Your sightglass observation seems right on. It would have to shatter, releasing all pressure before any boiler explosion!

Everything seems slower in August. The weather is good, the city has a relaxed atmosphere and somehow, sitting indoors in front of the computer is less attractive than it is in February. I do have some progress to report however. First, the brew reservoir is pretty close to completion. I tack welded on a couple of pretty hefty lugs that will carry the whole load of the boiler when it is attached to the frame. Then I brazed the G1/4" fitting in place. The fitting, by the way, was cut and tapped on the lathe from lead-free Ecobrass hex stock - which, though a little more difficult to machine than leaded brass, turns out to be pretty good. I have done quite a bit of soldering in the past but never any silver brazing - so I was exited to see how this would work. The white gunk in the before shot is flux which prevents atmospheric oxygen reaching the metals in the joint and oxidizing them as they are heated. Using an oxy-acetylene torch, the base part is heated evenly until the assembly reaches the melting point of the brazing metal (~1145 to 1650°F (618 to 899°C) depending on the brazing alloy). Just like with soldering, once the right temperature is reached, the liquified braze is drawn into the joint by capillary action. The hard part is actually to not overheat the joint - the oxy torch is a powerful beast - that and the cleanup afterwards which is a royal pita. This part just needs the final structural welds on the lugs and then it can be welded to the boiler tube.

So, on to the boiler tube!

This is a piece of 6" schedule 40 316 stainless pipe. It looks quite innocent and happy on the lathe in the picture. Cutting it, however, was not so easy! It is held at one end nicely by the six-jaw chuck from the inside. The other end, however, is completely unsupported. As it isn't even sort of round, it can't be clamped externally, so the only way to hold it is with an elaborate internal expanding plug - which would take a day or so to make. As this is a part which will likely be laser cut when I need more of them, I didn't want to go there. So, with the lathe on dead-slow and taking extremely light cuts (0.001" per revolution) with a carbide parting tool I cut it unsupported. As the setup is not even sort of rigid, the tool tends to chatter a bit, which has the effect of resonating and ringing the tube like a bell. REALLY LOUD! Even with heavy ear protection, I could feel my back teeth buzzing. Still, it did a good job and the end result is a very clean edge about as perpendicular to the walls as you can get.

Good job! Cutting unsupported work is difficult, and dangerous.

I have some high density Styrofoam that I use for making temporary plugs and jigs. It is 40#/cu.ft. so is similar to red oak in density and it turns well also. Turn it to make a big bullnose and slid it onto the small bullnose of the lathe.

Thanks for the interesting build info.

I would always use a pressure relief valve and not rely on some other part blowing out before the boiler does. Also think about where the hot water will be directed (at a person is not a good answer).

I expect that the thickness of stainless or copper is chosen to yield at roughly the same pressure.

Small bore piping and sight glasses will withstand over pressure events far in excess of boiler max code test pressures. The sight glass tube is usually the first to fail though in an over pressure event. The problem is in most but not all jurisdictions these small capacity low pressure boilers are not required to be certified so it is up to the seller and buyer to do their homework.

In the event of a sight glass failure due to an over pressure event there will be minimal to no liquid ejected, only vapor at high velocity and it will strip skin and muscle tissue off of bone almost instantly.

The normal human response when something like this happens is to inhale rapidly, end result is cooked lung tissue.

I think that this is what the insurance folks are worried about. Ah, pressure vessels, gotta love em.

http://www.bbc.com/news/uk-england-hampshire-11320258

Typical news article, no technical content at all. It is real hard to fail a spring relief valve. Obviously someone figured out how to do it.

+1, and if they did fail it would be more likely to fail into an open position with a broken spring.

Ok, the weeks just fly by. I have been busy with this project but don't seem to have found the time to post the progress.

I'll start here:



The latest batch of laser cut parts are two new ones, the stainless components that serve as bearing surface and lateral guidance for the bearings and a duplicate of the boiler flange as I was unhappy with the welding I did on the first one.

The first part is the bearing guide made from (roughly) 2mm 304 stainless. The slit is used to locate the bend line and to significantly weaken the part for bending as this would require a monster machine otherwise. Industrial origami! Prior to cleanup:



The parts are held in the vice using a couple of pieces of brass shim to elevate the fold line to the correct distance above the edges of the jaws.



A couple of taps with a heavy hammer using a scrap of aluminum to evenly spread the load and to avoid marring the surface and presto, instant folded parts without the $100k pneumatic press.