

http://WWW.amlinereast.com

 

[dicksewerrat]

Sorry, spiraled

Richard A. Cornelius, P.E.

http://WWW.amlinereast.com

 

[Pump2005]

An inside drop manhole seems like the way to go.