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Unread 04/06/2012, 02:49 PM   #23
BeanAnimal
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Join Date: Feb 2003
Location: Pittsburgh
Posts: 20,772
The point to walk away with here is that we can't cheat physics. As reality would have it, we often stumble upon something that works, even if we don't understand the physics. We often then assume that we can apply what we think we understand to other situations. Things don't always work out. The "air assisted" standpipes are a prime example in our hobby

When we talk about "durso", "stockman", "hoffer" or any other "air assisted" standpipe setup, we are talking about a system that is subject to numerous variables that affect its operation. What is quiet at a given flow rate in one environment may not be in another. There are simply too many variables that contribute to the operating mode of the system and subtle differences in standpipes can create rather dramatic differences in operation from one setup to the next.

These (and other) problems are much of the reason that I (and others) have designed standpipe systems that are predictable from one environment to the next.

Water flows through a standpipe due to the effect of gravity. The rate of flow of the system is function of the pipe volume and the resistance to flow created by the pipe configuration.

When we introduce air into the system, the air displaces what would otherwise be water. As the air to water ratio rises, the water begins to tumble and/or churn because the air does not fall as fast as the water around it. The net effect is very dependent on MANY factors (length of pipe, diameter of pipe, back pressure on pipe, roughness of pipe, viscosity of the fluid, etc) and is far from easily predictable.

A "durso" or "stockman" may fully open channel if the pipe is large enough in relation to the volume of flow. In this mode, the air entering the standpipe has no effect on the volume of flow and the only turbulence is that created by the walls of the pipe. The net effect is near silence, unless the walls of the pipe cause the water to tumble. In that case, you hear water running.

Sadly, MOST folks who run a "durso" or "stockman" push enough flow through them that they enter two-phase flow. In this scenario, the water falling through the pipe blocks enough of the diameter that a negative pressure (suction) is created by the falling mass of water. Allowing less air to enter the pipe allows more water to flow and allowing more air to enter the pipe allows less water to flow (again by displacement of water by air and by increased turbulence). At the upper end (almost no air) the system is usually silent. As the rate of air entrainment increases, the rate of ejected tiny (you folks call them microbubbles) increases and the noise of the system increases. At some critical point, the two phase flow becomes so turbulence that the air begins to chug and gurgle.

What one may consider "issues" another may not. Each of us has different expectations and goals with regard to system performance and each of us defines "acceptable" in a different manner.

With all due respect to Richard Durso and/or Ken Stockman, both of them stumbled upon a setup that worked for them and has helped many others move toward peace with regard to their overflow problems. That reality does not allow their standpipes to defy physics nor does it give scientific credence to their explanations of why they think their systems work as they do.

Certainly if and when everything works out, the system (air assisted standpipe) can run very well. This problem is that rarely does the return pump rate, pipe configuration head height match up to get a quiet, somewhat bubble free and stable system. For those that do stumble upon this Zen, life is wonderful For those that don't, the overflow is a nightmare.

From a safety standpoint, most folks do not understand that an air assisted standpipe running anywhere near its full siphon capacity will fail (flood the system) if there is any type of blockage or increased airflow (thus reducing standpipe throughput). That is, if you have to almost block the air off to get the standpipe to keep up, then it is a disaster waiting to happen. Air assisted standpipes are best (safest) used when the flow through them is open-open channel or on the two-phase side closer to open channel flow.

So can a 1" durso handle 600GPH... sure, in some setups where the head heaght, pipe diameter, back pressure, fitting configuration, etc. all play nice. In another system it may sound like a toilet or 2 headed slupring monster

You can't cheat the physics


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