Need help in the design of a Sewer Force Main Extension 1

[newbuck]

Hi,

I have a project wherein I have to extend an existing sewer force Main (which is connected to a gravity sewer)to a new location. The force main backs up whenever it tries to discharge raw wastewater to the gravity sewer.

I was wondering what are the primary concerns that I have to consider when trying to enginner the extension of the force main to a new location?

I would really appreciate any help I can get. Thankyou.

 

[BobPE]

what do you mean the force main backs up when it trys to work?


BobPE

 

[gibfrog]

Some things you might consider:
1) The line extension, air release valves at all highpoints and retrofit if necessary on existing lines.
2) Make sure that a flushing velocity of atleast 2 fps is achieved at least once a day.
3) Make sure that the recieving gravity system has adequate capacity.
4) Maintian minimum cover (36&quot on the pipes and plot a profile of all crossings. Preferable plot a profile of the whole force main, the existing and proposed.
5) Design you pumps for the maximum head condition but check the minimum to make sure that the pumps will not burn out (should not be a problem here)
6) If possible, keep the pumps submerged for extra cooling. Verify the minimum drawdown level for your pumps. Use atleast 2 pumps with alternator, but one pump should be able to handle max flow. If you use 3 or more pumps, consider varing pumps for min and max conditions.
7) Odor protection/H2S at gravity sewer, minimize splashing, submerged discharge or tee through side.
8) Is lift station current to code? (alarms, emergency generator receptacle, by-pass connection, lighting arrestor, etc...)

Clifford H Laubstein
FL Certified PE #58662

 

[newbuck]

Dear BobPE,

Sorry for the delay in the replying back to your question. I had to get some facts straight before I wrote back. I am a Junior engineer working for a city and under the supervision of a senior civil engineer. I am a recent hire and this Project is my first big one.

This is the scenario. There is a Junction box which has a 33" gravity sewer line carrying wastewater into it. Also there is a 16" Force main dumping waste water at 2000 gpm into the junction box. The outlet for this discharge is an inverted syphon which has two pipes 30" and 21" and which crosses a channel and comes out on the other side and then proceeds to the Waste Water Treatmet Plant through a 42" gravity sewer line.

The problem encountered is that there is backing up of water in the 33" sewer line for the duration that the force main dumps into the Junction box. The duration is approximately 6 minutes. The water in the 33"sewer line during this period backs up to the crown of the sewer line at the junction box (without surcharging). The concern are primarily of odor due to the backup and also some reversal of flow immediately upstream of the junction box.

The city now wants to disconnect the 16" force main at the junction box and wants to extend it as a separate line to the waste water treatment plant.

So I was wondering how to engineer this extension of the force main. what are the concerns I have to take into consideration. I just need some leads so that I can do a good job.

I will appreciate any help I can get. Also i hope I am clear in describing the scenario.

Thankyou.

The

 

[Gordonba]

Your forced main should have a check valve to stop backflow, located after the pump.

 

[dicksewerrat]

If you were to extend the force main outlet down the structure and into the flow of the 33" line and place the force main outlet pointed with the flow, you may eliminate bothe the odor and the backflow problem. Odors and H2S show up in the area of turbulence. if you want a little more info just ask.

 

[Chris73]

newbuck,
First of all I would check if the existing pumps can handle the extra head or if you need a new pumpstation near the junction box. Once you calculated the system curve check with the pump curves and make sure that the operating point is within an acceptable range. Sometimes little modifications to the pumps/motors will do it.
Another main issue I see with under-crossing the channel. Maybe consider trenchless technologies like pipe jacking. If you need assistance, let me know.

 

[stanier]

Newbuck,

You might like to consider adding variable speed drives to the pumps on the 16" force main. Operate these at a lower speed and reduce the flow but extend the period of discharge into the manhole. This may be sufficient to prevent the 33" main backing up.

Invaribly pumping stations are design to handle wet weather conditions that may be 4 times average dry weather flow. Some authorities use different values as a multiplier depending upon the state of their carriers, rainfall, illegal connections, metering etc. The problem comes then that the pumps can empty the wet well very quickly during ADWF.

If yoiu limit the speed in ADWF then there may still be a problem during Peak Wet Weather Flow.

I am sure that your company would have to hand the AWWA manuals on sewer design. I would start there. ASCE also put out some good books on pumping and pipelines. Sharing knowledge is a way to immortality

 

[semo]

What Gibfrog said and what Chris73 said.

Watch your hydraulics and you might or might not have to replace the lift station pumps. Probably will if the extension is very long or uphill. That will have a lot to do with your headloss (flow, elevations, and forcemain size).

Use a realistic C value in your calculations (depending on the pipe conditions). If you use one that is too conservative, you could run your pump out and have problems like Gibfrog refered to.