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Water Siphoning from Full Fresh Water Tank
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If we combine the observations made by Neil, the calculations and the vented loop ideas suggested by others . . . the solution to this becomes very simple . . .
The problem comes down to a 10" water column siphon action that generates enough vacuum to draw water out of the tank, collapsing the tank bottom and top, reducing tank volume.
Addition of a T and a second vent line to the existing first vent line should solve this problem. The second line must be a different length than the first, so that (if these two lines both happen to fill with water) the pressure/vacuum differential between two different vertical heights of water column will cause the longer one to drain the shorter one and then the shorter one will break the vacuum in the first one. See attached diagrams.
Now we just need someone (who has experienced this tank collapse draining) to install this simple modification to see if it works.
Rob
Water Tank Vent.pdfAttached FilesCate & Rob
(with Border Collies Molly & Angel + Kitties Hazel & Elsie)
2015 Reflection 303RLS
2022 F350 Diesel CC SB SRW Lariat
Bayham, Ontario, Canada
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Originally posted by Cate&Rob View PostNow we just need someone (who has experienced this tank collapse draining) to install this simple modification to see if it works.
In other words, at what amount of (measured) water can it be assumed the tank is starting to collapse due to the siphon effect?
Given the same camper orientation a test should be repeatable (the quantity of water siphoned from a full tank should be about the same). Does it matter how much water is in the tank at the start of the test?
Finally, below is how I configured my overflow (arrow points to where it attaches to the tank) with a shutoff valve. It would be relatively easy to add a horizontal extension since the line is accessible outside of the coroplast. Below is a crude drawing of what I could do--will it have the same effect as what you're proposing? I'd remove the valve, too.
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2017 Ford F-350 DRW, '19 315RLTSPlus
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Originally posted by TedS View Post
https://www.amazon.com/UKSAT-Locking...a-850101719941
Jim
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Cate&Rob howson I don't thine either of those setups will prevent the issue of siphoning as both outlets are below the level of water in the tank. If you eliminate the shorter down tube or make it such that the short down tube is even with the top of the tank the siphoning can then be broken as that will be the lowest pressure creating the differential needed.Joseph
Tow Vehicle: 2024 GMC K3500 Denali Ultimate Diesel
Coach: 303RLS Delivered March 5, 2021
South of Houston Texas
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Originally posted by Jlawles2 View PostCate&Rob howson I don't thine either of those setups will prevent the issue of siphoning as both outlets are below the level of water in the tank. If you eliminate the shorter down tube or make it such that the short down tube is even with the top of the tank the siphoning can then be broken as that will be the lowest pressure creating the differential needed.
RobCate & Rob
(with Border Collies Molly & Angel + Kitties Hazel & Elsie)
2015 Reflection 303RLS
2022 F350 Diesel CC SB SRW Lariat
Bayham, Ontario, Canada
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howson
Hi Howard,
The tank has to be filled 100% to submerge the vent/overflow connection which will start the siphon action. This siphon action has to be broken above the level of the water in the tank (essentially above the tank) My proposed second drain line will do this. This plan will stop the siphon draining but will not stop "slosh pumping" while driving . . . a positive shut off (on both drain lines in my design) would be required for that.
RobCate & Rob
(with Border Collies Molly & Angel + Kitties Hazel & Elsie)
2015 Reflection 303RLS
2022 F350 Diesel CC SB SRW Lariat
Bayham, Ontario, Canada
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Mike
2017 Imagine 2800BH pushing a 2019 Ford F150 Platinum
Formerly: 2002 Rockwood Popup
Location: Massachusetts
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The location that you have shown is outside the coroplast.
In any case . . . the vent in the loop has to be above the top of the tank to break the siphon action. In the location shown, the loop will likely see water flow (caused by the higher down tube) and not allow the vent to open.
Rob
Cate & Rob
(with Border Collies Molly & Angel + Kitties Hazel & Elsie)
2015 Reflection 303RLS
2022 F350 Diesel CC SB SRW Lariat
Bayham, Ontario, Canada
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Originally posted by Cate&Rob View Post
Hi Mike,
The location that you have shown is outside the coroplast.
In any case . . . the vent in the loop has to be above the top of the tank to break the siphon action. In the location shown, the loop will likely see water flow (caused by the higher down tube) and not allow the vent to open.
Rob
To have the second line work it needs to go up or it will still siphon and slosh vent. Thought - The line to the water pump must pass through the trailer floor, bring that second line through there and tie it up as far as possible, possibly beside the nautilus panel over the plastic tubs. That way if it does burp nothing important will get wet..
If you have a cabinet above this fitting you could come straight up into the cabinet , use the water tight vacuum breaker like is on the black tank flush line, or the U vent shown above, and then back down to below the belly fabric. To break the siphon the air needs to come from above to interrupt the flow through the factory U.
Just brainstorming here
Keith2018 Reflection 150 Series 220RK 5th whee, Star White 2022 F350 King Ranch CC Long bed (HAL) (CCC 4062lbs), B&W 25K OEM Companion,. SteadyFast system, Trailer reverse lights, rear receiver spare tire holder, storage tube, sumo springs, Victron MultiPlus 12/120/3000, Solar, Custom 6K axles upgrade, and other modifications.
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You could follow the tank fill line up to the Nautilus panel. Vent through the panel. That big hole in the bottom for hose and cable passthrough will drain any spill that comes out of the vent.Ted
2021 Reflection 310RLS
2020 F350 PS,CC,LB,SRW
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Originally posted by Yoda View Post
To have the second line work it needs to go up . . .
Slosh pumping will still occur (no matter which way the second hose goes) , but if a siphon action starts, it will be quickly broken as described above.
Rob
Last edited by Cate&Rob; 02-27-2021, 09:34 PM.Cate & Rob
(with Border Collies Molly & Angel + Kitties Hazel & Elsie)
2015 Reflection 303RLS
2022 F350 Diesel CC SB SRW Lariat
Bayham, Ontario, Canada
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Still need to admit air to the top of the water to prevent sides from bowing in forcing more water out. More water than natural draining of water that is above the vent port from a full or over-filled tank.Ted
2021 Reflection 310RLS
2020 F350 PS,CC,LB,SRW
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It seems this issue occurring only during a rapid drain from a full tank. If so, maybe just to use the water via the pump when full would prevent the the partial tank collapse. A valve on the overflow would prevent pumping from sloshing during travel and a high vent would help to prevent tank collapse when we forget to open the overflow valve for tank use.
Jim
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Cate&Rob With the 2 legs sticking down even at different heights you will still have the siphon, just now it has 2 exits and could have a higher flow rate than just the 1 drain. The shorter will allow it to "breathe" sooner than the longer leg, but both will pull water out of the tank until the tank has enough negative pressure to "inhale" from the sidewall strength.
I'm still a bit concerned that the tank will flex so many times that it actually looses strength to the point of complete collapse or failure in areas like the corners. I don't think its possible to "rotomold" or "blow mold" a one piece tank with internal baffles (the baffles will provide side wall strength to prevent suction deformation). Would be nice if they could make a tank with some form of internal support structure.Joseph
Tow Vehicle: 2024 GMC K3500 Denali Ultimate Diesel
Coach: 303RLS Delivered March 5, 2021
South of Houston Texas
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