How to solve the breather pipe pressurising the water tank

[VicS]

Not quite if diameter of the breather pipe is different to the filler pipe as there will be a different weight of fluid but I'm splitting hairs as the extra pressure of either weight is trivial compared to the weight of the fuel in the tank.

Forget this nonsense about weight. The pressure created by a column of water is purely a function of the height of the column . Diameter of the pipe does not come into it!

That is why/how you can see pressures quoted in "inches water gauge" or "feet head" etc.


( now wait for the pedants to mention density or "g".)

 

[RupertW]

Afraid you are wrong there - pressure is force over area. The area is the whole surface of the tank. The force is the weight of water in the pipe whose volume is determined by the pipe height x Pi x radius squared so a pipe with double the diameter will contain 4 times the fluid so 4 times the weight so 4 times the additional pressure on the fluid in the tank.

But that's 4 times nearly nothing as the weight of fluid in the tank is so many times more than the weight of the fluid in the pipe.

 

[Heckler]

Afraid you are wrong there - pressure is force over area. The area is the whole surface of the tank. The force is the weight of water in the pipe whose volume is determined by the pipe height x Pi x radius squared so a pipe with double the diameter will contain 4 times the fluid so 4 times the weight so 4 times the additional pressure on the fluid in the tank.

But that's 4 times nearly nothing as the weight of fluid in the tank is so many times more than the weight of the fluid in the pipe.

Pounds per SQUARE INCH!
Doesnt matter how big the pipe is the pressure is always quoted PER SQUARE INCH!
Stu

 

[RupertW]

Pounds per SQUARE INCH!
Doesnt matter how big the pipe is the pressure is always quoted PER SQUARE INCH!
Stu

Again you are missing the point - the area In question is the area of the main tank which is a constant - it is being acted upon by the additional force of the weight of the fluid in a pipe above it. Double the diameter of that pipe and you quadruple the additional force on the fixed area of the tank. Of course if we were answering a different question about the pressure at the bottom of a closed vertical pipe then the force increases by the square of the diameter but the pressure remains (as you say) exactly the same.

 

[Heckler]

Again you are missing the point - the area In question is the area of the main tank which is a constant - it is being acted upon by the additional force of the weight of the fluid in a pipe above it. Double the diameter of that pipe and you quadruple the additional force on the fixed area of the tank. Of course if we were answering a different question about the pressure at the bottom of a closed vertical pipe then the force increases by the square of the diameter but the pressure remains (as you say) exactly the same.

Keep digging!

Stu

 

[Tranona]

Here is a bad sketch, the filler is not sealed (at least until it backs up), the breather is shown exiting vertically but it actually is horizontal and has a slight dip due to the floor boards, hence the problem
View attachment 68601
It is actually a set up of 2 tanks with an interconnected breather via a T near the tanks as described in the post above. I hadn't considered the possibility of incomplete filling of one tank due to pressure imbalance of the breathers, I will try exiting them both at deck level. My colleague in the office just suggested a good solution beyond very slow filling, insert a second pipe down the filler (a length of hosepipe) into the air space then the filler will never block. I am curious now as to whether my tanks can take more than 300 litres.

Are you sure your filler pipe is 10cm diameter? That is enormous - the usual is a 38mm pipe. does it really go directly vertical into the tank? How deep is the tank at that point?

If it really is that diameter fail to see why any air from the tank cannot go back up the pipe if you are using a normal 15mm hose pipe - even with high mains pressure.

Your breather system as you describe does not help - better to have a breather for each tank (and a separate filler for each), preferably out of the top at the opposite end of the tank from the filler. although not essential. Out of the side is OK but it will cease to let air escape before the tank is full.

 

[Lon nan Gruagach]

Afraid you are wrong there - pressure is force over area. The area is the whole surface of the tank. The force is the weight of water in the pipe whose volume is determined by the pipe height x Pi x radius squared so a pipe with double the diameter will contain 4 times the fluid so 4 times the weight so 4 times the additional pressure on the fluid in the tank.

But that's 4 times nearly nothing as the weight of fluid in the tank is so many times more than the weight of the fluid in the pipe.

a wee thought experiment to help you out here (and ruin skipper_stu's fun)
So you have a tube full of water that is not flowing because of back pressure from a tank. ie pressure in tank = pressure at bottom of tube.
Now, instead of doubling the size of tube, you add 3 more at different locations (but all with their bottom ends at the same height in the tank)

 

[Lon nan Gruagach]

Here is a bad sketch, the filler is not sealed (at least until it backs up), the breather is shown exiting vertically but it actually is horizontal and has a slight dip due to the floor boards, hence the problem
View attachment 68601
It is actually a set up of 2 tanks with an interconnected breather via a T near the tanks as described in the post above. I hadn't considered the possibility of incomplete filling of one tank due to pressure imbalance of the breathers, I will try exiting them both at deck level. My colleague in the office just suggested a good solution beyond very slow filling, insert a second pipe down the filler (a length of hosepipe) into the air space then the filler will never block. I am curious now as to whether my tanks can take more than 300 litres.

And theres the problem.
There is water in the dip and, when the tank fills, there is enough water in the breather to form a column right up to deck level.
I suggest that (if your 10cm filler measurement is correct) then there is either a restriction in the filler tube or the filler tube goes some way down into the tank (possibly near the bottom)

Solutions:
address the filler tube so air can easily go back out that way
and/or
get rid of the dip or increase the diameter of the breather AFTER it turns upwards so that no matter how much water there is in it, there is not enough to form such a tall column.

 

[RupertW]

a wee thought experiment to help you out here (and ruin skipper_stu's fun)
So you have a tube full of water that is not flowing because of back pressure from a tank. ie pressure in tank = pressure at bottom of tube.
Now, instead of doubling the size of tube, you add 3 more at different locations (but all with their bottom ends at the same height in the tank)

Ok. So now you have 4 times the additional small force on the whole fixed surface area of the tank and therefore the pressure on the sides of the tank increases marginally. I wonder what the other people,on this thread happens, especially if the 3 extra pipes become a hundred and the weight of the water in all the combined pipes equals the weight of the water in the whole tank.

By their logic the pressure within the tank remains the same, and the same still if there were a thousand tanks worth of water in the pipes above, each one 1.5m in height.

 

[Lon nan Gruagach]

Ok. So now you have 4 times the additional small force on the whole fixed surface area of the tank and therefore the pressure on the sides of the tank increases marginally. I wonder what the other people,on this thread happens, especially if the 3 extra pipes become a hundred and the weight of the water in all the combined pipes equals the weight of the water in the whole tank.

By their logic the pressure within the tank remains the same, and the same still if there were a thousand tanks worth of water in the pipes above, each one 1.5m in height.

So, by that reason, you also have that additional force acting on the bottom of the first tube.

 

[RupertW]

So, by that reason, you also have that additional force acting on the bottom of the first tube.

Do you mean that the first tube exerts a force when it's the only one? In which case yes.

Or that in a multiple tube system the pressure within the whole system is affected at any given height below the surface by every other pipe, in which case yes as well.

So that within each identical pipe the pressure is exactly the same at any given height regardless of whether there is a tank of water connected underneath. But the pressure of water within the tank is increased slightly by each pipe full of water above it. Another thought experiment to demonstrate this would be imagining the tank not quite full. The water pressure increase half way down the tank is the same when the pipe full drops into the tank as it would be if the tank was full and the pipe full was sitting above it.

 

[NormanS]

The level of misunderstanding about pressure here is astounding.
Take a simple case of a tank with an upstand pipe from the top of the tank, open at the top. Fill the tank and the upstand pipe right up to the top. There is, by virtue of the column of water in the pipe, a pressure of "x" psi where the pipe is connected to the tank. It doesn't matter what diameter the pipe is, the pressure is the same, solely due to the "head".
That pressure is exerted on every square inch of the whole area of the top of the tank. Even the modest head from a deck filler and an average yacht tank, WHEN MULTIPLIED BY THE AREA OF THE TOP OF THE TANK exerts a considerable load on the tank. Yes, my tank gives a bang when it's full.

I can't really decipher the OP's drawing, but it looks as if his problems stem from having two tanks with a single breather, and the breather is installed in such a manner that there is a "U" bend in it.

 

[Lon nan Gruagach]

So, by that reason, you also have that additional force acting on the bottom of the first tube.

Do you mean that the first tube exerts a force when it's the only one?

The first tube must exert a force since:
The system is stable, nothing is moving, the column of water in the first tube is held there by pressure at its bottom (from the tank).

In which case yes.

Now, you agree that adding more identical tubes increases the pressure in the tank, and therefore at the bottom of the first tube.
How does that not push the water back up the tube?

 

[RupertW]

The level of misunderstanding about pressure here is astounding.
Take a simple case of a tank with an upstand pipe from the top of the tank, open at the top. Fill the tank and the upstand pipe right up to the top. There is, by virtue of the column of water in the pipe, a pressure of "x" psi where the pipe is connected to the tank. It doesn't matter what diameter the pipe is, the pressure is the same, solely due to the "head".
That pressure is exerted on every square inch of the whole area of the top of the tank. Even the modest head from a deck filler and an average yacht tank, WHEN MULTIPLIED BY THE AREA OF THE TOP OF THE TANK exerts a considerable load on the tank. Yes, my tank gives a bang when it's full.

I can't really decipher the OP's drawing, but it looks as if his problems stem from having two tanks with a single breather, and the breather is installed in such a manner that there is a "U" bend in it.

Brilliant - the combination of ignorance and confidence is wonderful.

The reason the tank bangs (mine does too even with unblocked filler pipes) is the pressure of water in the tank caused by the water in the tank, not the tiny pipes.

By that logic a millimetre square pipe 1m high will exert the same pressure on a metre square tank as a metre square pipe above the same tank, which is nonsense of course.

The pressure at the bottom of both pipes is of course the same but the force that is exerted by the weight of the pipe's water on the water in the tank is completely different as you need to DIVIDE by the area of the tank not MULTIPLY by it.

So a millimetre square pipe on a metre squared pipe will increase the pressure by height times ( pipe cross-sectional area / tank cross sectional area). So if pipe cross sectional area is equal to the tank cross sectional area then the pressure at the bottom of the tank is doubled if the pipe height and tank depth are the same, but if the pipe cross sectional area is only a thousandth of the tank cross sectional area then the pressure will be 1.001 x the pressure without a pipeful of water.

It's really not that hard.

 

[lw395]

Brilliant - the combination of ignorance and confidence is wonderful.

The reason the tank bangs (mine does too even with unblocked filler pipes) is the pressure of water in the tank caused by the water in the tank, not the tiny pipes.

By that logic a millimetre square pipe 1m high will exert the same pressure on a metre square tank as a metre square pipe above the same tank, which is nonsense of course.

The pressure at the bottom of both pipes is of course the same but the force that is exerted by the weight of the pipe's water on the water in the tank is completely different as you need to DIVIDE by the area of the tank not MULTIPLY by it.

So a millimetre square pipe on a metre squared pipe will increase the pressure by height times ( pipe cross-sectional area / tank cross sectional area). So if pipe cross sectional area is equal to the tank cross sectional area then the pressure at the bottom of the tank is doubled if the pipe height and tank depth are the same, but if the pipe cross sectional area is only a thousandth of the tank cross sectional area then the pressure will be 1.001 x the pressure without a pipeful of water.

It's really not that hard.

The only reason the pressure is less with a very small vertical pipe is capillary action/surface tension.
You are plain, stone cold wrong.

Imagine a pipe 1cm square and 1 metre tall full of water.
Each has a pressure at the bottom due to the weight of water in the tube.
100g of water, supported on 1sq cm of area. Presure = 100g/sq cm

Make the pipe 10cm x 10cm.
It now holds 10x 10 x100 = 10,000 cc of water.
Area = 100sqcm
Pressure= 10,000g/100sqcm = 100g/sqcm

Excuse the playschool units.

 

[NormanS]

Brilliant - the combination of ignorance and confidence is wonderful.

The reason the tank bangs (mine does too even with unblocked filler pipes) is the pressure of water in the tank caused by the water in the tank, not the tiny pipes.

By that logic a millimetre square pipe 1m high will exert the same pressure on a metre square tank as a metre square pipe above the same tank, which is nonsense of course.

The pressure at the bottom of both pipes is of course the same but the force that is exerted by the weight of the pipe's water on the water in the tank is completely different as you need to DIVIDE by the area of the tank not MULTIPLY by it.

So a millimetre square pipe on a metre squared pipe will increase the pressure by height times ( pipe cross-sectional area / tank cross sectional area). So if pipe cross sectional area is equal to the tank cross sectional area then the pressure at the bottom of the tank is doubled if the pipe height and tank depth are the same, but if the pipe cross sectional area is only a thousandth of the tank cross sectional area then the pressure will be 1.001 x the pressure without a pipeful of water.

It's really not that hard.

You are entirely wrong I'm afraid. I don't think you understand the difference between pressure and force exerted.

 

[VicS]

Brilliant - the combination of ignorance and confidence is wonderful.

The reason the tank bangs (mine does too even with unblocked filler pipes) is the pressure of water in the tank caused by the water in the tank, not the tiny pipes.

By that logic a millimetre square pipe 1m high will exert the same pressure on a metre square tank as a metre square pipe above the same tank, which is nonsense of course.

The pressure at the bottom of both pipes is of course the same but the force that is exerted by the weight of the pipe's water on the water in the tank is completely different as you need to DIVIDE by the area of the tank not MULTIPLY by it.

So a millimetre square pipe on a metre squared pipe will increase the pressure by height times ( pipe cross-sectional area / tank cross sectional area). So if pipe cross sectional area is equal to the tank cross sectional area then the pressure at the bottom of the tank is doubled if the pipe height and tank depth are the same, but if the pipe cross sectional area is only a thousandth of the tank cross sectional area then the pressure will be 1.001 x the pressure without a pipeful of water.

It's really not that hard.

"a millimetre square pipe 1m high will exert the same pressure on a metre square tank as a metre square pipe above the same tank,"

Absolutely ! That is correct, not nonsense at all.

 

[Dull Spark]

So.... is it all those bits of pressure times the area of the hull that keeps the boat afloat? Now I can see it! Regards, Archimedes

 

[Daydream believer]

Afraid you are wrong there - pressure is force over area. The area is the whole surface of the tank. The force is the weight of water in the pipe whose volume is determined by the pipe height x Pi x radius squared so a pipe with double the diameter will contain 4 times the fluid so 4 times the weight so 4 times the additional pressure on the fluid in the tank.

But that's 4 times nearly nothing as the weight of fluid in the tank is so many times more than the weight of the fluid in the pipe.

NO VicS is right

Sorry typed too slow

 

[knuterikt]

A tank with two pipes is just two communicating vessels, no need to make complicated
https://en.wikipedia.org/wiki/Communicating_vessels
best suggestion so far is to disconnect the hoses to locate the obstruction.