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Barometric Loops

January 22, 2013

There is a new movie out called The Dictator. I haven’t seen it yet but for some reason, I am attracted to it. Puzzling… Anyway, adoring underlings, it is time for your monthly indoctrination. Keep reading; there is no subliminal messaging going on that will turn you into my obedient henchmen. Bwu-ha-ha-ha. (I wonder if there is such a word as henchwomen?)

What is a barometric loop and why would you need one? Basically, it is taking your pipe vertically up out of the ground to the required height and then back down again. At the top, there is vent pipe or some sort of air valve or vacuum breaker. The situation that you would uses one is where you have a pumped main that has a high point somewhere along it that is higher than the outlet point at the end. When the pump turns off, the line drains down from the high point. So you put in the barometric loop to artificially raise the termination level so the line doesn’t drain. The one shown is on two parallel sewage rising mains and is fondly referred to as “Pete Hopman’s Erection”. The fifth leg is a vent pipe.

“I don’t care if my line drains”, you say. In some situations it doesn’t matter if it does drain. But there are a number of disadvantages of turning back on a pump on a partially drained line. This is particularly the case with raw sewage lines. There could be one or more of the following:

  1. The pump see reduced head and goes off the end of its curve with possible cavitation damage or motor overload;
  2. It takes a long time to clear all the air from a line, so stable flow is not established
  3. The air pushed out of the line on restart is malodourous (stinky) if it is a sewage line.
  4. The air pocket is a breeding ground for sulphur reducing bacteria that excrete sulphuric acid that eats away the cement lining of the pipe causing rust and failure (it has happened).

So assuming you do want one, how do you design it? It has been the practice to just make sure the top of the loop is some nominal distance above the high point, usually 2 to 5 metres so the air valve at the high point will hopefully seal. But this simplistic approach can result in a design that does not prevent air pocket forming at the high point. What happens when a pump stops is that flow stops entering the pipe but it continues to leave the pipe until the negative pressure wave reaches the end. If the line is short and made of steel or DI, then this time is short and manageable. If the pipe is long and plastic, then this becomes a problem. If the volume that has left the system is too much, then the loop will not have sufficient water in it to prevent an air pocket at the high point. You may be tempted to make the diameter of the up pipe larger than the pipeline to get more volume in it. This is OK for potable water, but don’t do it with sewage or sediment carrying water as the low velocity will mean all the grit will settle out at the base of the pipe and eventually block the line.

On the down leg you need to aerate it and then possibly put in a de-aeration chamber to remove the entrained air. Another alternative is to put in a vortex drop pipe, but I won’t open that can of worms at this point.

So if you want to put in a barometric loop, please give it some careful thought as it may not solve the problem you were hoping it would.

PS  A barometric loop isn’t the only way of dealing with this problem. You can also use an actuated valve.


From → Piping

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