Air/Vacuum Release Valves?

[Lexwater]

I had a small meeting yesterday with a pump rep, and one of his comments he said on some sanitary sewer force main's he recommends Engineers to place Air/Vacuum Release Valves at low points in the system. Does anyone know the reason as to why you would do this?

 

[StephenA]

At high points yes. At low points No.

At low points if anything you might need scour valves.

StephenA

 

[Lexwater]

Thanks. I haven't used scour valves before but I am guessing that is just a valve which could reduce the pipe ID thus increasing the veolocity to help clean the pipe.

 

[StephenA]

A scour valve would be on a T piece to empty the sludge / grit / sand that may collect at low points

StephenA

 

[BobPE]

Lexwater:

Was this person an engineer? There is no reason to put air/vacuum valves on ANY lowpoints....and you really need to understand air/vacuum valves to be putting them where they do belong...and this is not the responsibility of the pump vendor...

Let us know how you make out.

BobPE

 

[Lexwater]

Bob,
Thanks for the response. He was a vendor. I have designed several force main's that seem to be working just fine. It just scared me when he said that, beacuse I am still relatively new to the Engineering Professon. I started thinking and could not for the life of me think of situation where one would ever be needed at a low point.

After listening to you all and some here at the office I am starting to think that the vendor has a side busines with a air/vacuum rep.

Lexwater

 

[jonelv]

Lexwater

You and the rep may be at cross purposes. You don't necessarily just put air valves at high points - you may also put them where you have changes in the gradient either upslope or downslope or at regular intervals on long horizontal, ascending or descending sections. Its the pipeline profile together with the hydraulic gradient you need to look at - not just where the physical high points are.

 

[cvg]

however, you never place them at low points...

 

[Lexwater]

Yeah I have never placed them at low points.

 

[sjgordon]

Lexwater,

I know this response is a little late, but you know what they say.....

A pressurized sewer system could certainly benefit from air release valves placed at high points in the line. State regulations specify intervals based on change in elevation. Abrupt changes greater than say 10 vertical feet. Check the regulations applicable for your area.

Air bubbles actually form at low points in the line just upstream from high points because as the fluid (wastewater, water, whatever) flows downhill under pressure, high velocities may be encountered. The fluid is subjected to forces associated with pressure in the system and by forces associated with gravity. As the velocity increases, pressure decreases (within the fluid), creating bubbles.

After reaching the high point in the line, the air bubble will become lodged and physically restrict, thus decreasing the inside diameter of your forcemain. This restriction will increase your TDH. The higher head will possible force the pumps to operate outside of the range that they were designed for. Although this probably would not cause a catastrophic failure it would certainly decrease your efficiency and shorten the life of the pumps.

Hope this helps.