How to solve the breather pipe pressurising the water tank

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Polly1

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My water tanks are in the bilge. They have a breather at deck level. I thought it was blocked as at one point during filling they start to swell and block.
On investigation what is happening is a small amount of overflow in the breather is not draining back into the tank. As it gets lifted to the deck outlet the head of water that it represents pressurises the tank opposing further inflow.
Has anyone else had/solved this.
Thanks
Guy
 
easier said than done, but have you checked the route this breather pipe takes?
If there are substantial loops what you experience is a possibility (I'd have thought that the head is not going to be that big to make the plastic? tank swell...)
Have you removed, checked and cleaned the breather?

cheers

V.
 
Polly1 said:
My water tanks are in the bilge. They have a breather at deck level. I thought it was blocked as at one point during filling they start to swell and block.
On investigation what is happening is a small amount of overflow in the breather is not draining back into the tank. As it gets lifted to the deck outlet the head of water that it represents pressurises the tank opposing further inflow.
Has anyone else had/solved this.
Thanks
Guy
Click to expand...

Your breather must come from the bottom of the tank?

Or is there a non-return valve?
Some breathers only let air in, as the contents of the tank are used.
 
Polly1 said:
My water tanks are in the bilge. They have a breather at deck level. I thought it was blocked as at one point during filling they start to swell and block.
On investigation what is happening is a small amount of overflow in the breather is not draining back into the tank. As it gets lifted to the deck outlet the head of water that it represents pressurises the tank opposing further inflow.
Has anyone else had/solved this.
Thanks
Guy
Click to expand...

It's not possible for standing water in a breather pipe to affect the filling of the tank. A column of water over 30 feet high would be needed to produce even 1 atmosphere of pressure in the water tank so unless your breather exits at the top of the mast it can't be that. :confused:

Richard
 
DuncanHall said:
If they only let air in how would one fill the tank!
Click to expand...

The tank could be filled because the filling point would nomally be open whilst the hose pipe was filling it, although you might get quite a bit of blow-back whilst filling if the filling point was quite small.

It therefore sounds as if the OP is connecting the filling hose to a spigot on the tank so creating a sealed filling system. I think this is quite unusual but, if coupled with a blocked one-way valve in the breather, could cause the problem mentioned by the OP.

However, this is not a system that I have ever seen before.

Richard
 
RichardS said:
It's not possible for standing water in a breather pipe to affect the filling of the tank. A column of water over 30 feet high would be needed to produce even 1 atmosphere of pressure in the water tank so unless your breather exits at the top of the mast it can't be that. :confused:

Richard
Click to expand...

If the breather was completely full of water, and vented higher than the filler, and the filler was also full enough not to let air out, it could happen. But if you sealed the hose to the filler, even a small amount of pressure would blow the water out of the breather. Even the pressure from moving water. As you say 1 atm or 15 psi is 30+ feet of water so it's an unlikely story.
 
lw395 said:
If the breather was completely full of water, and vented higher than the filler, and the filler was also full enough not to let air out, it could happen.
Click to expand...

If the hose to the filler is not sealed, it's impossible to ever generate any pressure above atmospheric in the tank however high or full the breather is or how full the filler pipe is. :)

Richard
 
RichardS said:
If the hose to the filler is not sealed, it's impossible to ever generate any pressure above atmospheric in the tank however high or full the breather is or how full the filler pipe is. :)

Richard
Click to expand...

That was my thought too and in fact our breathers fill with water every time we fill a tank and it's the overflow from the breather that lets us know the tank is full as it it a few inches below desk level.

Our breather was blocked by a solitary bee nest but we did two checks to find that hidden problem. First took the breather off the tank and checked that water came out without obstruction as the tank filled, and then blew through the breather from that tank end of the breather pipe. As the breath was obstructed we then removed the breather pipe altogether onto the pontoon and used a mixture of hose pressure and rodding to completely clear it.
 
Thanks for the replies
The filler becomes sealed as there is a 1.5 metre drop to the tanks and it back fills as soon as it encounters any difficulty entering the tank. This comes from the air not leaving cleanly through the breather., this is located at the top of one of the tanks
As for the head of water not having much effect, 1 metre would be 0.1 atmospheres or 10,000 Pascals. The tanks have a surface area of maybe 2msq so 20, 000 Newtons of force or 2000 kg.
The tanks are stainless, 300 litres and swell enough the raise the floorboards if I dont turn off the tap.
I just wonder why no one else is having this problem, I imagine this is a common arrangement and I must have overlooked something.
 
Last edited:
Polly1 said:
Thanks for the replies
The filler becomes sealed as there is a 1.5 metre drop to the tanks and it back fills as soon as it encounters any difficulty entering the tank. This comes from the air not leaving cleanly through the breather., this is located at the top of one of the tanks
As for the head of water not having much effect, 1 metre would be 0.1 atmospheres or 10,000 Pascals. The tanks have a surface area of maybe 2msq so 20, 000 Newtons of force or 2000 kg.
The tanks are stainless, 300 litres and swell enough the raise the floorboards if I dont turn off the tap.
I just wonder why no one else is having this problem, I imagine this is a common arrangement and I must have overlooked something.
Click to expand...

A sketch showing how the tank filler is connected, how the tanks are interconnected and how they are vented might help to understand what the problem might be.

With a single tank one would expect the tank to vent through the filler while being filled, the tank vent only being there to allow air to enter as water is used.
 
Polly1 said:
Thanks for the replies
The filler becomes sealed as there is a 1.5 metre drop to the tanks and it back fills as soon as it encounters any difficulty entering the tank. This comes from the air not leaving cleanly through the breather., this is located at the top of one of the tanks
As for the head of water not having much effect, 1 metre would be 0.1 atmospheres or 10,000 Pascals. The tanks have a surface area of maybe 2msq so 20, 000 Newtons of force or 2000 kg.
The tanks are stainless, 300 litres and swell enough the raise the floorboards if I dont turn off the tap.
I just wonder why no one else is having this problem, I imagine this is a common arrangement and I must have overlooked something.
Click to expand...

Sorry, but your figures are not right - as the air pressure above and below is equal the only extra pressure is the weight of the water in the breather pipe which will be small enougn for 1.5m to be cleared by a simple couple of breaths as I know. It has to be an obstruction not the tiny weight of water.

Have you take off the lower end of the breather pipe off to check firstly that water flows freely from the tank and then that blowing into the breather pipe is unobstructed all the way to its upper exit?
 
Polly1 said:
Thanks for the replies
The filler becomes sealed as there is a 1.5 metre drop to the tanks and it back fills as soon as it encounters any difficulty entering the tank. This comes from the air not leaving cleanly through the breather., this is located at the top of one of the tanks
As for the head of water not having much effect, 1 metre would be 0.1 atmospheres or 10,000 Pascals. The tanks have a surface area of maybe 2msq so 20, 000 Newtons of force or 2000 kg.
The tanks are stainless, 300 litres and swell enough the raise the floorboards if I dont turn off the tap.
I just wonder why no one else is having this problem, I imagine this is a common arrangement and I must have overlooked something.
Click to expand...

The problem, as you describe it, is impossible I'm afraid.

Is your filling system sealed i.e. is the supply hose clamped to a spigot on the filling point ..... or is it just a hose pushed into a larger filling point like a petrol filler in a car fuel tank?

Richard
 
Polly1 said:
Thanks for the replies
The filler becomes sealed as there is a 1.5 metre drop to the tanks and it back fills as soon as it encounters any difficulty entering the tank. This comes from the air not leaving cleanly through the breather., this is located at the top of one of the tanks
As for the head of water not having much effect, 1 metre would be 0.1 atmospheres or 10,000 Pascals. The tanks have a surface area of maybe 2msq so 20, 000 Newtons of force or 2000 kg.
The tanks are stainless, 300 litres and swell enough the raise the floorboards if I dont turn off the tap.
I just wonder why no one else is having this problem, I imagine this is a common arrangement and I must have overlooked something.
Click to expand...

Don't think your problem is anything to do with the breather and a 1.5m drop is in theory good for filling. Your first sentence is the key to the problem. There is a restriction stopping the water from getting into the tank. Backfilling of the filler hose can only happen if the water cannot get into the tank. normally the water flows into the tank, but as the water does not fill the whole diameter air escapes up the hose. It is only when the tank is full that the breather does anything and it fills with water (assuming it also is not blocked) - as does the filler hose. Excess water then comes out of whichever is lower - usually the filler.

Some investigation needed as to what is obstructing the filler hose.
 
It is more about volume than pressure I think. I have exactly the same problem if I try to fill too fast. 38mm ID filler, 18mm ID vent. If I fill slowly the air can escape the vent comfortably, if I fill too fast the vent sounds like a jet and the (rigid plastic) tank swells with trapped air enough to lift the floorboards. It is a known Nauticat problem and referred to in the owners hand book. I have fitted an additional 18mm vent from the opposite end of the tank but if I get a really high output tap it still struggles.
 
Hadenough said:
It is more about volume than pressure I think. I have exactly the same problem if I try to fill too fast. 38mm ID filler, 18mm ID vent. If I fill slowly the air can escape the vent comfortably, if I fill too fast the vent sounds like a jet and the (rigid plastic) tank swells with trapped air enough to lift the floorboards. It is a known Nauticat problem and referred to in the owners hand book. I have fitted an additional 18mm vent from the opposite end of the tank but if I get a really high output tap it still struggles.
Click to expand...

Is yours a sealed filling system?

Richard
 
It is the same with my fuel tank. if I try to fill it too quickly ( from a jerry can) the volume of fuel fills the pipe & the vent cannot cope with the pressure build up. hence it sometimes causes the fuel to suddenly blow back on deck. presumably as the air is compressed then expands & pushes back.
the problem is solved by filling with care to give the breather a chance to compensate..
 
Daydream believer said:
It is the same with my fuel tank. if I try to fill it too quickly ( from a jerry can) the volume of fuel fills the pipe & the vent cannot cope with the pressure build up. hence it sometimes causes the fuel to suddenly blow back on deck. presumably as the air is compressed then expands & pushes back.
the problem is solved by filling with care to give the breather a chance to compensate..
Click to expand...

But in the cases being reported in this thread the problem is not that water (or fuel) blows back on deck. What is happening is that the tank itself if expanding fit to burst.

I've never heard of anything like it unless the filling system is sealed.

Richard
 
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