Gradient for the foul rising main?

[0mega]

I need to design rising main for combined foul/storm water pumping station. I was told by my manager that it is possible to design a rising main that is sloping away from the pumping station without air release valves if I lay the rising main under very slight gradient - air/gases released from water will be dragged by water during operation, therefore air release valves are not necessary.
Do you know how can I find/calculate this gradient and are there any references to it in British Standards/Specifications?
Thank you.

 

[bimr]

There is no such thing. It is impossible to always have the pipe sloping upward away from the lift station. What happens when you go over a hill? Sounds like your boss is giving you an early April 1st joke.

However,your boss may just be giving you advice to maintain a constant pipe elevation rather than having numerous dips and humps in the piping. Piping engineers label this piping arrangement as "No Pockets", which means exactly that. It is used more frequently in the chemical process industry to avoid potential fluid transfer problems such as the trapping of gases for example.

Force mains are usually buried just under the frost depth for economical construction.

You will have little air/gas released from the water. Where do you think the air/gas is going to come from if you keep the pumps under a flooded suction?

Air trapped in the pipeline is more of a concern with gravity systems. If you keep your pipeline velocity up, the air will be pushed through a pressure piping system. As long as you do not have significant elevation changes of over a hundred feet or so, you probably can get by with using manual air release valves or no air release valves. The air release valves are used more on longer pipelines and crossing over hills.

 

[0mega]

Thank you for the reply.

My rising main is about 460m and it is 150mm PE. I cannot maintain constant elevation, there will be some dips and peaks, but I can try to "smoothen" them up. Peaks, of course, will collect gases released from water. The water velocities will be about 1.5-2.0 m/s. According to the explanation I was given, water drags air/gas pockets at high velocities, if gradient of the pipe is small that allows the air to be dragged at small forces. Therefore, the pipe manages to clean itself out from these pockets, however, the force that keeps the air/gases in the pocket should not be higher than the force that drags them in the pipeline...

 

[bimr]

Did not at first understand what you meant by "foul" main.

Maybe this will help:

The Water Pollution Control Federation Manual of Practice No. FD-4 "Design of Wastewater and Stormwater Pumping Stations" states "A minimum velocity of 1.2 m/s (4 ft/sec) is required in the pipeline to shear the bubble and keep it moving downgrade."

http://www.eng-tips.com/viewthread.cfm?qid=202513

I would add that for sewage force mains, it is generally recommended to have manual air release blowoffs than automatic ones.