Air in Force Mains

[mou]

Hi all,

We've got a sewer pumping station where the high point is at the station itself and then it pumps downhill with several intermediate high points, then goes under a creek to the recieving sewer on the other side. Air Valves will be placed at hight points. There will be large amounts of air when the pumps are off. There is not enough volume in the wet well for the force main to ever pump full. Not a good design I know !!! My question is: How would you calculate the velocity in the main when the pump starts to account for the air in the main. I understand that 'normal' hydraulics/models do not apply in this instance. We need to work out the velocity to ensure self cleansing in the main. Any good web sites that show how to do this or can someone please advise.

Any other comments in regards the above would be most welcome.

Cheers

 

[dicksewerrat]

You will never get 'self-cleaning' in the main. That term is a misnomer. what the term means is that if the pipe is flowing continuously and if the suspended solids are all the same size and if the flow velocity is 2.5 to 3 fps, maybe the solids will stay in suspension. What you will have are solids dropping out at every low point and air gaps at the high ones. You may need to increase the size of the wet well to allow a longer pump cycle at higher pressure to push as much solids first and follow with a more liquid state after to flush. The intake for the pump should be very low, sucking the solids first.You may have to add water to the wet well also. Is there rock in the soils?

 

[mou]

Thanks dicksewerat,

Ive modelled the force main and the MAX velocities are very low (ie < 1 fps !!) in large sections of the main. The model showed large attenuations of flows in the mains (ie the pump was pumping 600 l/s while some areas were as low as 100 l/s in the lower ends !!!). I understand that to resuspend the setttled solids you actually need higher velocities than say the normal self cleansing velocities. The only way to achieve this is to have significantly smaller diameter pipes in the critical sections !!!. Pumping downhill certainly provides an interesting challenge. Even though the design has proceeded to such a point where it is now difficult to modify the design (other than modifying some sections of the diameter of the force main), we may have to rethink this.

Has anyone modelled (say using a package like SEWERCAD) a similar scenario where you are pumping downhill. I understand that the way you model such a scenario is to treat the downhill portion as gravity mains with bolted down manholes at the critical points.

Any additional comments/thoughts would be appreciated.

Cheers

 

[dicksewerrat]

Can you push/pull a smaller diameter HDPE line inside the old pipe? It would be smoother and if required, you could send a pig through. Also easier to run a jet cleaner

 

[rconner]

I have heard, and maybe even seen an article or two over the years, with regard to designers putting I guess where it is practical either "risers" (higher than the high points) or control valving etc. near the end of forcemains, to in effect do all they can from a hydraulic standpoint to assure that lines will "flow full" all the time (I guess as long as air valves etc. do their jobs). This, while apparently advantageous in many respects I guess including hydrogen sulfide problems etc., of course might not solve any problems due to improper line sizing or low velocity etc. Let me know by post if a reference to any of these articles might help, and I'll see if I can locate.