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HDPE floats

A project log for The Metabolizer

A living recycling center that eats trash and sunshine and poops recycled plastic objects

sam-smithSam Smith 09/20/2018 at 19:040 Comments

In order to collect and store the gas produced by the metabolizer's reactor, I designed this floating-drum style gas collector using two plastic barrels. I had originally seen this concept used for storing biogas from anaerobic digestion, and I was reminded of it back in June when Hackaday posted about YouTuber [NightHawkInLight]'s gasometer build.

The principle is pretty simple- people have been using this tech since the early days of coal gas to store low-pressure gas. It works sort of like a battery or capacitor stores and smooths out electrical demand- it accumulates a gas that is produced slowly or intermittently, so that it can be used all at once, on-demand. It's low-pressure, pretty safe, and cheap and easy to build.

[project link for photo]

One problem with the floating drum approach is that if the inner barrel gets too full of gas, and isn't supported in some way, it can get kind of tippy, which both looks bad (IMO), and can potentially lead to the gas escaping from the barrel. I wanted to do something to make sure that didn't happen. One night I had the idea that maybe I could use bulkhead fittings, and two different sized pipes, so that the drum could float up and down along a central pipe, keeping it neatly upright, and making the whole thing look cleaner and more stable. Here is the original concept drawing I did that night (sorry for the bad contrast):

A few weeks ago, I built a first prototype, and was pleased with the results. I was able to use 2 different sized threaded bulkheads (1.5" and 2"), which then thread to correspondingly-sized pipes (ABS, PVC, HDPE are all fine and very common in the waste stream. I got mine used at Rebuilding Center), and the two sizes sleeve together nicely. The smaller pipe goes into the bottom of the bottom barrel, the bigger one goes onto the top of the smaller floating barrel, turning it effectively into a big floating donut, and when the floating barrel fills with gas it slides up and down on the inner pipe, keeping it from tipping over, even when really full. This also increases the effective storage capacity slightly. Then I cut a big hole in the original barrel top, which stabilizes the inner barrel from the sides, and I think it looks cool.

Just the other day, I build a second iteration, making some minor improvements on the first. On the first attempt, I had tried to use the 2 bung fittings that the barrel already had as the two gas input/output holes, which seems intuitively obvious at first- why cut new holes when there are already 2? But I found them hard to seal, and they required special fittings from the hardware store. I ended up having to cover them in silicone caulk in order to get them air tight- and air-tightness is very important in this case. 

Not only do you not want to lose your gas, but wood gas also has a component of Carbon Monoxide, which can be toxic in large quantities. Best to not have any leaks. And additionally, if O2 can get into the barrel, the mixture inside can become combustible. The barrel-in-barrel design is such that if the gas does happen to combust/explode (which is rare), the barrel will simply pop off, dissipating the energy and preventing a true explosion. That's an inherent is a fail-safe of the design, which is cool I guess, but ideally we'll do everything we can to prevent that from happening. So, the point is, you want your thing to be truly gas tight.

So on my second iteration, I just flipped the inner barrel upside down, and used 1/2" bulkhead fittings I got at my local hardware store, which worked waaaaaaay better, and was way easier to do. This version was perfectly air-tight (the barrel doesn't fall at all after being filled with gas). I left the original bungs open on the submerged side, and now they just allow water to pass in and out from the bottom. Much better! 

One neat side effect of all this is that it makes a water-tight through-hole that goes through both barrels, right down to the ground. When the barrel is filled up with water, it ends up being several hundred pounds, so if you stick a piece of conduit or pipe into the center pipe, the weight of the water inside the barrel holds it perfectly upright, making a pretty secure anchor without actually having to attach anything to the ground. I'm curious if I can use this effect to anchor some sort of deployable structure or shelter, but that part comes later. For now, I feel pretty good about filling this with gas! Onward!

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