air is being suck by the distribution main

[archirodon]

hello everyone.
we have tried putting an air release valve at the highest point of our distribution main. what happened was instead of air being released, air is being sucked by the pipe into the distribution main. can somebody explain this?
we haven't put the ARV yet, only the ball valve at this point.
thanks

 

[BigInch]

Probably not enough backpressure is being held on that high point (too much water is being withdrawn from the distribution points of the downstream pipeline).

http://virtualpipeline.spaces.msn.com

 

[BRIS]

You have a siphon - the hydraulic grade line (HGL) is below the crown of the pipeline. This is not an acceptable situation for potable water supply: to prevent contamination you need to maintain a positive pressure. There will be statutory minimums for your location.

You either have to increase the system pressure or reduce the amount of water drawn off.

IF supply is not potable water then you can operate with a siphon (air valve closed sub atmospheric pressure), but you need to check that the pipe and joints are structurally able to sustain the vacuum pressure. You would also need to be sure that negative pressures particularly under transient (water-hammer/surge conditions) do not fall below vapour pressure.

 

[archirodon]

another thing, could you please give me an idea on how to determine the proper size of air release valve for a certain pipe size say 6" dia. PVC pipe for example.

 

[semo]

Are you sure you have an air release valve. Another similar valve is an air/vacuum valve that not only allows accumulated air to escape; but, it also allows air to enter in a vacuum situation.

 

[cvg]

if you contact your vendor, they can probably do the calcs for you.

 

[redbridge]

Does this only happen when you flush the lines? Or is this when a pump turns off? Are you pumping from one tank to another? Give me more details and I will try to help.

 

[semo]

I wasn't paying enough attention earlier. Bris is most likely correct the assessment and it isn't acceptable in potable water systems. Perfect place to bring contamination into a pipeline.

http://www.valmatic.com/technicalpapers.jsp

has papers on air in pipelines and on air valve sizing.

 

[archirodon]

thanks a lot! we've increased the pressure and already fixed the air release valve.