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A barometric loop is formed by having a section of the pipe in the shape of an inverted “U” upstream of a cross connection. Based on a physics principle, the height of a water column open to the atmosphere at the bottom will not be greater than 33.9 feet at sea level pressure. If the loop is greater than that height, backsiphonage cannot occur through it. However, the barometric loop is not effective against backpressure. http://www.nesc.wvu.edu/ndwc/article...4/TB_WI04.html
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The idea is that atmosphereic pressure can only lift a column of water about 34 feet.
Basically, when it comes to physics, there is no such thing as "suction". There is only creating a vacum. For something to move into that vacum, something must push it into the vacum.
So when you suck on a straw, what is really happening is that you are creating a vacum in the straw and atmospheric pressure is actually PUSHING your drink up the straw.
As sea level, air pressure is about 14psi. At that pressure, you can push a water column up 34 feet. So if your straw was 35 feet long (i.e. your mouth is 35 feet above your drink), it doesn't mater how hard you suck on that straw, you will never get your drink to your lips.
So the idea of of a barametric loop is should the source of the water pressure fail, the loss of pressure prevents a back siphon over the 35 foot loop.
The caviot is that the water on the far size of the loop can not be presurized (except by the atmosphere).
Of course the water flows through the barometric loop in the forward direction because the water is pressurized by the water company at 30psi or more, way more than enough to lift the water over the 35 foot high loop. The loop also doesn't loose water pressure (except the added resistance of more pipe to travel through) because what pressure it loses getting pushed up the 35 foot loop, it regains falling down the other side.
HooKooDooKu, not bad for someone who works in finance.
the same effect can be achieved with a check valve/backflow preventer and V-breaker or anti-siphon device.
I'm not completely sure, but I think, by and large, barometric loops (flagpoling) are somewhat obsolete, over 70' of pipe, structure to support it and the labor are easily outdone by the right fixtures, but maybe one of the commercial/industrial pipe fitters can give an example where they still have application.
My guess is Gear is looking for the info because it may have come up in a test or code book.
i'd been told they are used a lot on older buildings in europe. and i'd imagine if the bulding was at least 3 stories tall you could unistrut it to the wall for support. seems a bit archaic but if it works, why change it?
No, it's not rocket science, it's plumbing and unlike rocket science it requires a license.