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Popular Mechanics Boiler

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vtsteam:
I decided to take a detour before going farther on flash boilers and make a little boiler I've always been curious about. It was featured as a project in a 1963 Popular Mechanics magazine, and more recently there was a U-Tube video of one:

vtsteam:
I had most of the materials to make it on hand. Nothing is particularly difficult about the construction, except the original safety valve. This was shown in rather vague detail. And the builder in the Youtube video substituted what looks like a commercial valve.

I have one I can use as well, but I was curious about building the version on the plans. It looked to me like it wouldn't work mechanically, so I figured I'd try to build it to see, and then substitute the purchased safety when ready to actually steam.

So I built the thing today and I was right. It can't possibly work as designed. I kind of wonder how many original readers attempted it and found it was whacked!

Sorry I don't have any photos until tomorrow, but to describe it, it's a weight and lever relief valve, that uses the head and smooth shank portion of a #8 wood screw as the lifting plug. This fits into a hole in the boiler pipe with "only 1 or 2 thousandths" clearance. This in turn is soldered to a pivoting lever arm, which gets a 1 ounce weight on the end.

Well, of course, there is no way the plug can lift without jamming, since it's solidly fixed to the lever, rather than pivoted.

This failed device got me curious about safety valves, and though I have a ton of reading material about making them in Model Engineer, and many books, I also like to work out simple calculators for myself.

I had a question about what the pop off pressure of this particular valve would be (if it had been workable), so I decided to write a simple spreadsheet calculator (attached below). 

A #8 wood screw has a .164" diameter shank, so I used that in the spreadsheet, along with other plan dimensions.

(to be continued)

vtsteam:
Here's a the relief valve. Notice it says to solder the lever to the valve. Actually the arrow makes it almost look like the valve is soldered shut! Also note no dimensions for the lever other then overall length. And the width is indicated at 1/8", which also can't be correct. The weight is also specified as 1/2" brass rod -- well maybe that's depth but not the rod diameter!

Whoever the draftsman was for this project must have come in to work bleary-eyed the morning after, Maybe the copy editor had joined him!

vtsteam:
Here's the first trial fit. Nothing has been screwed down yet. The relief valve is immovable as specified (silver soldered to the lever arm and set in a hole with .001" clearance.) I didn't actually use a wood screw, instead I turned a valve from brass stock.




Later in the day I unsoldered the parts, and re-grooved the valve. It now lifts the lever freely, though there is no yoke to capture it if it lifts too high, and the part could be lost.

Using the spreadsheet I wrote earlier, I was able to calculate the the pop off pressure of the design as drawn was probably under 10 PSI.

vtsteam:
I don't particularly like the hole in the pipe body being the valve seat, and the sub 10 psi relief is lower than I'd like. That is with the 1 ounce weight out at the furthest notch. The only solution is more weight, or a smaller hole. More weight starts to get cumbersome -- it might contact the boiler side.

I decided I'd prefer .125" to the .164" present bore . so I started to make a replacement screw-in valve seat of brass. I can then drill out the present valve hole, tap and mount the seat. The seat was threaded for 1/8" pipe taper.

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