Diving Tank 4

[CostalGuy]

hello all, this is my first post after checking this forum for help several times. I stink at coming up with names so I am stuck with the current one... I am doing a side project for a volunteer organization I belong to and the grand scheme of things is this: We have a 1100 gallon poly water storage tank (5'-4" diameter, 7' tall) and the idea is to modify the tank to put windows in it so outsiders can observe SCUBA divers inside. See the attached sketch for the concept idea, note that the tank size has gotten much smaller.

the plan is to install 3'x3' windows at between 13" and 4' down from the top of the tank. I may have overthought this problem but the total force I have on the window would be about 1,500#, Scaling things up for a safety factor has me analyzing the windows at 5 psi (double the pressure); the resulting hoop stress for a 1/4" thick tank is 160 psi. Polycarbonate has a tensile strength of 9,500 psi, so I am not worried about that.

I was anticipating fastening the glass with 2 rows of 1/4" ss fasteners at 3" o.c. staggered and was curious to see if I could estimate the block shear strength based on the thickness of the materials. Would that even be a concern? I feel like I am overlooking something.

The material sizes and loads all seem kind of small for the application but I am also only holding back worse case scenario of 250 psf

any sort of insight is appreciated as I don't want to spill potentially 500 gallons of water

 

[TLHS]

I would likely just try to put the window on the inside of the tank with a gasket in there, and a few fasteners to compress the seal. Then I'd see if I could put some ratcheting shipping straps around the tank. If they're a couple of inches wide you could likely put a couple over the window and figure out a way to make them look like slightly classier framing, and put one just past the window top and bottom.

You pretension them and check the capacity against the tributary ring tension, then you do a real quick check that your tank material can span between them in bending.

Cheap and dirty, not complicated math, you pretension so your load path should be fairly controlled.

You could do something less visually intrusive by running a wire ripe or other strap above and below the window and then placing periodic vertically oriented beams under them to get load to them. Load path would be shell -> vertical beam -> ring tension in wire rope/strap. Something like the sketch I'm showing, where blue is your straps and green is wood blocking or something. You just check that the tank material and the window can span between the blocking. Still pretty easy to build without fancy techniques. If it's ugly, you can get some flexible plastic sheeting and curve it around over the whole thing, then finish it nicely.

Trying to do something where your fasteners between your window and tank restore the ring tension load path seems really annoying from a practical standpoint if you don't have experience working with these materials. You're also likely going to have to troubleshoot water-tightness by switching things up a few times, so making that joint integral to load carrying will complicate that.

 

[SwinnyGG]

Good lord y'all are putting a lot of effort into something that requires almost none

 

[LittleInch]

Why do you think that?

The OP is taking a huge chunk out of a plastic vessel holding a considerable amount of water and replacing it with a different material which may have a low pressure but a big area, hence a lot of force, bending a material hence imposing considerable stress or possibly changing its properties and a square edge hole.

Failure could release a considerable water volume onto members of the public, potentially including small children and a diver being ejected from this vessel. The tank will have forces repeatedly imposed on it from fill and drain cycles and transport plus adding new frames or steps.

Hence a reasonable amount of forethought and checking seems justified to me

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[SwinnyGG]

Why do you think that?

Look at any picture of a dunk tank with a clear window - which is a similar diameter vessel with a similarly sized square window at a similar water depth and ask yourself how many hours of calculation went into that design. Then ask yourself how many times you've heard of horrible child deaths resulting from a dunk tank window exploding out of the plastic dunk tank and dumping 200 gallons of water on the ground.

The failure mode for this is a leak that starts pissing water out of one of the seams - it isn't an entire small town being killed and maimed by shards of exploded lexan

 

[LittleInch]

Fair enough - I've never actually heard or seen one of those before now, but all the pictures appear to be remarkably similar design and manufacture. Hence why I've not see any failures as I've never seen one before now, but maybe these are more common in the US and Australia?

I actually think they did do quite a lot of design and have clearly got materials which are designed and manufactured to fit and of suitable thickness and strength. We have no real idea what this see through material is that is being proposed

They all also seem to sit flat on the ground, hence well supported and not on some home made structural support frame.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[SwinnyGG]

Dunk tanks of the carnival variety are relatively common-ish in the US... this is the type of thing that gets carried around in a van by carnies and gets zero maintenance for about 30 years and is generally fine. Manufacture is by whatever company in China can build it the cheapest from the least expensive, uncertified, unknown material. Still fine.

I think everyone thinking this requires in depth knowledge of material properties and detailed calculations is over estimating the consequences of a failure by a factor of about 100. If I was building something like this, I would never touch my calculator. The material (which do know plenty about for this application) that OP is going to use for the window is just about as strong as the base tank material is, and the base tank material has a safety factor of probably 500 when filled with water. When was the last time anyone heard of a standard poly agricultural tank which is full of water suddenly bursting? Never.

 

[LittleInch]

Dunk tank is clearly strengthened at the top rim to allow for lack of a top dome.

The OP is modifying quite dramatically a proven tank design which uses all the benefits of a circular barrel and two end domes.
So no a "standard poly tank" will last virtually for ever. A heavily modified and cut up tank? Different question.

That has some risks involved for what is a one off DIY modification.

Are those risks low enough to allow its use in a public arena?

Or even on moving vehicle with someone inside? "I may go as bold as setting it on a dual axle trailer to use in parades as well."..

So will it work? Probably, but there are some practical issues to solve around the flexing of the clear window I think and giving this some more serious thought than not seems like a good idea to me.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[canwesteng]

What risks are there? Seem like none at all to me. Who cares if it pops and dumps water on the ground.

 

[LittleInch]

Small children?
People with buggies?
Senior citizens?

This is going to be off the ground remember.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[dvd]

That tank is obviously used. Assuming that it is a cast-off - there is no way of knowing how much UV degradation, high/low temperature exposure, chemical exposure, etc. to which it has been subjected.

Apparently, poly tanks do fail - I would not put myself on record as stating that I would not calculate stresses. Even a small amount of diligence to CYA seems prudent.

How to check poly tank condition using three different methods

Time to Check Your Poly Storage Tanks

 

[SwinnyGG]

Small children?
People with buggies?
Senior citizens?

You're right, all those people are in extreme danger of instant death by getting their feet wet

 

[3DDave]

Shades of the Great Molasses Flood.

 

[IRstuff]

You're right, all those people are in extreme danger of instant death by getting their feet wet

No, a flash flood, however small, can knock over someone, who could then fall on crap that's lying around and get bone fractures, or concussions. My wife fell on a relatively low speed stop on a ski slope and dislocated her shoulder, which will likely cause a total of 6 months of not being able to use her arm. Falls are not always harmless.

TTFN (ta ta for now)
I can do absolutely anything. I'm an expert!

https://www.youtube.com/watch?v=BKorP55Aqvg

faq731-376 forum1529 Entire Forum list

http://www.eng-tips.com/forumlist.cfm

 

[LittleInch]

SwinneyGG,

We clearly disagree about the consequences of sudden failure and the probability of same.

That's OK. I'm just putting it out there so the OP can make their considered judgement on this based on a DIY, one off, significant modification of a used PE tank.

Putting yourself at risk is one thing. When you do it to others with no skin in the game you need to be about 1000 times less risk. 10^-3 to 10^-6.

As for getting any form of public liability insurance, I can't see anyone providing it which might be a deal breaker?

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[CostalGuy]

I am in the middle of both sides of the argument:

In a perfect world, without weakening of the plastics due to deterioration or abrasion;

1) The deformations from filling/emptieing will be present on a 'discontinuous' assembly material such as I am proposing. However by introducing similar property and thickness material, and not changing the original shape, i.e. similar thickness lexan and keeping the shape cylindrical then that limits the failure to the connections between the materials.

2) It seems to me that the major failure issues would be tearing of the plastic at the window opening. But by reinforcing the openings with fiberglass, that should change the properties at the window opening and negate the weaker plastic in turn for fiberglass

While I agree with SwinneyGG that in essence this will be a dunk tank, similar to a cheap Chinese made, tow behind a truck and heavily neglected carnival style dunk tank, I also have to agree the a sudden release of 500 gallons of water is bad, especially when you elevate the opening almost 4 feet above the ground with a crowd surrounding the tank. There is a reason that carnies have a reputation.

I could reinforce the platform with open diamond plate sheets to prevent bulging and deformation of the tank itself; however this is my course of action:

cut the top off, reinforce the lid with Azek and fiberglass, cut the window openings with a radius (hole saw in the corners), reinforce the window openings 4" all around with fiberglass, install the windows from the interior and fasten with adhesive AND sealant with stainless steel fasteners with fender washers.

At that point, I could always get a better understanding of how stiff it is by "pushing on it" and seeing how the deflection is effected. Also, I can always check it before final assembly by taking it to a pond, using a trash pump and filling the tank (would take approx 20 minutes). If all goes well, then routine checks of the tank will be required. I also plan on painting the tank so that will give it some UV protection as well.

 

[LittleInch]

I think you just need to try building this thing and see what happens....

Fill it to the brim for testing and then allow some lee way when using it for the volume of the driver and equipment but still have some free board.

My concern is still over the flexing of the window but maybe it will be fine.

Just be sure to send us some photos....

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[BridgeSmith]

I suggest including dynamic effects in the testing - maybe drop a 100 lb sack of potatoes, or a 32 gallon drum full of water, or something into it when it's full of water.