Cheap Hydrostatic Testing Chamber

[exceter]

"Cheap" Hydrostatic Testing Chamber

As background information, I'm a computer programmer by trade but I want to get into underwater ROVs as a hobby because I need a hobby that doesn't involve sitting in a chair all the time, because Bob Ballard was one of my heros as a child, and because there are a lot of fresh water lakes near my home. I've done a lot of reading about the existing state of RC submarine technology and they seem to come in the wet hull and dry hull varieties. Most people build wet hulls because they are simpler. Despite the added complexity, I want to build a dry hull because it has higher buoyancy and would allow me to carry extra batteries, servos, electronics etc. I'm in the process of researching what materials to use for the hull(PVC, steel, aluminum, etc) but to minimize the weight of the hull I need to be able to pressure test the design so that I don't risk losing my submersible/work at the bottom of a lake. The deepest lake within driving distance of my home is approximately 400ft deep so at .432 pounds per foot of fresh water depth, with a 50% safety factor, I want to be able to pressure test the hull to approximately 250psi. 400ft is the theoretical maximum that I'd ever want to go. At my skill level, that would be like landing on the moon. I may never get there but I enjoy the challenge of designing for it. Typically, my sub would operate at a depth of 20-100ft, because that's the depth of the lakes which are closest to me.

So, what I want is a cylindrical pressure vessel, that I can open from one end, which has an internal diameter of approximately 12" and a length of approximately 8' which can safely withstand an internal hydrostatic pressure of 250psi. Those internal dimensions should comfortably fit any ROV that I would ever attempt to build. I would then use a hand operated hydrostatic pump(typically used to hydrostatically test compressed air tanks) to put the desired pressure into the chamber. This would give me the confidence that my hull isn't going to buckle but, more importantly, that none of my shaft/wire seals, which penetrate the hull, leak.

Does anyone have any practical suggestions on how to build such a device?

 

[Enthalpy]

Well, yes, as I made hydraulics before.

Your chamber will cost money. Pressure means steel thickness. Sorry for that.

It will need round shapes at the ends, for instance hemispheres. You need a means to make / buy new / buy used / get them for free.

The ends must have seal rings, and the rings need a precise shape at the cylinder and the ends. Which does mean: turned parts. If you access a turning machine, fine. If not, you've lost.

The ends must be bolted to the cylinder with a force that exceeds the pressure on the ends. Which means: many big screws and a wrench.

Put together: if you have an industrial workshop, it's reasonably easy; if not, it's unreasonable.

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First solution: find an existing pressure vessel. They're rather common. For instance, balloons for hot water or for toilet flushers resist 7b and will probably resist 20b. Hydraulic pressure vessels exist in trucks, heavy vehicles... Get the part from a scrap yard.

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Second solution, which I prefer: don't spend time with a pressure vessel. Build the hull of your submarine, remove all equipment within, go with a noat to the centre of your lake, sink your hull under a line, recover the hull with the line, observe the result.

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How will you communicate with the submarine at 130m depth? For remote control for instance? Radio waves won't work.

 

[exceter]

The second solution does do a better job of following the KISS rule. My only concern with it is that I can't test the safety factor. In your first solution, how would you recommend that I add an opening through which the submarine could be inserted into the pressure vessel?

As for communication, I was aware that radio waves wouldn't work. Currently, I'm entertaining three different options.
1) Use cables which have neutral buoyancy and tether the submarine to a boat that's on the surface.
2) Use an acoustic modem which can modulate a digital signal over sound waves.
3) Make the sub autonomous.

As strange as it sounds, I think the 3rd option is the best one. 400 feet of cables/rope that have neutral buoyancy would be expensive. Acoustic modems are both low bandwidth and expensive. I can equip the submarine with short range sonar transceivers to avoid collisions with the bottom of the lake and with submerged objects. I can equip the sub with a pressure sensor to know what depth it is at. I can equip the sub with an IMU(these are available relatively cheaply) to allow the sub to approximate its position in 3D space. If I couple this with a Rasberry Pi I can write the guidance system in Java(programming is what I do for a living). My thought is that I would preprogram the sub to go take pictures of a specific section of the lake and download them when it came back. Obviously, this is a little dangerous before the bugs are worked out so my thought is to start out in lakes which are only 20-30 feet deep(to ease recovery) and to attach a line to the sub while I'm getting the system working.

 

[Pkruse]

I'd design the hull for whatever pressures I desired, and then use the lake to test it. Just weight the hull down and lower it to depth on a rope. At this point, the hull need not have all the expensive stuff installed. You are just testing the hull.

Designing for these pressures is very easy. I'd have no doubt but that my design was good for the desired pressure. What I'd really be testing would be for leaks. So your test vehicle must have realistic seals installed at all the hull penetrations.

 

[exceter]

This is a *slight* tangent but, on the topic of the hull design, one of the ideas that I've been toying with is the practicality of making the hull lighter by pressurizing the interior of the submarine to offset some of the external pressure. Is that even worth fooling with?

 

[Pkruse]

Probably not for depths as shallow as you are talking about here, at least from the perspective of hull design.

But depending on what sort of seals you select, it may be a good practice to prevent seals from leaking. If you are very concerned about leaking, you may try presurizing the system to well above your maximum working pressure. Then any leaks will be in the outward direction, and you can monitor for the development of leaks easily enough by keeping track of internal pressure.

 

[exceter]

That's a really good point about detecting leaks. That would also allow me to release additional pressure into the interior of the sub to compensate for leaks, once they were detected. HPA/C02 tanks from paintball guns(which I was planning to use in my ballast system anyway) could easily provide the necessary pressure.

Well, it sounds like my best/most productive option is to use the lake for leak testing. Thanks! I have other questions are hull design but I think I'll post them in an appropriately titled thread. :)

 

[maimonides]

You might be interested in http://openrov.com/.

 

[exceter]

Thanks! That link is interesting, although they're building a different type of submersible than the one that I'm attempting to build.

Rather than polluting this thread with submarine pressure vessel design I've started a new one: https://www.physicsforums.com/showthread.php?p=4150881

 

[Enthalpy]

Gas pressure inside would simplify the design in that the hull gets tensile stress instead of compressive one, hence won't buckle.

BUT:

- A gas burst is much more dangerous than a liquid burst
- Sealing against a gas is more difficult than against a liquid.