Forcemain pumping downhill

[Chris73]

Hi out there,

I got a similar but yet different question than a few threads earlier here: I am looking at a proposed sewage forcemain which will convey the wwater about 8 km to the nearest municipal system. To about 4 km down of the pumpstation, the forcemain has a continuous rising grade. After this high point, the forcemain will run for the remaining 4 km more or less downhill. I am wondering what the flow conditions would be in that downhill section - at some point the gravity flow would take over, wouldn't it? Why I bother - I probably wouldn't if the connecting point was proposed to atmosperic pressure (manhole, wet well etc), but it is proposed to another forcemain - and this looks fishy to me since I figure that the down-coming forcemain will not be under pressure anymore at this point. Anybody some good words? Thanx!

 

[stanier]

I have designed and also analysed designs of systems as you describe. The whole system needs evaluation, not only the forceman in that you are looking at. Yes there will be modes of operation where the downhill portion will operate under a gravity flow regime.

At the junction of the existing forcemain and the new main rather than being an atmospheric pressure wet well or vented manhole. Perhaps there is a need for a pressurised manhole. ie bolted cover. Fitted to this should be an automatic air/vacuum release valve. The valve should be suited to the sewage environment

http://WWW.ventomat.com

. Their RGX is recommended. There may also be a need for redundancy so the valve can be maintained without compromising the operations.

The other problem that you may encounter is odour emissions. The vent may need an odour scrubber.

As far as the hydaulics design is concerned all the possible modes of operations need to be analysed.