A question for the asylum physicists (or engineers)

[penberth3]

...Am I right in assuming that if the tank rated for 20kPa is 0.8m high, it already gets 80mbar of pressure at its bottom when filled with fuel, so it would be unsafe to pressurize it with air at 200mbar from the top, because the bottom would reach 280mbar (i.e. 28kPa, well above its max rated pressure)?
Or am I missing something again...?

Yes, you are right. An example, testing cross-country pipelines, pressure at the lowest point is the limiting factor. The theory is the same.

I think you've missed the different densities of fuel and water. 0.8m of water = 80mbar, fuel will be a bit less.

 

[MapisM]

I think you've missed the different densities of fuel and water. 0.8m of water = 80mbar, fuel will be a bit less.

Actually I didn't - see point (3) in my post #23.
But the difference is neither here nor there, in the debate context...
Thanks for the confirmation anyway! :encouragement:

 

[MapisM]

That tank rating 20kpa strikes me as surprisingly low. I'll check my equivalent data on Saturday when I'm on the boat.

Yup, I was thinking the same, because your previous comment on the need to withstand also dynamic loads on a boat, plus some safety factor, makes very good sense.
Just for the records, the similar plate on my DP shows 25kPa (also GRP tanks).
But according to your train of thought, that is still low, and it would be logical to expect MUCH more - like 40 or 50, if not even more.

Thanks in advance for checking your tanks, I'm curious to hear about them.
Though they are built in steel rather than structural GRP, as I recall?
Not that it makes any difference in terms of NEEDS for a boat tank, theoretically.
But it might make a difference in terms of technical FEASIBILITY - to make it stronger in steel, I mean...

 

[jim.howes]

I would be interested to know how you carried out the external checks. I suggested in old money 2 PSI not 2 bar(approx. 30PSI). As I see the situation there are 2 types of tanks one is portable fitted when the vessel was built and made of mild steel(heaven forbid) stainless steel ,aluminium or a grp tank and the other type of tank which is grp and is glassed in and becomes an integral part of the hull( non removable). I know it is easy for me to say and I know from past experience boat builders were never maintenance/ repair friendly but if the tank is portable would it be possible to remove or lift the tank giving you all round access and then test it? If it an integral tank not so easy but doable.

 

[jim.howes]

I am not sure Penberth3 understands the principle of a manometer but Compressed air(controlled properly) is a safe method because you have a shortish "U" tube with an OPEN end, impossible for the water in the "U" hold back 6 bar(approx. (95 psi old money) you would be lucky to attain 4psi(less than a quarter of 1 bar)

 

[MapisM]

I suggested in old money 2 PSI not 2 bar(approx. 30PSI)

Sorry for the confusion, I understood that your suggestion was in "old money" , but I jumped to bar just to say that I understand the risk of going way OTT and blow the thing, using a compressor (as opposed to your safer method). :encouragement:

Ref construction techniques, I'm afraid that the grp tank which inspired the thread is glassed in.
I suppose it's a bit arguable whether it can be called structural or not, when compared for instance to the central tank in the bottom of my boat, completely glassed inside the stringers, which for all intent and purposes makes the boat double hulled, in that section (and the same goes for the fresh water tank btw, which follows the same principle).
But portable/removable, that tank surely ain't.
Btw, I can't recall to have evet seen a grp tank which was't glassed to the hull, one way or another.
And how portable could be a 3400 liters tank, anyway...?

 

[vas]

P.,

trying to follow this thread whilst going up and down the country, not easy!

None has mentioned flexing and deformation of the bloody thing. I guess not a big issue on a glassed in tank, but definitely an issue on any stand alone metalic ones and possibly on static pumping them.

Cannot imagine these 20 and 25 silly values stamped on the Ferretti tank mean what we think they mean.
someone mentioned that slamming creates some serious values, I'd dare guess momentarilly enormous forces. Unfortunately I'm not into fluid dynamics so I cannot calculate anything but I'd doubt 3/4 full tank (say 60cm of diesel) slamming in an F5 to 6 wouldn't go 100times that value for splits...

good luck finding the pinhole (if it is) and as mentioned again, may be a hell of a distance from where the leak appears.
Isn't there some sort of material (like ground coffee was used to seal leaks in coolant circuits in the 50s and 60s) that you can dump in there and sort the leak without going to the engine and blocking filters/injectors/pumps?

cheers

V.

 

[penberth3]

I am not sure Penberth3 understands the principle of a manometer but Compressed air(controlled properly) is a safe method because you have a shortish "U" tube with an OPEN end, impossible for the water in the "U" hold back 6 bar(approx. (95 psi old money) you would be lucky to attain 4psi(less than a quarter of 1 bar)

Yes I do know what a manometer is. If you connect a workshop airline to the tank and it can only vent through a 6mm diameter manometer (after blowing out the water) you are pressurising the tank and you've got a dangerous situation. Don't mess about with compressed air.

 

[jfm]

Thanks in advance for checking your tanks, I'm curious to hear about them.
Though they are built in steel rather than structural GRP, as I recall?

Wil ldo, tomorrow. Mine are aluminium, non integral. Things of beauty, with radiused folded corners and only butt welds. There are big internal plates acting as baffles and of course they add significantly to the pressure the thing can take. My vents are at flybridge deck level and my bottom/centre tank sits on the keel, so there is 5m head before you start, if you fill them to the vents

None has mentioned flexing and deformation of the bloody thing. I guess not a big issue on a glassed in tank, but definitely an issue on any stand alone metalic ones and possibly on static pumping them.

Sure we have - its integral to any discussion on the pressure rating of the thing. It's LESS of an issue with stand alone metal tanks because you weld in baffle plates that significantly reduce deformation. Generally you can do this as you make a GRP tank too. you just do it before you stick the lid on. [/quote]

someone mentioned that slamming creates some serious values, I'd dare guess momentarilly enormous forces. Unfortunately I'm not into fluid dynamics so I cannot calculate anything but I'd doubt 3/4 full tank (say 60cm of diesel) slamming in an F5 to 6 wouldn't go 100times that value for splits...

No special maths needed. Pressure is gravitational. If you know the boat is decelerating at 3g when it hits the sea then you need a x3 factor. Likewise, while the boat is in free fall there is no pressure (relative to the atmosphere) at all in the tank.

 

[MapisM]

Wil ldo, tomorrow. Mine are aluminium, non integral. Things of beauty, with radiused folded corners and only butt welds. There are big internal plates acting as baffles and of course they add significantly to the pressure the thing can take. My vents are at flybridge deck level and my bottom/centre tank sits on the keel, so there is 5m head before you start, if you fill them to the vents

Tx in advance, I'm indeed curious to hear about the rating of your tanks.
As well as from any other which could contribute, maybe with other builders' boats, of course.

I'm not sure to get the point of bringing the vents up to f/b deck level, though.
Btw, I suppose it's impossible to fill the hose up to that point - aren't the filler(s) point(s) somewhere along the main deck...?

PS: ref. internal baffles, as I'm told there are some also inside the GRP tanks in my boat (and I suppose also in the Ferretti).
But how can they contribute to the pressure that can be taken? Well, why significantly, anyway?
My understanding is that those baffles are vertical, and meant to stop fuel from sloshing around.

 

[jfm]

I'm not sure to get the point of bringing the vents up to f/b deck level, though.
Btw, I suppose it's impossible to fill the hose up to that point - aren't the filler(s) point(s) somewhere along the main deck...?

PS: ref. internal baffles, as I'm told there are some also inside the GRP tanks in my boat (and I suppose also in the Ferretti).
But how can they contribute to the pressure that can be taken? Well, why significantly, anyway?
My understanding is that those baffles are vertical, and meant to stop fuel from sloshing around.

Fillers are on side decks, but on the sides of the saloon at your waist level. Thus it makes sense to take vents to flybr level. Also helps stop foam coming out of the vents, a bit.

Baffles contribute very significantly to the pressure that can be taken. Sure they stop fuel sloshing, but that's almost a side show. They also serve as cross braces to the tank structure. They're Usually bonded to bottom and sides of tank and possibly the top as well, depending on construction method. I'm surprised you're asking - it's kinda obvious that they play a huge role in strengthening the tank and stopping it bulging/ballooning, isn't it?

 

[MapisM]

Doh, yeah, 'course I can see why they can be concurrent in making the structure stronger - I was just thinking along the lines of how they can reduce the pressure created by the fuel inside...

PS: fwiw, as I'm told in my DP they are glassed on all sides, with just a small(ish) opening at the bottom, where the V of the hull is, because of course the fuel has to flow to the lower part of the tank where the senders are attached, eventually.
But don't ask me how they built them...
...though I can ask, if you're interested.