Siphon Effect with Plastic Water Tanks

[altmann]

Hi Folks, this past weekend I visited our mountain camp, with the purpose of changing out our water holding system. The current design is like this: there are 2 250 gallon cylindrical glass lined metal tanks in the rafters. We pump up lake water via a gas powered engine at the lake shoreline, ~40 feet from the camp. The fill line splits at the tanks and fills them simultaneously. Each tank has an overflow line at the top of the tanks, which join to one hose which leads to a shower 8 or so feet away from tanks. Our process is to pump up until water from overflow hose is heard dumping in the shower, and we signal the pumper at the shoreline to cut the engine/pump. This immediately stopped the overflow condition and all was well. The tanks developed leaks, and we decided to go to plastic rectangular tanks from Ca. They each have 2 openings on the top for fill and overflow, and an outlet on bottom to sinks and toilet. We similarly installed the plastic tanks, but did add some valves that we did not have in the old system, that allow for tank isolation. When the new system was installed, we filled up halfway and everything worked like a charm. As we went to test the overflow seals, we discovered a new phenomenon that was not occurring in the old system. Once the overflow condition was reached, and water spilled over to the shower, we cut the pump, but the water did not stop draining into the shower. Seems that a due to the nature of the plastic tanks (perhaps being pliable) it set up conditions for a siphon. So I have two questions....would really like to know a detailed explanation of what is occurring and why, and why the old system did not produce a siphon. The better the explanation (to a layman) the more appreciated! In addition, I am assuming we need to add some kind of a check valve, or something that will introduce air when the overflow occurs. What would be our options, and where would the placement options (for such a valve) be ? THANKS IN ADVANCE !!

 

[phishfood]

My first thought is that the overflow connections on the plastic tanks have a section inside that "dips" down into the water below the level that you actually attach the overflow pipe to. This won't be a problem normally, as there is nothing to start the siphon. However, when you fill the tanks up to a level that water flows out of the overflow, a siphon is created, and is not broken until the water level drops below the "dip tube".

If you are 100% certain that this is not the case, the only other thing that makes sense to me is that the overflow outlets are not capable of handling nearly as much water volume as the old ones were, so that by the time you get the pump shut down the level is built up above the overflow and it takes a little time to drain down below the overflow outlet.

 

[altmann]

Thanks for the reply Phish....I think I understand most, but when you talk about "level built up above the overflow", I am not quite getting that. The overflow holes in the rectangular cubical plastic tanks are on the top wall, so when we hear the water gushing in the shower, the tank is 100% full, and the level is to the top of the tank, with additional water in the overflow piping. The same was the case in the cylindrical tanks we had (overflow was a threaded hole in the topside of the cylinder wall). I do not know if there was a tube that jutted down in the plastic tank, i never saw one, and think it is just a threaded hole in the tank to which we connected a male thread in, which overflow tubing to shower connected to. I am almost thinking that you may have something in the size of the outlet hole. maybe it is smaller than the old steel tanks were, and then the tanks act as a bladder when completely filled, and all the air is pushed out in that case, helping to better create the siphon.