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Old 10-03-2016, 02:48 PM   #1
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Water tanks: filling and venting lines.

For background: This past spring, I replaced the "O" rings on all filler caps, both the water and fuel tanks. This summer, I noticed that there was a vacuum apparent when I opened the caps to fill the water tanks with air being sucked in upon opening. I interpreted this as a problem with my vent lines: they were blocked either at the thru hulls or somewhere down the line, causing negative pressure down the line. I have gone through an investigation of the vent lines and have concluded there are no blockages anywhere along the lines, either at the thru hull or the lines themselves to the tanks.

The only way I can think there will be a vacuum in the filler lines, is if there is a down-pipe into the tank, where the filler lines connect to the tank. So when the level of water in the tank falls with use, the water in the down-pipe remains above the water-line in the tank, creating a vacuum. The pressure is then equalized upon opening the filler cap. Is this interpretation correct?

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Old 10-03-2016, 03:51 PM   #2
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Jim, I think Krogen used the same fill caps on my Manatee, I have never had the problem you describe. Might be the vent hose collapses as the water is drawn out of the tank. My fill caps have the O ring seals so they seal air tight and are fairly new. The fill pipe theory may have merit, has there always been this negative pressure ?
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Old 10-03-2016, 04:07 PM   #3
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Quote:
Originally Posted by JDCAVE View Post
I have gone through an investigation of the vent lines and have concluded there are no blockages anywhere along the lines, either at the thru hull or the lines themselves to the tanks.
Jim
How did you confirm vents were OK?

I haven't heard of a FW tank w/ a dip tube - only holding tank drains??
Your scenario would only create a vac in the fill as the water level goes down - not in the tank

Can you pump the tank down without collapsing sides / top of the tank

HiDHo may be on to something

I've used a wet-dry vac to clear vents and may help diagnose the problem -

Blow into the fill and see if you get good venting & no tank expansion / back pressure

Reverse flow to Suck (vac) the fill and see if you get flow at vent

If vent is accessible you can repeat above from the vent to see if you get vac / pressure at the fill

Do you have a drain on the tank you can open (or pump feed line you can disconnect) to see if you get good flow or it stops as you draw a vac on the tank?
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Old 10-03-2016, 04:14 PM   #4
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Might be the vent hose collapses as the water is drawn out of the tank.
I have ruled out any problems with the vent thru-hulls or the lines themselves. There are no kinks and I have been able to blow air into the tanks through the lines and also out of the thru-hulls.

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The fill pipe theory may have merit, has there always been this negative pressure ?
I have not noticed it before, but it might have been because the old "O" rings had small cracks allowing air to pass through.

Jim
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Old 10-03-2016, 04:22 PM   #5
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Bacchus: I'm basically at a loss to explain my observations. As mentioned above, I have ruled out problems with the integrity of the vent lines. They are fine and show no signs of collapsing. As mentioned, I can blow air out of the vents and also into the tank through the lines. However, air rushes backwards...it does not go out the filler ports (which were open during this test).

The only "theory" I can come up with to explain my observations is there is some sort of pressure created by the fact that there is water in the filler tube, inside the tank. Because of the good "O" ring seal on the filler caps, a vacuum is created as the water level in the tank decreases, because of the "head" in the filler tube inside the tank. Think of the filler tube as a pipette filled with water. If you pull the pipette out of water, with your finger on the top, the water remains inside, kept there with the negative pressure inside the tube.
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Old 10-03-2016, 04:26 PM   #6
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A photo of the vent lines at the thru-hull location. I loosened the claps and pulled the lines off and checked the system at that point. I also traced the lines all the way to,the tanks and could not find any kinks or obstructions. Air could be blown into the tanks.
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Old 10-03-2016, 04:27 PM   #7
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Water tanks: filling and venting lines.

Oops! The photo!
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The lower lines are for the water tank. The upper line is for the waste tank.
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Old 10-03-2016, 04:48 PM   #8
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Jim, your theory is on the right track. You've got a good understanding of hydrostatics.

One other possibility is the water inlet line enters into the side of the tank rather than the top. This would have the same effect as having a down-pipe. Either way, its not a problem. It lets you know your o-rings are in good condition.
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Old 10-03-2016, 04:58 PM   #9
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Thanks A/C. As I think about it though, I not real happy with having the vent for the waste tank so close to the vents for the potable water tanks! Hmmm!
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Old 10-03-2016, 05:33 PM   #10
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There is no possible way for you to have a vacuum (or pressure) in the water tanks if the vent lines and fittings are open. No way. It would be like trying to create a vacuum in a bottle with the lid off.


Go back and check your testing.
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Old 10-03-2016, 05:39 PM   #11
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Quote:
Originally Posted by JDCAVE View Post
Bacchus: I'm basically at a loss to explain my observations. As mentioned above, I have ruled out problems with the integrity of the vent lines. They are fine and show no signs of collapsing. As mentioned, I can blow air out of the vents and also into the tank through the lines. However, air rushes backwards...it does not go out the filler ports (which were open during this test).

The only "theory" I can come up with to explain my observations is there is some sort of pressure created by the fact that there is water in the filler tube, inside the tank. Because of the good "O" ring seal on the filler caps, a vacuum is created as the water level in the tank decreases, because of the "head" in the filler tube inside the tank. Think of the filler tube as a pipette filled with water. If you pull the pipette out of water, with your finger on the top, the water remains inside, kept there with the negative pressure inside the tube.
I understand and agree a dip tube could create the condition you describe...

If you blow into the fill w/ the wet/dry vac (rag around to get a reasonably good seal) you should be able to confirm dip tube if you get a strong bubbling sound in the tank - if that's the case I don't see a problem do you?
If the vents are working properly the neg pressure is only in the fill line until you open it to add water - shouldn't be a problem I don't think

- if no bubbles & vent is exhausting I have no idea
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Old 10-03-2016, 06:02 PM   #12
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I may be seeing things, but those do not look like reinforced braided lines, but just soft plastic. Is it possible one or more is kinking when under pressure/vacuum?
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Old 10-03-2016, 08:36 PM   #13
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Water tanks: filling and venting lines.

Quote:
Originally Posted by WesK View Post
There is no possible way for you to have a vacuum (or pressure) in the water tanks if the vent lines and fittings are open. No way. It would be like trying to create a vacuum in a bottle with the lid off.

Go back and check your testing.

Sure there is:
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Let's say my finger represents the filler cap and the pipette represents the head of water in a filler tube that is inside the water tank, the surface of the water in the bucket represents the water level in the water tank at atmospheric pressure. The air space inside the pipette is under a vacuum that is released when I remove my finger. If the "O" ring gasket is a good seal, and there is a filler tube that goes inside the tank, you can theoretically get a vacuum, even though the vent lines are functional and at atmospheric pressure.

The vent lines are functional.
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Old 10-03-2016, 11:12 PM   #14
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Wink

JD, Interesting quandary you have there. To determine if the fill line is a stand pipe try placing your vacuum on that line see if you draw water out. If you do then you may well indeed have the configuration you are suggesting. Never have seen that in freshwater tank, but then again, I have seen little.
Sort of the reverse of what Don suggested. Suck it out Blow it in. Same Diff.
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Old 10-03-2016, 11:56 PM   #15
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Great idea, TB. I am unable to open the ports to the tanks that are under the floor. But I should be able to hook up a shop vac to the fill ports on the deck.
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Old 10-04-2016, 01:24 AM   #16
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Oops! The photo!
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The lower lines are for the water tank. The upper line is for the waste tank.
I hope there will never be a wastetank too full, there is a small chance of getting flavored water after overflow.
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Old 10-04-2016, 02:48 AM   #17
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Does your vent line have a dip forming a loop seal like a sink trap? Filled with water this would block vent flow until a certain vacuum is formed. The loop seal depth would determine how much vacuum needed to break over.
The vent line should slope continuously upwards from the tank to a high point then downhill continuously to the vent through hull.
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Old 10-04-2016, 08:30 AM   #18
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Well JD, here is a redneck solution, cut about 1/16" out of the "0" ring and let it pressure adjust naturally while you try suggestions one through fifty.
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Old 10-04-2016, 08:38 AM   #19
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Does your vent line have a dip forming a loop seal like a sink trap? Filled with water this would block vent flow until a certain vacuum is formed. The loop seal depth would determine how much vacuum needed to break over.
The vent line should slope continuously upwards from the tank to a high point then downhill continuously to the vent through hull.
That's a good possibility and could cause the problem the OP seems to have.
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Old 10-04-2016, 08:43 AM   #20
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Jim,
Thanks for this posting. Now that you mention it, I think I have also noticed a slight vacumn effect upon opening the fill.
However, I ignored it since I figured the vents had to be open otherwise the water pump would not be able to fully empty the tank.

I will investigate further upon my return to Dauntless in a few weeks.

Richard
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