Air in Force Mains
Air in Force Mains
(OP)
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
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





RE: Air in Force Mains
RE: Air in Force Mains
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
RE: Air in Force Mains
RE: Air in Force Mains