Safety valves

Of course, any boiler needs two safety valves, set to release at the working pressure, to avoid any risk of the boiler rupturing - heating water in an enclosed space really can get the pressure up!

Adapting a design used by W H Morewood ("Building the Raritan") and K Hiraoka ("Building the Shay"), I built safeties with adjustable release and resetting pressure in simple way - the above mentioned authors didn't bother with adjustable resetting pressure - instead, Morewood has a three-page description on how to make a jig and a reamer that together will determine the dimensions critical to resetting pressure... Why all that bother, when a threaded valve stem will make the resetting pressure adjustable?

The drawing above shows the parts needed - dimensions are not critical, except where noted - if you change these, keep the relative proportions constant. All the threads are the same size - I used what we here in Finland call R 1/8", which is a 28 tpi non-tapering tube thread, o.d. a little under 10 mm. (All my boiler fittings are R threads, enabling me to buy plugs and some other stuff off-the-shelf...) You can use any other suitable thread, by adjusting other dimensions to suit.

The machined parts are shown above, from left:

Left, the finished safety valves attached to the inner steam dome cover. I have added a cup that directs the steam blast upwards, to the top of the steam dome. I also drilled two additional venting holes, so there are now 4 holes equally spaced inside the cup. The valves are adjusted and locked so that one opens at 6.2 bars, closes at 6, and the other opens at 6.5, closes at 6.2. (6.2 bars is approx. 90 psi.) This should give an ample margin of safety for a boiler that withstood a hydro test of 14 bars (200 psi)...

Release pressure is set with the upper adjusting screw and locked with the ring, resetting pressure by screwing the valve body onto the stem, and locking it at the right position.

How to set it? I used an air compressor capable of 8 bars (120 psi) in the following way:

I was pleasantly surprised that this design worked flawlessly the first time I tried it, especially that I could adjust the resetting pressure to only 0.2 bar (3 psi) lower than the release pressure - both Morewood and Hiraoka say that it's supposed to be difficult to get that right...

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Any information presented on this website (especially any do-it-yourself instructions) is given without any acceptance of liability for damage or injury - so, always remember: SAFETY FIRST!

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