Pressure on water tank top

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ean_p

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If one started with a standard keel/bilge area water tank having a top surface area of say 2 square meters, filled from a cockpit filling point and vented through a high level vent and then added a second tank with its bottom level with the lower tanks top which was to be filled through the lower tank, to a height of 600mm above the lower tank,though it would have its own vent line. What would be the pressure exerted on the top skin of the lower tank and was it likely to be a problem?
 
Surface area is not the problem, makes no difference, the height of the top tank, (head of pressure) does make a difference, cant remember the formula now, but the pressure at .6 meters wont be very high.
Think of a lake and a barrel of water, the pressure at the bottom of a big lake at say 2 meters is the same as the pressure at 2 meters in a barrel, nowt to do with the surface area.
With this in mind, get a pipe .6 meters long and put a pressure gauge in the bottom, fill with water, what is the pressure? that's the pressure exerted on the bottom tank, excluding of course the height of your filler or breather, which also has an effect, but as they are already present, can be ignored, got it?
 
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If memory of school physics 50 years ago is correct, a column of water 30 ft high exerts 1 atmosphere of pressure or 14psi. A quick bit of mental arithmetic suggests the 600mm is one psi. To get the force exerted on the top surface multiply by the surface area.

If the vents and filling point of both tanks are at the same level then the max pressure on the lower tank will not change when the boat is static.
 
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Bosun Higgs said:
If memory of school physics 50 years ago is correct, a column of water 30 ft high exerts 1 atmosphere of pressure or 14psi. A quick bit of mental arithmetic suggests the 600mm is one psi. To get the force exerted on the top surface multiply by the surface area.
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The force on the top of the lower tank could be quite high, though.

If the surface area of the top of the tank is, say 5 sq feet this is 5 x 144 sq inches. The total (upwards) force on the bottom tank is therefore 720 lbs - surprising, isn't it. ;)
 
Playtime said:
The force on the top of the lower tank could be quite high, though.

If the surface area of the top of the tank is, say 5 sq feet this is 5 x 144 sq inches. The total (upwards) force on the bottom tank is therefore 720 lbs - surprising, isn't it. ;)
Click to expand...

Doesnt sound right somehow - I overfilled my bag tank last year and whilst it did lift the bunk board, It would have blown up like a balloon with that sort of force on it and it didnt. Well only half did.
 
the pressure = depth (head ) x density

in this case the depth is 60cm and the density of water is 1g/cm³

so the pressure is 60g/cm² or 0.06Kg/cm².

That converts to approx 0.85 lbs/sq inch ​

This then is the pressure at the bottom of the lower tank and the top of the lower tank.

The total force acting on the top of the lower tank (or the bottom of the upper tank) is pressure x area

the area in sq cm is 2 x 100 x 100 = 20,000 cm²

so the total force is 0.06 x 20000 = 1200 Kg !

That converts to approx 2600 lbs​


Unless he has corrected it while I have been typing that makes Playtime a bit out because he said 2m² is 5 ft². Its not 2 m² is approx 21.6 ft²!


BUT its too early after the loss of an hours sleep time last night for both brain cells to be communicating effectively. So please check my arithmetic
 
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VicS said:
the pressure = depth (head ) x density

in this case the depth is 60cm and the density of water is 1g/cm³

so the pressure is 60g/cm² or 0.06Kg/cm².

That converts to approx 0.85 lbs/sq inch ​

This then is the pressure at the bottom of the lower tank and the top of the lower tank.

The total force acting on the top of the lower tank (or the bottom of the upper tank) is pressure x area

the area in sq cm is 2 x 100 x 100 = 20,000 cm²

so the total force is 0.06 x 20000 = 1200 Kg !

That converts to approx 2600 lbs​


Unless he has corrected it while I have been typing that makes Playtime a bit out because he said 2m² is 5 ft². Its not 2 m² is approx 21.6 ft²!


BUT its too early after the loss of an hours sleep time last night for both brain cells to be communicating effectively. So please check my arithmetic
Click to expand...

I agree with Vics arithmetic (big of me!)

But what he has arrived at is the force acting over the entire base.
it may bulge a bit.
We don't know the construction of the tank and the actual dimensions but if you are concerned about popping at the seams (if it has seams):

area 2 square meters
length & breadth 1.414213562
seam 5.656854249 metres
total force 1200 kg
force per metre of seam 212.1320344 kg
force per cm 2.121320344 kg

Which doesn't appear excessive to me.
 
thanks for that Gents. I saw it as upwards of 1000kg and intuition had me running over it again and again to point were I've lost the ability to rationalise what the effect will be. The tank is stainless and bilge/wineglass shaped in section if that makes sense. The top has never had to withstand a 'pressure' before as it would always have had a free surface. Anyway its strapped across in 3 places to hold it down so we shall see if there is any deforming / sole-board lifting after the first fill !
I have another question but as its heads related I will put it in a new post!
Thanks again
 
VicS said:
the pressure = depth (head ) x density

in this case the depth is 60cm and the density of water is 1g/cm³

so the pressure is 60g/cm² or 0.06Kg/cm².

That converts to approx 0.85 lbs/sq inch ​

This then is the pressure at the bottom of the lower tank and the top of the lower tank.

The total force acting on the top of the lower tank (or the bottom of the upper tank) is pressure x area

the area in sq cm is 2 x 100 x 100 = 20,000 cm²

so the total force is 0.06 x 20000 = 1200 Kg !

That converts to approx 2600 lbs​


Unless he has corrected it while I have been typing that makes Playtime a bit out because he said 2m² is 5 ft². Its not 2 m² is approx 21.6 ft²!
Click to expand...

Vic - your arithmetic is fine. However, your assumption that I can't convert 2 sq m to sq ft is not correct. I used 5 sq ft as an example - I had not seen the 2 sq m that the OP suggested! ;)

I think you have re-inforced my comment that the total force on the tank is quite surprising. However, I agree that the force on the top seam is quite modest. :)
 
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ean_p said:
If one started with a standard keel/bilge area water tank having a top surface area of say 2 square meters, filled from a cockpit filling point and vented through a high level vent and then added a second tank with its bottom level with the lower tanks top which was to be filled through the lower tank, to a height of 600mm above the lower tank,though it would have its own vent line. What would be the pressure exerted on the top skin of the lower tank and was it likely to be a problem?
Click to expand...

VicS said:
Ah I see.
You know what they say to exam candidates. .... RTFQ... Read the full question :D
Click to expand...

Vic, what is the point you are trying to make , or maybe I should say score?

The OP posed a hypothetical question; it was phrased with "say 2 square metres". I gave an answer with a slightly different assumption about the surface area of the tank - 5 sq ft is a realistic size and I was following the previous poster using imperial (not metric) measures.

It is not an exam. All responses should be treated with respect unless they are factually incorrect and/or misleading. I don't believe mine was either!

Don't spoil your otherwise excellent posts by having a pop at someone else trying to give (what I hope are usually) helpful responses.
 
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Thanks Bosun.
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Playing Nicely, as we do,..

The short answer is that so long as the breather tube to the original lower tank works well and is of adequate size, say 12mm ID and no kinks, all will be fine.

If you really wanted to, it would be quite easy to 'blow' or rupture any water tank by connecting it to the marina mains directly at mains water pressure, which might be anything from 1 to 4 bar.
They are simply not designed to withstand internally generated pressures of that order.

Hence why we gravity feed them when filling. And always keep our vent pipes unobstructed.
 
Well ..... it seems that not all the rust is in the water tank!

Suppose for a minute that the filler/vent is 2m above the top of the bottom tank, and the water filler pipe is full. The pressure on the bottom tank top is then 2m of water.

If the second tank sits directly on the bottom tank's top there is no net pressure on the bottom tank's top : 2m - 2m = 0m head of water.

If however the second tank is not supported by the bottom tank's top, the net pressure is still 2m of water.
 
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