How to stop air getting into water tank

[hd002e]

I have a 50 gallon bladder type water tank that fills through a deck fitting. No matter what I seem to do, I tend to get an unpleasant amount of air in the tank so I have to empty it all out, disconnect the hoses and purge the tank by squeezing out the air every month.

What are some of the best practices to keep the air pit of bladder type tanks?

[Pete7]

The Plastimo flexible tanks I have previous used have the filler entering the top of the tank and water exiting at the bottom. I take it yours feed to the side or bottom. Can this be changed by moving the tank around or upside down?

Pete

[denverd0n]

Does it fill with air as you are using it? That is, after it has been filled and purged of air. Or while you are filling it?

[a64pilot]

I’m betting it either has a vent, or the fill port’s O-ring is leaking.

[billknny]

Assuming you have this properly set up, with a single fill, a single draw, and no vent. If it is correctly set up, there should never be any significant amounts of air in the tank.

You can reduce the amount of air that enters by filling slowly. That's really about the only control for the entry of air. If the tank is properly set up that should not be necessary.

The key to success is the geometry of how the tank fits in its space. If the fill pipe is at the highest point, (It needs to be!) then the tank will naturally vent itself as gravity pulls the top down.

The top surface of the tank needs to be free to collapse. I have seen installations where the tank upper surface was suspended from the fill pipe, and could not collapse as the tank emptied. If there is a accidental, or intentional, "U-Trap" in the fill line air will also be trapped in the tank with no way out.

It is also best to have both the fill AND the draw pull off of the upper surface of the tank. I know, other people tell you different, but that is not the way these should normally be installed. One of the issues with putting the draw pipe on the bottom of the tank is that it is always getting crushed with the weight of the water. The fitting can fail, or the output hose can kink.

The top draw works because the tank collapses as it empties. If the draw pipe exits from the top, it will also suck out any air that gets in and pass it out through the system.

If the only way to get air out of your tank is to empty it, and manually massage it out, your tank is not set up correctly and will not work with its current installation.

[GordMay]

Originally Posted by billknny

...
The key to success is the geometry of how the tank fits in its space. If the fill pipe is at the highest point, (It needs to be!) then the tank will naturally vent itself as gravity pulls the top down...
... It is also best to have both the fill AND the draw pull off of the upper surface of the tank. I know, other people tell you different, but that is not the way these should normally be installed...

Indeed.
From Plastimo:
Ø 38 mm inlet valve is situated on upper side of tank
Ø 12 mm outlet valve is situated on back side of tank (except on triangular model : inlet and outlet are both on upper side of tank).
➥ https://www.plastimo.com/en/reservoi...uce-10108.html
From Nauta:
ALWAYS mount fittings on the top of tanks to prevent wear on the tanks.
➥ https://www.imtra.com/COLLATERAL/DOC...IBLE_TANKS.PDF
From Vetus:
Both connector nipples must be fitted in the top of the tank, the 16 mm. outlet nipple should be fitted as low as possible.
➥ https://www.vetus.com/media/magentom...11133320_0.pdf

[hd002e]

Originally Posted by Pete7

The Plastimo flexible tanks I have previous used have the filler entering the top of the tank and water exiting at the bottom. I take it yours feed to the side or bottom. Can this be changed by moving the tank around or upside down?



Pete

I have the 200L plastimo model. The fill is on top and the draw is on the bottom.

[hd002e]

Originally Posted by denverd0n

Does it fill with air as you are using it? That is, after it has been filled and purged of air. Or while you are filling it?

I think air gets in primarily as I am filling it up. There may be more coming in afterwards but I don’t have a good method to detect that. As I am filling it up, I can actuwatch the inlet hose and see tge air bubbles get in, however the water supply does not have any air in it.

My fear is that the water hose from the dock being a smaller diameter than the deck fitting somehow traps air and carries that in. Not sure what the mechanism is.

Do I need a tight seal between the hose from the dock and the deck fitting?

[hd002e]

Originally Posted by billknny

Assuming you have this properly set up, with a single fill, a single draw, and no vent. If it is correctly set up, there should never be any significant amounts of air in the tank.

You can reduce the amount of air that enters by filling slowly. That's really about the only control for the entry of air. If the tank is properly set up that should not be necessary.

The key to success is the geometry of how the tank fits in its space. If the fill pipe is at the highest point, (It needs to be!) then the tank will naturally vent itself as gravity pulls the top down.

The top surface of the tank needs to be free to collapse. I have seen installations where the tank upper surface was suspended from the fill pipe, and could not collapse as the tank emptied. If there is a accidental, or intentional, "U-Trap" in the fill line air will also be trapped in the tank with no way out.

It is also best to have both the fill AND the draw pull off of the upper surface of the tank. I know, other people tell you different, but that is not the way these should normally be installed. One of the issues with putting the draw pipe on the bottom of the tank is that it is always getting crushed with the weight of the water. The fitting can fail, or the output hose can kink.

The top draw works because the tank collapses as it empties. If the draw pipe exits from the top, it will also suck out any air that gets in and pass it out through the system.

If the only way to get air out of your tank is to empty it, and manually massage it out, your tank is not set up correctly and will not work with its current installation.

Interesting point! The fill fitting is on top and the top is free to collapse. I do however noticed that there is some water sitting in the hose from the deck fitting to the bladder. That tells me there is an unintentional U somewhere. I will try to work that out tomorrow.

I also have an L “elbow” along the path from the deck fitting to the bladder tank. When I am filling the tank, I shove the hose from the dock all the way in there so it gets i to the elbow.

Due to the geometry of the tank I cannot do a top draw.

[guyrj33]

Add a vent line.

[Franziska]

We have the same issue with an in our case unavoidable U. A vent pipe with shut off valve at the upper end helped us.
By the way, we use the CAN type bladders. Big plus over Vetus is that the plastic connectors are welded in and do not rely on a pressure fit.
Use a thin layer of Teflon tape or a tiny amount of silicone on the threads when connecting them.

[Cheechako]

Add a vent line

[hd002e]

Originally Posted by Cheechako

Add a vent line

How does one go about doing that? Do I have to make another hole in the bladder?

[guyrj33]

Originally Posted by hd002e

How does one go about doing that? Do I have to make another hole in the bladder?

typically there is an additional fitting for a vent line added to the bladder.
I have a vent on the bladder I use for my holding tank.
I imagine you could add a T fitting in the fill line just before it enters the bladder, it could work.

[AndyEss]

My bladder water tanks (2) are filled and drained from the top. I have vent lines to both tanks.
As long as the bladder is allowed to completely deflate as you are using the water, no or little air should be trapped in the bladder.
Sounds like you need to add vents. A T fitting in the fill line as close to the tank should work best.