Water Siphoning from Full Fresh Water Tank

[Cate&Rob]

TedS mpking ncitro

Back to the "vented loop" idea suggested by both Ted & Mike . . .

In our earlier discussions of this problem, we made the assumption that the tank was ballooning from pressure fill and emptying this extra water was what extended the drain time. With this new information from Neil, we should reconsider the vented loop as used in marine plumbing. If this is siphon vacuum collapsing the tank, the vacuum break in a vented loop should stop this. This won't stop the surge pumping from expelling water while travelling, but would stop a siphon action started by surge pumping filling the vent line. These vented loops are available in different diameters from marine suppliers. When using water from the tank, the replacement air might come in through this vacuum break, so it would need to be located in a "clean" area . . . probably close to the tank and inside the frame rail & coroplast.

Rob

[mpking]

Something like this?
https://amzn.to/3ss09Oe

[Cate&Rob]

Something like this?
https://amzn.to/3ss09Oe

The challenge will be to get this thing as high above the tank as possible with a source of clean air for the air inlet. It might be possible to modify/replace the cap over the duckbill valve with a fitting that would take a hose routed to another area for the air inlet. 1/2" and 5/8" diameter vented loops are also available if one of these might be a better match to the tank fitting diameter.

Rob

[TedS]

Worrying too much about inhaled air quality. Current oem vent inhales from under the trailer, unfiltered.

[Cate&Rob]

Worrying too much about inhaled air quality. Current oem vent inhales from under the trailer, unfiltered.

My thought was not inhaled air quality, it was more towards avoiding contamination of the small duckbill valve in the vented loop. Even in boats, it doesn't take much to hold these open.

Rob

[Jlawles2]

Realistically the duck valve can be removed as long as there is a cap to prevent the water from shooting on the bottom of the floor. What's more important is the duck valve not get stuck closed as this would prevent it from opening and working as needed to break the vacuum stopping the siphon. Some creative person could make a similar device with a Tee, close nipple, cap and drill. Drill small holes pointing away from the floor in the cap after its put on the stem side of the TEE, and install with the stem UP.This may be a simple solution since the top of the tank is close to the bottom of the floor making it difficult to put that U bend in location at the proper orientation.

[Jlawles2]

Cate&Rob Brain working overtime here (funny but I think there is something burning).

https://www.amazon.com/dp/B06XCZ35NB...dDbGljaz10cnVl

Instead of using a NO or NC valve, use a ball valve. Need one with 3 wires though so there is no reversal of signal to activate. Could tie the close signal wire to the brake lights and the first time you press the brakes, it closes if you forgot. The issue is opening the valve and closing the valve. For this a SPDT momentary switch located on the fill panel that lets you select open and close the valve.

https://www.amazon.com/TWTADE-Moment...819S5T4Y&psc=1

Just comes down to the same factor as the manual valve, you have to remember to set it open when you start. The only other option I can think of is a reed switch https://www.amazon.com/WPERSUVV-12Pc...NsaWNrPXRydWU= which will force the valve open when you turn the fill port knob.

[Guest]

I think there are some great ideas being conveyed here. All worth a look. For my intentions to connect the gravity fill vent, I still wonder about sloshing even though I would run the line up through the floor, up to the top/underside of the kitchen cabinet and back down to the gravity fill vent. There can be a fair amount of energy in the sloshing with a half tank of water. If I see water pouring out the vent, I was thinking about an inline restriction (orifice).

Jim

[Yoda]

My thought was not inhaled air quality, it was more towards avoiding contamination of the small duckbill valve in the vented loop. Even in boats, it doesn't take much to hold these open.

Rob

The air vac in my post has a filter to prevent bugs. etc. If installed in a T above the tank it would work the same. The ball would also prevent surges from getting things wet too.

[Cate&Rob]

This was exactly right in my case. Once extended the vent line if it wasn’t pitched enough, then the water in the line would block the air flow. The tank was so weak (it should really have baffles for being so long and such thin material) that the tank would collapse rather than draw in the water that was in the vent line. I’ve seen it collapse so bad that when I would go out with my leaf blower and blow the water in the vent line up to burp the tank, it would read 50% on my Seelevel after it returned to normal.

With this "change in theory" from a while ago where we thought fill pressure was ballooning the tank . . . to overflow siphon vacuum collapsing the tank . . . I was wondering "how could this small a pressure difference lift the weight of the water in the tank" . So, on to some basic engineering.

Using my 60" x 30" x 7" 54 gal water tank as the example. Full, it carries 450 lbs of water.
The pressure differential is created by the column of water in the overflow/vent line. About the height of the tank plus 3" of drain line extending below = about 10".
1" water column = .0361 psi. 10" water column = 0.361 psi. (insignificant . . . right?)

Now, here is where it gets interesting . . . the floor of this tank is 1800 sq in. 1800 sq in x .361 lbs / sq in = 650 lbs of lift on the tank floor . . . more than enough to lift 450 lbs of water .

Of course, the original premise is also likely to be true. Filling the tank at city water pressure until there is a steady stream of water coming out the overflow could easily overpower the overflow line capacity and reach a pressure that would balloon the tank.

Rob

[Jlawles2]

Let' approach this a bit differently, 0.361 psi / in water * 3in extension below tank = 0.1 psi NEGATIVE PRESSURE. In reality its 10" water column or 0.361 psi again NEGATIVE pressure.

Any time the exit is below the water level it can siphon if the tank connection is submerged.

Now what happens to the tank at 0.36" of negative pressure. Is the tank so flexible that it deforms to the point that it lets that much water exit?

https://www.youtube.com/watch?v=lUFyCw_Nw3g

[Yoda]

With this "change in theory" from a while ago where we thought fill pressure was ballooning the tank . . . to overflow siphon vacuum collapsing the tank . . . I was wondering "how could this small a pressure difference lift the weight of the water in the tank" . So, on to some basic engineering.

Using my 60" x 30" x 7" 54 gal water tank as the example. Full, it carries 450 lbs of water.
The pressure differential is created by the column of water in the overflow/vent line. About the height of the tank plus 3" of drain line extending below = about 10".
1" water column = .0361 psi. 10" water column = 0.361 psi. (insignificant . . . right?)

Now, here is where it gets interesting . . . the floor of this tank is 1800 sq in. 1800 sq in x .361 lbs / sq in = 650 lbs of lift on the tank floor . . . more than enough to lift 450 lbs of water .

Of course, the original premise is also likely to be true. Filling the tank at city water pressure until there is a steady stream of water coming out the overflow could easily overpower the overflow line capacity and reach a pressure that would balloon the tank.

Rob

Rob
Another way to think of this. Tanks is 7" high (deep). 3/4' vent tub is installed at top of tank on the side, Installed the bottom lip of the vent is probably 1" below the top. So 1/7 of the tank can flow out if the tank is filled to the brim and maybe a little more. This starts the siphon action as the tank wants to initially return to base size. So as siphoning continues the big top and bottom squeeze in. This will continue until the suction pressure and outside air pressure equalize. In watching my tank flex I am guessing that is probably around 2" deflection. So when the tank equalizes the top 3" of water is gone, or 3/7 of the tank. This is 43% of the volume. I'm sure each tank behaves differently, but when folks sat they lost 1/3 to 1/2 a tank of water during travel I can believe it.

I have learned to fill slowly and then turn of the fill a few seconds after water comes out the overflow. I then wait about 30 seconds to see if it stops and I close my low point valve on my drain line. A while later I double check and I might get another cup out, but no siphon action. For me stopping the flow and with my air vac the level stabilizes with no fluid movement. Remember in a siphon action its the movement of the fluid to the lower level that keeps it going. The lower the outlet from the source the more and faster the flow.

The vented elbow you posted should work if it can be placed high enough.above the tank. The fact it uses a duck bill valve is better than the ball type air/vac's. All GD really needs to do the stop this is add a second vent line to the top of the tank and run it to the Nautilus panel with a screen on the end. It just needs to be a few inches higher with no low points.



BTW this should be part of the technical decision when our folks go to the factory.

Hope this helps
Keith

[howson]

All GD really needs to do the stop this is add a second vent line to the top of the tank and run it to the Nautilus panel with a screen on the end. It just needs to be a few inches higher with no low points.

That's brilliant, Keith.

What I read in your post is to manufacturer a tank with two vent lines with one-way valves attached. One line/valve for excessive water pressure (let the water out) and one with the valve to let air in (as water is consumed through use). Add a couple of baffles in the tank to break up the sloshing effect while towing and problem solved? We're talking about molded plastic material--shouldn't be too cost prohibitive to implement, right?

[Yoda]

That's brilliant, Keith.

What I read in your post is to manufacturer a tank with two vent lines with one-way valves attached. One line/valve for excessive water pressure (let the water out) and one with the valve to let air in (as water is consumed through use). Add a couple of baffles in the tank to break up the sloshing effect while towing and problem solved? We're talking about molded plastic material--shouldn't be too cost prohibitive to implement, right?

LOL "We're talking about molded plastic material--shouldn't be too cost prohibitive to implement, right?"

You would think - BUT...........................

Yep back to my corner

Keith

[ThePowells]

I wonder if the mothership is beginning to believe that they are playing to a hard audience?