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

 

[SwinnyGG]

Make the panel whatever size you want.. when you heat it up enough it's going to drape over the outside of the tank just fine. Radius this size, and with the tank and window being slightly flexible, the difference in radii between the inside and outside of the tank isn't going to matter.

 

[LittleInch]

OR just limit the size of the window to say 6 to 8 inches then you might get away with a bit of local deformation of the shell

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

 

[phamENG]

Let's back up a second. What is a diver going to be doing in that tank? Demonstrating equipment? It's not big enough to move around in. It'll be more like one of those preservation tanks in a sci-fi or super hero movie.

 

[BridgeSmith]

If you do the math it's actually 1.5 psi.

Ok, at 3.5' depth, which is approximately where the window is, that's about right. I was using the full 7' height, plus some dynamic effects. Oops.

 

[rb1957]

so form the window on the shell, then cut the hole. Make sure the width of the window is less than the diagonal of the cut-out.

"Hoffen wir mal, dass alles gut geht !"
General Paulus, Nov 1942, outside Stalingrad after the launch of Operation Uranus.

 

[SwinnyGG]

Let's back up a second. What is a diver going to be doing in that tank? Demonstrating equipment? It's not big enough to move around in. It'll be more like one of those preservation tanks in a sci-fi or super hero movie.

From OP's description, it sounds to me like this tank will live on a trailer and they'll roll it out to county fairs and the like so they can put a guy in it to say 'hey check out this scuba dude, also please donate to our nonprofit'

 

[rb1957]

sort of like a Guild Navigator in Dune ?

"Hoffen wir mal, dass alles gut geht !"
General Paulus, Nov 1942, outside Stalingrad after the launch of Operation Uranus.

 

[stevenal]

The curved window/water will magnify the diver. Might scare the kids.

 

[dvd]

Polycarbonate sheet has a forming temperature that is pretty high compared to what a polyethylene tank can withstand. I think that forming the window on the tank/cutout will result in unnecessary lessons learned. Also have reservations about the access ladder and platform as-drawn. The flexural stability of the bolted window joint seems dubious. This whole thing sounds like a lot of effort to find out if the public will be interested.

postscript - how long will the tank take to fill and drain? Are you hauling it full?

 

[CostalGuy]

Thank you all for the continued feedback; to answer questions/interject my own thoughts:

1) Yes the proposed loading is minimal so evaluating at 5 psi should cover the fudge factor. with that, the tank will have enough rigidity to it and I will reinforce the openings to add stiffness to it

2) The proposed window will be a 3'x3' but the plan is to install the window on the inside of the tank with an overlap of about 4 1/2" all around for fastening and sealing. My plan was to drill the fastening holes in the lexan, cut the hole for the window and use the lexan as a template to transfer the pattern to the tank. 1/4" lexan is flexible enough to cold bend the circumference with a little effort.

3) SwinnyGG has hit the nail on the head: The purpose of the tank is to display gear in action at our station, fundraising events, carnivals etc. I may go as bold as setting it on a dual axle trailer to use in parades as well. The fire is to get the tank ready before September when we have a marathon dive. Personally, I am aiming to dive for 4 hours continuously (imagine a walk-a-thon but underwater). I figure with that much time on my hands, I would finally figure out the patterns of a Rubik's cube.

4) the tank will be empty 90% of the time; it is intended to have a firetruck fill it and pump the water out when we need it filled/drained. For liability sake (and to keep the tank clean) it will be stored without water.

5) SwinnyGG again, hit the nail on the head: I am overthinking this, a dunk tank is similar construction with the same loading on it basically. I am not used to dealing with pressure vessels or plastics as materials at all so I want to make sure that the design intention is not out of this world; you'll notice that the steel platform is actually DESIGNED as that is my strong suit.

I am waiting on some money to come through as cutting into the tank will not be until after the tank base, access platform, and entry ladder are fabricated and fastened to the trailer itself. Again, thank you all for the input and for talking me off the ledge and giving me some tips on how to build this thing.

I will post updates on this when fabrication gets off the ground. Thank you all.

 

[rb1957]

would it be "better" (serve the purpose of seeing the diver better) to make the tank from 3 or 4 sectors of "window" joined together with (say) extrusions ?

"Hoffen wir mal, dass alles gut geht !"
General Paulus, Nov 1942, outside Stalingrad after the launch of Operation Uranus.

 

[CostalGuy]

would it be "better" (serve the purpose of seeing the diver better) to make the tank from 3 or 4 sectors of "window" joined together with (say) extrusions ?

I would think that having half of the tank as a window would be more than ample for the intended use. Also, half of the tank will be obstructed with the access stairs. I think that by reinforcing the cut edges with fiberglass that it should stiffen the tank significantly and by replacing the materials in kind that it should not effect the overall tank stiffness either.

Maybe the 'next' test tank that I design will have that built into it so that I can practice underwater welding a little more comfortably

 

[LittleInch]

You don't mention how exactly you're going to form the curved window you need?

To the exact dimensions you need. Prob within 5nm. The poly c is a lot stiffer than the tank.

Or get rid of the sharp corners.

But solve the issue of the curved window and it might just work. IMHO.

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

 

[3DDave]

I would likely contact some plastics forming companies to see what they might do. It would be simple to make a plywood form with a thin polished metal surfacing attached to keep the surface smooth. It could be a male mold for hand / drape forming and either a male or female mold if they want to vacuum form.

The trick is getting a large enough heating oven to get the polycarbonate uniformly soft and separately to ensure the polycarbonate is very dry to avoid forming steam bubbles while heating.

nm? Nautical miles or nanometers?

 

[LittleInch]

I do worry though about fatigue around the bolt holes and the PC sheet itself.

Multiple fill and empty cycles plus being transported around on a trailer is going to lead to fatigue issues - but knowing when is going to be problematic.

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

 

[SwinnyGG]

If it takes 1000 cycles for a fatigue crack to appear, I bet that's probably 250 years of life

 

[CostalGuy]

You don't mention how exactly you're going to form the curved window you need?

I would likely contact some plastics forming companies to see what they might do. It would be simple to make a plywood form with a thin polished metal surfacing attached to keep the surface smooth. It could be a male mold for hand / drape forming and either a male or female mold if they want to vacuum form.

I had anticipated just pushing it into position, 1/4" thick lexan is pliable enough to match the circumference of the tank. Being on the inside of the tank, the hydrostatic pressure should form the curve as well when filled. By reinforcing the window openings I am hoping to have the stiffness differences of the 2 materials not matter. Also, I thought about laying the tank on it's side, and "draping" the lexan over the top of it and bending with some heat.

I watched a video on youtube many years ago about someone who made their own backyard roller coaster from PVC pipe. In the vide he gets the pipe to bend into nice curves by filling the pipe with hot sand and then putting it into a mold. Maybe I can replicate the process if the curve becomes too difficult to make

I do worry though about fatigue around the bolt holes and the PC sheet itself.

I anticipate the bolts themselves will have a 'cushion' of sealant around them so direct stainless steel to PC interaction will have a buffer between them. Additionally, like a previous commenter discussed, treating the connection as a slip-critical type means that you rely on the compressive forces given by the bolt on the faying surfaces of the materials. So really, you rely on the friction between the plys of the materials and not just on the bolt bearing on the poly. In essence what I am proposing with the double layer of bolts is a nearly a 4" wide band of clamped adhesive all the way around the window openings.

 

[dvd]

I don't think you can have a slip critical connection with elastomeric material in the plies.

This tank is a one-off and will probably take some tinkering with. It sounds like there are some dissenting voices, but you are patiently discussing those. I hope you will give a final report.

 

[CostalGuy]

I don't think you can have a slip critical connection with elastomeric material in the plies.

elastrometric is a new word for me! I would consider an un-reinforced connection to be elastic, however, I am planning on reinforcing the openings with fiberglass and now anticipate making a fiberglass frame for the windows not as well.

It sounds like there are some dissenting voices, but you are patiently discussing those. I hope you will give a final report.

this is the whole reason that I am bringing this concept to the forum; I want to know what I could possibly be overlooking in regards to the deformation/performance of the tank. I think that I will be posting updates as they happen or at least a final product so stand by!

 

[LittleInch]

That would be good to see.

If the material is flexible enough to form a curve and seal against the tank then it is either going to be fairly highly stressed before you add water or is going to bulge out as the water level increases. Or both.

But then you run into the stress concentration effect at your corners. If you do nothing else make those square corners have a radius of at least 6 inches or a foot.

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